Livio Ruffine
email: livio.ruffine@ifremer.fr

Anh Minh Tang

Nick O'neill

Laurent M A A Toffin

Jean-Daniel Daniel Paris

Jinhai Yang

Peer Fietzek Valentin Georgiev

Michela Giustiniani

Environmental challenges related to methane hydrate decomposition from climate change scenario and anthropic activities: State of the art, potential consequences and monitoring solutions

Keywords: Continental margins, gas emissions, methane flux into the atmosphere, 41 natural gas hydrate deposits, permafrost and slope stability

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

magnitude and the duration of hydrate decomposition, the amounts of methane released 32 can affect to varying degrees the seafloor and the microbial communities that sustain the 33 methane cycle and regulate its transfer from the sediment to the water column; and that 34 aspect is addressed in this review. Here, we also considered the transfer of methane from 35 NGHDs and more broadly from marine emissions to the atmosphere, as it is assumed that 36 such transfer will likely increase in the future. Finally, multi-scale monitoring in space 37 and time is a key element to evaluate the impacts of natural and anthropic perturbations 38 by the development of excess pore pressure that, in turn, reduces the mechanical strength 199 of the sediment [START_REF] Le | Kinetics of methane hydrate formation and dissociation in sand sediment: 1604[END_REF][START_REF] Nguyen-Sy | A model to predict the elastic 1773 properties of gas hydrate-bearing sediments[END_REF][START_REF] Sultan | Detection of free gas and gas hydrate based on 3D seismic data and cone 2049 penetration testing: An example from the Nigerian Continental Slope[END_REF]. In the rest 200 of this section, we will focus on the consequences of hydrate decomposition as this 201 process can trigger sediment mass flow [START_REF] Sultan | Effect of gas hydrates melting on 2036 seafloor slope instability[END_REF], which can be prodigious in 202 scale and generate damaging tsunamis on open continental slopes [START_REF] Talling | Large submarine 2058 landslides on continental slopes: geohazards, methane release, and climate change[END_REF].

204

In nature, hydrate dissolution is more widespread than dissociation because dissolution 205 does not require the significant amount of energy needed to take the system outside its 206 stability field [START_REF] Ruppel | Timescales and Processes of Methane Hydrate Formation 1905 and Breakdown, With Application to Geologic Systems[END_REF]. It can result from the shutdown or the slowdown 207 of the gas flux supplying the NGHD, or an increase in either the pore water salinity or the 208 bottom water temperature. Very few works mention explicitly the consequences of 209 hydrate dissolution on the seafloor. [START_REF] Sultan | Hydrate dissolution as a potential 2042 mechanism for pockmark formation in the Niger delta[END_REF][START_REF] Sultan | Pockmark formation and evolution in deep water Nigeria: Rapid hydrate growth 2033 versus slow hydrate dissolution[END_REF] showed that the formation 210 of pockmarks in the Nigeria deep-water and the evolution of their morphology are 211 strongly related to hydrate formation and dissolution. The observed pockmarks can be 40 212 m depth and nearly a kilometer in diameter, making the installation of offshore oil and 213 gas production facilities in their close vicinity a risky task. The dissociation of natural 214 hydrates requires severe and more rapid changes in the (T, p) conditions. It can result 215 from an increase in the bottom water temperature or a decrease in the hydrostatic pressure 216 [START_REF] Andreassen | Massive 1231 blow-out craters formed by hydrate-controlled methane expulsion from the Arctic 1232 seafloor[END_REF][START_REF] Marin-Moreno | Estimates of future 1683 warming-induced methane emissions from hydrate offshore west Svalbard for a range 1684 of climate models[END_REF]. Most of the existing articles on the 217 impact of gas hydrate decomposition on the seafloor mentioned hydrate dissociation 218 explicitly.

220

The role of gas hydrate dissociation on submarine slope instability has been investigated 221 over decades but controversy persists [START_REF] Collett | Geologic implications of gas hydrates in the offshore of India: Results of the National 1324 Gas Hydrate Program Expedition 01[END_REF]Hassanpouryouzband et al., 222 2020). Dillon and Max (2000) have argued that dissociation of gas hydrates can be 223 considered as the main cause of submarine slope failure only when the three following [START_REF] Bugge | The storegga slide: Philosophical Transactions of 1300 the[END_REF]. The slide was most likely 231 triggered by a strong earthquake and developed as a retrogressive slide, where massive 232 sediments of about 150-200 m thick and up to 1030 km wide slid down over ~200 km, 233 causing huge tsunamis (Bryn et al., 2005). Besides, data presented few years ago (Geissler 234 et al., 2016) strongly indicates that gas hydrates, free gas and excess pore pressure may [START_REF] Lee | Timing of occurrence of large submarine landslides on the Atlantic Ocean 1609 margin[END_REF]. Gas hydrate dissociation during deglaciation phases 239 was also mentioned as one of the possible climate-driven changes that significantly 240 affected slope stability in the Mediterranean Sea [START_REF] Urgeles | Submarine landslides of the Mediterranean Sea: Trigger 2091 mechanisms, dynamics, and frequency-magnitude distribution[END_REF]. In Since the early works of [START_REF] Mciver | Role of naturally occurring gas hydrates in sediment transport[END_REF], several studies have argued for a causal link 261 between gas hydrate dissociation and the occurrence of submarine landslides (Bünz et al.,262 2005; [START_REF] Dawson | Relative timing of the Storegga submarine slide, 1347 methane release, and climate change during the 8.2 ka cold event[END_REF][START_REF] Elger | Submarine slope 1384 failures due to pipe structure formation[END_REF][START_REF] Grozic | Interplay between gas hydrates and submarine slope failure, Submarine mass 1437 movements and their consequences[END_REF][START_REF] Mciver | Role of naturally occurring gas hydrates in sediment transport[END_REF]Mienert et al., 263 2005). Such a link was supported by theoretical considerations based on the loss of 264 cementation coupled with the release of free gas, giving rise to the development of excess 265 pore pressure when changes in environmental conditions are no longer favorable for 266 stable gas hydrates [START_REF] Sultan | Effect of gas hydrates melting on 2036 seafloor slope instability[END_REF][START_REF] Tinivella | Variations in BSR depth due to gas hydrate stability versus 2075 pore pressure: Global and Planetary Change[END_REF]. Indeed, the 267 dissociation of hydrates at the basis of their stability field could liberate free gas and 268 increase the pore pressure just beneath the Bottom Simulating Reflector (BSR), and 269 consequently decrease the effective normal stress [START_REF] Hornbach | Critically pressured free-gas 1480 reservoirs below gas-hydrate provinces[END_REF]. Thus, such 270 focused dissociation weakens the strength of sediments due to loss of hydrate cementation 271 between grains [START_REF] Brown | Slide structure and role of 1297 gas hydrate at the northern boundary of the Storegga Slide[END_REF][START_REF] Tinivella | Variations in BSR depth due to gas hydrate stability versus 2075 pore pressure: Global and Planetary Change[END_REF]. In the last two 272 decades, the role of excess pore pressure (overpressure) on submarine slope instability 273 has been very often put forward. The overpressure could potentially (i) facilitate or trigger 274 submarine slides, (ii) form nearly vertical columns of focused fluid flow and gas 275 migration, and (iii) determine the failure of a permeable layer in presence of low-276 permeability zone [START_REF] Xu | Excess pore pressure resulting from methane hydrate 2171 dissociation in marine sediments: A theoretical approach[END_REF]. Recent analyses aiming to assess 277 quantitatively the role of gas hydrates on submarine slope instability have highlighted the 278 importance of sediment lithology [START_REF] Handwerger | Submarine landslides triggered by 1445 destabilization of high-saturation hydrate anomalies[END_REF] and drainage conditions 279 [START_REF] Priest | Stability of fine-grained sediments subject to gas hydrate 1844 dissociation in the Arctic continental margin, Submarine Mass Movements and their 1845 Consequences[END_REF], or fracture formation [START_REF] Yang | Effects of gas hydrates dissociation on 2180 clays and submarine slope stability[END_REF] Basin also suggested that excess pore pressure caused by gas hydrate dissociation could 295 have contributed to slope failures. Such an increase in the interstitial pore pressure 296 generated by gas hydrate dissociation was equally reported by [START_REF] Ker | Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in 1527 the Western Black Sea[END_REF] in the 297 Western Black Sea. They suggested that values of excess pore pressure of 1-2 MPa are 298 probably the result of the low permeability of hydrate-bearing sediments. This excess 299 pore pressure is expected to drastically reduce sediment stability and could be at the origin 300 of some of the observed landslides in the area. Such assumptions agree with the study of 301 [START_REF] Elger | Submarine slope 1384 failures due to pipe structure formation[END_REF] who used reflection seismic data from the Arctic Ocean and numerical 302 modeling to show a direct link between hydrates and slope instability. Their study showed 303 that hydrates reduce the sediment permeability and causes the build-up of overpressure at 304 the base of the GHSZ. This overpressure reaches the shallow permeable beds, through 305 hydro-fracturing forms like pipe structures, and then it is transferred laterally within the 306 sedimentary column, weakening the slope sediment and leading to its destabilization.

308

Hovland and Gudmestad discussed the engineering significance of gas hydrates and the 309 possible engineering consequences on seabed installations (Hovland and Gudmestad, conditions, but [START_REF] Mienert | Ocean 1733 warming and gas hydrate stability on the mid-Norwegian margin at the Storegga Slide[END_REF], thermal shock led by sea level rise (Maslin and 354 Thomas, 2003), and reduction in lithostatic pressure hence hydrostatic pressure due to 355 shrinking of subsea permafrost [START_REF] Paull | Natural gas hydrates: occurrence, distribution, and detection[END_REF][START_REF] Tinivella | Variations in BSR depth due to gas hydrate stability versus 2075 pore pressure: Global and Planetary Change[END_REF] 356 [START_REF] Tinivella | A Quick-Look Method for Initial 2079 Evaluation of Gas Hydrate Stability below Subaqueous Permafrost[END_REF][START_REF] Waage | Geological controls 2113 of giant crater development on the Arctic seafloor[END_REF][START_REF] Wallmann | Gas 2125 hydrate dissociation off Svalbard induced by isostatic rebound rather than global 2126 warming[END_REF]. However, the 357 environment in which hydrates are found is different from that encountered on continental 358 margins. In permafrost regions, gas hydrates and ice may coexist with salts, water, and 359 free gas in the offshore sediments, which leads to unique characteristics of the ice-360 hydrate-bearing sediments compared to those of the other marine sediments. However, 361 little work has been done to understand such properties of permafrost sediments 362 containing gas hydrates, although the mechanical properties of frozen soil and rock in the 363 absence of gas hydrates and unfrozen gas hydrate-bearing sediments were extensively 364 investigated (e.g. [START_REF] Arenson | The rheology of frozen soils[END_REF][START_REF] Tsytovich | Mechanics of frozen ground[END_REF][START_REF] Waite | Physical 2116 properties of hydrate-bearing sediments[END_REF]). The 365 mechanical properties of frozen sand, silt and clay containing tetrahydrofuran (THF) 366 hydrate were determined at -10 °C [START_REF] Yun | Mechanical properties of sand, silt, and clay 2183 containing tetrahydrofuran hydrate[END_REF], and they found that shearing results 367 in particle rotation, slippage, and rearrangement in the absence of THF hydrates. In 368 contrast, shear force leads hydrates to detach from the mineral surface in the presence of 369 low saturation (<40%) of THF hydrates, or to debond from the sediment grains in the 370 presence of high saturation (>40%) of THF hydrates. The mechanical properties of kaolin 371 clay-ice powder mixtures were investigated in the presence of carbon-dioxide (Liu et al.,372 2013) and methane hydrates [START_REF] Li | Mechanical behaviors of permafrost-1629 associated methane hydrate-bearing sediments under different mining methods[END_REF][START_REF] Li | Strength Behaviors of Remolded 1632 Hydrate-Bearing Marine Sediments in Different Drilling Depths of the South China Sea: 1633[END_REF][START_REF] Luo | Effect of sediment particle size on the 1656 mechanical properties of CH4 hydrate-bearing sediments[END_REF]Yun et al., 373 2007). The determined shear strength was unexpectedly low compared to the sediments 374 containing gas hydrates only, and this was mainly explained by the lack of cementation 375 between hydrate crystals, ice powders and clay grains. Recently, [START_REF] Yang | Gas hydrates in permafrost: Distinctive effect of gas hydrates and 2177 ice on the geomechanical properties of simulated hydrate-bearing permafrost 2178 sediments[END_REF] 376 reported a set of comprehensive experiments for methane hydrate-bearing frozen 377 sediments. Their results showed that gas hydrates can form micro frames or network 378 structures, resulting in distinctive effects on the mechanical properties (higher shear 379 strength and less brittle under compressional loading) of the simulated hydrate-bearing 380 permafrost sediments compared to frozen sediments in the absence of gas hydrates (Yang 381 et al., 2019).

382

Old estimation provided huge volumes (up 9.2 to 22.3x10 3 Pg (1 Pg = 10 15 g)) of methane 383 trapped as hydrates in permafrost of the Arctic regions [START_REF] Max | Oceanic methane hydrates: A frontier gas resource[END_REF].

384

However, more recent studies provided most lower estimates ranging from 27 (~1% of 385 global hydrate estimate) to 800 Pg of methane [START_REF] Knoblauch | Methane 1549 production as key to the greenhouse gas budget of thawing permafrost[END_REF]Miesner et al., 386 2023;[START_REF] Ruppel | Permafrost-associated gas hydrate: Is it really approximately 1% of the global 1898 system?[END_REF]. Gas hydrate-bearing permafrost sediments (GHBPS) are sensitive 387 to temperature changes that could be induced by global warming, seasonal changes, 388 geothermal fluxes, and human activities. Particularly, global warming is causing the shrinking of both terrestrial and subsea permafrost, leading to a reduction in lithostatic 390 pressure and hence hydrate decomposition [START_REF] Maslin | Gas hydrates: 1697 past and future geohazard?[END_REF]. Scientific research has 391 revealed that global warming has accelerated the rate of ice thawing and gas hydrate 392 decomposition in permafrost regions, and, consequently, led to the release of methane 393 and carbon dioxide into the atmosphere, exacerbating global warming (Andreassen et al., 394 2017;[START_REF] Portnov | Ice-sheet-driven 1842 methane storage and release in the Arctic[END_REF]. A more detailed review on gas release follows in the next 395 section. This adverse effect of greenhouse gases reaching the atmosphere becomes even 396 more severe in the Arctic regions due to the effect of polar amplification. In other words, 397 the atmosphere or an extensive ocean circulation can lead to heat transport toward the 398 poles. Consequently, climate warming would affect the areas adjacent to the Earth poles 399 at relatively greater magnitude compared to the rest of the Earth surface (Polyakov et al., 400 2002), accelerating decomposition of gas hydrates in permafrost regions such as the 401 Arctic. Observations of natural catastrophic events have demonstrated that this increasing 402 negative feedback could tip over the mechanical stability of GHBPS, and trigger a chain 403 of large-scale land and/or seafloor failures, and subsequently induce an abrupt greenhouse 404 gas venting from subsea permafrost in the East Siberian Arctic Shelf (Ruppel and Kessler,405 2017) and life-threating events such as tsunamis that could reach the shore of the UK 406 based on evidence of historic events [START_REF] Long | Lateglacial and Holocene relative sea-level changes and first evidence for the 1646 Storegga tsunami in Sutherland[END_REF][START_REF] Tinivella | A Quick-Look Method for Initial 2079 Evaluation of Gas Hydrate Stability below Subaqueous Permafrost[END_REF]. The 407 instability of GHBPS has been attributed to landslides in Yukon Territory in Canada 408 [START_REF] Maslin | Gas hydrates: 1697 past and future geohazard?[END_REF], the permafrost lava observed on the Tien Shan Mountains in China 409 [START_REF] Marchenko | Permafrost warming in the Tien Shan 1676 mountains, central Asia[END_REF], the formation of massive gas blow-out craters in the northern 410 Barents Sea [START_REF] Andreassen | Massive 1231 blow-out craters formed by hydrate-controlled methane expulsion from the Arctic 1232 seafloor[END_REF] and in the Yamal and Gydan Peninsulas in northern 411 Siberia in Russia [START_REF] Bogoyavlensky | Complex of 1273 geophysical studies of the Seyakha catastrophic gas blowout crater on the Yamal 1274 Peninsula[END_REF]. These destructive consequences in recent

419

Finally, when the overpressure reaches a certain threshold, gas hydrate pingos could be 420 formed, and their collapse creates craters in permafrost regions. Furthermore, 421 composition analysis of the gas from such craters showed particularly high concentration 422 of methane [START_REF] Kvenvolden | Thermogenic hydrocarbons in unconsolidated 1569 sediment of Eel River Basin, offshore northern California[END_REF]. Such high concentration of methane only 423 allows formation of structure I hydrate which makes the gas hydrate-bearing permafrost 424 more vulnerable to global warming. Furthermore, the occurrence of methane hydrates in 425 permafrost layers gives GHBPS more complex characteristics than the sediments 426 containing either ice (e.g. hydrate-free frozen soils) or gas hydrates (e.g. unfrozen 427 hydrate-bearing sediments). Such characteristics include fluid distribution in pores, 428 unfrozen water content, and ice/hydrate morphologies such as pore filling or grain 429 cementing, load bearing or micro frame structures. At present, there is little knowledge 430 about the physical properties of permafrost sediments that are saturated with variable ice, 431 hydrate, gas, and saline water (Farahani et al., 2021a, b;[START_REF] Hassanpouryouzband | Gas 1449 hydrates in sustainable chemistry[END_REF] 432 [START_REF] Yang | Gas hydrates in permafrost: Distinctive effect of gas hydrates and 2177 ice on the geomechanical properties of simulated hydrate-bearing permafrost 2178 sediments[END_REF]. 2.

449

Methane venting at the seafloor widely occurs on shelves and continental margins. There, 450 methane and hydrogen sulfide are generated, transported and transformed within the 451 sediment, and part of the former is discharged into the water column [START_REF] Suess | Marine cold seeps and their manifestations: geological control, biogeochemical 2024 criteria and environmental conditions[END_REF]. At 452 hydrate-bearing settings, the amount of methane that can potentially be released from 453 hydrate decomposition processes is variable and site-specific [START_REF] Baumberger | Editorial: Recent Advances in 1243 Natural Methane Seep and Gas Hydrate Systems[END_REF] 454 [START_REF] Braga | Modelling methane hydrate stability changes and gas release due to seasonal 1294 oscillations[END_REF][START_REF] De La | Gas origin linked to paleo BSR[END_REF][START_REF] Ketzer | Gas hydrate dissociation linked to 1530 contemporary ocean warming in the southern hemisphere[END_REF]Marcon et 455 al., 2022;[START_REF] Marin-Moreno | Estimates of future 1683 warming-induced methane emissions from hydrate offshore west Svalbard for a range 1684 of climate models[END_REF][START_REF] Merle | Distribution of methane plumes on Cascadia Margin and implications for the 1719 landward limit of methane hydrate stability[END_REF][START_REF] Meurer | Volumetric Mapping of Methane Concentrations 1725 at the Bush Hill Hydrocarbon Seep[END_REF]Riboulot et 456 al., 2018;[START_REF] Thatcher | Methane release from 2065 warming-induced hydrate dissociation in the West Svalbard continental margin: Timing, 2066 rates, and geological controls[END_REF]. However, judging from the number of articles published 457 in recent years, methane discharge from hydrate decomposition is not a sporadic event 458 [START_REF] Fu | Hydrate Formation on Marine Seep Bubbles and 1410 the Implications for Water Column Methane Dissolution[END_REF][START_REF] Hautala | Dissociation of 1455 Cascadia margin gas hydrates in response to contemporary ocean warming[END_REF][START_REF] Johnson | Analysis of bubble plume 1508 distributions to evaluate methane hydrate decomposition on the continental slope[END_REF][START_REF] Phrampus | Widespread gas hydrate 1822 instability on the upper U.S. Beaufort margin[END_REF] 459 [START_REF] Westbrook | Escape of methane gas from the seabed along the West 2152 Spitsbergen continental margin[END_REF], and it is likely that this process will further increase in the 460 decades and centuries to come due to climate change.

461

Once released from the hydrates, methane undergoes several physical and 462 biogeochemical processes, starting from its dissolution in the pore water followed by its 463 oxidation (Figure 2). The remaining methane is transferred into the hydrosphere where it 464 undergoes similar processes. These processes act as efficient barriers preventing methane 465 transfer to the atmosphere. However, the efficiency of these barriers depends on multiple 466 factors (e.g. water depth, upwelling flowrate, local current, water stratification, etc.); and, 467 at the end, only a small portion of the total amount of methane released from NGHD 468 decomposition may reach the atmosphere (de [START_REF] De Garidel-Thoron | Evidence for large methane 1350 releases to the atmosphere from deep-sea gas-hydrate dissociation during the last 1351 glacial episode[END_REF][START_REF] Ruppel | Methane hydrates and contemporary climate change[END_REF] 469 [START_REF] Ruppel | The interaction of climate change and methane hydrates[END_REF][START_REF] Westbrook | Escape of methane gas from the seabed along the West 2152 Spitsbergen continental margin[END_REF]. Such a transfer occurs particularly in 470 the case of catastrophic releases.

471

In the following sections, we discuss in detail the variability of the seafloor methane from 472 hydrate decomposition, followed by the impact of such a leakage on the seafloor 473 ecosystems, seawater chemistry, and the conditions that lead to its transfer to the 474 atmosphere. [START_REF] Archer | Methane hydrate stability and anthropogenic climate change[END_REF][START_REF] Biastoch | Rising Arctic Ocean temperatures cause gas hydrate 1258 destabilization and ocean acidification[END_REF][START_REF] Garcia-Tigreros | Estimating the impact of seep methane oxidation on ocean pH and dissolved 1417 inorganic radiocarbon along the US Mid-Atlantic Bight[END_REF]. To 1 and2). The rate of hydrate 492 dissolution has been determined in the Cascadia Margin [START_REF] Heeschen | Methane sources, distributions, and fluxes from cold vent 1462 sites at Hydrate Ridge[END_REF]Hester et 493 al., 2009;[START_REF] Lapham | Methane under-saturated 1590 fluids in deep-sea sediments: Implications for gas hydrate stability and rates of 1591 dissolution[END_REF][START_REF] Linke | In situ 1635 measurement of fluid flow from cold seeps at active continental margins: Deep Sea 1636[END_REF][START_REF] Riedel | synthesis: scientific findings[END_REF][START_REF] Römer | Tidally 1872 controlled gas bubble emissions: A comprehensive study using long-term monitoring 1873 data from the NEPTUNE cabled observatory offshore V ancouver I sland[END_REF] 494 [START_REF] Suess | Gas hydrate destabilization: enhanced 2028 dewatering, benthic material turnover and large methane plumes at the Cascadia 2029 convergent margin[END_REF][START_REF] Torres | Fluid and chemical fluxes in and out of 2083 sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I: Hydrological 2084 provinces[END_REF][START_REF] Wilson | Observing 2159 methane hydrate dissolution rates under sediment cover[END_REF], the Gulf of Mexico (Lapham 495 et al., 2014;Solomon et al., 2008) and the Monterey Bay [START_REF] Rehder | Dissolution rates of pure methane hydrate and carbon-dioxide hydrate in 1850 undersaturated seawater at 1000-m depth[END_REF]. The data, 496 either measured or calculated from other measured parameters, revealed high variabilities 497 not only from one region to another, but also within a given geological setting (Table 1).

498

Thus, for studies undertaken in the Cascadia Margin the dissolution rate ranges between 499 29 to 7.9x10 6 mmol.m -2 .yr -1 . Lowest values were obtained for deposits characterized by 500 thermogenic hydrates trapping multiple guest molecules, and/ or oil film covering the 501 outcropping specimens as the presence of such cover significantly mitigates the 502 dissolution rate. [START_REF] Lapham | Gas hydrate dissolution 1593 rates quantified with laboratory and seafloor experiments[END_REF] showed that the aforementioned range of 503 dissociation rates is similar to previously reported laboratory measurements (Bigalke et 504 al., 2009). In the Gulf of Mexico, [START_REF] Lapham | Gas hydrate dissolution 1593 rates quantified with laboratory and seafloor experiments[END_REF] measured a dissolution rate of 505 6.6x10 6 mmol.m -2 .yr -1 for thermogenic hydrates. Such a rate for thermogenic hydrates is 506 in the same order of magnitude than the value of 4.7x10 6 mmol.m -2 .yr -1 obtained by Hester 507 et al ( 2009) at Barkley Canyon, and it is in the same range than the rate range of 8-9.5x10 6 508 mmol.m -2 .yr -1 (141-167 cm.yr -1 ) measured from synthetic methane hydrates brought at 509 the seafloor for in situ decomposition experiments [START_REF] Rehder | Dissolution rates of pure methane hydrate and carbon-dioxide hydrate in 1850 undersaturated seawater at 1000-m depth[END_REF]. Furthermore, biofilms. This is supported by [START_REF] Wilson | Observing 2159 methane hydrate dissolution rates under sediment cover[END_REF] who investigated how that sediment 538 cover can affect hydrate dissolution by means of field (at Barkley Canyon) and laboratory 539 measurements. The dissolution rate of outcropping thermogenic hydrates covered with 540 sediment was around 3.4x10 3 mmol.m -2 .yr -1 (<1 cm.yr -1 ), whereas that of synthetic 541 hydrates exposed directly to bulk water was an order of magnitude higher. Their results

542

are in agreement with dissolution-rate values calculated by [START_REF] Riedel | synthesis: scientific findings[END_REF] for buried 543 hydrates at sites on the Clayoquot Slope of the Cascadia margin. Thus, as hydrate 544 dissolution is mainly controlled by diffusion, the presence of sediment slows the rate of 545 molecular diffusion via porosity/tortuosity effects.

547

For its part, hydrate dissociation has been investigated at the high latitude regions such as 548 the Western Svalbard [START_REF] Ferre | Reduced methane seepage from 1397 Arctic sediments during cold bottom-water conditions[END_REF][START_REF] Marin-Moreno | Estimates of future 1683 warming-induced methane emissions from hydrate offshore west Svalbard for a range 1684 of climate models[END_REF]Marin-Moreno et 549 al., 2013;[START_REF] Thatcher | Methane release from 2065 warming-induced hydrate dissociation in the West Svalbard continental margin: Timing, 2066 rates, and geological controls[END_REF][START_REF] Westbrook | Escape of methane gas from the seabed along the West 2152 Spitsbergen continental margin[END_REF], the U.S. Beaufort margin 550 [START_REF] Phrampus | Widespread gas hydrate 1822 instability on the upper U.S. Beaufort margin[END_REF] and the Laptev Sea [START_REF] Chernykh | Sonar estimation of methane bubble 1319 flux from thawing subsea permafrost: a case study from the Laptev sea shelf[END_REF]; as these regions are 551 very sensitive to temperature increase due to climate change [START_REF] Hassol | Impacts of a warming Arctic-Arctic climate impact assessment[END_REF]Hassol and 552 Corell, 2006). Investigations on the U.S. Atlantic margins also suggest that multiple 553 methane emission sites are partly related to hydrate dissociation due to the sea-bottom 554 temperature increase [START_REF] Phrampus | Recent changes to the Gulf Stream causing 1820 widespread gas hydrate destabilization[END_REF][START_REF] Skarke | Widespread methane 1996 leakage from the sea floor on the northern US Atlantic margin[END_REF]Weinstein et 555 al., 2016). However, as it is relatively usual to observe gas emissions associated with 556 hydrates at cold seeps, this coexistence makes it difficult to accurately assess the 557 contribution of hydrate dissociation in the total gas discharge. The dissociation process 558 requires bringing the pressure and temperature conditions of the deposits to the boundary 559 of the GHSZ to allow the formation of gas bubbles, and the common factor amongst the 560 aforementioned sites is the occurrence of intense gas emissions at the upper limit of the 561 GHSZ. Nevertheless, like for hydrate dissolution, Table 2 shows that the estimated

589

Moreover, it is important to recall that these methane fluxes are not constant over the 590 time; and therefore, there is a need to take into account processes and seasonal changes 591 that induced short to long-term variations that affect annual fluxes such as tides (Römer 592 et al., 2016;[START_REF] Sultan | Impact of 2045 tides and sea-level on deep-sea Arctic methane emissions[END_REF], seismic activities [START_REF] Fischer | Subduction zone earthquake as potential trigger of submarine 1401 hydrocarbon seepage[END_REF]Franek et al., 593 2017; [START_REF] Lapham | Temporal variability of 1586 in situ methane concentrations in gas hydrate-bearing sediments near Bullseye Vent[END_REF] and temperature [START_REF] Berndt | Temporal Constraints on Hydrate-1255 Controlled Methane Seepage off Svalbard[END_REF][START_REF] Ferre | Reduced methane seepage from 1397 Arctic sediments during cold bottom-water conditions[END_REF]. # for complex thermogenic hydrates, * for synthetic hydrates, π for buried hydrates (otherwise it is outcropping hydrates) 597 598 599 anoxic seafloor ecosystems [START_REF] Conrad | The global methane cycle: recent advances in understanding the microbial 1329 processes involved[END_REF]. Around 40% of the methane produced by 607 methanogenesis escapes to the atmosphere (Lyu et al., 2018). In marine sediments, 608 microbial methanogenesis produces methane through the terminal anaerobic breakdown 609 of organic matter deposited on the seafloor [START_REF] Parkes | A review of 1805 prokaryotic populations and processes in sub-seafloor sediments, including biosphere: 1806 geosphere interactions[END_REF][START_REF] Wellsbury | The geomicrobiology of deep marine 2145 sediments from Blake Ridge containing methane hydrate[END_REF] 610 (Fig. 2). Estimations suggest that 3-18% of the buried organic carbon within the seafloor 611 is converted to CH4 by methanogenesis in continental margin [START_REF] Egger | Global diffusive fluxes of 1379 methane in marine sediments[END_REF] observed from surface to deep sub-seafloor sediments (Table 3).

615 616

618

Methanogenesis

Anaerobic methanotrophic oxidation

Aerobic methanotrophic oxidation Sediments: 0.02 to 30 nmol cm -3 d -1***** 5.1 x 10 -8 to 7.0 x 10 4 nmol cm -3 d - 1**** 3.4 x 10 -8 to 1.1 nmol cm -3 d -1 **** -with hydrate 0.1-2.73 nmol cm -3 d -1*** < 4.25 x 10 -6 nmol cm -3 d -1* 10 2 to 5x10 3 nmol cm -3 d -1****** 0-12 mmol m -2 d -1****** -without hydrate 0.015-0.62 nmol cm -3 d -1** 0.001 to 10 4 nmol cm -3 d -1****** 0-289 mmol m [START_REF] Inagaki | Exploring deep microbial life in coal-bearing sediment 1497 down to~ 2.5 km below the ocean floor[END_REF][START_REF] Kurth | Structural insights into the 1564 methane-generating enzyme from a methoxydotrophic methanogen reveal a restrained 1565 gallery of post-translational modifications[END_REF]. The overall rates of methanogenesis in marine 646 sediments range from 0.02 to 30 nmol cm -3 d -1 (Table 3) (mole-to-mass conversion is 0.32 647 to 480 ng cm -3 d -1 reported in Figure 2). However, microbial rates are limited and 648 restricted to discrete habitats and mostly obtained in incubation with radiotracers in 649 laboratory conditions and ambient pressure. Thus, the extrapolation to estimates of global 650 microbial methane production and consumption per year may be subjected to high 651 uncertainties. All the methanogenesis pathways lead to the production rate of 5-33 Tg of 652 methane yr -1 in continental marine sediment at global scale [START_REF] Wallmann | The 2121 Global Inventory of Methane Hydrate in Marine Sediments: A Theoretical Approach[END_REF].

653

However, the transfer of this amount of methane generated from the seabed along 654 continental margins to the atmosphere is efficiently buffered and controlled by both 655 aerobic (AeOM) and anaerobic oxidation of methane (AOM), respectively at and below 656 the oxic-anoxic interface of the sediment and water columns [START_REF] Reeburgh | [END_REF] (Fig. 2).

657

These two oxidation processes can be expressed as follows: AeOM occurs into the geosphere at cold seeps and hydrothermal fields from a symbiotic 662 association between specific methane-oxidizing bacteria with marine animals (Petersen 663 and Dubilier, 2009). In the water column, AeOM represents the final sink for methane 664 before reaching the atmosphere. Within the anoxic sediment, AOM is a microbial-665 mediated process that occurs within the geochemical horizon commonly called the 666 sulfate-methane transition zone (SMTZ) where upward diffusion of methane reacts with 667 downward diffusing seawater sulfate [START_REF] Borowski | Marine pore-water sulfate profiles indicate in 1276 situ methane flux from underlying gas hydrate[END_REF][START_REF] Borowski | Global and local variations of interstitial sulfate 1278 gradients in deep-water, continental margin sediments: Sensitivity to underlying 1279 methane and gas hydrates[END_REF][START_REF] Gao | Significance of anaerobic oxidation of methane 1413 (AOM) in mitigating methane emission from major natural and anthropogenic sources: 1414 a review of AOM rates in recent publications[END_REF] 668 [START_REF] Hinrichs | The anaerobic oxidation of methane: new insights in 1467 microbial ecology and biogeochemistry: Ocean margin systems[END_REF][START_REF] Pohlman | Assessing 1833 sulfate reduction and methane cycling in a high salinity pore water system in the 1834 northern Gulf of Mexico[END_REF]. Laboratory experiments from natural 669 samples have shown that the overall rates of AOM in marine sediments coupled to sulfate, 670 metal oxides and nitrite/nitrate as electron acceptors range from 5.1 x 10 -8 to 7.0 x 10 4 671 nmol cm -3 d -1 (Gao et al., 2022, Table 3) (mole-to-mass conversion is 81.6 x 10 -8 to 112 x 672 10 4 ng cm -3 d -1 reported in Figure 2). Predicted global rates of AOM based on the 673 regression model with diffusive fluxes of CH4 suggest that 45-61 Tg of methane are 674 oxidized annually; corresponding to ~3-4% of the organic debris deposited on the seabed 675 [START_REF] Egger | Global diffusive fluxes of 1379 methane in marine sediments[END_REF]. On a global scale, 8 to 65 Tg of methane per year bypasses the two 676 aforementioned sedimentary biofilters and is released in the water column (Saunois et al.,677 2016; [START_REF] Steinle | Water column methanotrophy controlled by a rapid 2016 oceanographic switch[END_REF]. This estimate is not restricted to hydrate decomposition only 678 as it is difficult to decipher, but involves several methane pools, including free methane 679 generated either from thermal cracking at deep depth or shallow methanogenesis. 2) (mole-to-mass conversion 700 of methane production rates in hydrate bearing sediments were 1.6 to 43.68 ng 701 cm -3 d -1 reported in Figure 2). Thus, the cell numbers as well as the microbial 702 activities involved in the methane cycle (methanogenesis and anaerobic 703 methanotrophy) strongly increase within the sedimentary geochemical-horizons, 704 thus increasing the amount of hydrates and associated free gas (Dickens and 705 Quinby-Hunt, 1997; [START_REF] Wellsbury | The geomicrobiology of deep marine 2145 sediments from Blake Ridge containing methane hydrate[END_REF]. At the Black Ridge hydrate-setting, 706 the total microbial number is significantly higher around the Bottom Simulating 707 Reflector (BSR), especially beneath it within the free-gas or dissolved-gas 708 occurrence zone [START_REF] Wellsbury | The geomicrobiology of deep marine 2145 sediments from Blake Ridge containing methane hydrate[END_REF]. Indeed, the potential rates of microbial 709 activity increase at the interface between the BSR and the free-gas zone beneath, 710 mainly towards the free-gas zone [START_REF] Cragg | Bacterial 1334 populations and processes in sediments containing gas hydrates (ODP Leg 146: Cascadia 1335 Margin)[END_REF][START_REF] Cragg | The impact of fluid and gas venting on 1338 bacterial populations and processes in sediments from the Cascadia Margin 1339 accretionary system (sites 888-892) and the geochemical consequences[END_REF] 711 [START_REF] Wellsbury | The geomicrobiology of deep marine 2145 sediments from Blake Ridge containing methane hydrate[END_REF]. Thus, the presence of free gas associated with hydrate 712 deposits rather than free gas-depleted hydrates, impacts on the microbial activities 713 including methane and acetate metabolisms. At the SMTZ, pore water methane 714 and sulfate are consumed through the AOM by syntrophic consortia of anaerobic 715 methanotroph archaea (ANME) and sulfate reducing bacteria (SRB). The depth 716 of the SMTZ is strongly affected by the intensity of the methane flux, while its 717 state (steady or transient) is rather linked to the flux variations over time 718 [START_REF] Borowski | Global and local variations of interstitial sulfate 1278 gradients in deep-water, continental margin sediments: Sensitivity to underlying 1279 methane and gas hydrates[END_REF][START_REF] De Prunelé | Focused 1362 hydrocarbon-migration in shallow sediments of a pockmark cluster in the Niger Delta 1363[END_REF][START_REF] Jørgensen | Sulfur cycling and methane oxidation[END_REF] 719 [START_REF] Kasten | Processes and signals of nonsteady-state 1521 diagenesis in deep-sea sediments and their pore waters[END_REF]. A recent investigation of site located at the rim of a hydrate-

736

 Ecosystems-hosted methanotrophy. Microbial consortia of anaerobic 737 methanotroph archaea and sulfate-reducing bacteria are not restricted to gas 738 hydrate-bearing sediments [START_REF] Boetius | A marine microbial consortium 1267 apparently mediating anaerobic oxidation of methane[END_REF], but are widely distributed in 739 other marine environments including cold seeps [START_REF] Hinrichs | Methane-1469 consuming archaebacteria in marine sediments[END_REF]Vigneron 740 et al., 2013), mud volcano [START_REF] Lazar | Methanogenic diversity 1600 and activity in hypersaline sediments of the centre of the Napoli mud volcano, Eastern 1601[END_REF], deep hypersaline sediments (La 741 [START_REF] Cono | Unveiling microbial life in new deep-sea 1581 hypersaline Lake Thetis. Part I: Prokaryotes and environmental settings[END_REF] and hydrothermally impacted sediments [START_REF] Holler | Thermophilic anaerobic oxidation of methane by marine 1474 microbial consortia[END_REF] 742 [START_REF] Teske | Microbial diversity of hydrothermal sediments in the 2062 Guaymas Basin: evidence for anaerobic methanotrophic communities[END_REF]. The major controlling factors for the ANME population 743 distribution are the availability of methane and sulfate, or known alternative 744 electron acceptors such as nitrate, iron or manganese [START_REF] Wallenius | Anthropogenic and 2118 environmental constraints on the microbial methane cycle in coastal sediments[END_REF].

745

ANME is believed to use a process of "reverse methanogenesis" biochemical 746 pathway and are phylogenetically related to methanogenic lineages. The doubling 747 time of ANME/SRB consortia, estimated to approximately seven months under 748 controlled in vitro conditions [START_REF] Nauhaus | In vitro cell growth of 1766 marine archaeal-bacterial consortia during anaerobic oxidation of methane with sulfate: 1767[END_REF], suggests that the set-up of an 749 efficient AOM microbial filter in sediments would be on the order of decades 750 [START_REF] James | Effects of climate change on methane emissions from seafloor sediments in the Arctic 1503[END_REF]. The coupled process of AOM and sulfate-reduction increases 751 alkalinity, promoting the precipitation of authigenic carbonate by reacting with 752 the seawater calcium [START_REF] Luff | Fluid flow, methane fluxes, carbonate precipitation and 1652 biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia 1653 Margin: Numerical modeling and mass balances[END_REF]. This authigenic carbonate 753 precipitation results in a decreasing of the sediment porosity that changes the fluid 754 flow pattern, and in turn affects the supply of methane at the SMTZ (Steeb et [START_REF] Marlow | Carbonate-hosted microbial 1690 communities are prolific and pervasive methane oxidizers at geologically diverse marine 1691 methane seep sites[END_REF][START_REF] Marlow | Carbonate-hosted methanotrophy represents an unrecognized methane sink in the 1695 deep sea[END_REF]. Natural and 759 anthropogenic methane seepages into an oxygenated water column stimulate the 760 distribution, abundance and activity of aerobic methanotrophic bacterial 761 populations [START_REF] Rogener | Long-term impact 1865 of the Deepwater Horizon oil well blowout on methane oxidation dynamics in the 1866 northern Gulf of Mexico[END_REF][START_REF] Steinle | Water column methanotrophy controlled by a rapid 2016 oceanographic switch[END_REF]. The AeOM reaction 762 hinders the methane transfer to the atmosphere by converting it into biomass or 763 dissolved inorganic carbon; which in turn could affect the pH in the ocean 764 [START_REF] Biastoch | Rising Arctic Ocean temperatures cause gas hydrate 1258 destabilization and ocean acidification[END_REF]. Estimations of aerobic methanotrophic oxidation rates in 765 oxygenated sediments ranged from 3.4 x 10 -8 to 1.1 nmol of CH4 cm -3 d -1 (Table 766 2, [START_REF] Gao | Significance of anaerobic oxidation of methane 1413 (AOM) in mitigating methane emission from major natural and anthropogenic sources: 1414 a review of AOM rates in recent publications[END_REF] (mole-to-mass conversion is 54.4 x 10 -8 to 17.6 ng of CH4 cm - 767 3 d -1 reported in Figure 2). [START_REF] Ruff | In situ development of a methanotrophic microbiome in deep-sea sediments: The 1881[END_REF].

789

Predicting how climate change will affect the aforementioned microbial processes 790 remains elusive. Several studies have claimed that microbial methane production may be 791 enhanced by global warming. In fact, methanogens are active in most habitats where 792 temperature ranges from 0 to 122°C [START_REF] Hoehler | Environmental constraints 1471 that limit methanogenesis[END_REF]. In marine sediments, the 793 methanogenesis process is restricted to either the sediment layer deep below the SMTZ 794 (meter to tens of meters) and used acetate and hydrogen, or in surficial sediment (<1 m) 795 where non-competitive substrates such as methanol and other methylated compounds are 796 available. Interestingly, the coexistence of sulfate-reduction and methanogenesis also 797 occur in shallow sediment when methylotrophic pathway to produce methane takes place 798 [START_REF] L'haridon | Methanococcoides vulcani sp. 1575 nov., a marine methylotrophic methanogen that uses betaine, choline and N, N-1576 dimethylethanolamine for methanogenesis, isolated from a mud volcano, and emended 1577 description of the genus Methanococcoides: International journal of systematic and 1578[END_REF]. Methanogen distribution may be limited by competition for 799 acetate and dihydrogen and are rapidly out-competed by sulfate-reducers for these same 800 substrates. In shallow sulfate-rich sediments, methanogens utilizing non-competitive 801 substrates such as methylated amines could meet the substrate requirements of sulfate-802 reducing bacteria. Thus, variation of in-situ temperature in marine sediments can strongly 803 impact methane production pathways via biochemical, bioenergetic and/or ecological 804 mechanisms [START_REF] Conrad | Importance of hydrogenotrophic, aceticlastic and methylotrophic 1331 methanogenesis for methane production in terrestrial, aquatic and other anoxic 1332 environments: A mini review[END_REF][START_REF] Hoehler | Environmental constraints 1471 that limit methanogenesis[END_REF]. Temperature increase positively 805 impacts methanogenesis activity [START_REF] Maltby | Microbial methanogenesis in the sulfate-reducing zone of sediments in the 1668 Eckernförde Bay[END_REF], resulting in higher methane 806 production with potential higher emission budget to the hydrosphere. In fact, a warming seafloor and within water column at sites where free methane is released [START_REF] Levin | Manifestation, drivers, and emergence of open ocean deoxygenation[END_REF] 817 [START_REF] Steinle | Water column methanotrophy controlled by a rapid 2016 oceanographic switch[END_REF][START_REF] Sweetman | Major impacts of climate change on deep-sea 2053 benthic ecosystems: Elementa: Science of the Anthropocene[END_REF]. Accordingly, hydrate dissociation may affect 818 the water column even more than hydrate dissolution. If the methane hydrate dissociation 819 is fast enough and releases large quantities of gas, part of it will escape the function of 820 the microbial filter, as it will not be accessible by the microorganisms. However, 821 additional methane loss prior to reaching the atmosphere occurs as bubble stripping, i.e. 2006) studied methane emissions in the Black Sea and showed that significant transfer 832 to the atmosphere occurs at water depth shallower than 100 m. Other studies highlighted 833 the coastal region located in water depth ranging between ~0 and 50/ 75m, where gas 834 hydrates are not stable, as being by far the more important contributor of sedimentary 835 methane to the atmosphere [START_REF] Mao | Aerobic oxidation of methane significantly reduces global diffusive 1671 methane emissions from shallow marine waters[END_REF][START_REF] Michel | Observations of shallow 1728 methane bubble emissions from cascadia margin[END_REF][START_REF] Römer | Seafloor methane seepage related to salt 1869 diapirism in the northwestern part of the German North Sea[END_REF] 836 [START_REF] Weber | Global ocean methane emissions dominated by 2139 shallow coastal waters[END_REF]. In a recent study, [START_REF] Joung | Negligible atmospheric 1516 release of methane from decomposing hydrates in mid-latitude oceans[END_REF] Horizon blowout in the northern Gulf of Mexico, which has increased methane oxidation 852 rates in the water column compared to natural methane seepage [START_REF] Rogener | Long-term impact 1865 of the Deepwater Horizon oil well blowout on methane oxidation dynamics in the 1866 northern Gulf of Mexico[END_REF].

853

iii. Marine methane transfer to the atmosphere 854 After CO2, atmospheric methane is the second most important climate-forcing trace gas 855 driven by anthropogenic sources. Its burden in the atmosphere has drastically increased 856 by 150% since the pre-industrial era and is currently rising at a high rate (Nisbet et al.,857 2014; [START_REF] Nisbet | Rising 1785 atmospheric methane: 2007-2014 growth and isotopic shift: Global biogeochemical 1786 cycles[END_REF][START_REF] Platt | Methane at Svalbard and 1830 over the European Arctic Ocean[END_REF]. Our understanding on the respective 858 contribution from its different natural sources and sinks remains incomplete (Kirschke et 859 al., 2013;[START_REF] Saunois | The global methane budget 2000-2012[END_REF]Saunois et al., 2017;Saunois et al., 2020). Top-down and 860 bottom-up studies show that natural processes emit between 218 and 371 Tg CH4 yr -1 on 861 average, respectively (Saunois et al., 2020).

862

Aquatic methane enters the atmosphere either as bubbles rising from the seafloor or by predicted that the open ocean might become a sink at atmospheric concentrations 866 equivalent to today's concentrations [START_REF] Lambert | Reevaluation of the oceanic flux of methane: Uncertainties 1584 and long term variations[END_REF]. That prediction did 867 not materialize. Survey of the global ocean CH4 data combined with estimates of 868 ebullitive fluxes found a range of 6-12 Tg CH4 yr -1 [START_REF] Weber | Global ocean methane emissions dominated by 2139 shallow coastal waters[END_REF]. It is generally 869 admitted that seafloor emissions reach the atmosphere only at shallow depth (continental 870 shelf and upper slope), whereas in the open ocean methane flux correlates with net 871 primary production and would therefore be mainly produced in situ by organic matter 872 cycling [START_REF] Weber | Global ocean methane emissions dominated by 2139 shallow coastal waters[END_REF]. [START_REF] Rosentreter | Half of global methane emissions 1877 come from highly variable aquatic ecosystem sources[END_REF] further extended the study of 873 [START_REF] Weber | Global ocean methane emissions dominated by 2139 shallow coastal waters[END_REF] to sources from coastal ecosystems, and estimated a median flux of 874 8.4 Tg CH4 yr -1 (interquartile range 4.8-28.4 Tg CH4 yr -1 ) [START_REF] Rosentreter | Half of global methane emissions 1877 come from highly variable aquatic ecosystem sources[END_REF] 875 (Fig. 2). These estimates can separate emissions from seafloor reaching the atmosphere 876 on one side with in situ production on the other side. However, atmospheric estimates are 877 based on measurements that do not discriminate whether sedimentary methane has been 878 buffered in methane hydrates that eventually decomposed or is a direct emanation of 879 methane from seeps.

880

Focusing on submarine seeps only, [START_REF] Etiope | Gridded maps of geological methane 1386 emissions and their isotopic signature[END_REF], compiling the available literature, 881 estimated the methane transfer from the geosphere to the atmosphere at 3.9 Tg CH4 yr -1 882 with a maximum potential value of 9 Tg CH4 yr -1 . In this synthesis, all sources were 883 located at depths shallower than 500 m. The main emitting area is the East Siberian Arctic 884 Shelf with 2 Tg CH4 yr -1 (range 0-4 Tg CH4 yr -1 , [START_REF] Berchet | Atmospheric constraints on the methane 1250 emissions from the East Siberian Shelf[END_REF]). Etiope et al.'s 885 estimate is nearly twice lower than a previous geological estimates of 20 Tg CH4 yr -1 886 [START_REF] Judd | Natural seabed gas seeps as sources of atmospheric methane[END_REF][START_REF] Kvenvolden | Attention turns to naturally 1572 occurring methane seepage: Eos[END_REF]. There, offshore direct sources account for 5-20 Tg 887 CH4 yr -1 and hydrate dissociation around 0-5 Tg CH4 yr -1 .

888

Numerous investigations on the consequence of methane release from hydrate 889 decomposition during past climate change such as the Paleocene-Eocene thermal 890 maximum argued that these events have contributed to an increased injection of methane 891 into the atmosphere that moderately accelerated climate change (Dickens et al., 1997; 892 [START_REF] Kennett | Methane hydrates in 1524 Quaternary climate change: The clathrate gun hypothesis[END_REF][START_REF] Mcinerney | The Paleocene-Eocene Thermal Maximum: a 1713 perturbation of carbon cycle, climate, and biosphere with implications for the future[END_REF][START_REF] Norris | Carbon cycling and chronology of climate warming during the 1788 Palaeocene/Eocene transition[END_REF]. Such an impact 893 involves a huge amount of methane passing through the entire water column to reach the 894 atmosphere. Several modelling and simulation studies explored the interactions between 895 hydrate dissociation and climate change [START_REF] Archer | Methane hydrate stability and anthropogenic climate change[END_REF][START_REF] Ruppel | The interaction of climate change and methane hydrates[END_REF] and 896 references therein). Most of these studies found moderate release of methane to the 897 atmosphere under the worst climate scenario (RCP 8.5) [START_REF] Biastoch | Rising Arctic Ocean temperatures cause gas hydrate 1258 destabilization and ocean acidification[END_REF]Hunter et 898 al., 2013;[START_REF] Kretschmer | Modeling the fate of methane 1562 hydrates under global warming[END_REF]. [START_REF] Kretschmer | Modeling the fate of methane 1562 hydrates under global warming[END_REF] estimated that up to 4.73 Tg 899 CH4 yr -1 could be released on average, ignoring potential oxidation in the water column. showed that strong vertical density gradients in the water column developed during the 922 warm season might limit the ascent of methane gas to the sea surface and its release into 923 the atmosphere. During winter, the vertical mixing in the water column is enhanced and 924 consequently more methane is released into the atmosphere [START_REF] Gentz | A water 1423 column study of methane around gas flares located at the West Spitsbergen continental 1424 margin[END_REF]von 925 Deimling et al., 2011). Such a seasonal effect could also explain variations in the extent 926 of the gas hydrate stability zone (GHSZ) on the Svalbard margin and the distribution of 927 gas flares in the water column [START_REF] Ferre | Reduced methane seepage from 1397 Arctic sediments during cold bottom-water conditions[END_REF][START_REF] Riedel | Situ Temperature Measurements at the Svalbard Continental 1862 Margin: Implications for Gas Hydrate Dynamics[END_REF], but the net methane 928 budget still needs to be estimated. Apart from seasonal shifting, meteorological events 929 such as storms [START_REF] Shakhova | Ebullition and storm-induced 1987 methane release from the East Siberian Arctic Shelf[END_REF], chemical gradients and geological site-specific 930 parameters [START_REF] Ryerson | Atmospheric emissions 1912 from the Deepwater Horizon spill constrain air-water partitioning, hydrocarbon fate[END_REF]Schmale et al., 2010a;[START_REF] Schmale | Distribution of methane in the water column of the Baltic Sea[END_REF]Yvon-931 Lewis et al., 2011;[START_REF] Zhang | Methane escape from gas hydrate systems in marine environment, and 2194 methane-driven oceanic eruptions[END_REF] can also facilitate methane transfer from the sea to the 932 atmosphere.

933

As mentioned in section 4.ii, the methane injected in seawater is then submitted to 934 dissolution, gas exchange within the bubbles and oxidation. Modeling of gas bubble 935 ascent can predict the evolution of bubble sizes and the fractions of methane gas reaching 936 the sea surface, and then transferred to the atmosphere under idealized conditions 937 [START_REF] Mcginnis | Fate of rising 1710 methane bubbles in stratified waters: How much methane reaches the atmosphere?[END_REF][START_REF] Ruppel | The interaction of climate change and methane hydrates[END_REF]. For a water depth of 100m, methane 938 from only bubbles with a diameter larger than 2cm at the seafloor can reach the 939 atmosphere [START_REF] Ruppel | The interaction of climate change and methane hydrates[END_REF]. Furthermore, the internal dynamics of large-940 scale bubble plumes and the interactions with ambient bottom currents and density 941 stratification adds further complexity that is not yet fully resolved by numerical models 942 [START_REF] Leifer | The fate of 1617 bubbles in a large, intense bubble megaplume for stratified and unstratified water: 1618 Numerical simulations of 22/4b expedition field data[END_REF][START_REF] Schneider Von Deimling | Observation-based modelling of permafrost 1981 carbon fluxes with accounting for deep carbon deposits and thermokarst activity[END_REF]. Such an achievement 943 necessarily requires a sound knowledge, not only of its micro-biological state of the local 944 water column, but also its the physical properties (i.e. currents, temperature). Hence, 945 hydro-acoustic measurements and more additional field data are required to monitor and 946 forecast the fate of methane gas released at the seabed to improve the existing models. 947 Thus, in order to reliably evaluate methane contribution from hydrate decomposition to 948 the global atmospheric budget, it is necessary to (1) develop new bio-geochemical tools 949 to better identify and decipher all oceanic sources and understand the processes that 950 degrade this molecule in the water column, and (2) intensify observations and monitoring 951 of sites by increasing the number of devoted cruises as well as the deployment of 952 autonomous or remotely operated systems to achieve reliable estimates of the methane 953 fluxes across the interfaces sediment (geosphere)/ seawater (hydrosphere)/ atmosphere.

954

Once reaching the uppermost water layer, the diffusive transfer to the atmosphere is 955 strongly affected by ocean state and weather conditions [START_REF] James | Effects of climate change on methane emissions from seafloor sediments in the Arctic 1503[END_REF]Wanninkhof 956 et al., 2009). Wind speed modifies the exchange velocity across the interface. Wave 957 breaking further enhances gas exchange through bubble formation. Strong winds during 958 storms combine effects of increased water column vertical mixing and gas exchange. At 959 the high latitudes, sea ice acts as a barrier preventing the transfer of methane to the 960 atmosphere [START_REF] Kort | Atmospheric observations of Arctic Ocean methane emissions up to 82 1557 north[END_REF], and accordingly the reduction in sea ice is expected to 961 significantly increase the annual total methane flux to the atmosphere [START_REF] He | Sea ice in the Arctic Ocean: 1458 Role of shielding and consumption of methane[END_REF].

962

Currently, attempts to accurately measure atmospheric methane inputs from NGHDs, 963 especially in the Arctic where hydrate-bearing sediments are encountered at shallow 964 locations, failed due to widely varying numbers of factors that need to be considered.

965

Using a diffusive model [START_REF] Wanninkhof | Relationship between wind speed and gas exchange over the ocean[END_REF], [START_REF] Thornton | Methane fluxes 2069 from the sea to the atmosphere across the Siberian shelf seas[END_REF] estimated a 966 maximum flux range of 68-87 mmol.m -2 .yr -1 in the Laptev Sea and East Siberian Arctic 967 Shelf. Comparably, using inverse modelling of atmospheric measurements Myhre et al.

968

(2016) found a source not higher than 120 mmol.m -2 .yr -1 off the coast of Svalbard on a 969 known methane seep area [START_REF] Myhre | Extensive release of methane from Arctic seabed west 1760 of Svalbard during summer 2014 does not influence the atmosphere[END_REF]. When including the impact of storm, 970 [START_REF] Shakhova | Ebullition and storm-induced 1987 methane release from the East Siberian Arctic Shelf[END_REF] proposed higher annual average estimate reaching up to 506 971 mmol.m -2 .yr -1 for the East Siberian Arctic Shelf. For the same area, [START_REF] Berchet | Atmospheric constraints on the methane 1250 emissions from the East Siberian Shelf[END_REF] 972 used an atmospheric inversion method and propose that atmospheric observations in the 973 Arctic are compatible with a mean source ranging between 0-134 mmol.m -2 .yr -1 .

974

The estimates are different from each other from one site to another as well as from one 975 method to another for the same site, and such a result indicates that several challenges 976 still remain for a better quantification of these fluxes at the sea-air interface. Indeed, the 977 aforementioned studies assumed an atmospheric diffusive flux and estimated it through [START_REF] Berchet | Atmospheric constraints on the methane 1250 emissions from the East Siberian Shelf[END_REF]. Estimates from closed chamber 984 accumulation could be more accurate but is restricted to a very small spatial coverage, [START_REF] Edson | Direct covariance flux 1373 estimates from mobile platforms at sea[END_REF]. Measurements are 990 also needed to characterize the sub-seafloor origin of methane, and ground-checked on 991 whether or not it has been buffered in hydrates. In the previous sections, the hazards related to gas hydrate decomposition have been 1018 identified and discussed, whereas this section is devoted to the risk analysis related to 1019 hydrate decomposition. Risk analysis is essential to ensure safety of operations. Although 1020 the terms "risk" and "hazard" are often used interchangeably, "risk" and "hazard" have (https://www.quintessa.org/case-studies/coupled-process-modelling-for-co2-storage).

1041

A lightweight drillship using coiled tubing drilling over a short period of time may be 1042 used in NGHD exploration because the NGHD prospect will be at shallow depths in 1043 unconsolidated marine sediments. Production of NGHD is likely to be carried out by 1044 subsea infrastructure on the seafloor with real-time monitoring and control from a remote 1045 location [START_REF] Max | Commercial potential of natural gas hydrate, Exploration 1706 and Production of Oceanic Natural Gas Hydrate[END_REF]. Hydrate is stable and effectively inert in its reservoir 1046 host sediment. During depressurization, which seems the most promising method of 1047 production, pressure in the reservoir is lower than normal formation pressure which 1048 means that a blow-out is physically impossible and the likelihood of leakage of gas is area should be known, and qualitatively and quantitatively mapped [START_REF] Bayrakci | Acoustic monitoring 1246 of gas emissions from the seafloor. Part II: a case study from the Sea of Marmara[END_REF] 1076 [START_REF] Greinert | 1300-m-high rising 1426 bubbles from mud volcanoes at 2080m in the Black Sea: Hydroacoustic characteristics 1427 and temporal variability[END_REF][START_REF] Greinert | Hydroacoustic experiments to establish a method for the 1429 determination of methane bubble fluxes at cold seeps[END_REF][START_REF] Jerram | Split-beam echo sounder observations of natural 1505 methane seep variability in the northern G ulf of M exico: Geochemistry[END_REF][START_REF] Leblond | Acoustic monitoring of gas emissions from the 1606 seafloor. Part I: quantifying the volumetric flow of bubbles[END_REF] 1077 as well by appropriate acoustic devices and sensors (e.g. single-, split-and multi-beam 1078 echo sounder systems, dissolved gas sensors). The biological environment must also be The desktop study will confirm if the shallow geology and the environmental hazards 1084 generated by the production of gas hydrates is well understood. Invariably supplemental 1085 data acquisition will be required, and a site survey will be designed to fill the data gaps 1086 identified by the desktop study. Because NGHD E&P operations will involve deploying 1087 a seabed coring tool from a workboat, a seabed clearance survey will be required. An moorings and/ or lander from the site survey vessel to collect the data. However, although 1124 some gas (e.g. methane, carbon dioxide) sensors can be implemented on these platforms, 1125 most of the geochemical analyses of the gas bubbles and pore water will be performed 1126 onshore after sampling with appropriate tools such as corer and bubble sampler (Chen et 1127[START_REF] Chen | A novel mechanical gas-tight sampler for 1316 hydrothermal fluids[END_REF]Ruffine et al., 2018a;[START_REF] Ruffine | ): Strategy for 1893 site detection and sampling and first scientific outcome: Deep Sea Research Part II: 1894 Topical Studies in Oceanography[END_REF][START_REF] Wu | A Handheld Sampler for Collecting Organic 2162 Samples from Shallow Hydrothermal Vents[END_REF][START_REF] Wu | Development of an electric control gas-2165 tight sampler for seafloor hydrothermal fluids[END_REF].

1128

The environmental impact assessment will also need to assess the impact of NGHD E&P 1129 operations on human economic activity. Therefore, the site survey will also need to 1130 acquire information on the physical infrastructures, pipelines, telecommunication cables, 1131 fishing activity, etc., in the vicinity of the NGHD E&P operation area. A stakeholder 1132 engagement plan may also be necessary based on the human activity that may be 1133 impacted.

1134

With respect to the environmental monitoring plan for long-term NGHD production 1135 operations, it is advisable to deploy an instrumented lander and/ water column moorings 1136 to monitor essential environmental variables at an undisturbed reference site within a 1137 suitable distance from production operations where a similar lander/ and or water column 1138 moorings acquires information on the same essential environmental variables for 1139 comparison [START_REF] Klar | Marine gas hydrate technology: State of the art and 1543 future possibilities for Europe[END_REF]. The results of the baseline study based on the site survey 1140 data will form the input for modelling (geotechnical, geochemical, fluid flow, etc.) to 1141 predict reservoir conditions during production and to compare after decommissioning. In 1142 addition, the modelling can be used as a tool to plan the monitoring regime and to define 1143 the area that may be influenced both temporarily and spatially by the NGHD production. pre-production state) or the climate change-related impacts [START_REF] Liu | Monitoring and research on environmental impacts related to marine natural gas 1639 hydrates: Review and future perspective[END_REF]. Indeed, the 1152 strategy detailed in this section applied for assessing the impacts of disturbances due to 1153 both human activities and climate change. However, as mentioned previously the impacts 1154 due to climate change may be smoother and expand over a longer time period. Thus, the 1155 monitoring period should be adapted accordingly.

1156

The first step towards a suitable monitoring strategy is to generate a detailed project and lander deployments, as used for the baseline study [START_REF] Klar | Marine gas hydrate technology: State of the art and 1543 future possibilities for Europe[END_REF]. Multiple 1165 surveys should be conducted for full areal coverage. However, each survey should be 1166 conducted at a specific height above the seabed, along with permanent monitoring on and 1167 of the seabed by fixed installations such as observation wells and deployed sensors, to 1168 achieve optimal results. Additional targeted studies will have to be conducted, if active 1169 formation water seeps, gas seeps, geological features such as mud volcano and 1170 pockmarks, etc., connected to the NGHD producing reservoir are observed at the seabed.

1171

These sites must be revisited on a regular basis to determine emission rates of gases and 1194 Hydrate decomposition can be relatively fast, from a few months/ years to few decades, 1195 if it is voluntarily caused by human activities. On the other hand, the changing climate of 1196 our planet may also trigger the decomposition of vulnerable NGHDs located in the 1197 permafrost and the shallow sediments on the ocean margins. The kinetics of such a 1198 decomposition will likely be slower but will last over a longer time period. There are 1199 natural biogeochemical processes that degrade methane within both the sediment and the 1200 water column, preventing any large accumulation of it in the water masses and mitigating 1201 its transfer to the atmosphere. However, the question of the efficiency of these processes 1202 under a changing climate needs to be understood. How will the increase of sea bottom 1203 temperature affect the microbial communities that perform the aerobic and anaerobic 1204 oxidation of methane, and accordingly the amount of methane that will be released from 1205 hydrates? What will be the consequences of such releases over decades and centuries? 1206 These questions remain to be answered.

224

  criteria are met: (i) gas hydrates are widespread; (ii) slides are initiated in areas within the 225 hydrate phase boundaries; and (iii) sediment of low permeability (mainly fine-grained 226 sediments) is present at the base of the gas hydrate occurrence zones. There are two well-227 studied submarine slope failures that happened in interglacial periods that have been 228 potentially attributed to hydrate dissociation, although debate still remains: Storegga 229 Slide and Hinlopen Slide. The Storegga Slide is one of the largest submarine slope failures 230 known to have happened 8000 years ago

  235 have been the trigger or at least supportive factors for the slope instabilities north of 236 Svalbard, in the vicinity of the Hinlopen Slide. Considering other locations, Lee argued 237 that hydrate dissociation might have played a role in the large submarine landslides on 238 the US Atlantic margin

241

  another recent work, by considering a seafloor temperature increase of 2 or 4 °C, 242 Alessandrini et al (2019) simulated the impact of future global warming over the next 50 243 and 100 years on the Gas Hydrate Stability Zone (GHSZ) off the Chilean margin. They 244 suggested that, despite an increase in hydrostatic pressure due to sea level rise, seafloor 245 temperature increase would induce the dissociation of the entire NGHD located along the 246 upper slope in the next century. The dissociation will cause slope instability in some areas, 247 and could seriously damage coastal cities if this dissociation triggers tsunamis. Using a 248 large geophysical dataset including bathymetry and 2D/3D P-cable seismic data, Hillman 249 et al. (2018) investigated the potential relationship between gas migration pathways, gas 250 vents observed at the seafloor and submarine slope failures near the S2 Canyon in the 251 Danube fan (Northwestern Black Sea). They found abundant evidence for gas migration 252 in the area that suggests a connection between slumps and gas migration, although it is 253 not clear whether gas migration facilitates slope failure through pressure and pre-254 conditioning of the sediments, or vice versa. From 2D seismic reflection profiles along 255 the active margin of northern Colombia in the western Caribbean Sea, Leslie and Mann 256 defined a series of three giant, previously unidentified, submarine landslides (Leslie and 257 Mann, 2016). They suggested that an over-pressurized zone of weakness located at the 258 boundary of the hydrate stability has likely facilitated slope failure.

  on the development 280 of excess pore pressures during hydrate dissociation. Mountjoy et al. (2014) analyzed 281 seismic reflection data of the Tuaheni landslide complex (New Zealand East Coast) and 282 proposed that, gas hydrates can directly destabilize the seafloor via three mechanisms 283 (Mountjoy et al., 2014): (1) excess pore pressure induced by gas hydrate dissociation; (2) 284 hydro-fracturing in the gas hydrate zone; (3) and time-dependent plastic deformation of 285 hydrate-bearing sediment. Recently, unlike previous studies, Gross et al. (2018) 286 performed a 3D analysis of the distribution of gas and gas hydrate indicators and did not 287 show evidence of gas hydrates within the upper landslide unit. However, their 288 observations support the hypotheses of Mountjoy et al. (2014) that argue that build-up of 289 overpressure below low permeability gas hydrate-bearing sediments could cause hydro-290 fracturing in the gas hydrate zone and valving excess pore pressure into the landslide 291 body. The hypotheses also suggested that gas is migrating into the critical mechanical 292 zone at the base of the landslide, and thus building up overpressure that can lead to the 293 reactivation of the landslide. Results obtained by Horozal et al. (2017) in the Ulleung 294

412

  years are also attributed to the global warming of anthropogenic activities. Based on 413 numerical modeling, Andreassen et al. (2017) developed an understanding of how the 414 craters and mounds were formed in the northern Barents Sea and postulated that climate 415 warming leads to the ice sheet retreating, and this is accompanied with catastrophic 416 reduction of the sediment pore pressure. Consequently, gas hydrates initially in 417 permafrost or sediments dissociate. The dissociation of gas hydrates at depth, particularly 418 those underlying impermeable layers, results in building up of overpressure in pores.

Figure 2

 2 Figure 2 illustrates the functioning of cold seep areas where the main methane-involving

Fig. 2 :

 2 Fig.2: Conception scheme of biogeochemical processes taking place at hydrate-bearing

475i.

  Fluxes of methane discharged at the seafloor 476 Due to the sensitivity of the NGHD to environmental changes, concerns have been 477 raised regarding a massive and prolonged release of methane from hydrate decomposition 478 and its consequences in terms of climate feedback, ocean acidification and ecosystem 479 vulnerability

  480provide some answers to these societal issues, it is necessary to consider the fluxes of 481 methane released from hydrate decomposition. These fluxes are related to the rate of 482 hydrate decomposition, which depends on multiple factors. For outcropping hydrates, the 483 main factors are the flow velocity of the contacting methane-free water, the extent of 484 partial-sediment cover, and the presence of oil film or biofilms and non-methane guest 485 molecules. Considering hydrate-bearing sediments, the porosity, thickness and thermal 486 properties of the sediment are key factors affecting build-up and flow of methane-charged 487 fluids until its release at the seabed; and these flow-driving factors depend on both the 488 free-gas and hydrate saturation levels that evolve over the course of the decomposition. 489 Although hydrates are widespread on continental margins and in the permafrost, 490 and were intensively investigated in the former location, there are few natural sites where 491 hydrate decomposition has been quantified (Tables

510

  Bigalke et al. (2009) performed laboratory-based experiments at temperature and pressure 511 conditions within the hydrate stability field to investigate the dissolution of methane 512 hydrates in undersaturated seawater. The dissolution rate was found to be strongly 513 dependent on the friction velocity and the temperature of the seawater turbulent flow, 514 showing that hydrate dissolution in undersaturated seawater is a diffusion-controlled 515 process. They suggested that additional experimental work is required to address the 516 dissolution kinetics of non-pure methane hydrates, as both, the solubility limit and the 517 understanding of the molecule transfer processes at the phase boundary during 518 dissolution, are less well constrained. It is also important to note that the measurement 519 principles, the instruments and the computing approaches used for these aforementioned 520 studies of in situ hydrate-dissolution may differ from one research group to another, 521 leaving bias in the comparison. 522 Moreover, methane hydrate dissolution rate depends on the thickness of the 523 diffusion boundary layer and the methane concentration in water, and this second 524 parameter is in turn controlled by the solubility of methane. Rehder et al. (2004) showed 525 that gas hydrate dissolution rate depends on the difference between the local solubility in 526 the presence of gas hydrates and the local dissolved-phase concentration of the hydrate-527 forming gas. Lapham et al. (2010 and 2014) measured the methane concentration in 528 sediments immediately surrounding outcropping deep-sea gas hydrates for two sites in 529 Barkley Canyon and the Gulf of Mexico, respectively. Their results showed that pore-530 fluids are significantly under-saturated with respect to expected values at equilibrium 531 with methane hydrates, indicating that the hydrates should continue to dissolve. They also 532 showed that dissolution rates calculated from in situ methane concentration gradients are 533 significantly lower than those predicted for methane-undersaturated pore water in direct 534 contact with pure methane hydrates if equilibrium methane concentrations exist 535 immediately adjacent to the hydrate surface. Lapham et al. (2010) suggested that the 536 factors that retard hydrate decomposition in sediments here could be oil coating or 537

562

  methane fluxes and the potential discharge trajectory are highly heterogeneous. In one of 563 the first estimates, Wesbrook et al (2009) estimated that ~20 Tg.yr -1 of methane can be 564 released from hydrates over an area of 22,300 km 2 corresponding to a swath within 360-565 400 m water depth nearly all around the Svalbard archipelago. Marin-Moreno et al (2013 566 and 2015) also investigated the sensitivity to climate change of the hydrate deposit present 567 in the Western Svalbard continental margin by considering a temperature-increase path 568 following either the lower (2.6) or the upper (8.5) cases of the Representative 569 Concentration Pathways (RCP). Their projection indicated that 0.029-0.053 Tg.yr -1 of 570 methane can be released from hydrate dissociation over an area of ~134 km 2 during the 571 next three centuries. Such a projection is also in agreement with the estimates of 22.4 Tg. 572 yr -1 proposed by Ferré et al (2020) along the 360 m isobaths at the Norwegian-western 573 Svalbard Margin. By extending to the swath width of Svalbard archipelago corresponding 574 to 400-500 m water depth (swath width complementary to the one considered by 575 Wesbrook et al., 2009), Marin-Moreno et al. (2013 and 2015) showed that the methane 576 discharge may reach 1.7-9 Tg.yr -1 over 41,400 km 2 . Thus, their estimates of vulnerable 577 hydrates are more than twice smaller than the estimate obtained by Wesbrook et al (2009) 578 for a surface area three times larger but located at deeper depth. Their results also showed 579 that hydrates remain stable beyond 500 m water depth. Such studies illustrate the 580 importance of the water depth (hydrostatic pressure) on the stabilization of hydrates. It is 581 very interesting to mention that they found that the choice of the RCP scenario has a small 582 impact on the total discharge. The calculated methane fluxes off Svalbard ranges from an 583 estimated average of 2 mmol.m -2 .yr -1 (Thatcher et al., 2013) to 13.5 mol.m -2 .yr -1 584 calculated by Marin-Moreno et al (2013). The fluxes calculated by Phrampus et al. (2014) 585 fall within this range of values for the U.S. Beaufort margin (0.88-4.4 mol.m -2 .yr -1 ), 586 whereas values up to two and three order of magnitude higher were obtained by Skarke 587 et al (2014) and Chernykh et al (2020) on the U.S. Atlantic margin and in the Laptev Sea, 588 respectively.

  ) + 2O2 (aq)  CO2 (aq) + 2H2O (AeOM) 660 CH4 (aq) + SO4 2-(aq)  HCO3 -(aq) + HS -(aq) + H2O (AOM) 661

680

  Methane released from hydrate decomposition affects the transport of chemical species 681 within the sediment overlaying the deposits by generating a continuous carbon flux that 682 represents an energy supply for the benthic chemosynthetic assemblages(Niemann et al., 683 2006). Such an energy supply stimulates the seepage-associated ecosystems located in 684 the sediment as well as in the water column[START_REF] Boetius | Hydrate Ridge: a natural laboratory for the study of microbial 1270 life fueled by methane from near-surface gas hydrates[END_REF] Marcon et al., 685 2014;[START_REF] Sahling | Macrofaunal community 1915 structure and sulfide flux at gas hydrate deposits from the Cascadia convergent margin[END_REF][START_REF] Suess | Marine cold seeps and their manifestations: geological control, biogeochemical 2024 criteria and environmental conditions[END_REF] and gives rise to the following consequences:686 Formation of interface horizons shaped by redox processes. The microbial 687 processes and the community diversity that characterized them are often 688 stimulated at discrete subsurface geochemical interfaces[START_REF] Parkes | A review of 1805 prokaryotic populations and processes in sub-seafloor sediments, including biosphere: 1806 geosphere interactions[END_REF], 689 creating a zonation within the sedimentary column. From a biogeochemical 690 viewpoint, anaerobic methane oxidation, methanogenesis from acetate and 691 Hydrogen/ Carbon Dioxide, acetate oxidation, sulfate-reduction, and bacterial 692 productivity were stimulated in both the hydrate and the free-gas sediment 693 horizons beneath compared to hydrate-free sediments (from 1.5 to 15 times 694 higher) at the Black Ridge setting in NW Atlantic Ocean[START_REF] Wellsbury | The geomicrobiology of deep marine 2145 sediments from Blake Ridge containing methane hydrate[END_REF] 695 and surface sediment of the Cascadia Margin in the North Pacific Ocean(Cragg 696 et al., 1995); and distinct microbial communities dominated the former 697 sedimentary environments. Estimations of microbial methane production rate 698 ranged from 0.015 to 0.62 nmol cm -3 d -1 in sediments without hydrate to 0.1 to 699 2.73 in methane hydrate bearing sediments (Table

  720 bearing zone off Svalbard has shown that nonvertical methane transport can 721 significantly affect the gas migration pathway, leading to the occurrence of double 722 SMTZs and unusual microbial distribution within the sediment (Treude et al., 723 2020). Previous studies have shown that the settlement of the SMTZ starts around 724 100 to 1000 years after the inception of gas hydrate dissociation (Argentino et al., 725 2021; de Prunelé, 2015; de Prunelé et al., 2017; Hong et al., 2016; Pape et al., 726 2020; Sultan et al., 2016). The SMTZ depth is controlled by the diffusive fluxes 727 of methane and sulfate, which depend on continuous decrease in the reactivity of 728 organic matter with the increasing sediment depth and therefore the age of the 729 sediment[START_REF] Borowski | Global and local variations of interstitial sulfate 1278 gradients in deep-water, continental margin sediments: Sensitivity to underlying 1279 methane and gas hydrates[END_REF]. The sedimentation rate and the associated 730 organic matter burial flux are the key control factors to define the depth of the 731 SMTZ[START_REF] Egger | Global diffusive fluxes of 1379 methane in marine sediments[END_REF]. These two factors depend in turn on the water depth, 732 the distance from land and surface biological productivity in the overlying water 733 column. Human alteration of nutrient loading and oceanographic regimes due to 734 global warming can also impact on the SMTZ distribution in sediment (Egger et 735 al., 2018).

768

  Temporal and spatial dynamics of (micro)-biological communities. The 769 methane concentration and flux strongly affect the diversity of the chemosynthetic 770 communities such as sulfur bacterial mats, tubeworms, and clams that are settled 771 on the seafloor[START_REF] Fischer | Interaction 1403 between hydrocarbon seepage, chemosynthetic communities and bottom water redox 1404 at cold seeps of the Makran accretionary prism: insights from habitat-specific pore 1405 water sampling and modeling[END_REF][START_REF] Pop Ristova | Bacterial diversity and biogeochemistry of different chemosynthetic habitats of 1839 the REGAB cold seep (West African margin, 3160 m water depth[END_REF]. These 772 chemosynthetic seep fauna are therefore bioindicators of AOM activity and 773 methane flux. Thus, macrofauna colonies such as vesicomyid clam populating the 774 Beggiatoa mats (filamentous microbes) settled on methane hydrate-bearing 775 sediments consist nearly exclusively of endemic species and are found in higher 776 density at seeps and hydrothermal vents compared to non-seep environments 777 (Sahling et al., 2002). The methane fluxes may vary on very large spatial scales 778 as a consequence of factors such as the variation of the rate of hydrate 779 decomposition or carbonate precipitations, or after a submarine mud eruption, a 780 continental slope sliding, and even an earthquake. Thus, large expulsions of fluids 781 and mud from active submarine volcanos may strongly affect the distribution of 782 communities; sometimes causing their extinction. However, long-term 783 observation after such large expulsions of fluids also indicates a resilience of the 784 ecosystem to recover the main microbial function. For instance, the re-785 colonization of complex pioneer microbial populations composed of aerobic and 786 anaerobic methanotrophs were observed several years after a subsurface mud 787 eruption at the active center of the Häkon Mosby mud volcano

  807of bottom water may result in increasing the amount of methane contained within the 808 shallow sediments and eventually lead to the shoaling of the SMTZ in vast areas of the 809 continental shelf. Even if representative isolated microbes grow within a wide range of 810 temperatures in the laboratory, changing global temperatures may also shift the diversity 811 of the microbial community towards more responsive and efficient microbial 812 metabolisms at higher in situ temperature. 813 Moreover, the lifetime of methane in the water column is longer when it is transferred 814 from the seafloor as bubbles, partly because microbes use dissolved methane. The aerobic 815 oxidation of methane by free-living and symbiont-associated bacteria occurs on the 816

  822dissolution in the surrounding water, or replacement by oxygen, nitrogen or other 823 dissolved gases in the water masses. Such a gas exchange may not be complete over a 824 short distance, and thus a significant methane fraction would be transferred to the 825 atmosphere at shallow water depth. The amount of methane bypassing sedimentary 826 benthic biofilters, entering in the water column, and transfer to the atmosphere can vary 827 widely depending on the type of water and specific environmental conditions. For coastal 828 and marine environments, estimation is challenging due to complex interactions between 829 sediments, water column and the atmosphere. Therefore, it is difficult to provide depth 830 threshold from which sedimentary methane is transferred to the atmosphere. McGinnis et 831 al. (

  investigated the transfer of 837 methane from the seafloor to the sea surface along the US margins at seep areas including 838 hydrate-bearing sites, both in the Atlantic and Pacific sides. They results showed no 839 occurrence of such a vertical transfer at water depth deeper than 430m, which is shallower 840 than the upper boundary of the hydrate-stability field (Joung et al., 2020; Joung et al., 841 2022). In addition, during the aerobic oxidation of methane in the water column, bacteria 842 produce biomass and/or carbon dioxide, which latter is dissolved in the seawater. Large 843 production of carbon dioxide may contribute to the decrease of the water pH, and lead to 844 local ocean acidification as mentioned in section 4.i. Biastoch et al (2011) estimated that 845 the oxidation of methane discharged in the Arctic Ocean within the next centuries will 846 result in a drop of 0.25 pH units and a local decrease of up to 25% of the oxygen 847 concentration. A bloom of aerobic methanotroph communities in response to the large-848 scale methane release may happen, creating a vertical stratification of aerobic 849 methanotrophs consuming the dissolved oxygen and depleting the amount of oxygen in 850 the water column. Such a microbial bloom has been observed from the Deep Water 851

  863diffusion from saturated waters across the sea-air interface. Early estimates by Lambert864&[START_REF] Lambert | Reevaluation of the oceanic flux of methane: Uncertainties 1584 and long term variations[END_REF] provided a marine source to the atmosphere of 3.5 Tg CH4 yr -1 and 865

900

  This is comparable to today's global ocean methane sources, a minor term in the global 901 annual methane budget. They also noted the importance of the uncertainty associated with 902 the inventory of hydrates in the sediment within the GHSZ. Serov et al. (2017) highlighted 903 the importance of understanding the post-glacial history of present-day gas hydrate 904 provinces to better predict the trajectory of their destabilization. It is also important to 905 assess the role of anthropic activities on the vulnerability of NGHDs. Indeed, although 906 several studies investigated the monitoring of marine methane emissions from oil and gas 907 wells (Böttner et al., 2020), there is no study related to human-induced destabilization of 908 NGHD from seafloor or sub-seafloor resource extraction. To our knowledge there is no 909 literature dealing with the quantification of methane leaks during production tests from 910 hydrates and its fate, nor on its impact on the associated ecosystems over time. As 911 mentioned previously, the water column acts as a relatively efficient but still poorly 912 characterized oxidative-barrier to prevent seafloor methane from reaching the 913 atmosphere. Therefore, continuous efforts driven by field-based quantitative approaches 914 are needed to achieve an in-depth understanding of the transfer and fate of gases in the 915 different Earth compartments, as well as to identify under which circumstances marine 916 methane reaches the atmosphere. Thus, the assessment of the physical and 917 biogeochemical processes related to methane in the water column and its probability to 918 reach the atmosphere remains timely and challenging. 919 Besides, like seafloor methane emissions, methane transfer from the ocean to the 920 atmosphere is influenced by environmental factors such as seasonality. Experiments 921

  978the gradient method, which does not represent adequately ebullitive fluxes. Atmospheric 979 inversions as top-down methods are unable to unambiguously quantify specifically 980 methane released from hydrate dissociation distinctly from that originating from other 981 marine sources. They are further limited by model transport error, embedded sources and 982 atmospheric data coverage. However, top-down methods may provide valuable upper 983 constraints to marine emissions

985

  weakening the spatial representativity and therefore remain difficult to operate in open 986 ocean and large areas. The eddy covariance technique deployed on ships is technically 987 adequate (Thornton et al., 2020) but challenging in marine environment, especially 988 because it requires a careful correction of the measured high-frequency, three-989 dimensional wind for ship motion and attitude

1008 •• 1012 ••

 10081012 sections, we have shown the scarcity of methane flux measurements at 996 hydrate-bearing sites and the need to intensify such measurement campaigns. We also 997 stressed the need to undertake investigations at different spatial and temporal scales to 998 gain insights into the impacts of hydrate decomposition on the seafloor stability and the 999 fate of the permafrost. The outcomes of such studies will help to clarify the evolution path 1000 of NGHDs and its impact upon climate change or/ and human activity, and how this 1001 evolution path affects the ecosystems colonizing the sediment as well as the water 1002 column. Addressing the aforementioned environmental challenges requires both the 1003 definition of an environmentally sound monitoring strategy and the assessment of the 1004 legal framework regulating the release of gases and particles in the water column. The 1005 current legal framework must be adjusted to account for the specific environmental risks 1006 associated with natural gas hydrate dynamics. Therefore, the following actions should be 1007 considered: Assess how slope stability may be compromised by either global warming or gas 1009 production from hydrates under different geological boundary conditions; 1010 Identify suitable precursors/changes for slope failure to be targeted in monitoring 1011 program; Develop a generic strategy for environmental baseline studies and the 1013 environmental monitoring of hazards related to gas hydrate dynamics; 1014 Evaluate whether national and international legal frameworks are suitable to 1015 protect the environment.1016 1017

  1021well defined meanings in risk assessment procedure. Hazard is a potential source of harm 1022 or damage. Harm or damage is always the consequence of an accident, whereas risk is a 1023 combination of the likelihood (L) of the harm occurring and the severity of that harm 1024 (damage D). Thus, risk can be formulated as follows:1025 R= f (D, L) = D x L 1026Where R represents the risk matrix.

  strategy related to gas production from NGHDs 1029 The production of gas hydrates, whether or not coupled with carbon dioxide 1030 sequestration, can create environmental hazards by affecting seafloor stability and 1031 delicate microbial/benthic ecosystems and by releasing methane and particles into the 1032 water column. Therefore, countries that are considering such resources are committed to 1033 developing an appropriately focused environmental baseline survey prior to gas hydrate 1034 production operations to quantify those risks. An environmental risk assessment of gas 1035 hydrate exploration and production (E&P) operations will be generated from the baseline 1036 survey and will include proposed mitigation measures to minimize the impacts of the 1037 E&P operations. During E&P operations, an appropriate monitoring program to assess 1038 the effectiveness of mitigation measures is necessary. There are precedents for baseline 1039 and monitoring studies from oil and gas E&P (IOGP, October 2017) and CO2 storage 1040

1049

  highly reduced. If gas leakage does occur methane gas bubbles are dissolved in ambient 1050 seawater during their ascent through the water column and oxidized by aerobic 1051 microorganisms using dissolved oxygen as terminal electron acceptor (Atlas and Hazen, 1052 2011) with the potential consequences of oxygen depletion in the water column as 1053 detailed in section 4.ii. 1054 Prior to drilling an NGHD exploration borehole, a site survey to establish baseline 1055 environmental conditions and identify drilling hazards at the proposed drill site is needed. 1056 Site surveys are performed to minimize the risk of harm to personnel and equipment, and 1057 to protect the natural environment. Therefore, the site survey must acquire sufficient data 1058 over an appropriate period of time to adequately describe the physical, chemical, 1059 geological and biological environments in the operational area. Initially a desktop study 1060 is carried out to inform the design of the site survey. With respect to the specific hazards 1061 presented by NGHDs the integrated analysis of exploration 3D and/or 2D seismic data, 1062 offset well data (logs, operation reports, industry databases, etc.), geotechnical borehole 1063 information, offset site surveys and relevant public domain data will allow an initial 1064 ground model of the seabed and shallow section to be developed. This geological 1065 characterization will include detailed bathymetry, geochemical, mineralogical and 1066 petrophysical characteristics of the sediment and geological structures. The 1067 biogeochemical characteristics of the seafloor and its surroundings (bottom water, near 1068 seafloor sediment characteristics as well as pore water composition, together with the 1069 dissolved benthic fluxes) must be measured. Further measurements will be required to 1070 establish baseline environmental conditions. These include the physical and chemical 1071 characteristics of the entire water column especially in relation to oxygenation and 1072 acidification, i.e. dissolved gases such as O2, CO2, and CH4 as well as other parameters 1073 of the carbonate system (pH, DIC, alkalinity), nutrients, SO4 2-, H2S, Ca 2+ , and total 1074 Hardness. Potential gas emissions from the seafloor into the water column in the target 1075

1079

  described including the biological community of the seafloor and near seafloor sediments 1080 (biocenosis) with reference to conservation of protected areas, habitats and species.1081Economic activity that might be impacted by NGHD production operations should be 1082 described including archaeological heritage.

  1083

1088

  indicative site survey area is 200 m below seabed and 500 m radius around the borehole 1089 site location. 1090 The baseline studies can be carried out by an autonomous underwater vehicles (AUV) 1091 and a seafloor and water column generic instrument module or lander deployed from a 1092 site survey vessel with seabed sampling equipment and a towed magnetometer (cf. Klar, 1093 2019 for information on monitoring technology). The seabed sampling using box cores 1094 and piston cores deployed from the site survey vessel will acquire geotechnical 1095 information on the strength of near seabed sediments and biological habitats. The 1096 magnetometers will provide archaeological information. The AUV and lander should be 1097 equipped with suitable instruments to monitor the water column, the water-sediment 1098 interface and partially the sub-seafloor as well (e.g. side scan sonar and synthetic aperture 1099 sonars (SAS), echo sounders, sub-bottom profilers, hydrophones, physical sensors, 1100 chemical sensors, still camera, sediment traps, benthic chambers, etc.). It may be 1101 necessary to deploy an autonomous, fully instrumented lander for several months at the 1102 proposed site to acquire adequate baseline data and capture variabilities related to season, 1103 tide, storm or other aforementioned environmental and anthropogenic events.

1104

  Geological characterization is necessary to assess the risk and likely impacts of seafloor 1105 deformation caused by NGHD E&P operations. The geological characterization will 1106 include the aforementioned of surveys and analyses proposed for the site surveys, 1107 together with data from OBS (Ocean-Botton Seismometer) or OBC (Ocean-Botton 1108 Cable), as well as high-resolution multibeam bathymetry/backscatter mapping of the 1109 seabed and hydro-acoustic imaging of shallow gas accumulations in the seabed it they are 1110 not already available from site surveys. A paleo-geological history study will also be 1111 required. The combination of multibeam backscatter data with the results of the analyses 1112 of box cores will allow the characterization of the benthic biotopes of the operational area. 1113 Likewise, to assess the likely impacts of methane gas release on the environment, several 1114 parameters will need to be measured in the baseline site survey. In addition to all 1115 aforementioned measurements, these include detection and quantification of gas bubbles 1116 in the water column through acoustic systems; sampling and analyses, including both 1117 isotopic and molecular composition determination of any ascending gases; video/photo 1118 imaging of biota at the seabed; biological integrity; oceanographic measurements to 1119 model the transport of sediments and gases; chemical detection of dissolved gases; 1120 chemical and isotopic compositions of key pore-water elements and dissolved gases; 1121 habitat mapping. Several of these measurements can be achieved by deploying an 1122 autonomous seafloor and water column generic instrument module, water column 1123

1144

  The modelling can help to understand if an additional dataset is necessary to complete the 1145 baseline study. Commercial software is only available for partially modelling site 1146 evolution, so one suggestion would be to evaluate the possibility to develop new software 1147 to enable complete modelling of site evolution.

1148b.

  Generic strategy to monitor gas seepages from hydrates 1149After the baseline study has delivered the comprehensive characterization of the site, 1150 monitoring is necessary in order to identify any anomalies relative to the anthropic-(e.g.

  1151

1157

  description based on the baseline study and modelling, in order to identify the main risks 1158 for the environment and assess the potential impacts. From the environmental impact 1159 assessment, a recommendation of mitigation measures and a plan for environmental 1160 management and spatiotemporal monitoring should be established. The next step is to 1161 plan the management and monitoring strategy based on the risk and impact assessment.

1162

  Surveys carried out during the baseline study should be repeated. It could include 1163 autonomous underwater vehicles (AUVs), monitoring vessels, water column moorings

  1164

  1172 fluids. Such periodic visits are key to check for instance, if any seepage has strengthened 1173 and/or seafloor geological structure formation is (re-)activated by the NGHD production 1174 operation. If new seeps develop during the production operational phase, they must be 1175 investigated and sampled in detail to determine the origin and chemical composition of 1176 the seeping fluids and gases and their emission rates. 1177 Especially during the NGHD production it is also important to accurately monitor seabed 1178 deformation, since deformation and subsidence processes around the center of activity 1179 could already be an imminent hazard precursor. This type of monitoring relies on lander-1180 deployed high precision, high stability pressure sensors together with inclination 1181 measurements at several positions within the production field. Acoustic Long Baseline 1182 (LBL) methodology can be used to monitor relative positions in between the measuring 1183 nodes at the seafloor[START_REF] Klar | Marine gas hydrate technology: State of the art and 1543 future possibilities for Europe[END_REF].

  have been investigated for multiples purposes for decades, whether 1187 to understand their role in the oceanic methane (carbon) cycle, as a geohazard, or as a 1188 potential energy resource. Whatever the purpose of their study, the question of their 1189 decomposition, -the main factors that trigger this process, its speed and duration, as well 1190 as the methane flux released-, are highly relevant. In the present review, the 1191 environmental impacts related to hydrate decomposition have been discussed and 1192 monitoring strategies have been proposed to identify and better characterize those 1193 impacts.

1207

  Coastal regions are amongst the most populated areas on Earth. The decomposition of 1208 hydrates, mainly due to climate change, may reduce seafloor stability and cause 1209 submarine landslides with associated tsunamis impacting coastal communities. The 1210 potential occurrence of such catastrophic socio-economic events that threaten human life, 1211 natural resources and key infrastructure in populated coastal regions highlight the need to 1212 monitor the dynamics of NGHD. This review has shown that field observations and 1213 measurements are limited in space and time. Field observations are crucial to achieving 1214 an in-depth and comprehensive understanding of the operating environment, and to 1215 developing an appropriate environmental monitoring and management plan. Currently, 1216what we observe on a given site is not representative for other sites and an intensification 1217 of the measurements of key parameters, both spatially and temporally, is urgently needed.

Impact of hydrate decomposition on permafrost 351

  probably not sufficiently safe to allow the implementation of 348 infrastructure for hydrate production at the seabed without taking specific mitigation 349 measures into account (e.g. relocation options, design criteria).It has been generally accepted that global warming tends to destabilize gas hydrates

	352
	353	present in permafrost through different mechanisms, such as directly melting top layers
		of gas hydrates

350

ii.

Table 1 :

 1 Dissolved methane fluxes measured from field study of hydrate dissolution

	596
		Hydrate dissolution
	Location	Dissolution rate/cm.yr -1	Dissolution rate/ mmol.m -2 .yr -1	Reference
	Monterey Bay (Monterey Canyon-MontC)	187 * -220 *	1.07 -1.26 x 10 7	Rehder et al. (2004)
	Gulf of Mexico (Green	15 # (GC)	8.57 x 10 5	Lapham et al. (2014)
	Canyon-BC and Mississippi
	Canyon-MC)
		0.001 #π (MC)	63	Lapham et al. (2010)
	Northern Cascadia Margin:	62 # and 104 (BC)	4.7 x 10 6# and 7.9 x 10 6	Hester et al. (2009); Lapham et al. (2014)
	(Barkley Canyon-BC),
	South Hydrate ridge, Hydrate ridge (regional	0.0005/ 0.03 # π (BC) 29/ 1800	Lapham et al. (2010)
	average-RA), Clayoquot	3.5 # (BC)	0.2 x 10 6	Lapham et al. (2010)
	Slope (CS)	0.06-0.33 (CS)	3.43 x 10 3 -1.88 x 10 4	Romer et al. (2016) Riedel et al. (2010)
		0.06 # (BC)	3.4 x 10 3	Wilson et al. (2015)

Table 2 :

 2 Free methane fluxes measured from field study of hydrate dissociation and over a hydrate deposit

	600
		Hydrate dissociation
	Location	Methane flux from gas discharged at hydrate sites	Reference
		0.027 Tg yr -1 (30-km-long plume-area)	Westbrook et al.
	Svalbard Continental margin;	potentially 20 Tg yr -1 (22300 km 2 )	(2009)
	West Svalbard
		2 mMol.m -2 yr -1	Thatcher et al (2013)
		0.0053-0.029 Tg yr -1 over the next 300	Marín-Moreno et al
		years; e.g 2.5-13.5 mol yr -1 m -2	(2013)

processes and methane discharge at the 602 seafloor 603

Currently global methane emissions are about 500-600 Tg per year

(Ehhalt et al., 2001;

604

Saunois et al., 2020)

(see section 4.iv "Transfer to the atmosphere"), with around 70% 605 (350-400 Tg) of which is due to methanogenesis from a wide variety of habitats including 606

Table 3 :

 3 Rates of methanogenesis, anaerobic methanotrophy (AOM) and aerobic methanotrophy (AeOM)

	617
	in various microbial marine habitats.

  Recycle Reactor coupled to quantitative PCR of mcrA gene abundances(Colwell et al., 2008). ** Vigneron 621 et al., 2015, Cold seeps at Guaymas Basin in Sonora margin; *** Wellsbury et al., 2000, Blake Ridge; **** (Gao 622 et al., 2022). *****[START_REF] Parkes | Biogeochemistry and biodiversity of methane cycling in subsurface marine sediments 1810[END_REF] and[START_REF] Newberry | Diversity of prokaryotes and methanogenesis in deep subsurface sediments from the 1770 Nankai Trough[END_REF]) ******Boetius and Wenzhöfer, 2013. The

	623
	624	< sign indicate underlying uncertainties.
	625	The formation of methane involves a group of strictly anaerobic archaea called
	626	methanogens. However, smaller amounts of methane are produced from
	627	methylphosphonates in oxic waters by both phytoplanktons with mainly Cyanobacteria
	628	and Thaumarchaea (Bižić et al., 2020; Klintzsch et al., 2019; Metcalf et al., 2012).
	629	Methanogens use various low-molecular-weight catabolic substrates for biosynthesis of
	630	methane, and thus the rate of methanogenesis can be difficult to estimate. The Redox
	631	reaction of dihydrogen (H2) with carbon dioxide (CO2) and/or with formate are
	632	considered as the main substrates for methanogenesis in marine sediments (Newberry et
	633	al., 2004; Parkes et al., 2007). However, it has been shown that acetate can become the
		dominant substrate in deeper sediments (Reeburgh, 2007). Methanogens also performed
		-2 d -
		1******

* Estimation of microbial methane production rates in deep marine sediment at hydrate ridge is based on 619 representative species of methanogen Methanoculleus submarinus starved in culture with Biomass 620 634 methanogenesis with methylated compounds. Methylotrophic methanogenesis may be an 635 important methane source in coastal shallow sediment

[START_REF] Conrad | Importance of hydrogenotrophic, aceticlastic and methylotrophic 1331 methanogenesis for methane production in terrestrial, aquatic and other anoxic 1332 environments: A mini review[END_REF]

. Acetate and other 636 potential substrates for microbial methanogenesis could derive from either thermal 637 alteration or biological degradation of organic matter. Recent developments in high 638 throughput sequencing techniques have revealed an unexpected larger diversity within 639 the methanogens and suggest that this metabolic pathway is one of the most ancient on 640 Earth

[START_REF] Vanwonterghem | Methylotrophic methanogenesis discovered in the archaeal phylum[END_REF][START_REF] Wang | A methylotrophic origin of methanogenesis and early divergence of anaerobic 2133 multicarbon alkane metabolism[END_REF]

. Metagenome assembled genomes 641 (MAGs) of potential methanogens suggest the acquisition of both novel functions in 642 activation of alternative catabolic substrates such as short and medium chain of alkanes 643 and volatile fatty acids (Laso-Pérez et al., 2019; Wang et al., 2021), and in derivation of 644 cellular energy from methoxylated aromatic compounds from lignin occurring in coal bed 645

  al.,

	755
	756	2014). At Hydrate Ridge in the Cascadia margin subduction zone, carbon volume
	757	estimations suggest that 14% of the oxidized methane is sequestered into this
	758	seafloor mineral. Thus, carbonate mounds may represent a significant methane
		sink in deep-sea

Acknowledgements 1220