Giovanni Aloisi
email: aloisi@ipgp.fr

Tatiana V Pogodaeva

Jeffrey Poort

Andrey V Khabuev

Andrey V Kazakov

Grigorii G Akhmanov

Oleg M Khlystov

Giovani Aloisi

Jerrfey Poort

Biogeochemical processes at the Krasniy Yar seepage area (Lake Baikal) and a comparison with oceanic seeps

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Introduction

Lake Baikal (eastern Siberia) is both the largest lake in terms of water volume and one of the lakes with the oldest and thickest sedimentary pile in the world [START_REF] Logachev | History and geodynamics of the Baikal rift Russian[END_REF]. The organic matter productivity in the water column is low and comparable to that of oligotrophic water masses of the ocean [START_REF] Weiss | Deep-water renewal and biological production in Lake Baikal[END_REF][START_REF] Yoshida | Seasonal dynamics of primry production in the pelagic zone of southern Lake Baikal[END_REF][START_REF] Yoshioka | Distribution of dissolved organic carbon in Lake Baikal and its watershed[END_REF]. The organic content of Lake Baikal sediments (Corg = 1.5-2.5 wt-%; ), however, is similar to that of oceanic sediments of highly productive continental margins [START_REF] Prokopenko | Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed Quaternary[END_REF][START_REF] Qiu | Biogenic silica accumulation and paleoproductivity in the northern basin of Lake Baikal during the Holocene[END_REF][START_REF] Vologina | Types of Holocene deposits and regional pattern of sedimentation in Lake Baikal[END_REF]. This is due to transport of terrestrial organic matter (mainly humic acids and cellulose produced by landplants) from land, and to a minor component derived from organic matter produced by phytoplankton in the water column [START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF]. The hydrocarbon system of this lacustrine basin bears many resemblances to marine hydrocarbon systems [START_REF] Hovland | Methane and minor oil macro-seep systemstheir complexity and environmental significance[END_REF][START_REF] Moore | Climate Change and the World's ''Sacred Sea''-Lake Baikal[END_REF][START_REF] Yoshida | Seasonal dynamics of primry production in the pelagic zone of southern Lake Baikal[END_REF]. Methane of both biological and thermogenic origin has been detected in Lake Baikal sediments [START_REF] Kalmychkov | Geochemical characteristics of methane from sediments of the underwater high Posolskaya Bank (Lake Baikal)[END_REF]. The resemblance of the hydrocarbon system with those of oceanic basins extends to shallow sediments and the benthic boundary, because, similar to the ocean, the expulsion of methane-rich fluids to bottom waters is widespread in Lake Baikal. Shallow methane seeps in sediments of Lake Baikal have been known since the 17 century due to visual observation of free gas bubbling at the lake surface or trapped in the ice cover [START_REF] Granin | Gas hydrates and gas venting in Lake Baikal[END_REF]. The expulsion at larger water depths (>400 m) of lowtemperature methane-charged fluids to bottom waters has been observed for the first time in Lake Baikal in the Frolikha bay using temperature and heat flow measurements [START_REF] Crane | Hydrothermal vents in Lake Baikal[END_REF][START_REF] Golubev | A high-resolution diatom record of the paleoclimates of East Siberia for the last 2.5 My from Lake Baikal[END_REF]. Since this first discovery, cold seeps areas with acoustic free gas flares have been discovered more than 20 locations in the southern, central and northern basins [START_REF] Granin | Gas seeps in Lake Baikaldetection, distribution, and implications for water column mixing[END_REF]. The seepage of methane-charged fluids is accompanied by venting of methane-gas bubbles that produce characteristic flares in echosounder images (Makarov et al. 2016b). Cold seeps occur in normally-sedimented portions of the seafloor, but also in correspondence of mud volcanoes [START_REF] Khlystov | Gas hydrate of Lake Baikal: Discovery and varieties[END_REF]. Gas hydrates are widespread in cold seep areas located at a water depth greater than 380 m meters [START_REF] Khlystov | Gas hydrate of Lake Baikal: Discovery and varieties[END_REF][START_REF] Khlystov | The experience of mapping of Baikal subsurface gas hydrates and gas recovery[END_REF][START_REF] Matveeva | Gas hydrate accumulation in the subsurface sediments of Lake Baikal (Eastern Siberia)[END_REF]. The picture that emerges from the geological and geophysical observations carried out in Lake Baikal over the past 15 years, is that of cold seep systems comparable in size and frequency to those of oceanic, high-productivity continental margins.

But while the geological and sedimentary pre-requisites for the presence of cold-seep systems might be similar in Lake Baikal and in the ocean, little attention has been given to possible differences in biogeochemical processes. In fact, the very different chemical composition of Lake Baikal water masses and seawater, likely has important consequences on cold-seep biogeochemical processes. In particular, the every low dissolved SO4 2-concentration of Lake Baikal, where SO4 2-is 500-fold less abundant than in seawater [START_REF] Falkner | The major and minor element geochemistry of Lake Baikal[END_REF][START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF], will strongly limit the anaerobic oxidation of methane with sulfate (AOM) [START_REF] Boetius | A marine microbial consortium apparently mediating anaerobic oxidation of methane[END_REF], a major methane sink at oceanic cold seeps. This is bound to have consequences on the chemical environment of cold seep shallow sediments. With the geophysical and geological investigations of Lake Baikal cold seeps being recently complemented with geochemical and microbiological investigations [START_REF] Lomakina | Diversity of Archaea in Bottom Sediments of the Discharge Areas With Oil-and Gas-Bearing Fluids in Lake Baikal[END_REF][START_REF] Zemskaya | Bacterial communities in sediments of Lake Baikal from areas with oil and discharge[END_REF][START_REF] Zemskaya | Geochemical and microbiological characteristics of sediments near the Malenky mud volcano (Lake Baikal, Russia), with evidence of Archaea intermediate between the marine anaerobic methanotrophs ANME-2 and ANME-3[END_REF], a first comparison of biogeochemical processes of Lake Baikal and oceanic seeps can be made.

The goal of this paper is to describe the biogeochemical processes taking place in the Krasniy Yar deep water cold seepage area of lake Baikal and compare them with the processes active at seepage areas in high-productivity continental margins of the ocean. Because of the very different chemical composition of Lake Baikal and oceanic water masses, also the biogeochemical processes in sediments unrelated to methane seepage will differ. We therefore include oceanic continental slope sediments and Selenga river delta slope sediments in our comparison, with the goal of providing a comprehensive view of the similarities and differences between Lake Baikal and oceanic cold seeps that will help guide future research in Lake Baikal.

The Krasniy Yar seepage area

The Krasniy Yar gas and fluid seepage area was discovered during a geophysical survey in the delta front of the Selenga River in 2007 at a water depth between 740 and 780 meters (figure 1). On side scan sonar records it consists of several areas of strong backscatter compared with the surrounding seabed. Gas-bearing feeder channels were imaged on profiler records. The first geological sampling campaign for «Krasny Yar» structures was conducted in 2012. Nearby the seepage structures, upward along the flank of the Selenga delta, a scarp is located at a water depth between 360 and 380 m; it is characterized by a gently concave upwards shape in cross-section. On multibeam data downslope from the escarpment, this scarp appears as an amphitheatre-shaped scour that probably formed by submarine slumping.

Based on cores and subbottom seismic profiles, it seems that the propagation of the slumped sediments has been stopped by a seabed sediment ridge. Towards the north, the ridge is breached and the slumped material funnels out through a narrow opening spreading down the slope by sediment gravity flows.

Sediment cores and heat flow measurements were obtained during cruise VER-15-02 from the central and peripheral parts of the Krasniy Yar seep area, both within and outside of the high backscatter areas seen on the side scan sonar records (figure 1). Five cores (092G, 093G, 094G, 096G and 102G) were taken outside of the strong sidescan backscatter areas; four cores (088G, 097G, 099G and 100G) were taken near the border of strong sidescan sonar backscatter areas and one core (095G) was taken at the centre of the largest sidescan sonar strong backscatter area.

Methods

The side scan sonar image shown in figure 1 was obtained during cruise VER-07-01 as detailed in [START_REF] Khabuev | Gas hydrate resources estimation by geophysical methods in "Krasny Yar" underwater gas seep of Lake Baikal Reports of the Siberian Branch of teh Acadey of[END_REF]. Gravity cores were obtained with a core barrel assembled at the Limnological Institute, Irkutsk. The 128 mm diameter steel core barrel was used with 100 mm diameter plastic liner. The barrel has an upper valve and a leaf-type core catcher to keep the sediment inside the barrel during core retrieval. The maximum sub-bottom penetration of the gravity cores was 350 cm.

Bottom sediments for chemical analysis of pore water were taken from the middle part of the core every 2-3 cm and every 5-10 cm from cores with an undisturbed structure. The sediments were centrifuged at 8,000 rpm followed by another ultracentrifugation at 14,000 rpm to remove microparticles from pore waters. The solutions were filtered with disposable syringes through a 0.20 μm filter (cellulose acetate, Vladisart, Russia) and then injected into chromatography tubes. Pore water anion concentrations were measured onboard the ship immediately after preparation by means of liquid chromatography, on a Milichrom-2А chromatograph with a relative error of 5-10% [START_REF] Baram | Application of high-efficiency micocolumn liquid chromatography with UV detection for determining anions in environmental objects[END_REF]. The most important advantage of this special Baikal water method is that it uses potassium hydrogen phthalate as an eluent, enabling SO4 2-, Cl -and NO3 -to be determined simultaneously, along with bicarbonate (НСО3 -). The detection limits are 0.1 mg/L for all anions. The measurement of one sampling point was accomplished in a maximum of 10 min. The samples for cation analysis were stored in polystyrene vials pre acidified with 50 μL of ultrapure concentrated HNO3 and stored at 4 °C prior to analysis. Cation (Ca 2+ , Mg 2+ , Fe 2+ , Mn 2+ ) concentrations were determined in the laboratory LIN SB RAS within a week by means of atomic absorption on an AAS-30 Carl Zeiss Jena spectrophotometer, as well as by flame emission methods (for Na + and K + ; [START_REF] Fomin | Water. Control of Chemical, Bacteriological and Radiation Safety According to International Standards[END_REF]) with a relative error of 3-5%. The detection limits were 0.01 mg/L (for Na + , K + , Ca 2+ , and Fe 2+ ) and 0.001 mg/L (for Mg 2+ and Mn 2+ ).

For the majority of the coring stations, the corer has been equipped with 5 autonomous highprecision temperature loggers (type THP/NKE®, temperature resolution of better than 5/1000K) to measure thermal gradients in the sediments at four intervals of 0.5 m. The corer has to stay at least 7-10 minutes in sediment without tension on the corer in order to allow the main dissipation of heat friction. An S2IP logger (NKE®) monitored water depth and tilt angle of the corer. The sediment equilibrium temperatures are obtained using the THPread software (F. Lucazeau). Heat flow is determined as the product of the vertical temperature gradient in the surface sediments and the thermal conductivity of the sediments. For the latter a regional average of 0.8 W:m/K from previous studies is applied [START_REF] Golubev | A high-resolution diatom record of the paleoclimates of East Siberia for the last 2.5 My from Lake Baikal[END_REF][START_REF] Poort | Absence of a regional surface thermal high in the Baikal Rift; new insights from detailed contouring of heat flow anomalies[END_REF].

Results

A summary of the core positioning with respect to the side-scan record and of the lithological, heat flow and pore water characters of the sediments included in this study is given in table 1.

Lithology of sediments and heat flow in the Krasniy Yar seepage area

The lithologies sampled from the Krasniy Yar seepage area show features that are characteristic of both normal background sedimentation in the southern Baikal basin, as well as evidence for the seepage of methane-charge fluids. These features correlate well with the position of the cores with respect to patches of strong backscatter on the side-scan sonar. The five cores taken outside of the areas of strong backscatter (cores 092G, 093G, 094G, 096G and 102G) contain the normal background recent sedimentary sequence of the southern Baikal basin. This consists of a 100 to 170 cm thick layer of clayey diatomic ooze with an upper, <10 cm thick, oxidised, watery interval containing a soft Mn-and Fe-oxide crust. The portion of this top layer beneath the oxidised interval often contains hydrotroilite laminae and some pure diatomic laminae a few cm in thick. The clayey diatomic ooze is the typical Holocene sediment of lake Baikal (Grachev et al. 1998). Below the clayey diatomic ooze, the cores have retrieved butter-like, consolidated clay sediments, often bearing hydrotroilite laminae, that are the typical Pleistocene sediment of lake Baikal. The four cores taken near the border of strong side-scan sonar backscatter areas (088G, 097G, 099G and 100G) also contain the Holocene clayey diatomic ooze and the underlying Pleistocene clay typical of the background southern Baikal sedimentary succession. However, compared to the cores that were taken outside from the area of strong backscatter, the cores taken near the border of this area contain clear evidence for the seepage of methane charged fluids. These include sediments which are extremely gas-saturated (core 088G), sediments bearing gas escape structures (cores 097G and 100G) and sediments containing lenticular gas hydrate accumulations (core 097G). The core taken at the centre of the largest area of strong backscatter recorded by the side-scan sonar (095G) contains 58 cm of clay with abundant lenticular gas hydrate accumulations.

In the corer stations characterised by low back-scatter and normal lithological stratification, a heat flow is measured in the range of 49-78 mW/m² (average 61±10 mW/m², which is close to the background heat flow of 55-70 mW/m² reported for all Baikal [START_REF] Poort | Absence of a regional surface thermal high in the Baikal Rift; new insights from detailed contouring of heat flow anomalies[END_REF]. In station 95G where the side-scan backscatter is high, a heat flow is measured that is three times higher than normal (170 mW/m²).

Pore waters in the Krasniy Yar seepage area

We present the pore water geochemical composition (HCO3 -, Ca 2+ , Mg 2+ , K + , Na + , Cl -, SO4 2- and NO3 -) of 10 sediment cores taken from the Krasniy Yar seepage area. Three patters of down-core pore water geochemistry can be observed: (1) the typical Baikal slope sediments diagenesis pattern, (2) geochemical profiles suggesting advection of fluids from below, and

(3) geochemical profiles suggesting inflow of bottom lake waters into the sediment. These three patterns will be described below.

Baikal slope sediments

Seven cores (088G, 092G, 093G, 094G, 096G, 097G and 102G) show a pore water geochemical signature that is very similar to that of typical Baikal slope sediments not influenced by fluid seepage or other sedimentological (e.g. turbidites) or geochemical (e.g. hydrothermalism) processes [START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF][START_REF] Pogodaeva | Chemical compostiion of pore waters of bottom sediments in different Baikal basins[END_REF]) (Fig. 2). These cores were taken either outside of the area of strong backscatter in the side scan sonar images (cores 092G, 093G, 094G, 096G and 102G) or near its border (cores 088G and 097G). The cores taken outside from the strong backscatter area all contain normal background Baikal sediments (see section 4.1); those taken near the border contain normal background Baikal sediments that are gas-charged. In addition, core 097G contains gas hydrates in its lower part.

These observations suggest that in the Krasny Yar seepage area, the seepage of methanecharged fluids is limited to the zone of strong backscatter seen on the side scan sonar records.

The presence of gas in cores 088G and 097G and of gas hydrates in core 097G does not modify the pore water signature that remains very similar to that of background Baikal slope sediments. We therefore conclude that no advection of methane-charged fluids is taking place in these two cores, because this would create convex-upwards pore water profiles that are not observed in these cores.

In the top 0-30 cm, that corresponds to the oxidised portion of sediments, all of the mentioned cores show a minimum in the concentrations of HCO3 -, Ca 2+ and Mg 2+ , suggesting a sink of these elements at this shallow depth. In the same interval, instead, the concentration of K + and Na + increases downcore (Fig. 2), suggesting a shallow source of these elements. Below 30 cm, the concentration of HCO3 -and of all the above cited cations (Ca 2+ , Mg 2+ , K + and Na + ) increases linearly downcore, suggesting diffusion from deeper sediment intervals. Dissolved SO4 2-is depleted in the top 20 cm while dissolved Cl -varies irregularly downcore with concentrations that are often larger than those of bottom waters. Note that the pore water geochemical signature of Baikal slope sediments differs somewhat from that of typical Baikal pelagic sediments [START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF] because in the latter dissolved Mg, K, Na and Cl stay constant with depth in the first few meters of sediments. As explained in the discussion section, this might be due to silicate weathering processes taking place at depth in Baikal sediments.

Concave upwards porewater profiles

Core 095G was taken near the centre of the area with strong backscatter on the side scan sonar record; it contains gas hydrates immersed in a clay matrix throughout its length (58 cm) and lacks completely the oxidised layer and Fe-rich and Mn-rich layers of background Baikal sediments. In this core, the pore water concentration profiles of HCO3 -, Ca 2+ , Na + and K + show a convex upwards shape that is typical for upwards advection of fluids from deeper sediment intervals (Aloisi et al. 2004a;[START_REF] Chatterjee | Pore water sulfate, alkalinity, and carbon isotope profiles in shallow sediment above marine gas hydrate systems: A numerical modeling perspective[END_REF][START_REF] Luff | Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances[END_REF][START_REF] Reitz | Sources of fluids and gases expelled at cold seeps offshore Georgia, eastern Black Sea Geochim[END_REF]Wallmann et al. 2006b) (Fig. 3). The shape of the pore water profiles of HCO3 - and Ca 2+ , in particular, is in evident contrast with that of these elements in background Baikal sediments where a clear minimum in concentration is observed in the 0-30 cm depth interval. 4.2.3 Pore waters with Lake Baikal bottom water composition Two cores have a peculiar pore water signature in that the top 100 cm (in core 100G) and 150 cm (in core 099G) contain pore waters that have the same chemical composition of Lake Baikal bottom waters (Fig. 4). These two cores were taken near the border between the area of strong backscatter on the side scan sonar and surrounding sediments; they contain normal background Baikal sediments (see section 4.1). Below this top layer with bottom water composition, chemical profiles are very similar to those of cores showing Baikal slope sediment diagenetic processes below the oxidized top layer, with HCO3 -, Ca 2+ , Mg 2+ , Na + and K + increasing downcore and the near absence of SO4 2-.

Discussion

Background biogeochemical processes in Lake Baikal slope sediments

The background biogeochemical processes in Baikal sediments have been investigated by many authors [START_REF] Granina | Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal[END_REF][START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF][START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF][START_REF] Pogodaeva | Chemical compostiion of pore waters of bottom sediments in different Baikal basins[END_REF]). Most of the efforts have been directed to understand the early diagenetic processes in the oxidised surface sediments ( 0-25 cm depth) responsible for the formation of indurated Fe(III)-and Mn(IV)-layers between 3 and 8 cm depth, and their subsequent remobilisation between 15 and 20 cm depth [START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF]. The gravity corer used to retrieve the cores presented in this work is not adapted to sample the top 30 cm of sediments without disturbing the lithological layering. Furthermore, the pore water sampling resolution used in the present study (1 sample every 20-30 cm) is too low to highlight the Mn and Fe cycling in the top 30 cm of the sediment column. Nevertheless, for the sake of completeness, we introduce these Mn-and Fe-related shallow biogeochemical processes in our discussuon, taking the relevant geochemical trends from [START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF], [START_REF] Pogodaeva | Chemical compostiion of pore waters of bottom sediments in different Baikal basins[END_REF] and [START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF]. The discussion on Mn-and Fe-related shallow diagenesis is completed with the indications of additional shallow ( 0-25 cm depth) and deeper (> 25 cm) biogeochemical processes deduced based on the data presented in this work (Fig. 2).

Figure 5a is a simplification of the main biogeochemical processes taking place in background Baikal sediments as deduced from [START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF]), Pogodeava et al. (2017;[START_REF] Prokopenko | Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed Quaternary[END_REF] and the present work. The nomenclature of geochemical environments follows that proposed by ( 2009). In the near-absence of dissolved SO4 2-( 50 mol kg -1 ) and with only minor concentrations of NO3 -(< 10 mol kg -1 ) (see table 2), organic matter diagenesis in Baikal sediments takes place mainly via oxidation with O2 (reaction 1), Mn(IV)-oxides (reaction 2), Fe(III)-oxides (reaction 3) and via methanogenesis (reaction 4) which define the relevant organic matter respiration processes (Fig. 5a): CH2O + O2 => CO2 + H2O

(1)

CH2O + 2MnO2 + 3CO2 + H2O => 4HCO3 -+2Mn 2+ (2) CH2O + 4FeOOH + 7CO2 + H2O => 8HCO3 -+4Fe 2+ (3) CH2O => CO2 + CH4 (4)
The chemical changes produced by these respiration processes give rise to the oxic, manganous, ferruginous and methanic geochemical zones [START_REF] Canfield | Towards a consistent classification scheme for geochemical environments, or, why we wish the term 'suboxic' would go away[END_REF] (Fig. 5a). Note that since dissolved Mn 2+ and Fe 2+ are not consumed below the Mn-and Fecrust in the considered sediment interval, the manganous and ferruginous geochemical zones coincide (Fig. 5a).

The HCO3 -released by reactions 2 and 3 is converted to CO2 when upwards diffusing Mn 2+

and Fe 2+ react with dissolved oxygen, precipitating as solid oxides and hydroxides in the upper portion of the Mn-and Fe-crust:

2Mn 2+ + O2 + 4HCO3 -=> 2MnO2 + 4CO2 + 2H2O (5) 4Fe 2+ + O2 + 8HCO3 -+ 2H2O => Fe(OH)3 + 8CO2 (6)
Dissolved HCO3 -is also converted to CO2 by reaction of solid Mn(IV)-oxides with dissolved Fe 2+ :

MnO2 + 2Fe 2+ + 3HCO3 -+ 2H2O => 2Fe(OH)3 + Mn 2+ + 4CO2 (7)
Upwards diffusing methane is possibly consumed anaerobically by reaction with manganese and iron oxides [START_REF] Beal | Manganese-and Iron-Dependent Marine Methane Oxidation[END_REF][START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF]):

CH4 + 4MnO2 + 7CO2 + 2H2O => 8HCO3 -+ 4Mn 2+ (8) CH4 + 8Fe(OH)3 + 15CO2 + 13H2O => 16HCO3 -+ 8Fe 2+ (9)
Reactions 1 to 9 explain the downwards changes in concentration of O2, Mn 2+ and Fe 2+ in the oxidised sediment layer, and the appearance of CH4 at the boundary between the oxic and anoxic sediments (Fig. 5a).

The major dissolved ions that analysed in the present work (HCO3 -, Ca 2+ , Na + , K + , and Mg 2+ ) (Fig. 5b) are involved in biogeochemical processes different from the ones introduced above.

Dissolved HCO3 -, Mg 2+ and Ca 2+ are consumed in the oxidised layer, while this layer is a source of Na + and K + (Fig. 5b). Net consumption of HCO3 -in the oxidised layer can take place if reactions 1 to 9 are a net sink of HCO3 -. But the cation trends need to be explained too. Reverse weathering reactions [START_REF] Michalopoulos | Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage[END_REF][START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF] could explain consumption of HCO3 -, Ca 2+ and Mg 2+ . In this process, biogenic opal reacts with metal hydroxide, cations and HCO3 -are taken up in the neo-formation of authigenic silicate phases and CO2 is released:

Biogenic opal + metal hydroxides + Ca 2+ + Mg 2+ + HCO3 -=> authigenic silicates + CO2 (10)

The availability of biogenic opal and iron hydroxides is critical for this process to take place.

These constituents are abundant in surface Baikal sediments [START_REF] Torres | Early diagenetic processes generate iron and manganese oxide layers in the sediments of Lake Baikal[END_REF].

Cation absorption-desorption processes on the surface of clay minerals can complicate this picture. The charged surfaces of clay particles transported by rivers are occupied by Na + , Mg 2+ , Ca 2+ , and K + ions [START_REF] Charlet | Fe(II)-Na(I)-Ca(II) cation exchange on montmorillonite in chloride medium: evidence for preferential clay adsorption of chloride-metal ion pairs[END_REF][START_REF] Sayles | The equilibration of clay minerals with seawater Geochim[END_REF][START_REF] Sayles | Cation-exchange characteristics of Amazon river suspended matter and its reaction with seawater Geochim[END_REF][START_REF] Von Breymann | Magnesium adroption and ion exchange in marine sediments: a multiple component model[END_REF]). In sediments, the NH4 + and H + produced by organic matter degradation processes (reactions 1 to 4), may replace the cations adsorbed on the surface of clay minerals, resulting in their release to pore fluids [START_REF] Von Breymann | Magnesium adroption and ion exchange in marine sediments: a multiple component model[END_REF][START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF]. Note that although NH4 + does not appear in the reactions above, it is produced by reactions 2 to 4, while H + ions are produced by the release of CO2 via the relevant reactions cited above. Adsorption/desorption processes will compete with other processes in defining the concentration of dissolved cations. Therefore, the increase in dissolved Na + and K + could take place if desorption from the surface of clay minerals is faster than their consumption in reverse weathering processes like reaction 10. Deeper, in the anoxic sediments, the dowcore increasing concentrations of Ca 2+ , Mg 2+ , Na + and K + could be due to adsorption/desorption from the surface of clay minerals. The marked down-core increase in HCO3 -, however, needs to be explained.

In the anoxic zone of marine sediments, detrital silicate minerals can be altered, converting methanogenic CO2 to HCO3 -, releasing cations and resulting in the formation of secondary silicate minerals:

Cation-rich silicate + CO2 => cation-depleted silicates + HCO3 -+ Ca 2+ + Mg 2+ + Na + + K + (11)
This process is known to take place in the methanogenic zone of marine sediments where the rates of organic matter production are high [START_REF] Scholz | Submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins[END_REF][START_REF] Solomon | Gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the Krishna-Godavari Basin[END_REF][START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF]). In the marine environment [START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF], silicate weathering processes dominate over absorption/desorption processes in the production of cations in the metahneogenic zone of sediments. The existence of this process in lakes was first proposed by [START_REF] Mizandrontsev | Groundwater solutions of lakes[END_REF]. [START_REF] Pogodaeva | Chemical compostiion of pore waters of bottom sediments in different Baikal basins[END_REF] proposed that this process is taking place in the reduced sediments of Lake Baikal.

The HCO3 -and cation trends in the top few meters of the anoxic zone of Baikal sediments are coherent with submarine weathering processes taking place in deeper sediments. Indeed, submarine weathering of silicates takes place over sediment depths of tens to hundreds of meters and the geochemical pore-water signature of this process is transported to shallower sediment depths by molecular diffusion [START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF]. Indeed, the downcore linear trends in the concentration of HCO3 -and of the major dissolved cations (Fig. 2) suggest transport from deeper sediment intervals rather than production in the top few meters of the Baikal sediment section.

The downcore gradient of dissolved HCO3 -in Baikal sediments can be compared to the downcore gradient in total alkalinity (TA) in the methanogenic zone of sediments of the Sakhalin slope (Sea of Okhotsk) where silicate weathering is currently taking place. HCO3 -in the major contributor to TA accounting for approximatively 75 % of TA in seawater [START_REF] Zeebe | CO2 in Seawater: equilibrium, kinetics, isotopes[END_REF]. In the methanogenic sediments of the Sakhalin slope, the submarine weathering of detrital silicates currently result is an increase of 3 mmol kg -1 of total alkalinity (TA) per meter of depth. In the methanogenic zone of Baikal sediments, the increase in HCO3 -is roughly half this amount (Fig. 2), suggesting that detrital silicate weathering is an important process also in the sediments of Lake Baikal. More work is necessary in the Baikal sediments to fully characterise the weathering of detrital silicates which, according to the HCO3 -and major cation trends discussed here, is a process likely to take place in methanogenic sediments.

Geochemical evidence for a hydrogologically active cold seep system at Krasniy Yar

We interpret the convex upwards pore water profiles in core 095G as indicative of active seepage of fluids to bottom waters. The presence of gas hydrates near the seafloor and the steep geochemical gradients in the top 10 cm of sediments suggest that upwards seepage of fluids is currently active or very recent. By assuming that dissolved HCO3 -and Ca 2+ behave conservatively in their transit through the top 56 cm of sediments, their pore water profiles can be used to estimate the rate of fluid advection by applying the following transport equation:

d C i x d x D i e ff d 2 C i x d x 2 w d C i x d x ( 12
)
where Ci (in mol cm -3 ) is the concentration of solute i (HCO3 -or Ca 2+ ), D eff (in cm 2 yr -1 ) is the effective molecular diffusion coefficient of solute i, calculated from the free solution molecular coefficient (D, in cm 2 yr -1 ) as a function of temperature and salinity using equations given in [START_REF] Boudreau | Diagenetic Models and their Implementation[END_REF] and applying Archie's law to consider the effects of tortuosity on diffusion. The applied coefficient (m = 2 in Archie's law) results in:

D eff = *D (13)
where (-) is porosity and w (in cm yr -1 ) is the upwards fluid advection rate. Porosity is set constant and equal to 0.7, to account of the fact that gas hydrates in the core decrease the porosity that is equal to 0.8-0.9 in surface surface Baikal sediments [START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF]. By imposing upper and lower boundaries equal to the measured concentrations and a sediment column length equal to the length of the core (58 cm), equation 12 is integrated in time from linear initial conditions until steady state is attained. The rate of upwards advection of fluids (w) was varied in successive model runs until the best fit of the HCO3 -and Ca 2+ model profiles to the measured concentration was obtained for and advection rate of 15 cm yr -1 (Fig. 3). To our knowledge this is the first estimate of fluid advection rates at cold seeps in the Baikal basin. This advection rate is in the range of typical fluid advection rates at oceanic colds seeps [START_REF] Luff | Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances[END_REF].

Next to core 095G where fluids are advected upwards to bottom waters (Fig. 3), cores 099G and 100G contain pore waters in the top 150 cm that have a chemical composition equal to that of bottom waters (Fig. 4). Vertically constant pore water concentrations in the top sediment layer can only be maintained if bottom waters are actively mixed in the sediment to counter the effect of molecular diffusion that tends to produce a vertically linear concentration profile (supposing a constant porosity with depth). Mixing of bottom waters into the sediment is a common process at cold seeps where it is driven by downwards entrainment of bottom waters into the sediment as a response to bubble streams rising in the sediment [START_REF] Haeckel | Bubble-induced porewater mixing: A 3-D model for deep porewater irrigation Geochim[END_REF]. Gas flares in echo-sounder records have been observed above the Krasniy Yar seepage area, (Makarov et al. 2016a) indicating that bubble streams rising in the sediments are present at Krasniy Yar. In the cold seeps of the Sakhalin Slope (Sea of Okhotsk), rising bubble streams in the sediment result in inflow of bottom waters into the sediment down to a depth of 0.5 -3 meters [START_REF] Haeckel | Bubble-induced porewater mixing: A 3-D model for deep porewater irrigation Geochim[END_REF]), a range which includes the thickness of the sediment interval bearing pore waters with a chemical composition equal to that of Baikal bottom waters (sites 099G and 100G) (Fig. 4). Together with the evidence for active expulsion of warm fluids at site 095G, the indications for convection of bottom waters in the sediment imply a hydrologically active cold seep system at Krasniy Yar.

Convection of bottom waters into the sediment and mixing with fluids advected upwards from deeper sediment intervals explains a peculiarity of the geochemical signature of the fluids expelled at site 095G: while the concave upwards shape of the HCO3 -and Ca 2+ pore water profiles clearly indicates upwards advection of fluids, the concentrations of these components at the base of the 60 cm core are lower than at 300 cm depth in the pore waters of normal Baikal sediments (Fig. 2). If upwards advection of fluids alone is responsible for the geochemical signature of pore fluids at site 095G, one would expect HCO3 -and Ca 2+ concentrations equal, or higher, than those at the base of cores from background Baikal sediments (Fig. 2). This can be expected because weathering of detrital silicates (equation 11)

will continue in the methanic zone of sediments below the cores depth of 350 cm (Fig. 2), producing further increasing the concentration of HCO3 -, Ca 2+ , Mg 2+ , Na + and K + . The relatively HCO3 -and cation-poor character of fluids at the base of core 095G, therefore, suggests that the upwards advecting fluids at the base of core 095G have mixed with Baikal bottom waters convecting in the sediment, with the result that deep fluids are diluted. This mixing-and-dilution process is a typical consequence of shallow convection at cold seeps (Aloisi et al. 2004b;[START_REF] Haeckel | Bubble-induced porewater mixing: A 3-D model for deep porewater irrigation Geochim[END_REF]) and confirms the hydrologically active nature of the Krasniy Yar cold seep system.

The potential oxidants (electron acceptors) for methane oxidation in aquatic environments are: O2 [START_REF] Hanson | Methanotrophic Bacteria[END_REF], NO3 - [START_REF] Haroon | Anaerobic oxidation of methane coupled to nitrate reduction in a novel archaeal lineage[END_REF]; Mn(IV)-and Fe(III)-oxides, [START_REF] Beal | Manganese-and Iron-Dependent Marine Methane Oxidation[END_REF]) and SO4 2-(Boetius et al. 2000). In Lake Baikal sediments, anaerobic oxidation of methane with NO3 -and SO4 2-(reaction 17) can only have a limited importance due to the very small concentration of these electron acceptors in Baikal bottom waters (Table 2). The obvious electron acceptor in Lake Baikal sediments is O2:

CH4 + O2 => CO2 + 2H2O (14)
In addition, there are indications that anaerobic oxidation with Mn(IV)-and Fe(III)-oxides (reactions 8 and 9) [START_REF] Beal | Manganese-and Iron-Dependent Marine Methane Oxidation[END_REF] might take place at seep sediments of lake Baikal [START_REF] Pimenov | Microbial oxidation of methane in the sediments of central and southern[END_REF]). These authors have measured methane concentrations and methane oxidation rates (both aerobic and anaerobic) at methane seeps in southern and central Baikal;

they show that the rates of aerobic (9 -3485 mol m -2 day -1 ) and anaerobic (24 -4125 mol m -2 day -1 ) oxidation of methane are comparable. Although the electron acceptor for the anaerobic oxidation process has not been determined, based on the low concentrations of NO3 -and SO4 2-in Lake Baikal waters, it leaves only Mn(IV)-and Fe(III)-oxides as electron acceptors. Mn(IV)-and Fe(III)-oxides are indeed very abundant in the upper centimetres of the sedimentary section of Lake Baikal [START_REF] Granina | Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal[END_REF][START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF].

Comparison with biogeochemical processes in oceanic continental slope sediments

In this section we will outline the main differences in the biogeochemical processes taking place in Baikal cold seep sediments and their background sediments of the Selenga delta slope, compared to those taking place in oceanic cold seep and continental slope sediments.

Although biogeochemical processes in background Baikal sediments are relatively homogeneous throughout the lake [START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF], biogeochemical processes in oceanic sediments vary widely, mainly reflecting differences in the concentration of organic matter and the composition of the solid phase of sediments [START_REF] Berner | Global Environment: Water, Air, and Geochemical cycles[END_REF]). In the present comparison, our "oceanic sediment" reference broadly represents sediments of continental margins with organic matter contents comparable to those of Baikal sediments (1.5-2.5 wt-%) [START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF]. In particular, we will consider biogeochemical processes in the sediments of the Sakhalin slope (Sea of Okhotsk) (Wallmann et al. 2006a) where silicate weathering processes such as those we propose for Baikal sediments have been observed [START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF].

Baikal water vs oceanic water composition

Sedimentary biogeochemical processes are much affected by the composition of bottom waters, which both defines the upper boundary of the biogeochemical system and is the source of pore waters trapped in the sediment during the accumulation of the solid particles.

We therefore start by comparing the chemical composition of Lake Baikal water with that of ocean water. This comparison is summarised in table 2. The salinity of Lake Baikal water (Salinity = 100 mg litre -1 ) is two orders of magnitude smaller than that of ocean water (Salinity = 35 g litre -1 ). The most abundant dissolved ions in Lake Baikal are HCO3 -(1.09 mM) and Ca 2+ (0.4 mM). With regards to cold seep biogeochemical processes, the most important difference with seawater is the very small concentration of dissolved SO4 2-in lake Baikal waters (57.4 M), compared to seawater (28.9 mM). As we will see further on, the lack of dissolved SO4 2-in Baikal water decreases the microbial filter resulting in enhanced methane fluxes to bottom waters compared to methane seeps in oceanic settings. All other ions that form the largest part of the dissolved salts in seawater (Cl -, Na + , Mg 2+ and K + ) are present in Lake Baikal waters in only very small concentrations ( 150 M).

Biogeochemistry of background Baikal slope sediments vs ocean sediments

The nearly total absence of SO4 2-and NO3 -, in Baikal lake waters (table 2) has the effect that, compared to oceanic sediments, in Baikal sediments the nitrogenous and sulfidic geochemical zones are relatively less important than the other geochemical zones (Fig. 5 a and5c). This has important consequences for the alkalinity balance in sediments and the formation of authigenic mineral phases. In fact, due to the scarcity of electron acceptors SO4 2-and NO3 -, a greater portion of the DIC produced by organic matter degradation (reactions 1 to 4) and subsequent red-ox reactions is in the form of CO2 rather than HCO3 -. The first source of missing HCO3 -in lake Baikal is that produced by the oxidation of organic mater with dissolved SO4 2-, which, in the ocean, is a major pathway of organic matter degradation [START_REF] Bowles | Global rates of marine sulfate reduction and implications for sub-sea-floor metabolic activities[END_REF]):

C(H2O) + 0.5 SO4 2-=> 0.5 HS -+ 0.5 CO2 + 0.5 HCO3 -+ 0.5 H2O

Therefore, also missing, or extremely limited, are the HCO3 --producing redox processes that rely on HS -produced by reaction 14 as a reducing agent: 2Fe(OH)3 + HS -+ 5 CO2 => Fe 2+ + S° + 5 HCO3 -+ H2O ( 16)

MnO2 + HS -+ 3 CO2 + H2O => Mn 2+ + S° + 3 HCO3 - (17)
As well as the anaerobic oxidation of CH4 with dissolved SO4 2-that is an important source of HCO3 -in marine sediments [START_REF] Egger | Global diffusive fluxes of methane in marine sediments[END_REF]):

CH4 + SO4 2-=> HCO3 -+ HS -+ H2O (18) Thus, compared to ocean sediments, Baikal sediments (i) contain only limited amounts of dissolved sulfide and (ii) can be expected to produce relatively more CO2 than HCO3 -, leading to diagenetic environments characterised by a lower pH. The absence of H2S and the low pH have important consequences on the fate of the solid phase of sediments : (1) sulfide minerals such a pyrite (FeS2) are less abundant in Baikal sediments than in oceanic sediments, [START_REF] Boudreau | Diagenetic Models and their Implementation[END_REF] authigenic carbonate minerals are less likely to form in Baikal sediments than in oceanic sediments and (3) the low pH will stimulate both adsorption/desorption processes on the surface of clay minerals [START_REF] Charlet | Fe(II)-Na(I)-Ca(II) cation exchange on montmorillonite in chloride medium: evidence for preferential clay adsorption of chloride-metal ion pairs[END_REF][START_REF] Sayles | The equilibration of clay minerals with seawater Geochim[END_REF][START_REF] Sayles | Cation-exchange characteristics of Amazon river suspended matter and its reaction with seawater Geochim[END_REF][START_REF] Von Breymann | Magnesium adroption and ion exchange in marine sediments: a multiple component model[END_REF]) and the weathering of detrital silicate minerals [START_REF] Wallmann | Silicate weathering in anoxic marine sediments Geochim[END_REF]. The scarcity of neo-formed carbonate and sulfide minerals in Baikal sediments, which are a sink or Mn 2+ and Fe 2+ , has the effect that dissolved Fe 2+ and Mn 2+ concentrations remain relatively important in the methanic zone of sediments, such that in Baikal sediments there is not net separation of the manganous, ferruginous and methanic geochemical zones (Fig. 5a), which is a characteristic of oceanic sediments (Fig. 5c.)

Biogeochemistry of Baikal cold seeps vs oceanic cold seeps

The most evident different between cold seep biogeochemical processes in Lake Baikal, compared to those of the oceanic realm, is the negligible role of sulfate in Lake Baikal sediments as an electron acceptor for the anaerobic oxidation of methane (reaction 18). In oceanic cold seeps, the anaerobic oxidation of methane with sulfate is a very effective benthic filter for methane advecting from deeper sedimentary intervals. [START_REF] Luff | Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances[END_REF] show that up to fluid advection rates of 50 cm yr -1 , 99 % of the deep methane flux is consumed by reaction with sulfate. Only at advection rates greater than about 100 cm yr-1, is a significant methane flux to bottom waters produced. At the cold seeps of Lake Baikal, where only O2, and possibly Mn(IV)-and Fe(II)-oxides, are used as elector acceptors, it is likely that a larger percentage of the deep methane flux passes to bottom waters: the microbial filter in Lake Baikal is less effective than in the ocean.

The negligible importance of methane oxidation with sulfate has the consequence that carbonate mineral precipitation is very limited in Lake Baikal cold seep sediments. At oceanic cold seeps, the large HCO3 -flux produced by the oxidation of methane with sulfate (reaction 18) triggers the widespread precipitation of auhtigenic carbonates in the form of nodules in the sediments and carbonate crust pavements on the seafloor [START_REF] Aloisi | party Ms Methane and gas hydrate-related authigenic carbonate crusts in the mud volcanoes of the eastern Mediterranean Sea[END_REF][START_REF] Bohrmann | Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability[END_REF][START_REF] Luff | Numerical modeling of carbonate crust formation at cold vent sites: significance for fluid and methane budgets and chemosynthetic biological communities Earth and[END_REF]). These well-recognisable methane-related carbonates are absent in Baikal cold seep sediments, where only a very limited amount of authigenic siderite and rhodocrosite have been observed [START_REF] Krylov | Authigenic rhodochrosite from a gas hydrate-bearing structure in Lake Baikal[END_REF][START_REF] Krylov | Crystallization of authigenic carbonates in mud volcanoes at Lake Baikal[END_REF]. Of the other methane sinks discussed above, the aerobic oxidation of methane (reaction 14) produces CO2, a weak acid that inhibits carbonate mineral precipitation. The oxidation of methane with Mn(IV)-and Fe(III)-oxides (reactions 8 and 9), instead, produce HCO3 -, but the lack of authigenic carbonates suggests that the rates of these processes are not high enough to significantly shift dissolved inorganic carbon equilibrium in favour of carbonate mineral precipitation. Finally, limited carbonate mineral precipitation in Lake Baikal cold seep sediments is also due to a concentration of dissolved calcium 20 times lower than that of the ocean (table 2). No extensive carbonate pavements have been observed during MIR submersible dives on Baikal cold seeps in 2008-2010 (O. Khlystov, personal communication) (O. Khlystov, personal communication).

In support of the above arguments, the rate of fluid flow (15 cm yr -1 ) constrained at site 095G with the transport model (section 5.2) is in the range of fluid flows that results in the widespread formation of authigenic carbonate crusts at cold seeps [START_REF] Luff | Numerical modeling of carbonate crust formation at cold vent sites: significance for fluid and methane budgets and chemosynthetic biological communities Earth and[END_REF]).

However, we have found no macroscopic evidence of authigenic carbonate in core 095G.

This confirms that the lack of AOM in Baikal seep sediments radically changes the TA/DIC balance near the benthic boundary. By extension, the TA/DIC balance of the fluids emitted at cold seeps will also be different in Baikal seep compared to the ocean, with Baikal seeps emitting fluids with a lower TA/DIC ratio that oceanic cold seeps where AOM is important.

In conclusion, in comparison with oceanic methane seeps, biogeochemical processes at methane seeps in Lake Baikal: (1) have a reduced capacity to decrease the deep methane flux, likely resulting in larger benthic methane fluxes for a given fluid advection rate; (2) result in a larger CO2/HCO3 -ratio when methane is oxidised; (3) result in very limited carbonate mineral precipitation.

Directions for future research on Baikal cold seep biogeochemistry

The investigation of cold seep biogeochemical processes in Lake Baikal sediments is in its infancy. As geophysical and geological investigations are showing that cold seepage is widespread in the Baikal basin [START_REF] Khlystov | Gas hydrate of Lake Baikal: Discovery and varieties[END_REF][START_REF] Khlystov | The experience of mapping of Baikal subsurface gas hydrates and gas recovery[END_REF], geochemical and microbiological investigations suggest that, biogeochemically, Lake Baikal cold seeps are distinct from their oceanic counterpart. Based on the results of the present study, and of the work by [START_REF] Pogodaeva | Chemical compostiion of pore waters of bottom sediments in different Baikal basins[END_REF], [START_REF] Pogodaeva | Background composition of pore waters in Lake Baikal bottom sediments[END_REF] and [START_REF] Och | New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments[END_REF], we propose the following directions for future research to better understand the biogeochemical processes active at Lake Baikal cold seeps:

A full set of pore water geochemical measurements that include dissolved O2, CH4 and TA concentrations, but also measurements of sedimentation rate and porosity, will enable to apply more sophisticated transport-reaction models [START_REF] Luff | Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances[END_REF]) to estimate the rates of methane consumption in the sediment and evaluate the efficiency of the benthic microbial filter in mitigating methane fluxes to Lake Baikal bottom waters;

Microbial experiments to investigate the anaerobic oxidation of methane using Mn(IV)and Fe(II) oxides as electron acceptors, will mechanistically inform on the existence of these processes in Lake Baikal cold seep sediments. The abundance of these minerals in the very shallow sediments of Lake Baikal, together with the very low sulfate concentration, make this a particularly promising setting to investigate these processes;

Finally, spatially-resolved video-observations and bottom sampling operations with video-guided multicorer methods will make it possible to associate specific geochemical signatures (upwards advection vs convection, for example) to macroscopic features of cold seep areas such as bubble streams -an essential step to extend investigations from process-oriented to the estimate of local and regional geochemical methane budgets.

Conclusion

The new pore water geochemical profiles obtained from the Krasniy Yar seepage of Lake

Baikal, together with published data on biogeochemical processes in Lake Baikal slope sediments not affected by cold seepage, allows to draw the following conclusions with regards to the differences with biogeochemical processes at oceanic cold seeps: (1) the very different chemical composition of Lake Baikal water mass and the oceanic water mass has an important impact on biogeochemical processes; (2) in sediments not affected by seepage low bottom water SO4 2-and NO3 -concentrations result in a very low relative importance of the nitrogenous and sulfidic geochemical zones in Baikal compared to the ocean; (3) processes involving silicate minerals (shallow reverse weathering and deeper, detrital silicate weathering), instead, seem to act in a similar way in Lake Baikal and oceanic sediments; (4) in Lake Baikal cold seep sediments the deep methane flux is mitigated by reaction with O2, and possibly Mn(IV)-and Fe(III)-oxides, whereas in oceanic sediments the main methaneconsuming process is the anaerobic oxidation of methane with sulfate. As a consequence, Lake Baikal cold seep sediments experiencing advection rates similar to ocean seeps are nearly devoid of authigenic carbonate minerals, while extensive carbonate crust pavements cover large portions of the seafloor in oceanic cold seep systems. Tables Table 1 -

Figure 1 -

 1 Figure 1 -Location map of the Krasniy Yar seepage area discussed in this work. (A) geographical location; (B) side-scan sonar backscatter map showing the location of cores and heat-flow measurements and (C) sub-bottom profiler across the Krasniy Yar seepage area .

Figure 2 -

 2 Figure2-Pore water concentrations of (a to g) HCO3 -, Ca 2+ , Mg 2+ , Na + , Cl -, SO4 2-and K + in cores 088G, 092G, 093G, 094G, 096G, 097G, 102G from the Krasniy Yar seepage area that show the typical geochemical signature of Baikal sediments not influenced by seepage; (h) and (i) show the concentrations of HCO3 -and Ca 2+ in the top 80 cm of cores 088G and 096G.Note: the blue vertical dashed lines indicate the composition of lake Baikal bottom waters.

Figure 3 -

 3 Figure 3 -Measured (dots) and modelled (red and dashed black lines) concentration profiles of HCO3 -, Ca 2+ , Mg 2+ , Na + , Cl -, SO4 2-and K + in core 095G from the Krasniy Yar seepage area that shows the typical geochemical signature of upwards advection of fluids. Notes: numbers in panels HCO3 -and Ca 2+ represent the advection rates (in cm yr -1 ) that correspond to the model curves; the blue vertical dashed lines indicate the composition of lake Baikal bottom waters.

Figure 4 -

 4 Figure 4 -Pore water concentrations of HCO3 -, Ca 2+ , Mg 2+ , Na + , Cl -, SO4 2-and K + in cores 099G and 100G from the Krasniy Yar seepage area that show the typical geochemical signature of downflow of bottom Baikal waters into the sediments. Note: the light brown shaded area represents the maximum depth of inflow of bottom waters in core 099G.

Figure 5 -

 5 Figure 5 -Summary of biogeochemical processes in Lake Baikal sediments (a, b) and oceanic continental margin sediments (c, d) not affected by seepage of methane-charged fluids.

Figure 6 -

 6 Figure 6 -Summary of biogeochemical processes in Lake Baikal sediments (a) and oceanic sediments (b) affected by the seepage of methane-charged fluids.

Table 2 -

 2 Summary of main sedimentological and geochemical features of cores from the Krasniy Yar seepage area taken during cruise VER-15-02 Notes: a see section 4.1 for a description of background Baikal sediments; b see section 4.2 for a description of background sediment pore water composition. Comparison of the most abundant dissolved ions in seawater and in Lake Baikal waters

	Gravity Core	Side-scanner backscatter	Lithology	Heat flow (mW/m²)	Pore water
	92	Low	background Baikal a background (49) background Baikal b
	93	Low	background Baikal background (72) background Baikal
	94	Low	background Baikal + gas	background (55) background Baikal
	96	Low	background Baikal background (64) background Baikal
	102	Low	background Baikal background (52) background Baikal
	88	Border	background Baikal + gas		background Baikal
			background Baikal
	97	Border	+ gas + gas		background Baikal
			hydrates
	99	Border	background Baikal background (78) inflow
	100	Border	background Baikal + gas	background (60) inflow
	95	Inside strong backscatter area	clay + gas hydrates elevated (170)	upwards advection of fluids

Acknowledgements

This work was supported by a bilateral French-Russian funding: PRC-CNRS programme (Shy@Baikal, nr. 1072) and the RFBR research project №16-55-150005. The cruise on R/V Vereshagin took place in the framework of the Class@Baikal TTR expeditions, and was organized by LIN and MSU. We thank the captain and its crew for their professional assistance.