José Seco
email: jseco@ua.pt

José C Xavier

Joao Coelho

Bárbara Pereira

Geraint Tarling

Miguel A Pardal

Paco Bustamante

Gabriele Stowasser

Andrew S Brierley

Maria E Pereira

João P Coelho

Spatial variability in total and organic mercury levels in Antarctic krill Euphausia superba across the Scotia Sea

Keywords: Food-web, Eggs, Organic Mercury, Southern Ocean, Antarctica

établissements d'enseignement et de recherche

Introduction

Mercury contamination in the environment has been acknowledged as a global problem, and the production and use of this element is nowadays very strictly regulated and limited [START_REF] Selin | Global Biogeochemical Cycling of Mercury: A Review[END_REF]UNEP, 2013). Pathways of dispersion through ecosystems, including in the Antarctic, of this long-range contaminant are complex [START_REF] Streets | Projections of Global Mercury Emissions in 2050[END_REF]. Interplay between the distinctive Antarctic atmosphere and the seasonal sea-ice cycle in the Southern Ocean generates a unique combination environmental factors that can explain why the remote Southern Ocean has some of the highest reported concentrations of organic mercury (i.e. compounds containing covalent bonds between carbon and mercury) in open waters [START_REF] Cossa | Mercury in the Southern Ocean[END_REF].

Due to its high affinity for proteins [START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North Eastern Atlantic waters: Influence of geographical origin and feeding ecology[END_REF], organic mercury is the most toxic form of the element [START_REF] Clarkson | Mercury: Major Issues in Environmental Health[END_REF]. It accumulates in aquatic organisms and biomagnifies within food webs, being toxic for top predators [START_REF] Ackerman | Mercury in birds of San Francisco Bay-Delta, California: trophic pathways, bioaccumulation, and ecotoxicological risk to avian reproduction[END_REF][START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF][START_REF] Coelho | Mercury bioaccumulation in the spotted dogfish (Scyliorhinus canicula) from the Atlantic Ocean[END_REF][START_REF] Dehn | Trophic relationships in an Arctic food web and implications for trace metal transfer[END_REF]) with consequences at the population level (Goutte et al., 2014a;2014b). Wandering albatrosses are an example of this biomagnification effect in Antarctica, as it was found that they had some of the highest concentration of total mercury (from now on noted as mercury) in marine birds (up to 24.80 ± 8.61 µg g -1 dry weight) (Cherel et al., 2018;[START_REF] Tavares | Influence of age, sex and breeding status on mercury accumulation patterns in the wandering albatross Diomedea exulans[END_REF].

In the Southern Ocean, Antarctic krill, Euphausia superba, is a key species in the marine food webs connecting primary producers and higher predators [START_REF] Everson | Krill: Biology, Ecology and Fisheries[END_REF]. It has an estimated biomass of around 379million tonnes [START_REF] Atkinson | A reappraisal of the total biomass and annual production of Antarctic krill[END_REF] and being the main food for many vertebrates [START_REF] Murphy | Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web[END_REF][START_REF] Xavier | Life Beyond the Ice, in: Exploring the Last Continent: an Introduction to Antarctica[END_REF]. For example, minke whales, Balaenoptera acutorostrata and Crabeater seals, Lobodon carcinophaga, feed almost exclusively (>95 %) on Antarctic krill [START_REF] Adam | Lobodon carcinophaga[END_REF][START_REF] Armstrong | Consumption of Antarctic krill by Minke whales[END_REF][START_REF] Croll | Penguins, fur seals, and fishing: prey requirements and potential competition in the South Shetland Islands, Antarctica[END_REF][START_REF] Dimitrijević | Isotopic niches of sympatric Gentoo and Chinstrap Penguins: evidence of competition for Antarctic krill?[END_REF][START_REF] Perrin | Encyclopedia of marine mammals[END_REF]. Chinstrap penguins, Pygoscelis antarctica, Gentoo penguins, Pygoscelis papua, and other species of penguins, in the Southern Ocean, also feed mostly on Antarctic krill [START_REF] Dimitrijević | Isotopic niches of sympatric Gentoo and Chinstrap Penguins: evidence of competition for Antarctic krill?[END_REF][START_REF] Xavier | Seasonal prey switching in non-breeding gentoo penguins related to a wintertime environmental anomaly around South Georgia[END_REF] with values around 1.2 kg d -1 [START_REF] Croll | Penguins, fur seals, and fishing: prey requirements and potential competition in the South Shetland Islands, Antarctica[END_REF]. Finally, Antarctic krill is the most harvested species in the Southern Ocean, with > 260 000 tonnes fished in 2016, regulated under the Convention for the Conservation of Antarctic Living Resources (Nicol et al. 2000;Tou et al. 2007;CCAMLR 2017).

In the context of environmental change [START_REF] Constable | Climate change and Southern Ocean ecosystems I: how changes in physical habitats directly affect marine biota[END_REF][START_REF] Cossa | Marine biogeochemistry: Methylmercury manufacture[END_REF][START_REF] Gutt | The Southern Ocean ecosystem under multiple climate change stresses--an integrated circumpolar assessment[END_REF], it is important to evaluate the impact of contaminants like mercury, particularly in a remote and presumably less impacted environments such as Antarctica with the associated risk to Southern Ocean top predators. This approach will contribute to a more in-depth knowledge of mercury bioaccumulation dynamics, in an effort towards the preservation of Antarctica ecosystems into the future [START_REF] Rintoul | Choosing the future of Antarctica[END_REF][START_REF] Seewagen | Threats of environmental mercury to birds: knowledge gaps and priorities for future research[END_REF]. Despite the major role of Antarctic krill in the Southern Ocean, there are only a few studies reporting mercury concentrations in this region [START_REF] Bargagli | Biomagnification of mercury in an Antarctic marine coastal food web[END_REF]Brasso et al., 2012b;[START_REF] Locarnini | Trace element concentrations in Antarctic Krill, Euphausia superba[END_REF][START_REF] Moren | Element concentrations in meals from krill and amphipods, -Possible alternative protein sources in complete diets for farmed fish[END_REF]. Indeed, to our knowledge, no studies have ever analysed organic mercury content in Antarctic krill. Assessing the levels of organic mercury in such an important prey as Antarctic krill is crucial to better understand the pathway of this contaminant through Southern Ocean food webs. In this context, this study compares the total and organic mercury of Antarctic krill from three different locations: the South Orkney Islands, an Antarctic island group which experiences winter sea ice [START_REF] Murphy | Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web[END_REF]; South Georgia, a sub-Antarctic island free of sea ice [START_REF] Rogers | A Biophysical and Economic Profile of South Georgia and the South Sandwich Islands as Potential Large-Scale Antarctic Protected Areas[END_REF]; and the Antarctic Polar Front, a transition area from the Southern Ocean to the Atlantic Ocean with warmer waters [START_REF] Dong | Location of the Antarctic polar front from AMSR-E satellite sea surface temperature measurements[END_REF]. Under this context, differences among life stages (eggs, juveniles, adults) and sexes (males and females), were assessed and interpreted in the scope of a possible biomagnification of mercury in the Antarctic trophic web.

Material and methods

Sampling

Antarctic krill Euphausia superba were collected from the British research vessel RRS James Clark Ross during the austral summers of 2007/08, 2015/16 and 2016/17 (cruises JR177, JR15004 and JR16003 respectively). The three cruises sampled three areas of the Scotia Sea (Figure 1) with different oceanic characteristics. JR16003 had one sampling point at the Antarctic Polar Front. Both JR16003 and JR177 sampled predominantly around South Georgia, and JR15004 sampled around the South Orkney Islands.

Samples were collected from the water column using an 8 m 2 mouth-opening Rectangular Midwater Trawl (RMT8; mesh size reducing from 4.5 mm to 2.5 mm in the cod end) [START_REF] Roe | A new multiple rectangular midwater trawl (RMT 1+8M) and some modifications to the institute of oceanographic sciences' RMT 1+8[END_REF]. The net was rigged with two nets that could be remotely opened and closed at different depths. The RMT8 was used to target particularly Antarctic krill swarms and other layers of interest (e.g. fish layers) identified by the vessel scientific echosounder system (i.e. Simrad EK60/EK80 operating between 38 and 200 kHz).

Antarctic krill in the catches were identified and total length (TL) of each individual was measured, from the anterior edge of the eye to the tip of the telson and rounded down [START_REF] Morris | An assessment of the merits of length and weight measurements of Antarctic krill Euphausia superba[END_REF]. Sex and maturity stage were determined with reference to the presence of a petasma (males), thelycum (females) or absent (juveniles; individuals without visible external sexual characteristics) [START_REF] Ross | Reproduction in Euphausiacea[END_REF]. Samples were either preserved in sample bags at -20ºC (JR15004 andJR16003) or on vials in ethanol (for JR177) [START_REF] Fort | Does temporal variation of mercury levels in Arctic seabirds reflect changes in global environmental contamination, or a modification of Arctic marine food web functioning?[END_REF].

Laboratory procedures

Prior to the mercury analysis, all samples were freeze-dried for at least 24 hours. The eggs of females (Maturity stage III) [START_REF] Ross | Reproduction in Euphausiacea[END_REF] from JR177 (South Georgia) were removed under the microscope before freeze-drying. Dried individuals and tissues were homogenized and analysed for total mercury by thermal decomposition atomic absorption spectrometry with gold amalgamation, using a LECO AMA-254 (Advanced mercury analyser) following [START_REF] Coelho | Pattern and pathways for mercury lifespan bioaccumulation in Carcinus maenas[END_REF].

Organic mercury was determined through digestion with a mixture of 18 % potassium bromide (KBr) in 5 % sulfuric acid (H 2 SO 4 ), followed by extraction of organic mercury into toluene as described in [START_REF] Válega | Determination of Organic Mercury in Biota, Plants and Contaminated Sediments Using a Thermal Atomic Absorption Spectrometry Technique[END_REF]. Analytical quality control was performed using certified reference material (CRM; in this case National Research Council,Canada]). The obtained values (mean ± SD) for the whole of the CRM analyses ranged from 81 to 102 % (TORT-2: 87 ± 3 %, n = 41; TORT-3: 90 ± 8 %, n = 27), results were corrected using the daily recovery efficiency of CRMs. The mass of CRM used for quality control analyses was adjusted to be within the range of total mercury (in ng) present in the samples.

Analyses were performed in duplicate, blanks were analysed at the beginning of each set of samples and the coefficient of variation between replicates never exceeded 10%. CRMs were also used to validate organic mercury analyses, with an extraction efficiency of 80 ± 2 % and 98 ± 5 %, respectively. The limit of detection for this analytical method is 0.00001 µg g -1 of absolute mercury and 0.004 µg g -1 for organic mercury. All concentration data are expressed subsequently in µg g -1 dry weight.

Statistical analysis

Wilcoxon test were used to investigate whether there were any differences in mercury concentrations between females and males, between eggs and females, or between sampling sites. Kruskall-Wallis were performed to examine if there were statistical differences between sex/maturity and location. Linear regressions were calculated to examine possible relationships between Antarctic krill length and individual mercury concentration. All analyses were performed using the R software version 3.4.2 [START_REF] Core | R: A language and environment for statistical computing[END_REF]. All values are presented as mean ± SD.

Results

Total mercury concentrations in Antarctic krill according to geographic areas

Total mercury concentrations varied between 0.054  0.018 µg g -1 in females, 0.048  0.011 µg g -1 in males and 0.071  0.023 µg g -1 in juveniles from the South Orkney Islands to 0.006  0.002 µg g -1 in females, 0.007  0.002 µg g -1 in males and 0.014  0.005 µg g -1 in juveniles from the South Georgia and 0.017  0.006 µg g -1 in juveniles from the Antarctic Polar Front.

There was a clear differentiation in mercury concentrations between the three locations (Figure 2): Adult Antarctic krill from the South Orkney Islands had concentrations of mercury about 7 times higher in females (Wilcoxon rank sum test, W = 120, p< 0.001) and males (Wilcoxon rank sum test, W =120, p< 0.001) than adult Antarctic krill from South Georgia, and juveniles showed concentrations around 5 times higher in the South Orkney Islands (Kruskall-Wallis, H 3 = 41.03, p< 0.001) than those collected at South Georgia and the Antarctic Polar Front. Juveniles from the northern locations (South Georgia and Antarctic Polar front) had similar mercury concentrations (Wilcoxon rank sum test, W = 192, p= 0.093).

Total mercury concentrations in Antarctic krill according to life stage

There were significant differences (Wilcoxon signed rank test, Z= -3.351p = 0.001) between the mercury concentrations in the eggs (0.015  0.002 µg g -1 ) and the corresponding female somatic tissue (0.008  0.003 µg g -1 ) from South Georgia (Figure 2). There were no significant differences (Wilcoxon rank sum test, W = 189, p = 0.071) between the females sampled in 2007/08 and 2016/17 at South Georgia (0.007  0.002 µg g -1 ). Juveniles caught around South Georgia (0.014  0.005 µg g -1 ) had significantly higher mean concentration of mercury than adults (0.007  0.002 µg g -1 ; Kruskall-Wallis H = 41.031, p < 0.01 ) from the same region. Juveniles and eggs from South Georgia also had similar concentrations (Wilcoxon rank sum test, W = 205, p = 0.254). Like in juveniles from South Georgia, juveniles caught at the South Orkney Islands (0.071  0.024 µg g -1 ) also had significantly higher mercury concentrations than adults (0.051  0.015 µg g -1 ; Kruskall-Wallis H = 10.048, p =0.07).

Significant negative correlations of mercury concentration with body size was common to both the South Orkney Islands and South Georgia (Y = -0.0124*X -

Total mercury concentrations in Antarctic krill according to sex

Concentrations of mercury in adult females (0.054  0.018 µg g -1 ) and males (0.048  0.011 µg g -1 ) from South Georgia were similar (t 28 = 0.9323, p= 0.4; Figure 2). There were also no differences in mercury concentration between sexes in the samples collected from the South Orkney Islands (t 27 = 0.917, p= 0.4; Figure 2).

Organic mercury in Antarctic krill

Adult Antarctic krill from the South Orkney Islands had higher concentrations of organic mercury than adults from South Georgia (Table 1) (for both males and females), but concentrations in juveniles were similar between the two locations.

While no significant differences between juveniles, males and females were observed in the South Orkney Islands, juveniles in South Georgia had higher organic mercury concentrations than adults.

Organic mercury percentages in Antarctic krill were lower in the South Orkney Islands (15% in juveniles, 16% in females and 21% in males) than at South Georgia (29% in juveniles, 37% in females and 36% in males) and the Antarctic Polar Front (35% in juveniles; Table 1). Adults had slightly higher organic mercury percentages than juveniles (Table 1).

Discussion

Despite some studies reporting mercury levels in Antarctic krill [START_REF] Bargagli | Biomagnification of mercury in an Antarctic marine coastal food web[END_REF]Brasso et al., 2012b;[START_REF] Locarnini | Trace element concentrations in Antarctic Krill, Euphausia superba[END_REF][START_REF] Moren | Element concentrations in meals from krill and amphipods, -Possible alternative protein sources in complete diets for farmed fish[END_REF], there has remained a gap in knowledge regarding variability in mercury concentration by size, gender and location. Furthermore, to our knowledge this is the first study to determine organic mercury concentrations in Antarctic krill.

Total mercury concentrations according to geographic areas

We found Antarctic krill from South Orkney Islands had mercury body burdens 5 to 7 times higher than those from South Georgia and from the Antarctic Polar Front.

Habitat differences may explain the differences in contamination levels between [START_REF] Barnes | Shallow benthic fauna communities of South Georgia Island[END_REF][START_REF] Clarke | The seasonal cycle of phytoplankton, macronutrients, and the microbial community in a nearshore antarctic marine ecosystem[END_REF] and at the Antarctic Polar Front. This temperature gradient leads to an important ecosystem difference, promoting the presence of more winter ice in the South Orkney Islands [START_REF] Atkinson | South Georgia, Antarctica: a productive, cold water, pelagic ecosystem †[END_REF]. Ice formation can act as a buffer for mercury and other elements [START_REF] Lindberg | Dynamic Oxidation of Gaseous Mercury in the Arctic Troposphere at Polar Sunrise[END_REF]. Furthermore, the ice may act as a trap for contaminants precipitating from the atmosphere [START_REF] Beyer | Long-range transport potential of semivolatile organic chemicals in coupled air-water systems[END_REF][START_REF] Cossa | Mercury in the Southern Ocean[END_REF], which are released into the water column upon ice melting [START_REF] Brierley | Ecology of Southern Ocean pack ice[END_REF][START_REF] Geisz | Melting Glaciers: A Probable Source of DDT to the Antarctic Marine Ecosystem[END_REF][START_REF] Mastromonaco | Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)[END_REF]. In the Arctic, for instance, higher concentrations of mercury were measured in seawater under sea-ice, when compared with ice-free regions [START_REF] Hintelmann | Methylated Mercury Species in Canadian High Arctic Marine Surface Waters and Snowpacks[END_REF] and higher concentrations of mercury were found under ice during spring [START_REF] Mastromonaco | Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)[END_REF]. Additionally, depletion events promote higher precipitation rates of atmospheric mercury in colder areas, mainly during springtime, when halogen radicals oxidize the mercury [START_REF] Ebinghaus | Antarctic Springtime Depletion of Atmospheric Mercury[END_REF][START_REF] Lindberg | Dynamic Oxidation of Gaseous Mercury in the Arctic Troposphere at Polar Sunrise[END_REF]. Indeed, these depletion events have been reported along and between regions of Antarctic sea-ice [START_REF] Dommergue | Overview of mercury measurements in the Antarctic troposphere[END_REF].Thus, higher depletion rates, sea ice formation and its melting may explain why there were more contaminants available to Antarctic krill around the South Orkney Islands than around South Georgia. Comparing our data with previous records of mercury in Antarctic krill, we see that samples from the Ross Sea, an area with winter sea ice [START_REF] Bargagli | Biomagnification of mercury in an Antarctic marine coastal food web[END_REF], had higher concentrations than South Georgia and the Antarctic Peninsula (Brasso et al., 2012a;[START_REF] Cipro | Mercury in the ecosystem of Admiralty Bay, King George Island, Antarctica: Occurrence and trophic distribution[END_REF][START_REF] Locarnini | Trace element concentrations in Antarctic Krill, Euphausia superba[END_REF], but similar to those at the South Orkney Islands (Table 2).

Other possible explanations for the higher mercury contamination in Antarctic krill from the South Orkney Islands could be the proximity to active volcanoes, which are well-known sources of mercury [START_REF] Varekamp | Mercury emissions from Mount St Helens during September 1980[END_REF][START_REF] Zambardi | Mercury emissions and stable isotopic compositions at Vulcano Island (Italy)[END_REF]. Several volcanoes have recently been reported in the Antarctic Peninsula (van Wyk de [START_REF] Van Wyk De Vries | A new volcanic province: an inventory of subglacial volcanoes in West Antarctica[END_REF], which is closer to the South Orkney Islands than to the other two sampling sites in the present study. Nevertheless, the uptake of mercury from such sources is likely to be variable given that previous studies measuring mercury concentrations in Antarctic krill from the Antarctic Peninsula measured levels that were lower than those specifically in the South Orkney Islands Antarctic krill population reported here (Brasso et al., 2012a;[START_REF] Locarnini | Trace element concentrations in Antarctic Krill, Euphausia superba[END_REF][START_REF] Moren | Element concentrations in meals from krill and amphipods, -Possible alternative protein sources in complete diets for farmed fish[END_REF] (Table 2). Mercury body burdens in Antarctic krill may also be related to food availability [START_REF] Chen | High Plankton Densities Reduce Mercury Biomagnification[END_REF]. Phytoplankton blooms, which are a main source of mercury to krill, are spatially and temporally variable in the Southern Ocean and have a large influence on Antarctic krill growth [START_REF] Atkinson | Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage[END_REF][START_REF] Cuzin-Roudy | Seasonal reproduction, multiple spawning, and fecundity in northern krill, Meganyctiphanes norvegica, and Antarctic krill, Euphausia superba[END_REF]. Accordingly, the dynamics and availability of food between locations will probably have a significant effect on the mercury bioavailability, intake and bioaccumulation in Antarctic krill.

In comparison with other krill species around the world (Table 2), there are examples where the concentration of mercury is lower, for instance, species from the Order Euphausiacea in the Hudson bay (Canada) [START_REF] Foster | Mercury Biomagnification in Marine Zooplankton Food Webs in Hudson Bay[END_REF] and

Euphausia pacifica in the Californian Current [START_REF] Sydeman | Trace metals in seabirds, Steller sea lion, and forage fish and zooplankton from central California[END_REF] than in some of our samples. Mercury concentrations in euphausiids from more industrialized European regions [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF][START_REF] Leatherland | Concentrations of some trace metals in pelagic organisms and of mercury in Northeast Atlantic Ocean water[END_REF][START_REF] Minganti | Total and organic mercury concentrations in offshore crustaceans of the Ligurian Sea and their relations to the trophic levels[END_REF] and the Arctic [START_REF] Ritterhoff | Trace metals in field samples of zooplankton from the Fram Strait and the Greenland Sea[END_REF] are nevertheless considerably higher than in Antarctic krill (Table 2). Higher concentrations are also evident in euphausiid populations in the sub-Antarctic Kerguelen Islands [START_REF] Cipro | Trace elements in invertebrates and fish from Kerguelen waters, southern Indian Ocean[END_REF] which, like the Southern Ocean, is likely to result from remote atmospheric sources [START_REF] Cossa | Mercury in the Southern Ocean[END_REF]. otherwise expect burdens to be lower in juveniles through a growth dilution effect, as reported for Daphnia pulex [START_REF] Karimi | Stoichiometric controls of mercury dilution by growth[END_REF]. Furthermore, juveniles have more frequent molting cycles compared to adults [START_REF] Buchholz | Moult cycle and growth of Antarctic krill Euphausia superba in the laboratory[END_REF], and excretion ratios will probably be more efficient at these early stages. Somatic growth of Antarctic krill is pre-programmed to slow once a certain age or maturity has been reached [START_REF] Tarling | Natural growth rates in Antarctic krill (Euphausia superba): I. Improving methodology and predicting intermolt period[END_REF], in order to divert considerable resources to reproductive tissue when reaching adulthood [START_REF] Atkinson | Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage[END_REF][START_REF] Cuzin-Roudy | Seasonal reproduction, multiple spawning, and fecundity in northern krill, Meganyctiphanes norvegica, and Antarctic krill, Euphausia superba[END_REF]. Adults also prey on higher trophic levels compared to juveniles [START_REF] Atkinson | Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter-II. Juveniles and adults[END_REF] which should mean higher bio-magnification potential, and therefore contrary to what was observed. The higher contaminant load of juveniles when compared with adults has, however, been reported in previous studies on Antarctic krill [START_REF] Locarnini | Trace element concentrations in Antarctic Krill, Euphausia superba[END_REF] as well as the subantarctic krill Euphausia vallentini [START_REF] Cipro | Trace elements in invertebrates and fish from Kerguelen waters, southern Indian Ocean[END_REF]. One mechanism that may explain this phenomenon is through egg laying, which has been reported as an important elimination route for mercury in several organisms such as birds (Brasso et al., 2012a;[START_REF] Pedro | Mercury accumulation in gentoo penguins Pygoscelis papua: spatial, temporal and sexual intraspecific variations[END_REF] and fish [START_REF] Johnston | Intra-and interpopulation variability in maternal transfer of mercury to eggs of walleye (Stizostedion vitreum)[END_REF][START_REF] Schofield | The mercury cycle and fish in the Adirondack lakes[END_REF], and also previously hypothesized for crustaceans species [START_REF] Coelho | Pattern and pathways for mercury lifespan bioaccumulation in Carcinus maenas[END_REF]. In the present study, the higher mercury concentrations were found in Antarctic krill eggs when compared to corresponding somatic tissue, suggesting that egg laying maybe an elimination mechanism.

Species

However, males also have lower mercury burdens compared to juveniles which either rules out this hypothesis or indicates that males also eliminate mercury through their own gonadic tissue. Spermatophores are regularly produced and passed out of the body throughout the lifespan of males, although concentrations of mercury in these structures has yet to be measured.

Organic mercury

We found concentrations of the highly toxic, organic form of mercury of between 0.002 and 0.008 ug g -1 dw, with the higher concentrations being found in both the South Orkneys and South Georgia, particularly in juveniles. Antarctic krill is the main prey for several Southern Ocean predators and it is estimated that more than half of its total biomass of 379 Mt is eaten by whales, seals, seabirds, squid and fish [START_REF] Atkinson | A reappraisal of the total biomass and annual production of Antarctic krill[END_REF]. Assuming the lowest individual mercury concentrations measured by the present study, this would mean 1.33 t of mercury will be passed on from the consumption of Antarctic krill, of which 0.57 t will be in the organic form.

However the 1.33t of mercury potentially transferred in the trophic web is a conservative number, as it was calculated from the lowest concentration levels found in the present study, that is, at the same time the lowest concentration ever measured in the literature. So it can be considered an underestimation. This organic mercury will be potentially bioaccumulated in the tissues of Antarctic krill predators and transferred towards upper food web predators leading to its biomagnification.

Thus, it may reach concentrations that can affect the behaviour, reproductive success and even to reduce the survival of the top predators (Tan et al. 2009;Eagles-Smith et al. 2018). Such bioaccumulation of organic mercury from Antarctic krill consumption can explain how some Antarctic seabirds have particularly high concentrations of mercury [START_REF] Tavares | Influence of age, sex and breeding status on mercury accumulation patterns in the wandering albatross Diomedea exulans[END_REF].

Conclusions

The accumulation of mercury in Antarctic krill decreases with increasing body size and maturity. Juveniles have higher concentrations than adults which may be the result of a growth dilution effect and also elimination through gonadic tissue (eggs and spermatophores).

The observed spatial differences suggest that Antarctic krill reflects differential contaminant bioavailability in the Southern Ocean, while further studies are needed to discern the most significant variables governing site-specific mercury bioaccumulation.

The range of mercury concentrations reported in Antarctic krill are within the same range, or even higher, than other euphausiids from areas closer to the industrialized part of the world, highlighting mercury as a global pollutant.

Overall, our results stress the need to put into action pollutant monitoring programs to evaluate the sources, pathways and effects of contaminants in remote ecosystems.

Figure 1 -

 1 Figure 1 -Sampling sites of Antarctic krill (white squaresamples of juveniles, females and males; white dotsamples of juveniles ) and general positions of the

Figure 2 -

 2 Figure 2-Total mercury concentrations (µg g -1 dw) in Antarctic Krill (Euphausia superba) collected around South Georgia and at the Antarctic Polar Front in the austral summer of 2016/17, and around the South Orkney Islands during the austral summer of 2015/16. Bars show the mean. Error bar is 1 standard deviation.

Figure 3 -

 3 Figure 3 -Total mercury concentration (µg g -1 dw) on a log10 scale versus total length (mm) for individual Antarctic krill (Euphausia superba) by maturity stage and sex respectively. Data are shown separately for krill collected around South Georgia (Y = -0.0124*X -1.525, R 2 = 0.46, F 1, 43 = 36.41, p < 0.001), the Antarctic Polar Front (both in the austral summer of 2016/17) and the South Orkney Islands (Y = -0.01072*X -0.8675, R 2 = 0.2746, F 1, 52 = 19.69, p < 0.001; summer of 2015/16).

Table 1 -

 1 Organic mercury (OHg) and total mercury (THg) concentrations in samples of Antarctic krill (Euphausia superba) collected from different locations in the Scotia Sea during the austral summers of 2015/16 and 2016/17. Average  Standard deviation

  in the Southern Ocean. The average sea surface temperature around the South Orkney Islands is lower than in South Georgia

	these three areas
	Location	Year Sex / Maturity	Number	OHg (ug g -1 dw)	THg (ug g -1 dw) %O
	South Orkney Islands	2016	Juvenile	20	0.008  0.003	0.051  0.016	1
	South Orkney Islands	2016	Female	20	0.008  0.002	0.052  0.022	1
	South Orkney Islands	2016	Male	20	0.008  0.003	0.040  0.014	2
	South Georgia	2017	Juvenile	20	0.008  0.002	0.024  0.006	2
	South Georgia	2017	Female	20	0.002  0.0002 0.006  0.0003	3
	South Georgia	2017	Male	20	0.003  0.0001 0.007  0.0004	3
	Antarctic Polar Front	2017	Juvenile	20	0.005  0.001	0.014  0.005	3

Table 2 -

 2 Total mercury concentrations (µg g -1 dw) in different species of Antarctic krill around the world from published data and this study (mean  standard deviation).Total mercury concentration according to life stage and sexMercury concentration in Antarctic krill unexpectedly decreased with age (see results). Since juveniles have a faster rate of growth compared to adults, one would

	0
	1
	2
	3

Acknowledgments

We thank the crew, officers and scientists aboard the RSS James Clark Ross during the cruises JR 177, JR15004 and JR16003 for their assistance in collecting samples. We also thank to Peter Enderlein, Carson McAfee, Daniel Ashurst and Scott Polfrey for assembling and maintaining the net gear. We acknowledge the Portuguese Foundation for the Science and Technology (FCT) through a PhD grant to José Seco (SRFH/PD/BD/113487). Acknowledgments are due also to the Integrated Program of SR&TD 'Smart Valorization of Endogenous Marine Biological Resources Under a Changing Climate' (reference Centro-01-0145-FEDER-000018),

co-funded by Centro 2020 program, Portugal 2020, European Union, through the European Regional Development Fund, for personal funding to J.P.Coelho. The IUF (Institut Universitaire de France) is acknowledged for its support to P. Bustamante as a Senior Member. This research was also within José Xavier strategic program of MARE (MARE -UID/ MAR/04292/2013).


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Yves Guéguen

Physical and Mechanical Properties of Thermally Cracked Andesite Under Pressure

Keywords: Andesite, Thermal crack, Ultrasonic velocity, Crack density, Onset of dilatancy, Brittle Strength, Partial melting, Crack sealing Aspect ratio C' Onset

The efects of thermal crack damage on the physical properties and rupture processes of andesite were investigated under triaxial deformation at room temperature. Thermal cracking was induced by slowly heating and cooling samples. The efects of heat treatment temperatures ranging between 500 °C and 1100 °C on the P-wave velocities and on the microstructure were investigated. Then, the mechanical properties of andesite samples treated at 930 °C were investigated under triaxial stress at room temperature using constant strain rate tests and conining pressures ranging between 0 and 30 MPa. Similar triaxial experiments were conducted on non-heat-treated samples. Our results show that: (1) for heat treatments at temperatures below 500 °C, no signiicant changes in the physical properties are observed; (2) for heat treatments in the temperature range of 500-1100 °C, crack density increases; and (3) thermal cracking has no inluence on the onset of dilatancy but increases the strength of the heat-treated samples. This last result is counterintuitive, but seems to be linked with the presence of a small fraction of clay (3%) in the non-heat-treated andesite. Indeed, for heat treatment above 500 °C, some clay melting is observed and contributes to sealing the longest cracks.

List of Symbols

Introduction

The physical properties and mechanical behavior of andesite are of interest for the understanding of several natural processes and for engineering design. Natural processes include ground deformation due to magma rising below volcanoes (Jaupart 1998;Costa et al. 2009;[START_REF] Heap | Microstructural controls on the physical and mechanical properties of ediice-forming andesites at Volcán de Colima, Mexico[END_REF], eruption activity and cessation of dikes (Gudmundsson 2006(Gudmundsson , 2011;; 1 3 Browning et al. 2015), or fault activity in volcano systems (Rowland and Sibson 2001). Industrial contexts are geothermal reservoir engineering [START_REF] Siratovich | Physical property relationships of the Rotokawa Andesite, a signiicant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand[END_REF] and CO 2 sequestration [START_REF] Trias | High reactivity of deep biota under anthropogenic CO2 injection into basalt[END_REF].

In volcanic areas, where hot luids circulate, temperature is a parameter that should be taken into account when investigating mechanical behavior. Temperature variations can induce intragranular cracks and intergranular cracks or even melting of the rock [START_REF] Friedman | Strength and ductility of four dry igneous rocks at low pressures and temperatures to partial melting[END_REF][START_REF] Wong | Crack aperture statistics and pore space fractal geometry of Westerly granite and Rutland quartzite: Implications for an elastic contact model of rock compressibility[END_REF], leading to large changes in the microstructure. A common way to investigate the efect of temperature on the mechanical behavior is to use heat-treated samples.

Many studies have been performed on the role of thermal efects in the mechanical behavior of igneous rocks, including intrusive and extrusive rocks [START_REF] Heap | Exploring the scale-dependent permeability of fractured andesite[END_REF][START_REF] Heap | Microstructural controls on the physical and mechanical properties of ediice-forming andesites at Volcán de Colima, Mexico[END_REF][START_REF] Heap | Fracture and compaction of andesite in a volcanic ediice[END_REF][START_REF] Vinciguerra | Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts[END_REF][START_REF] Fortin | Inluence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure[END_REF]Stanchits et al. 2006;[START_REF] Vinciguerra | Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts[END_REF][START_REF] Wang | Physical properties and brittle strength of thermally cracked granite under coninement[END_REF]. Increasing temperature can modify luid transport properties [START_REF] Vinciguerra | Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts[END_REF][START_REF] Nasseri | Common evolution of mechanical and transport properties in thermally cracked westerly granite at elevated hydrostatic pressure[END_REF][START_REF] Faoro | Linking permeability to crack density evolution in thermally stressed rocks under cyclic loading[END_REF][START_REF] Darot | Efect of pore and conining pressures on VP in thermally pre-cracked granites[END_REF]Geraud 1994), elastic wave velocities [START_REF] Walsh | The efect of cracks on the uniaxial elastic compression of rocks[END_REF][START_REF] Nasseri | Coupled evolutions of fracture toughness and elastic wave velocities at high crack density in thermally treated Westerly granite[END_REF], and mechanical properties [START_REF] Faulkner | The efect of temperature, the nature of the pore luid, and subyield diferential stress on the permeability of phyllosilicate-rich fault gouge[END_REF][START_REF] Wang | Physical properties and brittle strength of thermally cracked granite under coninement[END_REF]. Thermal cracking has been shown to weaken diferent rocks, including granite [START_REF] Homand-Etienne | Thermally induced microcracking in granites: characterization and analysis[END_REF][START_REF] Chaki | Inluence of thermal damage on physical properties of a granite rock: porosity, permeability and ultrasonic wave evolutions[END_REF], gabbro [START_REF] Keshavarz | Damage and changes in mechanical properties of a gabbro thermally loaded up to 1000 C[END_REF], calcarenite (Brotóns et al. 2013), and carbonate [START_REF] Sengun | Influence of thermal damage on the physical and mechanical properties of carbonate rocks[END_REF]). P-wave velocity changes due to thermal cracking are expected. Such changes are important for seismic tomography inversion of volcano systems and may be useful for investigating magma chamber activity and dike eruption. According to Griith et al. (2017) and Browning et al. (2016), more cracks are created during the cooling stage than during the heating stage.

In contrast with intrusive rocks, extrusive rocks have a diferent structure resulting from fast cooling: a ine groundmass and phenocrysts. Note that the ultrasonic velocities and transportation properties of andesite have been investigated in previous studies [START_REF] Fortin | Inluence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure[END_REF]Fatquharson et al. 2015;[START_REF] Heap | Microstructural controls on the physical and mechanical properties of ediice-forming andesites at Volcán de Colima, Mexico[END_REF][START_REF] Heap | Fracture and compaction of andesite in a volcanic ediice[END_REF][START_REF] Siratovich | Experimental thermal stimulation of the Rotokawa Andesite Smith DL, Evans B (1984) Difusional crack healing in quartz[END_REF][START_REF] Heap | Exploring the scale-dependent permeability of fractured andesite[END_REF].

The selected rock for this study is an andesite from Guadeloupe, France. Guadeloupe is located in the Lesser Antilles arc. This arc results from the subduction of the North American Plate under the Caribbean Plate [START_REF] Bouysse | Subduction of Atlantic aseismic ridges and Late Cenozoic evolution of the Lesser Antilles island arc[END_REF]). The Bouillante (Guadeloupe, France) geothermal power plant has been operating for 30 years [START_REF] Jaud | The Bouillante geothermal power-plant[END_REF]. GEOTREF (A project aimed at improving understanding fractured geothermal reservoirs) is planning a new geothermal project in the area from which the samples were taken. Faults and active luids are present in the volcano system and cause signiicant seismicity activity [START_REF] Feuillet | Active arc-transverse normal faulting in Guadeloupe (French Lesser Antilles)[END_REF][START_REF] Feuillet | Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles[END_REF]. Because induced seismicity is possible during geothermal exploration, it is important to systematically determine the geomechanical behavior and to characterize its evolution resulting from thermal stresses. This paper aims to investigate the efect of heat treatment on the mineralogical composition, microstructural parameters (porosity, crack density), and petrophysical parameters (permeability, elastic wave velocities) of andesite. For this purpose, we selected an andesitic block from a quarry located in Guadeloupe, France, and samples were heat-treated at diferent temperatures. Then, petrophysical and mechanical properties were measured.

Materials and Methods

Materials

Rock Samples

The samples of andesite were cored from a single block, which was obtained from a quarry located in Déhaies (Guadeloupe, France). Cylindrical samples were cored with diameters of 40 mm and lengths of 80 mm. No bedding related to volcanic features was identiied in the block. After coring, boundary surfaces were polished to ensure smooth horizontal surfaces.

The samples have a mean density of 2670 kg/m 3 , and the porosity varies between 1.5% and 2.3% with a mean porosity of 1.9%. The porosity is estimated by comparing the weight of a water-saturated sample with the weight of the same sample dried under a vacuum at 40 °C for 4 days. The permeability of the non-heat-treated andesite is approximately 10 -21 m 2 . The P-wave velocity of the non-heat-treated andesite samples is 5400 m/s with a measurement error of 200 m/s.

The andesite is composed of a ine groundmass with large phenocrysts (Fig. 1). The matrix is composed of quartz and plagioclase (50-60%). The mean grain size of the matrix is 2 µm. Phenocrysts are plagioclase (25-35%), pyroxene (15-20%), and oxides (4-5%). The mean grain size of phenocrysts is 100 µm (Fig. 1).

Figure 1 shows that this andesite is naturally microcracked. Cracks can be observed inside the phenocrysts (Fig. 1c). Figure 2 also shows that the andesite sample has a few long cracks in the matrix, with lengths on the order of 300 µm-500 µm.

Procedure of The Heat Treatment

Samples were heat treated to four diferent temperatures: 500 °C, 800 °C, 930 °C, and 1100 °C. Samples were placed in a programmable Meker MHT-3 furnace [START_REF] Nicolas | Brittle and semi-brittle behaviours of a carbonate rock: inluence of water and temperature[END_REF], and the temperature was increased at a rate of 140 °C/h to the target temperature. The furnace atmosphere was not controlled by air-oxygen fugacity. The target temperature was held constant for 2 h. Samples were slowly cooled to room air temperature. The cooling rate was controlled by the thermal inertia of the oven and remained slow (~ 10 h to decrease from 930 °C to room temperature). The efects of the heat treatment amplitude on the petrophysical properties of the andesite are presented in Sect. 3.1. The particular temperature of 930 °C was selected for a detailed triaxial deformation test. This temperature corresponds to that of the second magma chamber temperature in La Souière, Guadeloupe [START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal luids[END_REF], which is where the dikes begin [START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal luids[END_REF]. The mechanical properties of the heat-treated andesite samples are investigated and compared to the mechanical properties of the non-heat-treated samples.

Experimental Methods

Experimental Apparatus

Triaxial deformation experiments are performed on the triaxial cell installed at the Laboratoire de Géologie at École Normale Supérieure in Paris (Fig. 14 in Appendix 1). The axial load is applied by an axial piston with a maximum load of 680 MPa and is measured with a sensor with an accuracy of approximately 10 -3 MPa. The conining pressure is applied by a volumetric servo pump that can provide pressure up to a maximum of 100 MPa and is measured by a pressure transducer with an accuracy of 10 -3 MPa [START_REF] Ougier-Simonin | Cracks in glass under triaxial conditions[END_REF][START_REF] Nicolas | Brittle and semi-brittle behaviours of a carbonate rock: inluence of water and temperature[END_REF]. The pressure-conducting medium is silicone oil.

Strain and Ultrasonic Instrumentation

Four groups of strain gauges are glued at diferent positions directly on the sample. (Fig. 15 in Appendix 1). Each group is composed of one axial gauge and one radial gauge. Strain gauges Tokyo Sokki TML FCB 2-11 are employed. The axial strain a , and radial strain r are both averaged across the four strain gauges in each orientation. The volumetric strain is deduced as v = a + 2 r . Neoprene tubing is used to separate the sample from the oil of the conining medium. Then, 12 P-wave sensors (PI 255 PI Ceramics, 1 MHz resonance frequency) and four polarized S-wave sensors (Shear PZT plate) are glued directly on the surface of the rock and sealed with a two-component epoxy (Fig. 15 in Appendix 1). The network of ultrasonic sensors used allows us to record (i) acoustic emissions (passive mode) and (ii) the evolution of the ultrasonic velocities in diferent directions (active mode). For the ultrasonic wave velocity survey, a 250 V high-frequency signal is pulsed every 2 min on each sensor while the others are recording. In passive mode, these sensors can record the acoustic emissions (AE) that take place in the sample. The AEs are ampliied at 40 dB and can be discretely recorded with a maximum rate of 12 AE/s. [START_REF] Schubnel | Quantifying damage, saturation and anisotropy in cracked rocks by inverting elastic wave velocities[END_REF][START_REF] Schubnel | Fluid-induced rupture experiment on Fontainebleau sandstone: Premonitory activity, rupture propagation, and aftershocks[END_REF][START_REF] Brantut | Efect of dehydration reactions on earthquake nucleation: stable sliding, slow transients, and unstable slip[END_REF][START_REF] Ougier-Simonin | Cracks in glass under triaxial conditions[END_REF][START_REF] Nicolas | Brittle and semi-brittle behaviours of a carbonate rock: inluence of water and temperature[END_REF].

Experimental Procedure

For all the samples, the P-ultrasonic wave velocity is measured at room temperature and atmospheric pressure. Permeability is measured with water using transient pulse or steady low methods at 2.5 MPa efective conining pressure.

For non-heat-treated samples and samples heat-treated to 500 °C, permeability is measured using the transient pulse [START_REF] Brace | Dilatancy in fracture of crystalline rock[END_REF]) (two symmetrical measures are performed by switching the low direction in the sample); for samples heat-treated to 800 °C and 930 °C, permeability is measured with steady state low.

For non-heat-treated andesite samples and heat-treated (at 930 °C) andesite samples, experiments are performed under dry conditions at room temperature, with conining pressures of 5, 15 and 30 MPa (non-heat treated samples) and 0, 15 and 30 MPa (heat-treated samples). Diferential loading is applied up to peak stress using a controlled constant strain loading rate of 10 -6 /s. During triaxial deformation, ultrasonic measurements are made on all samples every 2 min.

In the following, we adopt the convention that compressive stresses and compactive strains are positive. The differential stress is noted by Q = 1 -3 , and the mean stress by P =( 1 + 2 3 )∕3 , where 1 and 3 = P c are the vertically applied (maximum) stress and the conining pressure (minimum), respectively.

Results

Properties of Samples Heat-Treated at Diferent Temperatures

P-Wave Velocity, Crack Density and Permeability

The samples were heat-treated at diferent temperatures: 500 °C, 850 °C, 930 °C and 1100 °C. The evolution of the P-wave velocity and permeability as functions of the temperature of the heat treatment are shown in Fig. 3. In addition, we invert the P-wave velocity to obtain the crack density, as deined by = ∑

N l 3 i V
, where l i is the length of the i-th crack and N is the total number of cracks embedded in the representative elementary volume V. When the crack length is smaller than the wavelength, efective medium theory is an appropriate method [START_REF] Ougier-Simonin | Characterisation of hydraulic fractures in limestones using X-ray microtomography[END_REF]. The efective elastic properties are mainly controlled by cracks (Gueguen and [START_REF] Guéguen | Efective elastic properties of cracked rocks-an overview[END_REF]. A noninteractive assumption is employed [START_REF] Kachanov | Effective moduli of solids with cavities of various shapes[END_REF]. Noninteractive efective medium theory has been shown to be valid when cracks are distributed randomly and crack density does not exceed 0.2-0.3 [START_REF] Kachanov | Effective moduli of solids with cavities of various shapes[END_REF][START_REF] Sayers | Microcrack-induced elastic wave anisotropy of brittle rocks[END_REF]Gueguen and;[START_REF] Guéguen | Crack-induced anisotropy in crustal rocks: predicted dry and luid-saturated Thomsen's parameters[END_REF]. The details of the inversion in crack density are given in the appendix "Crack density inversion method-Isotropic case" [START_REF] Ougier-Simonin | Characterisation of hydraulic fractures in limestones using X-ray microtomography[END_REF][START_REF] Mallet | Evolution of the crack network in glass samples submitted to brittle creep conditions[END_REF][START_REF] Fortin | Inluence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure[END_REF][START_REF] Nicolas | Brittle and semi-brittle behaviours of a carbonate rock: inluence of water and temperature[END_REF]) A small decrease in the P-wave velocity of 70 m/s (1.3%) is recorded (Fig. 3) for a heat treatment of 500 °C. However, the decrease reaches 1000 m/s (18%) for a temperature of 800 °C and 2250 m/s (41%) for a temperature of 1100 °C, indicating a major change at approximately 500 °C, which should be associated with a signiicant change in microstructure. The crack density inverted from the P-wave velocity increases from 0.01 for a heat treatment of 500 °C to 1.4 for a heat treatment of 1100 °C. This last value is outside the range in which efective medium theory is valid. However, the value points to very intense cracking. The induced crack density reaches the percolation threshold of 0.1 [START_REF] Guéguen | Transport properties of rocks from statistics and percolation[END_REF] for a heat treatment between 500 °C and 800 °C. This result is in good agreement with the permeability evolution, which shows no change below 500 °C, but shows an increase by four orders of magnitude for a heat treatment of 930 °C (Fig. 3).

Microstructural Observations of The Heat-Treated Samples

The microstructures are shown in Fig. 4. In the sample heattreated at 500 °C (Fig. 4a,b), few cracks can be observed. In the samples heated at 800 °C (Fig. 4c,d) and 930 °C (Fig. 4e, f), cracks surrounding the large inclusions with lengths of 50-200 µm can be observed, including intergranular and intragranular cracks. Small cracks begin to appear in the matrix. Partial melting occurs above 500 °C; the details and efects of partial melting are presented in Sect. 3.2. The sample treated at 1100 °C shows the highest crack density (Fig. 4g,h). In particular, in the matrix (Fig. 4h), the number of cracks with lengths ranging between 1 µm and 20 µm is greatly increased, and most cracks are located at the boundaries of the crystal grains of quartz, tridymite, and plagioclase. Overall, the evolution of the microstructure is in good agreement with the evolution of the P-wave velocity and the permeability (Fig. 3).

Mineralogical Efects of Heat Treatment on Andesite

Mineralogical Composition

The mineralogical compositions of non-heat-treated andesite samples and heat-treated andesite samples were analyzed with EBSD (electron backscatter diffraction) and XRD (X-ray difraction).

For non-heat-treated andesite samples, the ratio of diferent phases is shown in Table 1. A more detailed composition is given in Appendix A. The non-heat-treated andesite sample is composed of 70.8% plagioclase (including bytownite, sanidine and albite), 14.1% pyroxene (including diopside, hypersthene and augite), 9.2% other silicates (including tridymite and quartz) and 2.5% iron oxides. The clay content is 3.4%, mainly smectite due to plagioclase alteration (Table 2).

The mineralogical composition is slightly changed for samples heat-treated at 500 °C and 930 °C. In particular, the smectite (clay) disappears, and another form of clay appears: illite. The samples heat treated at 930 °C and 1100 °C are similar (disappearance of smectite); in addition, the proportion of plagioclase decreases by 2%, whereas the proportion of silicate increases by 2%, but this last result could be due to the heterogeneity existing in the samples. Irreversible dehydration and loss of swelling properties of smectite have been shown (e.g., [START_REF] Greene-Kelly | Irreversible dehydration in montrnorillonite[END_REF]Russel and Farmer 1964) to occur at temperatures between 350 °C and 550 °C, while dihydroxylation of smectite occurs at temperatures between 550 °C and 800 °C [START_REF] Malek | The study of the dehydration and dehydroxylation of smectites by emanation thermal analysis[END_REF].

Partial Melting

The non-heat-treated andesite contains smectite (clay) related to the alteration of plagioclase, the smectite is mainly located in the vicinity of plagioclase (Fig. 5a).

As the heat treatment reaches 930 °C, partial melting is observed in the samples from SEM images (Fig. 5a). Glass is visible in the porosity (Fig. 5b). Smectite melting results in voids with sizes ranging from 5-10 µm, mainly located near plagioclase (where the smectite was located). In the matrix, small pores of 0.5-1 µm are also observed, mostly at the boundaries of oxides. These pores may also be due to the melting of smectite.

Triaxial Deformation of Non-Heat-Treated

Andesite and Heat-Treated Andesite

Stress-Strain Behavior of Non-Heat-Treated Andesite

Triaxial deformation tests were performed at conining pressures of 5 MPa, 15 MPa and 30 MPa under dry conditions at room temperature. The diferential stress and mean stress are plotted versus axial strain and volumetric strain, respectively, in Fig. 6. The samples under conining pressures varying between 5 MPa and 30 MPa are deformed in the brittle regime. The diferential stress reaches a peak stress followed by macroscopic failure. The peak stress is observed to increase with conining pressure: the maximum diferential stress increases by 20% as the conining pressure increases from 5 MPa to 15 MPa and increases by 27% as the conining pressure increases from 5 MPa to 30 MPa. Note that at the beginning of the loading, the stress-strain curves (Fig. 6) are almost linear, which indicates that the number of preexisting crack is very small.

Another result of interest is the identiication of the D' point, which is the point where volumetric strain reverses (Fig. 6). Beyond D', dilatancy dominates over compaction [START_REF] Heap | Microstructural controls on the physical and mechanical properties of ediice-forming andesites at Volcán de Colima, Mexico[END_REF]. The diferential stress for D' increases as the conining pressure is increased from 5 MPa to 30 MPa. The stress state of D' increases by 22% as the conining pressure increases from 5 MPa to 15 MPa and increases by 50% as the conining pressure increases from 5 MPa to 30 MPa.

Ultrasonic Velocity Evolution of Non-Heat-Treated Andesite

The P-wave velocity evolution during the triaxial deformation of andesite samples under conining pressures of 5 MPa, 15 MPa and 30 MPa is shown in Fig. 7b. In Fig. 7a, the rays of the diferent P-wave velocities are plotted. The P-wave velocities were measured at four ixed angles (0°, 22°, 48°, 53°). During the irst phase of loading, in the quasi-elastic regime, the P-wave velocity is almost constant. This observation agrees with the almost linear stress-strain curve and indicates that the amount of crack porosity is very low [START_REF] Walsh | The efect of cracks on the uniaxial elastic compression of rocks[END_REF]. The P-wave velocity decreases at the onset of dilatancy, marked as point C' [START_REF] Brace | Dilatancy in fracture of crystalline rock[END_REF], as cracks nucleate and propagate. The stress state C' corresponds to the beginning of the P-wave velocity decrease (acoustic data) and to the stage where the volumetric strain curve deviates from linearity (mechanical data) [START_REF] Brace | Dilatancy in fracture of crystalline rock[END_REF], as shown in Fig. 7b. For the three experiments, P-wave anisotropy is observed as the sample is deformed, with the P-wave velocity for rays at 90° with respect to the axial stress decreasing most.

Stress-Strain Relation of Heat-Treated Andesite

Triaxial experiments were performed on andesite samples heat-treated (at 930 °C) under conining pressures of 0 MPa (ultrasonic sensor and strain gauges are glued directly on the sample, measured inside the triaxial cell), 15 MPa and 30 MPa under dry conditions. The diferential stress and mean stress are plotted versus axial strain and volumetric strain, respectively, in Fig. 8.

As in the case of the non-heat-treated andesite samples, the mechanical behavior of the heat-treated samples is brittle. The peak stress increases by 56% as the conining pressure increases from 0 MPa to 15 MPa and by 200% as the conining pressure increases from 0 MPa to 30 MPa.

A clear efect of pre-existing crack closure is shown on the stress-strain curve in Fig. 8, which is denoted by the concave curve at the beginning of loading.

Ultrasonic Velocity Evolution of Heat-Treated Andesite

The P-wave velocity evolution during the triaxial deformation of heat-treated andesite samples under conining pressures of 0 MPa, 15 MPa and 30 MPa is shown in Fig. 9. In Fig. 9, the evolution of P-wave velocities along diferent traces from 0° to 53° (Fig. 7.a) is plotted. A clear diference can be observed in comparison with Fig. 7b for heat-treated samples, the P-wave velocity increases at the beginning of loading, indicating the closure of pre-existing cracks (Stage I). This observation agrees with the stress-strain curve of heat-treated andesite samples, which is concave at the beginning of loading.

During stage II (from C' to D'), the P-wave velocity reaches a plateau. There is competition between pre-existing crack closure and new crack nucleation and propagation. The onset of dilatancy (point C') corresponds to new crack nucleation. Point C' is determined from both acoustic data and mechanical data, as shown in Fig. 9. Regarding the acoustic data, point C' corresponds approximately to the stage where the radial P-wave velocity stops increasing. At stage III, from point D' to the failure point, a clear decrease in P-wave velocities is observed as well as clear P-wave anisotropy indicating the propagation and nucleation of mainly axial cracks.

Discussion

Efect of The Heat Treatment on The Crack Density

Heat Treatment and Crack Density

Cracks formed in heat-treated andesite are mainly due to thermal stresses because minerals or aggregates have different thermal expansion coeicients (Friedrich and Wong 1986). According to theoretical considerations based on thermoelasticity and fracture mechanics, the total surface of crack S (per unit volume) created during heat treatment and due to diferent thermal expansion of minerals is given by [START_REF] Davidge | The strength of two-phase ceramic/glass materials[END_REF][START_REF] Fredrich | Micromechanics of thermally induced cracking in three crustal rocks[END_REF]:

where E is Young's modulus of the (intact) andesite (E = 64.7 GPa), υ is Poisson's ratio of intact andesite (υ = 0.28) and

(1)

S = 1 8G IC E 1 -2v (Δ ΔT) 2
Δα is the thermal expansion contrast between plagioclase and quartz/tridymite or glass, which can be estimated as 14 × 10 -6 /°C. ΔT is the temperature of the heat treatment. G IC is the energy release rate of andesite under mode I and is estimated to be 5-6.5 J/m 2 [START_REF] Fredrich | Micromechanics of thermally induced cracking in three crustal rocks[END_REF][START_REF] Tutluoglu | Mode I fracture toughness determination with straight notched disk bending method[END_REF][START_REF] Talukdar | Correlating mode-I fracture toughness and mechanical properties of heat-treated crystalline rocks[END_REF] In addition, the relationship between the total surface of the crack S and the crack density, = Nl 3 ∕V , is S = 2 π /c, where c is the mean radius of the crack in the volume V (l = 2c).

Following [START_REF] Wang | Physical properties and brittle strength of thermally cracked granite under coninement[END_REF], and assuming that the average thermal crack size is that of the grain size, l, (i.e., c = l/2), we can estimate the crack density created (i) in the matrix or (ii) at the interface between the large inclusions and the matrix. In the matrix, grains of plagioclase and quartz/tridymite have an average length of 1 µm, which leads to a crack density of 0.04-0.053. However, the large inclusions have average lengths of 100-200 µm, which lead to crack densities of 0.4-0.53, values much higher than the crack density generated in the matrix. The estimated initial crack density is ρ = 0.6 (Fig. 3) for andesite samples heattreated at 930 °C. Note that this igure is out of the range of validity for efective medium theory. However, this value quantitatively indicates a high degree of cracking.

In addition, we can deine the aspect ratio = w∕c , where w is the crack half aperture. The total crack porosity is equal to = 2 , which is 3.7% after the heat treatment. This value leads to a crack aspect ratio of approximately 10 -2 , an order of magnitude that agrees with those in previous studies [START_REF] Wang | Physical properties and brittle strength of thermally cracked granite under coninement[END_REF][START_REF] Adelinet | Dispersion of elastic moduli in a porous-cracked rock: theoretical predictions for squirt-low[END_REF].

Crack Density and Anisotropy

The evolution of the crack density during loading can be deduced from the evolution of the elastic P-wave velocities [START_REF] Fortin | Inluence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure[END_REF]. More precisely, assuming a transversely isotropic symmetry, noninteraction and dry conditions, the two components of the crack density tensor can be deduced from the ultrasonic measurements (Gueguen and [START_REF] Guéguen | Efective elastic properties of cracked rocks-an overview[END_REF]). An alternative and equivalent method is to estimate, from the ultrasonic measurements, the vertical crack density and the randomly oriented crack density (Gueguen and [START_REF] Guéguen | Efective elastic properties of cracked rocks-an overview[END_REF][START_REF] Fortin | Inluence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure[END_REF][START_REF] Nicolas | Brittle and semi-brittle behaviours of a carbonate rock: inluence of water and temperature[END_REF]). See also the appendix "Crack density inversion method-Transversely isotropic symmetry".

The evolution of the crack density for the experiments performed at Pc = 30 MPa on the non-heat-treated samples and heat-treated samples is shown in Fig. 10. In the non-heat treated samples, the crack density evolution is directly correlated with the initiation and propagation of cracks at the onset of dilatancy. These induced cracks are mainly axial, and the density of the randomly oriented cracks is almost equal to zero during the loading (Fig. 10a).

3

In heat-treated samples that contain pre-existing thermal cracks, the randomly oriented crack density starts to decrease at the beginning of diferential loading, while the axial crack density increases signiicantly beyond the onset of dilatancy. The axial crack density in Fig. 10b increases even before the onset of dilatancy. As the axial stress increases, the pre-existing randomly oriented cracks close while the axial cracks open. Crack opening causes a decrease in the P-wave velocity and an increase in the crack density.

Note that the crack density inverted in Fig. 3 is based on the measurement of the P-wave velocity at ambient pressure, whereas the crack density inverted in Fig. 10 is based on the measurement of the P-wave velocity under a conining pressure of 30 MPa. Due to the efect of pressure, the P-wave velocities increase as the pressure increases; thus, the crack density decreases as the conining pressure increases. This phenomenon explains why the initial crack density in Fig. 10 is lower than that reported in Fig. 3.

Efect of Heat Treatment on The Microstructure: Partial Melting

The question is: where does the melt go? Fig. 11 shows that the melt seals some cracks, especially the cracks with the largest apertures. We also observe that bubbles are distributed in the melt in agreement with the observations of Simmons and Richter (1976), [START_REF] Swanenberg | Fluid inclusions in high-grade metamorphic rocks from SW Norway[END_REF][START_REF] Roedder | Origin of luid inclusions and changes that occur after trapping[END_REF], indicating that the bonding strength is recovered.

The melt lows into the cracks, and as it cools, the glass bonds the two surfaces of cracks. Bubbles are clearly observed in the SEM (Fig. 6). Since the spherical bubbles represent relatively stable pore shapes, we refer to the bubble planes as being strength recovered. SEM and EBSD reveal that the melt content is an amorphous phase that contains Fe, K, and Mg. Partial melting probably results from the presence of smectite, which has lowered melting point of other minerals. Indeed, the melting point of pyroxene and plagioclase is 1400 °C, whereas smectite melts at temperatures of 400-500 °C. The stress states at the onset of dilatancy (C') during constant strain rate triaxial deformation experiments of intact and heat-treated andesite samples are plotted in Fig. 12. Heat treatment does not induce any change in the stress state at the onset of dilatancy despite the dramatic increase in damage (crack density). This result can be explained by the fact that C' is sensitive to crack nucleation or propagation regardless of the initial crack density.

Wing crack damage models are currently used to quantitatively analyze the dilatancy evolution under axisymmetric loading. Brittle faulting can be modeled by a sliding wing crack model [START_REF] Cotterell | Slightly curved or kinked cracks[END_REF]. Following [START_REF] Ashby | The damage mechanics of brittle solids in compression[END_REF], wings are expected to start to grow from pre-existing laws at an axial stress equal to:

where 1 and 3 are the maximum and minimum principal stresses, µ is the friction coeicient on pre-existing laws, K IC is the fracture toughness and a is the length of pre-exist- ing laws. Note that this criterion (Eq. 2) is not directly sensitive to the crack density. The length of pre-existing laws, a, is estimated to be in the range of 100-300 µm, which corresponds to the average length of large inclusions. Fitting the onset of dilatancy for triaxial deformation experiments performed at diferent conining pressures on intact samples, the friction coeicient µ is estimated to be equal to 0.57, and the fracture toughness K IC is in the range from 1.1 MPa m 1/2 to 1.9 MPa m 1/2 . The friction coeicient is in good agreement with the mean values found in rocks [START_REF] Byerlee | Friction of rocks. Rock friction and earthquake prediction[END_REF], and the fracture toughness is in the range of values obtained for andesite (1-2 MPa m 1/2 ), as measured by [START_REF] Ouchterlony | Fracture toughness testing of rock with core based specimens[END_REF], [START_REF] Tutluoglu | Mode I fracture toughness determination with straight notched disk bending method[END_REF][START_REF] Tutluoglu | Mode I fracture toughness determination with straight notched disk bending method[END_REF][START_REF] Nara | Inluence of relative humidity on fracture toughness of rock: implications for subcritical crack growth[END_REF].

The friction coeicient µ HT is estimated to be 0.57, the same value as for non-heat-treated samples. The fracture toughness for heat-treated andesite samples K IC-HT is estimated in the range 1-1.7 MPa m 1/2 , values slightly lower than those for non-heat-treated samples.

Efect on The Peak Strength

Heat-treated samples of andesite have higher peak stresses than intact samples (Fig. 13).

The 2D damage model previously presented to analyze the onset of dilatancy can also be used to quantitatively model the failure of the samples [START_REF] Ashby | The damage mechanics of brittle solids in compression[END_REF] (2) [START_REF] Kemeny | Micromechanics of deformation in rocks[END_REF][START_REF] Baud | Efects of porosity and crack density on the compressive strength of rocks[END_REF][START_REF] Mallet | Evolution of the crack network in glass samples submitted to brittle creep conditions[END_REF]. This model leads to a brittle failure envelope that can be described as:

1 = (1 + 2 ) 1∕2 + (1 + 2 ) 1∕2 - 3 + √ 3 (1 + 2 ) 1∕2 - K IC √ a
where A and B are constants that depend on the friction coeicient on pre-existing laws and on the initial damage (D 0 ) deined as D 0 = 4 3 ( a) 3 N V , where N v is the number of sliding cracks initially present. According to the model prediction, peak strength decreases with initial damage, assuming a is a constant, which may seem to contradict our results showing an increase in peak stress after heat treatment despite an increase in initial damage.

However, the previous model is probably too simple for at least two reasons. First, the enhancement of heat-treated sample strength relative to those of intact samples could be related to a crack sealing process. Microstructural observations show that the longest and largest fractures passing through the matrix are sealed. During the temperature increase, partial melting of some minerals occurs, and the melt lowed along the cracks that had already formed irst due to thermal stress (This interpretation implies that parameter a is not a constant and is likely to decrease). The melt illed part of the cracks and inally led to amorphous medium illing and sealing of cracks during cooling.

A second reason is that the most dangerous cracks are the longest ones. Crack sealing of the longest cracks implies an efective crack length decrease. The longest cracks are segmented into smaller length cracks. In contrast with elastic moduli, which depend on crack density, the mechanical strength is controlled by the longest cracks.

Conclusions and Perspectives

This investigation aimed to quantify thermal treatment efects on the microstructure (cracking) and the physical and mechanical properties of andesite. In situ conditions (high temperature) and the presence of luid might modify our observed results. At elevated temperatures during triaxial deformation, the high temperature range (500-1100 °C) could lead to a brittle-ductile transition and thus more complicated mechanical behavior [START_REF] Violay | Porosity evolution at the brittle ductile transition in the continental crust: implications for deep hydro-geothermal circulation[END_REF].

The main results of this work are as follows:

1. Due to thermal treatment, the P-wave velocity decreases by up to 50% as the temperature of heat treatment varies from room temperature to 1100 °C. The crack density increases to 0.6. 2. A clay content as low as 1.5% in the intact rock leads to partial melting for a temperature of heat treat- 4. Thermal treatment could have a strengthening efect on a rock depending on the original microstructure of the rock and the mineral content. This result is counterintuitive but might be linked to the presence of clay in the andesite.

(3) 1 = A( , D 0 ) 3 + B( , D 0 ) K IC √ a , 525 526

Crack Density Inversion Method: Isotropic Case

In the noninteraction approximation, the efective elastic properties in the dry case are given by Bristow 1960:

K is the efective bulk modulus that can be directly inverted from a combination of the P-and S-wave velocities, and G is the efective shear modulus that can be directly inverted from the S-wave velocities. K o and G o are the bulk and shear moduli of the crack-free matrix, and o is the corresponding Poisson's ratio of this state. The value h is a factor given by

K o K = 1 + c h 1 -2 o (1 -o 2 ) G o G = 1 + c h 1 + o (1 -o 5 ) h = 16(1 -2 o ) 9

Fig. 1 aFig. 2

 12 Fig. 1 a SEM images of a non-heat-treated andesite sample at low magnitude. b High magniication view of the matrix. c High magniication view of the phenocrysts

Fig. 3 PFig. 4

 34 Fig.3P-wave velocity (green curve), crack density (blue curve). Permeability (red curve). Note that the percolation threshold (crack density of 0.1) is indicated as a blue dashed line

Fig. 5 a

 5 Fig. 5 a Partial melting, in samples heat-treated at 930 °C. Melting occurs at the boundaries of the phenocrysts, i.e., where the clay (smectite) is located. b Glass is spotted in the porosity, indicating partial melting

Fig. 6 Fig. 7 a

 67 Fig. 6 Conventional triaxial experiments on non-heat-treated andesite samples under dry condition. a Diferential stress versus axial strain under conining pressure of 5 MPa, 15 MPa and 30 MPa. b Mean stress versus volumetric strain

Fig. 8

 8 Fig. 8 Conventional triaxial experiments on heat treated (at 930 °C) andesite samples under dry condition. a Diferential stress versus axial strain under conining pressure of 0 MPa, 15 MPa and 30 MPa. b Mean stress versus volumetric strain

Fig. 9 P

 9 Fig. 9 P-wave velocity and diferential stress measured during triaxial deformation experiments on heat treated (at 930 °C) andesite samples under conining pressure of 0 MPa, 15 MPa and 30 MPa. P-wave velocity and diferential stress are plotted versus axial strain. P-wave

  °C. Partial melting modiies the microstructure and the characteristics of the rocks. 3. A small amount of crack sealing changes the distribution of the crack size. The ultrasonic velocity is weakly modiied, but the mechanical strength is enhanced.

Fig. 10

 10 Fig. 10 Axial crack density and randomly oriented crack density evolution versus axial strain during triaxial deformation experiments on (a) non-heat-treated andesite samples under conining pressure of 30 MPa and (b) heat-treated (at 930 °C) andesite samples under conining pressure of 30 MPa ◂

Fig. 11

 11 Fig. 11 Crack sealing observed in intergranular cracks. Bubbles observed in the melt which seals the cracks

  Young's modulus of the crack-free matrix v 0 Poisson's ratio of the crack-free matrix Propagation angles V p ( ) Wave phase velocities along the propagation angles M The least square value K Efective bulk modulus

Fig. 12

 12 Fig. 12 Stress states C' (onset of dilatancy) in non-heat-treated samples and heat-treated samples (at 930 °C)

Table 1

 1

	Chemical composition of andesite samples heat treated	Sample	Plagioclase(%) Pyroxene (%) Silicon (%) Ironoxides (%) Smectite (%) Illite (%)
	to diferent temperatures	Andesite	70.8	14.1	9.2	2.5	3.4	0.0
		HT500	70.9	15.5	7.8	2.2	0.0	3.5
		Andesite HT930	73.2	14.4	8.0	1.9	0.0	2.5
		Andesite HT 1100 68.7	13.6	11.0	2.9	0.0	3.9
	Table 2 Chemical composition of andesite samples heat treated to diferent temperatures	Classiication	Mineral (%)	Andesite (%)	Andesite 500 (%)	Andesite 930 (%)	Andesite 1100 (%)
		Plagioclase	Bytownite	7.4		24.7	23	14.8
			Sadine	10.6		11.4	12.7	14.3
			Albite	52.8		34.8	37.5	39.6
		Pyroxene	Diopside	1.7		2.3	3.3	0.8
			Hypersthene	6.4		8.4	9.1	6.4
			Augite	6.1		4.8	2.1	6.5
		Silicon	Tridymite	8.0		7.2	5.0	9.2
			SiO2	1.2		0.6	3.1	0.8
		Iron-oxides	Ilmentite	0.2		0.0	0.0	1.9
			Magnetite	2.4		2.2	1.9	0.9
		Clay	Smectite	3.4		0.0	0.0	0.0
			Illite	0.0		3.5	2.5	3.9
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Author Proof

Acknowledgements

We thank Léa Lévy for help with SEM observation and XRD analysis, and thanks to Dr. Thomas Ferrand and Dr. Sarah Incel for discussion. This work is supported by the French State through the Future Investments funds under GROTREF project, which is a multidisciplinary platform for innovation and demonstration activities for the exploration and development of high geothermal energy in fractured reservoirs.

with the minimum distance between predicted and measured velocities [START_REF] Fortin | Inluence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure[END_REF][START_REF] Nicolas | Brittle and semi-brittle behaviours of a carbonate rock: inluence of water and temperature[END_REF].

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