Gas seepage related to permafrost degradation at the southern Kara Sea: geochemical evidence

1. Gas seepage related to permafrost degradation at the southern Kara Sea: geochemical evidence Pavel Serov1,Petr Semenov1, B.Vanshtein1, P.Ilatovskaya1, A.Portnov1,2 1- «VNIIOkeangeologia», Saint-Petersburg, Russia2-University of Tromsø, Norway

2. Presentation structure Regional approach Local approach 1. Areal distribution of volumetric gas content and methane concentration in near-bottom water layer. 2. Geochemistry of the gas seepage zone located Goals: Goals: • Construction of areal distribution maps, anomaly zones detection; • Calculation of total methane mass in near-bottom water layer of study area; • Preliminary estimation of methane budget in water column; • Analysis of geochemical parameters downcore distribution ; • Methane genesis investigation; • Study of geochemical processes associated with gas seepage as result of permafrost degradation;

3. Study area Area 1 Sampling stations:132 Distance between sampling stations: 2 km Total square: 8584,2 km2 Gas seepage located T-04 Area 2 Sampling stations:191 Distance between sampling stations: 2 km Total square: 11472,8 km2 Map of the Kara Sea (left) and study area with sampling points and seismoacoustic profiles (right)

4. Methods Field Laboratory • bottom sampling using boxcorer and gravity tube corers; water sampling using bathometers • hydrocarbon gases (C1-C5) analysis using Shimadzu GC 2014 FID gas chromatography Methane concentration values in area 1 and 2 • TOC analysis using Shimadzu TOC-Vscan element analyzer • Carbon isotopic analysis ThermaFinnigan Delta V CF-IR mass spectrometer • sedimentary and water gas extraction using SUOK-DG and SUOK degassing sets Examples of gas samples gross composition • Ion analysis using Classic chemistry methods: gravymetric, titrimetric methods • Bottom sediments grain-size analysis

5. Bottom sediments Bottom sediments map Fine sand Clayey silt Silty clay Clay

6. Near-bottom water total gas content Bottom sediments map Bathymetry Gas content in near-bottom water layer Lithologically controlled (low values of gas content) Gas discharge (high values of gas content ) Scarse permafrost • The areal distribution of values ​​of gas content of water reflects two trends: • higher values ​​of gas content of water in the area of ​​sandy sediments distribution • not connected with sandy sediments high gas content areasmay be associated with scarse permafrost zone.

7. Near-bottom water methane content distribution with the presence of the bottom maximum Methane content µmol Total square: 8584,2 km2 Total amount of methane in 1m water layer: 4057,59 t. Types of distribution of dissolved methane in the water column (Sergienko et. Al, 2010) Methane Methane in water (CTD profiling data) Total square: 11472,8 km2 Total amount of methane in 1m water layer : 7322,35 t. Total amount of methane in 1 m near-bottom layer of water for two areas is 11400 t.According to our data, the upper layer of water column contains about 47% of methane towards the near-bottom layer. Consequently, the top 1m layer of water contains 5400 t. of methane.

8. 2. Geochemistry of the gas seepage zone located T-01 T-02 T-03 T-04 Fluent oxidized mud Black clayey mud Grey sandy mud Fine-grained grey clayey sand Silty clayey mud Hydrotroilite nests Shelly detritus Near-bottom water gas content aerial distribution Gas seepage

9. δC13-CO2 1 -10 0 3 -20 2 -30 0 -40 -50 δC13-CH4 SO42-, mg/L -60 -50 -70 -80 -90 -100 CH4, ppm Cl-, g/L Ʃ C2-C5, ppm Fluent oxidized mud Black clayey mud Grey sandy mud Fine-grained grey clayey sand Hydrotroilite nests Shelly detritus

10. CH4+SO42- = HCO3-+HS-+H2O T-04 The dramatic methane sink is probably related to intensive AOM coupled with sulfate reduction Methane concentrations are high (~3%) in the lower part of the sediment section and reduced by~ 500 times in the very upper part According to δC13 (>-78‰) values, methane is of microbial or mixed origin TOC concentration within the sulfate depleted zone is too low for significant methane production in the upper sediment layer Total salt content gradient is indicative of upward freshened water influx

11. methane sink TOC sink TOC sink TOC sink TOC sink residual insufficient methane discharge terrigenic OM CH4 + 2O2 = CO2 + 2H2O aerobic methane oxidation water H2S CO2 Sulphate-rich zone anaerobic methane oxidation CH4+SO42- = HCO3-+HS-+H2O intensive degradation residual OM influx sediments methane saturated fresh water influx methanogenesis CO2 + 4H2 = 2H2O + CH4 degassing of thawing permafrost OM degradation 2CH2O = CO2 + CH4\ OM permafrost permafrost degradation

12. Conclusions Regional approach The areal distribution of ​​gas content values in water reflects correlation with bottom sediment types; High gas content areas of fine sediments may be associated with permafrost degradation as source of gas discharge; Total methane amount estimated is relatively low, which corresponds to the data of average methane flux for Western Arctic seas available Local approach • Methane upward migration in the sediment layer is evident from • geochemical data • Sufficient amount of microbial derived methane is discharged as result of permafrostdegradation (captured gas release and/or permafrost OM decomposition) • Upward migrating methane is intensively oxidized by sulfate-reducing • microorganisms in upper anoxic sediments and insufficient amount of methane enters water column

13. Acknowledgements • To “Rosnedra” for funding this work; • To crew of “Neotrazimy” marine rescue tug for the assistance in onboard works; • To “Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academу of Sciences”