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Temporal Constraints on HydrateControlled Methane Seepage off Svalbard
should be easier to observe here than elsewhere (11). The continental margin of Svalbard is characterized by abundant contourite deposits (12) that consist of fine-grained sediments with high water content which cover most of the margin between water depths of 800 and 3000 m. It is likely that these contourites are underlain by Miocene sed1 2 1 1 1 3 C. Berndt, * T. Feseker, T. Treude, S. Krastel, † V. Liebetrau, H. Niemann, V. J. iments with 3%wt of total organic 1 1 1 4 5 1 1 Bertics, ‡ I. Dumke, K. Dünnbier, § B. Ferré, C. Graves, F. Gross, K. Hissmann, V. carbon as found at ODP Site 909 (13) and that the emanating gas was proHühnerbach,5‖ S. Krause,1 K. Lieser,1 J. Schauer,1 L. Steinle3 duced by these sediments. Proximally, 1 GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany. 2MARUM - Center for Marine i.e shallower than 700-800 m water Environmental Sciences and Faculty of Geosciences, University of Bremen, 28359 Bremen, Germany. depth, Pleistocene and Pliocene highly 3 4 Department of Environmental Sciences, University of Basel, 4056 Basel, Switzerland. Department of heterogeneous, terrigeneous glacial Geology, University of Tromsø, 9037 Tromsø, Norway. 5National Oceanography Centre, Southampton deposits overlie the contourites (14, SO14 3ZH, UK. 15). In the glacial deposits there is only *Corresponding author. E-mail: [email protected] limited evidence for free gas and there is no clear geophysical evidence for gas †Present address: Institute of Geosciences, University of Kiel, 24118 Kiel, Germany. hydrate such as a Bottom Simulating ‡Deceased. Reflector. Yet, seismic evidence for gas §Present address: Institut für Angewandte Geowissenschaften, Technische Universität Berlin, 10587 Berlin, hydrate occurrence is conclusive for the Germany. contourite deposits farther west (16) ‖Present address: GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany. where gas hydrate has also been sampled at a vent site in approximately 900 Methane hydrate is an icelike substance that is stable at high pressure and low m water depth (17). temperature in continental margin sediments. Since the discovery of a large number Several oceanographic expeditions of gas flares at the landward termination of the gas hydrate stability zone off were able to corroborate the location of Svalbard, there has been concern that warming bottom waters have started to the gas flares discovered in 2008. Durdissociate large amounts of gas hydrate and that the resulting methane release may ing the MSM21/4 survey in August possibly accelerate global warming. Here, we can corroborate that hydrates play a 2012, we collected a series of role in the observed seepage of gas, but we present evidence that seepage off PARASOUND 18 kHz parametric Svalbard has been ongoing for at least three thousand years and that seasonal echosounder profiles with 40 m spacing fluctuations of 1-2°C in the bottom-water temperature cause periodic gas hydrate around the site of the MASOX observaformation and dissociation, which focus seepage at the observed sites. tory (Fig. 1). A ~40 m footprint of the PARASOUND system at 390 m water depth allowed us to obtain a complete Large quantities of methane, a powerful greenhouse gas, are present in coverage of the flare locations within the area of this survey, which the continental margin West off Svalbard where they are stored as ma- means these data are no longer biased by selection of ship tracks as prerine gas hydrate (1–3). As hydrate stability is temperature dependent vious surveys. Our results show that the gas flares align between 380 and Arctic warming is a potentially major threat to both the environment and 400 m water depth corresponding to the upper limit of the gas hydrate the global economy. If even a fraction of the methane contained in Arc- stability zone (GHSZ) considering present day bottom water temperature tic hydrates were released to the atmosphere the effect on climate could of around 3°C (16). Geological structures that may focus gas from deepbe dramatic (4, 5). er parts of the plumbing system are absent (16). Thus, we interpret this Water column temperature measurements and mooring data suggest match of gas flare origination depth and the calculated landward termia one-degree centigrade bottom water temperature warming for the past nation of the gas hydrate stability zone in the sediments as strong cirthirty years (6, 7). Numerical modeling of hydrate stability predicts that cumstantial evidence for a link between gas hydrate dynamics and gas such warming would result in the dissociation of hydrates in the shal- seepage. At the gas flares, significant amounts of methane are liberated lowest sediments (6–9). Therefore, the discovery of numerous gas flares, into the water column, leading to bottom water CH concentrations of up 4 i.e., trains of gas bubbles in the water column, precisely at the water to 825 nM and a net flux of methane to the atmosphere (18). depth where gas hydrate is expected to dissociate was interpreted as the One objective of this study was to deduce a minimum age for the ononset of submarine Arctic gas hydrate dissociation in response to global set of marine methane release from the sea floor. For detailed sampling warming which may potentially lead to large-scale escape of methane of the gas seeps, we carried out ten dives with the manned submersible into the water column and ultimately into the atmosphere (6). In order to JAGO. Our observations substantiate the presence of more than 5 m assess the consequences of methane venting on ocean and atmosphere wide and typically more than 20 to 40 cm-thick outcropping carbonate composition it is necessary to establish how the rates of methane emis- crusts at the Polarstern (246 m) and the HyBIS (385 m) (Fig. 2) sites; sions from hydrate systems change through time (10). small carbonate nodules at the MASOX site (395 m) were found in gravThe margin of Svalbard (Fig. 1) can be considered a model system to ity cores. We analyzed carbonates from the HyBIS and the MASOX site. study a temperature-related gas hydrate destabilization scenario, as water The mineralogical composition of the carbonates was heterogeneous and temperature in the Fram Strait oceanographic gateway will be more af- admixed with high amounts of detrital silicates. They were characterized fected by changes in global atmospheric temperature than elsewhere in by low δ13C isotope values between -27.1 and -41.4 ‰ V-PDB (17). the Arctic and therefore any corresponding changes to a hydrate system Consequently, these carbonates can be regarded as an archive of micro-
bially-induced, methane-related authigenic precipitation processes (19). The most reliable single age data were obtained from aragonite dominated surface samples. U/Th isotope measurements and resulting minimum seepage age for the MASOX site imply that significant methane-related precipitation was already occurring at 3 kys BP (18). For comparison, the derived ages for the HyBIS site are overlapping or older, e.g., sample SV-2: 8.2 ± 0.5 kys BP or sample SV-3: 4.6 ± 0.5 kys BP. The youngest isochron-based age of approximately 0.5 kys BP was deduced from carbonates that were found in sediments at the MASOX site at 40 to 50 cm depth below sea floor. Due to changes in the path of methane bearing fluids, inclusion of impurities, and alteration of sample material it was not possible to decipher potential on/off-stages or chemical variation of the seeping fluids beyond the results presented in this paper. Hence, it is possible that seepage strength and transport of methane from the sediment to the water column and atmosphere varied over time. We propose that carbonate formation in this area continues until today, because surface sediments (0-10 cm depth below sea floor) at gas vents both at the HyBIS and the MASOX sites were characterized by high rates of anaerobic oxidation of methane (AOM; max 11.3 μmol CH4 cm−3 d−1), which is the driver for carbonate precipitation at methane seeps (19). AOM correlated with high concentrations of methane (max 14800 nM), sulfide (max 11000 nM) and total alkalinity (max 29 mEq l−1) in the sediment. Chemosynthetic communities (sulfur bacteria mats and frenulate tubeworms) were present at both sites (18). Observations of old carbonate crusts imply that seepage must have been ongoing at all three sites for more than 0.5 kys. Detailed paleoceanographic reconstructions for the Svalbard area (11) show a pronounced warming since the end of the 19th century. However, even this 100 year-time span seems too short to explain the observed thicknesses. The ages of the recovered carbonate crusts, which are all significantly older than 100 years support this conclusion. Thus, it is unlikely that an anthropogenic decadal-scale bottom water temperature rise is the primary reason for the origin of the observed gas flares, although it may contribute to keeping gas pathways open longer and further. During the cruise, we recovered the MASOX observatory, which had been deployed twice for a total of 22 months within a cluster of flares between 390 and 400 m water depth. The observatory contained a bottom water temperature sensor sampling every 15 min during both deployments. The recorded time series reveals fluctuations of bottom water temperature between 0.6 and 4.9°C with lowest temperatures between April and June and highest temperatures between November and March (Fig. 3). In both years, the temperature difference between spring and fall/winter was around 1.5 degrees centigrade, but during the second year, the average bottom water temperature was generally about 0.5 degrees higher than that recorded during the first deployment. The time series implies that there is a strong seasonal change of sea floor temperature. In order to obtain better constraints on the heat exchange between the sediment and the bottom water, we conducted in-situ sediment temperature and thermal conductivity measurements using a 6 m-long heat flow probe along transects down the slope. Between 500 and 360 m water depth, our measurements revealed a landward increase in thermal conductivity from 1.5 to 2.6 Wm−1K−1 with a maximum around the position of the MASOX observatory. High sediment thermal conductivity, large temporal variability in bottom water temperature, and possibly formation and dissociation of gas hydrates resulted in very irregular sediment temperature profiles, which made it difficult to determine the heat flow along the transect line from our data. Based on our measurements at 500 m water depth, we estimate the regional heat flow to be around 0.05 Wm−2. Given the comprehensive evidence for seepage this value is likely modulated by convective heat transport. Based on the recorded bottom water temperature time series and the acquired thermal conductivity data, we developed a two-dimensional
model of the evolution of the GHSZ along the transect line. As illustrated in Fig. 3, the seasonal changes in bottom water temperature are accompanied by large lateral shifts of the GHSZ at least within the top 5 m of surface sediments. During the cycle of a year in which bottom water temperature varies as observed in 2011/12, the volume of the GHSZ varied between a maximum value in summer and a minimum value in winter. During the time period covered by our measurements, the GHSZ was at its maximum in June 2011, when it extended to 360 m water depth. Increasing bottom water temperatures from June until December were accompanied by a retreat of the GHSZ at the seafloor to more than 410 m water depth. In the sub-surface, the GHSZ retreated further until it reached its minimum in March 2012. The modeling shows that persistent supply of dissolved methane from below the GHSZ in this section of the slope would lead to the formation of hydrate from winter until summer. The newly formed hydrate would dissociate again during the second half of the year and thus augment methane emissions from the seabed both by opening pathways to gas ascending from underneath and by releasing gas from the hydrate phase. The total volume of sediment that was affected by seasonal shifts of the GHSZ amounted to between 3000 and 5000 m3 per meter of the margin. Assuming a gas hydrate concentration of 5 percent of the pore space and a porosity of 50 percent, the seasonal GHSZ has the potential to periodically store and release between 9 and 15 tons of CH4 per meter of the margin. However, these amounts represent the upper limits of the seasonal buffering capacity, as the latent heat of hydrate kinetics was not included in the simulation. Depending on the concentration and distribution of gas hydrates in the sediment, alternating formation and dissociation would dampen the oscillation of the GHSZ and thus reduce its volume. While the modeling shows clearly that seasonal bottom water temperature variations are capable of modulating the observed gas emissions, we find no direct evidence in the heat flow data that would suggest that the slope sediments experienced decadal-scale warming. The combined data demonstrate that hydrate is playing a fundamental role in modulating gas seeps between 380 and 400 m water depth at the upper limit of the gas hydrate stability zone, while ascending gas would be trapped or deviated up along the base of the GHSZ further seaward. Long-term variations in seepage may exist but presently available data are insufficient to document annual, decadal or centennial changes in seepage. Our data suggest that shallow hydrate accumulations are sensitive to bottom water temperature changes and therefore significant anthropogenic warming will impact the shallow parts of the hydrate system. This sensitivity demonstrates the need for quantifying the total amount of gas hydrate in the shallowest part of the gas hydrate stability zone if climate feedback mechanisms are to be assessed beyond simple global models (20, 21). In summary, our observations show that methane seepage west off Svalbard has been ongoing for much longer than anthropogenic warming. Therefore, observations of large contemporary emissions reported in other studies cannot be considered proof of accelerating hydrate destabilization, although neither do they prove that catastrophic destabilization is not accelerating. References and Notes 1. G. K. Westbrook, S. Chand, G. Rossi, C. Long, S. Bünz, A. Camerlenghi, J. M. Carcione, S. Dean, J.-P. Foucher, E. Flueh, D. Gei, R. R. Haacke, G. Madrussani, J. Mienert, T. A. Minshull, H. Nouzé, S. Peacock, T. J. Reston, M. Vanneste, M. Zillmer, Estimation of gas hydrate concentration from multicomponent seismic data at sites on the continental margins of NW Svalbard and the Storegga region of Norway. Mar. Pet. Geol. 25, 744–758 (2008). doi:10.1016/j.marpetgeo.2008.02.003 2. A. Chabert, T. A. Minshull, G. K. Westbrook, C. Berndt, K. E. Thatcher, S. Sarkar, Characterization of a stratigraphically constrained gas hydrate system along the western continental margin of Svalbard from ocean bottom seismometer data. J. Geophys. Res. 116, B12102 (2011).
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(German Baltic). Limnol. Oceanogr. 50, 1771–1786 (2005). doi:10.4319/lo.2005.50.6.1771 43. T. Treude, A. Boetius, K. Knittel, K. Wallmann, B. B. Jørgensen, Anaerobic oxidation of methane above gas hydrates at Hydrate Ridge, NE Pacific Ocean. Mar. Ecol. Prog. Ser. 264, 1–14 (2003). doi:10.3354/meps264001 44. H. Niemann, T. Lösekann, D. de Beer, M. Elvert, T. Nadalig, K. Knittel, R. Amann, E. J. Sauter, M. Schlüter, M. Klages, J.-P. Foucher, A. Boetius, Novel microbial communities of the Haakon Mosby mud volcano and their role as a methane sink. Nature 443, 854–858 (2006). Medline doi:10.1038/nature05227 Acknowledgments: This manuscript is dedicated to the memory of our beloved colleague and friend Victoria Bertics. We are grateful to Captain K. Bergmann and the officers and crew of R/V MARIA S. MERIAN for their help at sea. The German Research Foundation DFG, the Swiss National Science Foundation, and the Cluster of Excellence “The Future Ocean” supported the project financially. Further support came from the PERGAMON project (COST) and the Alexander von Humboldt Foundation. Figure 1 was drafted using GMT (22). Supplementary Materials www.sciencemag.org/cgi/content/full/science.1246298/DC1 Materials and Methods Fig. S1 to S4 Tables S1 and S2 Movie S1 References (24–44) 23 September 2013; accepted 16 December 2013 Published online 02 January 2014 10.1126/science.1246298
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Fig. 1. The Svalbard gas hydrate province is located on the western margin of the Svalbard archipelago (inset). At water depths shallower than 398 m, numerous gas flares have been observed in the water column (color-coded dots for different surveys) using EK60 echo sounders and high-resolution side scan sonar. The gas flares are located between the contour lines at which gas hydrate is stable in the subsurface at 3 (brown) and 2 (blue) degrees C average bottom water temperature. The black lines show the location of PARASOUND profiles with 40 m separation, i.e., complete coverage, for flare mapping. The black arrows point to the location of submarine dives discussed in the text. The red line shows the location of the modeling transect (bold section shown in Fig. 3). The large cluster of seeps at the northern limit of the gas flare line at a water depth of 240 - 260 m can be explained by the presence of an elsewhere absent glacial debris flow deposit that is deviating gas laterally within the prograding debris flow deposits and cannot have anything to do with gas hydrate dynamics (16, 23).
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Fig. 2. Photograph of the massive authigenic carbonate crusts observed at the HyBIS site in 385 m water depth. For scale, the total length of the larger white sessile ascidia (white stalk-like animal on the crest of the uplifted carbonate plate) is approximately 15 cm. Carbonate crusts such as these take at least several hundred years to develop through anaerobic oxidation of methane.
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Fig. 3. The top panel shows daily means of bottom water temperature recorded by the MASOX observatory. The times when the extent of the GHSZ was at its maximum and minimum are marked by the solid red and dashed blue lines, respectively. The bottom panel shows the seasonal dynamics of the GHSZ. Driven by changes in bottom water temperature, the GHSZ advances and retreats in the course of the year. The solid red lines and the dashed blue lines indicate the maximum and minimum extent of the GHSZ, respectively. The area in which gas hydrates are stable in the longterm is shaded in yellow. The difference between the maximum and minimum extent of the hydrate stability zone is shaded in orange and corresponds to the seasonal GHSZ, in which gas hydrate dissociation and formation alternate periodically. The triangles filled in magenta represent the projected locations of all flares detected within 1000 m of the transect line. The green diamond shows the position of the MASOX observatory. An animated illustration of the modeling results is provided in the supplements.
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Temporal Constraints on HydrateControlled Methane Seepage off Svalbard
should be easier to observe here than elsewhere (11). The continental margin of Svalbard is characterized by abundant contourite deposits (12) that consist of fine-grained sediments with high water content which cover most of the margin between water depths of 800 and 3000 m. It is likely that these contourites are underlain by Miocene sed1 2 1 1 1 3 C. Berndt, * T. Feseker, T. Treude, S. Krastel, † V. Liebetrau, H. Niemann, V. J. iments with 3%wt of total organic 1 1 1 4 5 1 1 Bertics, ‡ I. Dumke, K. Dünnbier, § B. Ferré, C. Graves, F. Gross, K. Hissmann, V. carbon as found at ODP Site 909 (13) and that the emanating gas was proHühnerbach,5‖ S. Krause,1 K. Lieser,1 J. Schauer,1 L. Steinle3 duced by these sediments. Proximally, 1 GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany. 2MARUM - Center for Marine i.e shallower than 700-800 m water Environmental Sciences and Faculty of Geosciences, University of Bremen, 28359 Bremen, Germany. depth, Pleistocene and Pliocene highly 3 4 Department of Environmental Sciences, University of Basel, 4056 Basel, Switzerland. Department of heterogeneous, terrigeneous glacial Geology, University of Tromsø, 9037 Tromsø, Norway. 5National Oceanography Centre, Southampton deposits overlie the contourites (14, SO14 3ZH, UK. 15). In the glacial deposits there is only *Corresponding author. E-mail: [email protected] limited evidence for free gas and there is no clear geophysical evidence for gas †Present address: Institute of Geosciences, University of Kiel, 24118 Kiel, Germany. hydrate such as a Bottom Simulating ‡Deceased. Reflector. Yet, seismic evidence for gas §Present address: Institut für Angewandte Geowissenschaften, Technische Universität Berlin, 10587 Berlin, hydrate occurrence is conclusive for the Germany. contourite deposits farther west (16) ‖Present address: GEOMAR Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany. where gas hydrate has also been sampled at a vent site in approximately 900 Methane hydrate is an icelike substance that is stable at high pressure and low m water depth (17). temperature in continental margin sediments. Since the discovery of a large number Several oceanographic expeditions of gas flares at the landward termination of the gas hydrate stability zone off were able to corroborate the location of Svalbard, there has been concern that warming bottom waters have started to the gas flares discovered in 2008. Durdissociate large amounts of gas hydrate and that the resulting methane release may ing the MSM21/4 survey in August possibly accelerate global warming. Here, we can corroborate that hydrates play a 2012, we collected a series of role in the observed seepage of gas, but we present evidence that seepage off PARASOUND 18 kHz parametric Svalbard has been ongoing for at least three thousand years and that seasonal echosounder profiles with 40 m spacing fluctuations of 1-2°C in the bottom-water temperature cause periodic gas hydrate around the site of the MASOX observaformation and dissociation, which focus seepage at the observed sites. tory (Fig. 1). A ~40 m footprint of the PARASOUND system at 390 m water depth allowed us to obtain a complete Large quantities of methane, a powerful greenhouse gas, are present in coverage of the flare locations within the area of this survey, which the continental margin West off Svalbard where they are stored as ma- means these data are no longer biased by selection of ship tracks as prerine gas hydrate (1–3). As hydrate stability is temperature dependent vious surveys. Our results show that the gas flares align between 380 and Arctic warming is a potentially major threat to both the environment and 400 m water depth corresponding to the upper limit of the gas hydrate the global economy. If even a fraction of the methane contained in Arc- stability zone (GHSZ) considering present day bottom water temperature tic hydrates were released to the atmosphere the effect on climate could of around 3°C (16). Geological structures that may focus gas from deepbe dramatic (4, 5). er parts of the plumbing system are absent (16). Thus, we interpret this Water column temperature measurements and mooring data suggest match of gas flare origination depth and the calculated landward termia one-degree centigrade bottom water temperature warming for the past nation of the gas hydrate stability zone in the sediments as strong cirthirty years (6, 7). Numerical modeling of hydrate stability predicts that cumstantial evidence for a link between gas hydrate dynamics and gas such warming would result in the dissociation of hydrates in the shal- seepage. At the gas flares, significant amounts of methane are liberated lowest sediments (6–9). Therefore, the discovery of numerous gas flares, into the water column, leading to bottom water CH concentrations of up 4 i.e., trains of gas bubbles in the water column, precisely at the water to 825 nM and a net flux of methane to the atmosphere (18). depth where gas hydrate is expected to dissociate was interpreted as the One objective of this study was to deduce a minimum age for the ononset of submarine Arctic gas hydrate dissociation in response to global set of marine methane release from the sea floor. For detailed sampling warming which may potentially lead to large-scale escape of methane of the gas seeps, we carried out ten dives with the manned submersible into the water column and ultimately into the atmosphere (6). In order to JAGO. Our observations substantiate the presence of more than 5 m assess the consequences of methane venting on ocean and atmosphere wide and typically more than 20 to 40 cm-thick outcropping carbonate composition it is necessary to establish how the rates of methane emis- crusts at the Polarstern (246 m) and the HyBIS (385 m) (Fig. 2) sites; sions from hydrate systems change through time (10). small carbonate nodules at the MASOX site (395 m) were found in gravThe margin of Svalbard (Fig. 1) can be considered a model system to ity cores. We analyzed carbonates from the HyBIS and the MASOX site. study a temperature-related gas hydrate destabilization scenario, as water The mineralogical composition of the carbonates was heterogeneous and temperature in the Fram Strait oceanographic gateway will be more af- admixed with high amounts of detrital silicates. They were characterized fected by changes in global atmospheric temperature than elsewhere in by low δ13C isotope values between -27.1 and -41.4 ‰ V-PDB (17). the Arctic and therefore any corresponding changes to a hydrate system Consequently, these carbonates can be regarded as an archive of micro-
bially-induced, methane-related authigenic precipitation processes (19). The most reliable single age data were obtained from aragonite dominated surface samples. U/Th isotope measurements and resulting minimum seepage age for the MASOX site imply that significant methane-related precipitation was already occurring at 3 kys BP (18). For comparison, the derived ages for the HyBIS site are overlapping or older, e.g., sample SV-2: 8.2 ± 0.5 kys BP or sample SV-3: 4.6 ± 0.5 kys BP. The youngest isochron-based age of approximately 0.5 kys BP was deduced from carbonates that were found in sediments at the MASOX site at 40 to 50 cm depth below sea floor. Due to changes in the path of methane bearing fluids, inclusion of impurities, and alteration of sample material it was not possible to decipher potential on/off-stages or chemical variation of the seeping fluids beyond the results presented in this paper. Hence, it is possible that seepage strength and transport of methane from the sediment to the water column and atmosphere varied over time. We propose that carbonate formation in this area continues until today, because surface sediments (0-10 cm depth below sea floor) at gas vents both at the HyBIS and the MASOX sites were characterized by high rates of anaerobic oxidation of methane (AOM; max 11.3 μmol CH4 cm−3 d−1), which is the driver for carbonate precipitation at methane seeps (19). AOM correlated with high concentrations of methane (max 14800 nM), sulfide (max 11000 nM) and total alkalinity (max 29 mEq l−1) in the sediment. Chemosynthetic communities (sulfur bacteria mats and frenulate tubeworms) were present at both sites (18). Observations of old carbonate crusts imply that seepage must have been ongoing at all three sites for more than 0.5 kys. Detailed paleoceanographic reconstructions for the Svalbard area (11) show a pronounced warming since the end of the 19th century. However, even this 100 year-time span seems too short to explain the observed thicknesses. The ages of the recovered carbonate crusts, which are all significantly older than 100 years support this conclusion. Thus, it is unlikely that an anthropogenic decadal-scale bottom water temperature rise is the primary reason for the origin of the observed gas flares, although it may contribute to keeping gas pathways open longer and further. During the cruise, we recovered the MASOX observatory, which had been deployed twice for a total of 22 months within a cluster of flares between 390 and 400 m water depth. The observatory contained a bottom water temperature sensor sampling every 15 min during both deployments. The recorded time series reveals fluctuations of bottom water temperature between 0.6 and 4.9°C with lowest temperatures between April and June and highest temperatures between November and March (Fig. 3). In both years, the temperature difference between spring and fall/winter was around 1.5 degrees centigrade, but during the second year, the average bottom water temperature was generally about 0.5 degrees higher than that recorded during the first deployment. The time series implies that there is a strong seasonal change of sea floor temperature. In order to obtain better constraints on the heat exchange between the sediment and the bottom water, we conducted in-situ sediment temperature and thermal conductivity measurements using a 6 m-long heat flow probe along transects down the slope. Between 500 and 360 m water depth, our measurements revealed a landward increase in thermal conductivity from 1.5 to 2.6 Wm−1K−1 with a maximum around the position of the MASOX observatory. High sediment thermal conductivity, large temporal variability in bottom water temperature, and possibly formation and dissociation of gas hydrates resulted in very irregular sediment temperature profiles, which made it difficult to determine the heat flow along the transect line from our data. Based on our measurements at 500 m water depth, we estimate the regional heat flow to be around 0.05 Wm−2. Given the comprehensive evidence for seepage this value is likely modulated by convective heat transport. Based on the recorded bottom water temperature time series and the acquired thermal conductivity data, we developed a two-dimensional
model of the evolution of the GHSZ along the transect line. As illustrated in Fig. 3, the seasonal changes in bottom water temperature are accompanied by large lateral shifts of the GHSZ at least within the top 5 m of surface sediments. During the cycle of a year in which bottom water temperature varies as observed in 2011/12, the volume of the GHSZ varied between a maximum value in summer and a minimum value in winter. During the time period covered by our measurements, the GHSZ was at its maximum in June 2011, when it extended to 360 m water depth. Increasing bottom water temperatures from June until December were accompanied by a retreat of the GHSZ at the seafloor to more than 410 m water depth. In the sub-surface, the GHSZ retreated further until it reached its minimum in March 2012. The modeling shows that persistent supply of dissolved methane from below the GHSZ in this section of the slope would lead to the formation of hydrate from winter until summer. The newly formed hydrate would dissociate again during the second half of the year and thus augment methane emissions from the seabed both by opening pathways to gas ascending from underneath and by releasing gas from the hydrate phase. The total volume of sediment that was affected by seasonal shifts of the GHSZ amounted to between 3000 and 5000 m3 per meter of the margin. Assuming a gas hydrate concentration of 5 percent of the pore space and a porosity of 50 percent, the seasonal GHSZ has the potential to periodically store and release between 9 and 15 tons of CH4 per meter of the margin. However, these amounts represent the upper limits of the seasonal buffering capacity, as the latent heat of hydrate kinetics was not included in the simulation. Depending on the concentration and distribution of gas hydrates in the sediment, alternating formation and dissociation would dampen the oscillation of the GHSZ and thus reduce its volume. While the modeling shows clearly that seasonal bottom water temperature variations are capable of modulating the observed gas emissions, we find no direct evidence in the heat flow data that would suggest that the slope sediments experienced decadal-scale warming. The combined data demonstrate that hydrate is playing a fundamental role in modulating gas seeps between 380 and 400 m water depth at the upper limit of the gas hydrate stability zone, while ascending gas would be trapped or deviated up along the base of the GHSZ further seaward. Long-term variations in seepage may exist but presently available data are insufficient to document annual, decadal or centennial changes in seepage. Our data suggest that shallow hydrate accumulations are sensitive to bottom water temperature changes and therefore significant anthropogenic warming will impact the shallow parts of the hydrate system. This sensitivity demonstrates the need for quantifying the total amount of gas hydrate in the shallowest part of the gas hydrate stability zone if climate feedback mechanisms are to be assessed beyond simple global models (20, 21). In summary, our observations show that methane seepage west off Svalbard has been ongoing for much longer than anthropogenic warming. Therefore, observations of large contemporary emissions reported in other studies cannot be considered proof of accelerating hydrate destabilization, although neither do they prove that catastrophic destabilization is not accelerating. References and Notes 1. G. K. Westbrook, S. Chand, G. Rossi, C. Long, S. Bünz, A. Camerlenghi, J. M. Carcione, S. Dean, J.-P. Foucher, E. Flueh, D. Gei, R. R. Haacke, G. Madrussani, J. Mienert, T. A. Minshull, H. Nouzé, S. Peacock, T. J. Reston, M. Vanneste, M. Zillmer, Estimation of gas hydrate concentration from multicomponent seismic data at sites on the continental margins of NW Svalbard and the Storegga region of Norway. Mar. Pet. Geol. 25, 744–758 (2008). doi:10.1016/j.marpetgeo.2008.02.003 2. A. Chabert, T. A. Minshull, G. K. Westbrook, C. Berndt, K. E. Thatcher, S. Sarkar, Characterization of a stratigraphically constrained gas hydrate system along the western continental margin of Svalbard from ocean bottom seismometer data. J. Geophys. Res. 116, B12102 (2011).
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(German Baltic). Limnol. Oceanogr. 50, 1771–1786 (2005). doi:10.4319/lo.2005.50.6.1771 43. T. Treude, A. Boetius, K. Knittel, K. Wallmann, B. B. Jørgensen, Anaerobic oxidation of methane above gas hydrates at Hydrate Ridge, NE Pacific Ocean. Mar. Ecol. Prog. Ser. 264, 1–14 (2003). doi:10.3354/meps264001 44. H. Niemann, T. Lösekann, D. de Beer, M. Elvert, T. Nadalig, K. Knittel, R. Amann, E. J. Sauter, M. Schlüter, M. Klages, J.-P. Foucher, A. Boetius, Novel microbial communities of the Haakon Mosby mud volcano and their role as a methane sink. Nature 443, 854–858 (2006). Medline doi:10.1038/nature05227 Acknowledgments: This manuscript is dedicated to the memory of our beloved colleague and friend Victoria Bertics. We are grateful to Captain K. Bergmann and the officers and crew of R/V MARIA S. MERIAN for their help at sea. The German Research Foundation DFG, the Swiss National Science Foundation, and the Cluster of Excellence “The Future Ocean” supported the project financially. Further support came from the PERGAMON project (COST) and the Alexander von Humboldt Foundation. Figure 1 was drafted using GMT (22). Supplementary Materials www.sciencemag.org/cgi/content/full/science.1246298/DC1 Materials and Methods Fig. S1 to S4 Tables S1 and S2 Movie S1 References (24–44) 23 September 2013; accepted 16 December 2013 Published online 02 January 2014 10.1126/science.1246298
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Fig. 1. The Svalbard gas hydrate province is located on the western margin of the Svalbard archipelago (inset). At water depths shallower than 398 m, numerous gas flares have been observed in the water column (color-coded dots for different surveys) using EK60 echo sounders and high-resolution side scan sonar. The gas flares are located between the contour lines at which gas hydrate is stable in the subsurface at 3 (brown) and 2 (blue) degrees C average bottom water temperature. The black lines show the location of PARASOUND profiles with 40 m separation, i.e., complete coverage, for flare mapping. The black arrows point to the location of submarine dives discussed in the text. The red line shows the location of the modeling transect (bold section shown in Fig. 3). The large cluster of seeps at the northern limit of the gas flare line at a water depth of 240 - 260 m can be explained by the presence of an elsewhere absent glacial debris flow deposit that is deviating gas laterally within the prograding debris flow deposits and cannot have anything to do with gas hydrate dynamics (16, 23).
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Fig. 2. Photograph of the massive authigenic carbonate crusts observed at the HyBIS site in 385 m water depth. For scale, the total length of the larger white sessile ascidia (white stalk-like animal on the crest of the uplifted carbonate plate) is approximately 15 cm. Carbonate crusts such as these take at least several hundred years to develop through anaerobic oxidation of methane.
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Fig. 3. The top panel shows daily means of bottom water temperature recorded by the MASOX observatory. The times when the extent of the GHSZ was at its maximum and minimum are marked by the solid red and dashed blue lines, respectively. The bottom panel shows the seasonal dynamics of the GHSZ. Driven by changes in bottom water temperature, the GHSZ advances and retreats in the course of the year. The solid red lines and the dashed blue lines indicate the maximum and minimum extent of the GHSZ, respectively. The area in which gas hydrates are stable in the longterm is shaded in yellow. The difference between the maximum and minimum extent of the hydrate stability zone is shaded in orange and corresponds to the seasonal GHSZ, in which gas hydrate dissociation and formation alternate periodically. The triangles filled in magenta represent the projected locations of all flares detected within 1000 m of the transect line. The green diamond shows the position of the MASOX observatory. An animated illustration of the modeling results is provided in the supplements.
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