ALBERT

Description: Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH4) in sediments as gas hydrate and free gas. Although release of this CH4 to the ocean and atmosphere has become a topic of discussion, the region remains sparingly explored. Here we present pore water chemistry results from 32 sediment cores taken during Leg 2 of the 2014 joint Swedish–Russian–US Arctic Ocean Investigation of Climate–Cryosphere–Carbon Interactions (SWERUS-C3) expedition. The cores come from depth transects across the slope and rise extending between the Mendeleev and the Lomonosov ridges, north of Wrangel Island and the New Siberian Islands, respectively. Upward CH4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO42−), alkalinity, and the δ13C of dissolved inorganic carbon (DIC), is negligible at all stations east of 143° E longitude. In the upper 8 m of these cores, downward SO42− flux never exceeds 6.2 mol m−2 kyr−1, the upward alkalinity flux never exceeds 6.8 mol m−2 kyr−1, and δ13C composition of DIC (δ13C-DIC) only moderately decreases with depth (−3.6 ‰ m−1 on average). Moreover, upon addition of Zn acetate to pore water samples, ZnS did not precipitate, indicating a lack of dissolved H2S. Phosphate, ammonium, and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment and supports very low organic carbon turnover rates. A single core on the Lomonosov Ridge differs, as diffusive fluxes for SO42− and alkalinity were 13.9 and 11.3 mol m−2 kyr−1, respectively, the δ13C-DIC gradient was 5.6 ‰ m−1, and Mn2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that abundant CH4, including gas hydrates, do not characterize the East Siberian Sea slope or rise along the investigated depth transects. This contradicts previous modeling and discussions, which due to the lack of data are almost entirely based on assumption.

Description: The Eastern Siberian Margin (ESM), a vast region of the Arctic, potentially holds large amounts of methane in sediments as gas hydrate and free gas. Although this CH4 has become a topic of discussion, primarily because of rapid regional climate change, the ESM remains sparingly explored. Here we present pore water chemistry results from 32 cores taken during Leg 2 of the 2014 SWERUS-C3 expedition. The cores come from depth transects across the continental slope of the ESM between Wrangel Island and the New Siberian Islands. Upward CH4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO42−), alkalinity, and the δ13C-dissolved inorganic Carbon (DIC), is negligible at all stations east of where the Lomonosov Ridge abuts the ESM at about 143° E. In the upper eight meters of these cores, downward sulfate flux never exceeds 9.2 mol/m2-kyr, the upward alkalinity flux never exceeds 6.8 mol/m2-kyr, and δ13C-DIC only slowly decreases with depth (−3.6 ‰/m on average). Additionally, dissolved H2S was not detected in these cores, and nutrient and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment. A single core on Lomonosov Ridge differs, as diffusive fluxes for SO42− and alkalinity were 13.9 and 11.3 mol/m2-kyr, respectively, the δ13C-DIC gradient was 5.6 ‰/m, and Mn2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that significant quantities of CH4, including gas hydrates, do not exist in any of our investigated depth transects spread out along much of the ESM continental slope. This contradicts previous assumptions and hypothetical models and discussion, which generally have assumed the presence of substantial CH4.