Pore water geochemistry along continental slopes north of the East Siberian Sea: inference of low methane concentrations

Abstract. 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.