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

Abstract

Abstract. Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH 4 ) in sediments as gas hydrate and free gas. Although release of this CH 4 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 CH 4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO 2− 4 ), alkalinity, and the δ 13 C of dissolved inorganic carbon (DIC), is negligible at all stations east of 143 • E longitude. In the upper 8 m of these cores, downward SO 2− 4 flux never exceeds 6.2 mol m −2 kyr −1 , the upward alkalinity flux never exceeds 6.8 mol m −2 kyr −1 , and δ 13 C composition of DIC (δ 13 C-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 H 2 S. 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 SO 2− 4 and alkalinity were 13.9 and 11.3 mol m −2 kyr −1 , respectively, the δ 13 C-DIC gradient was 5.6 ‰ m −1 , and Mn 2+ 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 CH 4 , 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.

High-resolution underwater laser spectrometer sensing provides new insights into methane distribution at an Arctic seepage site

“…Because the instrument assembly lacked an inertia measurement unit, the stability during towing is unknown but we did not observe any effect on the measurements from wobble and/or rotation. Gentz et al (2014) and Myhre et al (2016) suggested that a pronounced pycnocline is a prerequisite to limit the vertical transport of dissolved CH 4 towards the surface. One should note that this hypothesis is based on discrete sample data rather than high-resolution data.…”

“…Methane (CH 4 ) release from gas-bearing ocean sediments has been of high interest for many years (e.g. Westbrook et al, 2009;Ferré et al, 2012;Ruppel and Kessler, 2016;Jørgensen et al, 1990;Boetius and Wenzhöfer, 2013;Myhre et al, 2016;Platt et al, 2018). Once released and dissolved in the water column, the CH 4 gas diffuses and is partly oxidized in the water column (Reeburgh, 2007), contributing to minimum oxygen zones (Boetius and Wenzhöfer, 2013) and possibly to ocean acidification (Biastoch et al, 2011).…”

“…However, most studies of shallow CH 4 seepage sites have found no or little CH 4 flux to the atmosphere (e.g. Miller et al, 2017;Platt et al, 2018;Myhre et al, 2016;Gentz et al, 2014).…”

High-resolution under-water laser spectrometer sensing provides new insights to methane distribution at an Arctic seepage site

“…Methane (CH4) release from gas bearing ocean sediments has been of high interest for many years (e.g. Jørgensen et al, 1990;25 Westbrook et al, 2009;Ferré et al, 2012;Boetius and Wenzhöfer, 2013;Myhre et al, 2016;Ruppel and Kessler, 2016;Platt et al, 2018). Once released and dissolved in the water column, the CH4 gas diffuses and is partly oxidized in the water column (Reeburgh, 2007), contributing to ocean acidification (Biastoch et al, 2011) and minimum oxygen zone formation (Boetius and Wenzhöfer, 2013).…”

“…However, most studies of shallow CH4 seepage sites have found no or little CH4 flux to the atmosphere (e.g. Gentz et al, 2014;Myhre et al, 2016;Miller et al, 2017;Platt et al, 2018).…”