Methane distribution in porewaters of the Eastern Siberian Shelf Sea – chemical, acoustic, and video observations

Wednesday, 16 December 2015: 14:25
3007 (Moscone West)
Volker Bruchert1, Joanna E Sawicka2, Vladimir Samarkin3, Riko Noormets4, Gabrielle Jarvik Stockmann2, Lisa Bröder5, Jayne Rattray6 and Julia Steinbach6, (1)Stockholm University, Geological Sciences and Bolin Centre for Climate Research, Stockholm, Sweden, (2)Stockholm University, Geological Sciences, Stockholm, Sweden, (3)University of Georgia, Athens, GA, United States, (4)University Centre in Svalbard, Longyearbyen, Norway, (5)Stockholm University, Analytical Scemistry and Environmental Sciences, Stockholm, Sweden, (6)Stockholm University, Stockholm, Sweden
We present porewater methane and sulfate concentrations, and the isotope composition of carbon dioxide from 18 sites in areas of reported high methane water column concentrations on the Siberian shelf. Echosounder imaging and video imagery of the benthic environment were used to detect potential bubble emission from the sea bottom and to locate high methane emission areas. In areas where bubble flares were identified by acoustic echsounder imaging, recovered sediment cores provided evidence for slightly elevated porewater methane concentrations 10 cm below the sediment surface relative to sites without flares. Throughout the recovered sediment depth intervals porewater concentrations of methane were more than a factor 300 below the gas saturation limit at sea surface pressure. In addition, surface sediment video recordings provided no evidence for bubble emissions in the investigated methane hotspot areas although at nearby sites bubbles were detected higher in the water column. The conflicting observations of acoustic indications of rising bubbles and the absence of bubbles and methane oversaturation in any of the sediment cores during the whole SWERUS cruise suggest that advective methane seepage is a spatially limited phenomenon that is difficult to capture with routine ship-based core sampling methods in this field area.

Recovery of a sediment core from one high-activity site indicated steep gradients in dissolved sulfate and methane in the first 8 cm of sediment pointing to the presence of anaerobic methane oxidation at a site with a high upward flux of methane. Based on the decrease of methane towards the sediment surface and the rates of sulfate reduction-coupled methane oxidation, most of the upward-transported methane was oxidized within the sediment. This conclusion is further supported by the stable isotope composition of dissolved carbon dioxide in porewaters and the precipitation of calcium carbonate minerals only found in sediment at this site. Overall, these results from the SWERUS expedition point to the well-described presence of sedimentary anaerobic microbial methane oxidation acting as a lid to prevent methane emission to the bottom waters of the Siberian shelf. Caution has to be exercised to apply these results in a wider spatial context for the Siberian shelf.