Methane emissions from the East Siberian Arctic Shelf - Production and removal pathways

Abstract

Methane is an important greenhouse gas that enters the atmosphere via anthropogenic emissions and natural processes. The response of natural methane sources to a changing climate is difficult to predict as uncertainties arise from strong climatic feedbacks. The Arctic regions are especially important because above-average warming may trigger extensive methane release from its vast carbon reservoirs. The East Siberian Arctic Shelf (ESAS) is one of the world’s largest continental shelves and contains carbon-rich permafrost underneath a shallow shelf sea. Subsea permafrost is thawing as a result of seawater intrusion or geothermal activity. Some years ago it was discovered that large quantities of methane are bubbling from the ESAS seafloor, reaching the atmosphere.

The processes driving this methane flux are currently not well understood. In thawing permafrost, organic material can be mineralized to methane as a result of geothermal heat (thermogenic) or microbial activity (biogenic) or removed by microbial oxidation or diffusion to the water column. Furthermore, large quantities of methane can be stored for thousands of years in permafrost reservoirs – gas hydrates and gas pockets – and released in a short period of time once the sealing layer of frozen sediment thaws. If we want to predict how emissions of ESAS methane will develop as Arctic warming continues, we need to identify its key sources and sinks.

Here we present a unique dataset of mixing ratios and stable isotopes (deuterium and carbon-13) of methane from different depths in the ESAS sediment and water column. Sampling took place between 2007 and 2012 during several winter (ice drilling) and summer (cruise) expeditions at locations where methane is actively bubbling (hotspots) and diffusing (background) towards the surface. With the dataset we have identified several production and removal pathways of methane in the sediment and water column. We find that methane emissions originate from a mixture of sources that may be part of a larger marine carbon cycle. Furthermore, isotopic fractionation processes associated with active methane cycling may obscure a hydrate degassing event in stable isotope measurements of atmospheric methane in the region.