Methane and carbon dioxide release from the East Siberian Arctic Shelf: The role of subsea and coastal permafrost and other controlling factors as inferred from decadal observational efforts
The Arctic region contains a huge amount of organic carbon (OC) buried inland and within the Arctic Ocean sedimentary basin, that is extremely sensitive to increased global temperatures because of the ice content of both on-land and submarine permafrost. The most pronounced warming is currently registered in the East Siberian part of the Arctic, where surface air temperature increased during the 2000-2005 period by about 5°C compared to 20th century temperature average. It is reasonable to expect that under conditions of continuing warming the regional carbon pool, which consists of OC in Siberian soil and sediments and seabed reservoirs of methane (CH4), will be disturbed and signs of this disturbance will occur over the East Siberian Arctic shelf (ESAS).
The ESAS is employed as an integrator of ongoing changes in surrounding land, creating a terrestrial or exogenous signal which is carried by fresh water, and of in situ changes, creating a marine or endogenous signal, which is generated by submarine permafrost destabilization, increasing coastal erosion, and involvement of old carbon in the modern biogeochemical cycle. Eighty percent of all subsea permafrost underlies the ESAS. Increasing rates of coastal erosion and ongoing destabilization of both on-land and subsea permafrost have been altering the marine carbon cycle, and this could have a significant effect on global, not just Arctic, climate. Misbalance in the carbon cycle causes severe ESAS oversaturation regards to atmosphere by the main greenhouse gases: CH4 and carbon dioxide (CO2).
Sustained CH4 release from thawing Arctic permafrost to atmosphere may be a positive, major feedback to climate warming. ESAS atmospheric CH4 venting was reported as on par with flux from Arctic tundra (Shakhova, Semiletov et al., Nature Geoscience, 7, 64-70, 2014, NGEO2007). Unlike release when ancient carbon in thawed on-land permafrost is mobilized, ESAS CH4 release is not determined by modern methanogenesis. Pre-formed CH4 largely stems from seabed deposits. Our investigation, including observational studies using hydrological, biogeochemical, geophysical, geo-electrical, microbiological, and isotopic methods, and modeling efforts to assess current subsea permafrost state and the ESAS’ contribution to the regional CH4 budget, have clarified processes driving ESAS CH4 emissions. Subsea permafrost state is a major emission determinant; rates vary by 3-5 orders of magnitude (Shakhova, Semiletov et al., Phil. Trans. R. Soc. A, 373: 20140451). Progressive subsea permafrost thawing and decreasing ice extent could significantly increase ESAS CH4 emissions.
Subsea permafrost drilling results reveal modern recently submerged subsea permafrost degradation rates, contradicting previous hypotheses that thousands of years required to form escape paths for permafrost-preserved gas. Recently detected warmer water temperatures near the seabed may impact the stability of subsea permafrost and the carbon pool buried in the ESAS. As a result of these processes, subsea permafrost is degrading at higher rates than previously thought. Dissolved outer ESAS CH4 takes ≤1000 days to be oxidized because oxidation rates are low. Storms could release some aqueous CH4 to atmosphere; dissolved CH4, captured beneath ice in winter, can spread via currents and escape to atmosphere through breaks in the ice.
We here use decadal data to show that extreme and extensive Ocean Acidification (OA) in the ESAS is caused not by direct uptake of atmospheric CO2 but rather by naturally-driven processes: carbon mobilization (and its oxidation to CO2) from thawing coastal permafrost/coastal ice complexes, and freshening due to growing Arctic river runoff and ice melt, which transport carbon along with freshwater to the ESAS (Semiletov, Pipko et al., Nature Geoscience, 9,361–365, NGEO2695). These processes compose a unique acidifying phenomenon that causes persistent, and potentially increasing, aragonite under-saturation of the entire water column. Extreme aragonite under-saturation in the western near-shore ESAS is associated with >80% depression of the total calcifying benthic biomass. Massive OA on the ESAS, the largest sea shelf system of the World Ocean, illustrates the complexity of the Earth system interacting with increasing anthropogenic pressure.
The significance of this research for world science is illustrated by publication > 40 papers in top journals, including Biogeosciences, Geochimica et Cosmochimica Acta, Geophysical Research Letters, J. Geophysical Research, Marine Chemistry, Nature Communications, Nature Geoscience, Permafrost Pereglacial Processes, and the opening remarks by Garry Nicholas-Roth, Editor in Chief of “G7 Climate Change: The New Economy” about Dr. Shakhova’s Foreword (P. 16-17), which precedes transcripts of speeches by the G7 leaders: President Van Rompay (European Council), President Hollande (France), Chancellor Merkel (Germany), Prime Minister Abe (Japan), Prime Minister Cameron (United Kingdom) and President Obama (United States): “This year’s Foreword by Dr. Natalia Shakhova (key scientist in the project) is compelling and should be read with great trepidation and foresight” /The G7 Summit, Brussel, Belgium, 4-5 June, 2014 (http://cloud.digipage.net/go/g7climatechange2014/).