The Deep Permafrost Carbon Pool of Siberia and Alaska

An estimation of the carbon stored in Arctic permafrost and its biogeochemical characteristics are essential topics in today’s permafrost research. While the uppermost cryosoil horizons are well-studied and recorded in the Northern Circumpolar Soil Carbon Database (NCSCD), there are large uncertainties concerning the quantity and distribution of permafrost deep organic carbon. We studied the organic carbon content of the Yedoma region, where substantial quantities of organic carbon are sequestered. This region is unique because of its organic carbon, which was deeply incorporated into permafrost during the late Quaternary. Rapid inclusion of labile organic matter into permafrost halted decomposition and resulted in a deep long-term carbon sink. Organic carbon in the Yedoma region occurs manly as peat inclusions, twigs and root fragments, other solid plant remains, and finely distributed plant detritus, but also as fossil mammal remains, insects, plankton and soil microorganisms, and finally its decomposition and metabolic products in terms of particulate and dissolved organic matter. With our study we show that two major sub-reservoirs compose the Yedoma region deep frozen organic carbon; Yedoma deposits (late Pleistocene ice- and organic-rich silty sediments) and deposits formed in thaw-lake basins (generalised as thermokarst deposits). Thaw-lake basins result when lake formation degrades Yedoma deposits, then the lakes drain and deposits refreeze. Therefore, the deep Yedoma region organic carbon pool is far from homogeneous and strongly linked to depositional and permafrost dynamics as well as the ecological and climatic history. There are significant differences to former estimates of the Yedoma coverage area, thickness of the relevant frozen deposits, ground ice content and finally in organic carbon content that lead to a reassessment of the deep permafrost carbon pools of the northern high latitude Yedoma region. Using a dataset of approximately 1000 frozen samples from 23 Siberian and Alaskan study sites, we measured and modelled the primary parameters necessary to quantify the overall frozen OC pool, including thickness, spatial coverage, bulk density, wedge ice volume, and total organic carbon content. Because of high inherent (spatial) heterogeneity and non-normal input parameter distributions, we used median values (rather than means) and bootstrapping statistics for carbon budget calculation and error estimation. We quantified the organic carbon pool to 83 +61/-57 Gt for Yedoma deposits and to 128+99/-96 Gt for thermokarst deposits. The total Yedoma region 211+160/-153 Gt is a substantial amount of thaw-vulnerable organic carbon that must be accounted for in global carbon-cycle models