Organic-matter quality of deep permafrost carbon – a study from Arctic Siberia

The organic-carbon (OC) pool accumulated in Arctic permafrost (perennially frozen ground) equals the car- bon stored in the modern atmosphere. To give an idea of how Yedoma region permafrost could respond under future climatic warming, we conducted a study to quantify the organic-matter quality (here defined as the intrinsic poten- tial to be further transformed, decomposed, and mineralized) of late Pleistocene (Yedoma) and Holocene (thermokarst) de- posits on the Buor-Khaya Peninsula, northeast Siberia. The objective of this study was to develop a stratigraphic clas- sified organic-matter quality characterization. For this pur- pose the degree of organic-matter decomposition was es- timated by using a multiproxy approach. We applied sed- imentological (grain-size analyses, bulk density, ice con- tent) and geochemical parameters (total OC, stable carbon isotopes ( 13 C), total organic carbon : nitrogen (C= N) ra- tios) as well as lipid biomarkers (n-alkanes, n-fatty acids, hopanes, triterpenoids, and biomarker indices, i.e., average chain length, carbon preference index (CPI), and higher-plant fatty-acid index (HPFA)). Our results show that the Yedoma and thermokarst organic-matter qualities for further decom- position exhibit no obvious degradation-depth trend. Rela- tively, the C= N and 13 C values and the HPFA index show a significantly better preservation of the organic matter stored in thermokarst deposits compared to Yedoma deposits. The CPI data suggest less degradation of the organic matter from both deposits, with a higher value for Yedoma organic mat- ter. As the interquartile ranges of the proxies mostly over- lap, we interpret this as indicating comparable quality for further decomposition for both kinds of deposits with likely better thermokarst organic-matter quality. Supported by prin- cipal component analyses, the sediment parameters and qual- ity proxies of Yedoma and thermokarst deposits could not be unambiguously separated from each other. This revealed that the organic-matter vulnerability is heterogeneous and depends on different decomposition trajectories and the pre- vious decomposition and preservation history. Elucidating this was one of the major new contributions of our mul- tiproxy study. With the addition of biomarker data, it was possible to show that permafrost organic-matter degradation likely occurs via a combination of (uncompleted) degrada- tion cycles or a cascade of degradation steps rather than as a linear function of age or sediment facies. We conclude that the amount of organic matter in the studied sediments is high for mineral soils and of good quality and therefore susceptible to future decomposition. The lack of depth trends shows that permafrost acts like a giant freezer, preserving the constant quality of ancient organic matter. When undecom- posed Yedoma organic matter is mobilized via thermokarst processes, the fate of this carbon depends largely on the en- vironmental conditions; the carbon could be preserved in an undecomposed state till refreezing occurs. If modern input has occurred, thermokarst organic matter could be of a better quality for future microbial decomposition than that found in Yedoma deposits.