Assessing Organic Matter Characteristics in Ancient Permafrost: A Biogeochemical Study at the Batagay Megaslump, East Siberia

Abstract. The Batagay megaslump, a permafrost thaw feature in northeastern Siberia, provides access to ancient permafrost up to ~650 ka old. We aimed to assess the permafrost-locked organic matter (OM) quality and to deduce paleoenvironmental information on glacial-interglacial timescales. We sampled five stratigraphic units exposed on the 55-m-high slump headwall and analyzed lipid biomarkers. Our findings revealed similar biogeochemical signatures for the glacial periods: the Lower Ice Complex (Marine Isotope Stage (MIS) 16 or earlier), the Lower Sand Unit (some time between MIS 16-6) and the Upper Ice Complex (MIS 4-2). The OM in these units has a terrestrial character, and microbial activity was likely limited. Contrarily, the n-alkane and fatty acid distributions differed for the units from interglacial periods: the Woody Layer (MIS 5), separating the Lower Sand and the Upper Ice Complex, and the Holocene Cover (MIS 1), on top of the Upper Ice Complex. The Woody Layer, marking an permafrost degradation disconformity, contained markers of terrestrial origin (sterols) and high microbial decomposition (iso- and anteiso-fatty acids). In the Holocene Cover, biomarkers pointed to wet depositional conditions and we identified branched and cyclic alkanes, which are likely of microbial or bacterial origin. Higher OM decomposition characterized the interglacial periods. As climate warming will continue permafrost degradation in the Batagay megaslump and in other areas, large amounts of deeply buried, ancient OM with a variable composition and degradability are mobilized, likely significantly enhancing greenhouse gas emissions from permafrost regions.