Thermokarst Landscape Evolution Recorded by n-Alkanes in a Deep Sediment Core from Bykovsky Peninsula, Northeast Siberia

Warming of the Arctic triggers deep permafrost thaw, which has a strong impact on permafrost organic carbon (OC) storage. To identify the sedimentation history and organic matter (OM) characteristics of thermokarst-affected permafrost landscapes, we carried out an expedition in spring 2017 to the Bykovsky Peninsula. This is a remnant of a late Pleistocene accumulation plain on the Laptev Sea coast, northeastern Siberia. We retrieved a 31-m-long sediment core from underneath a thermokarst lake (water depth: 5.1 m) and analyzed the sediments for n-alkanes, total organic carbon content (TOC) and grain size. From the bottom upwards, the core contained 3 m of frozen sediments from underneath the thaw bulb (Unit I: 36.6-33 m), 25 m of unfrozen Yedoma (taberal) sediments (Unit II: 33-18 m, Unit III: 18-10 m) and 4 m of unfrozen lake sediments (Unit IV: 10-5.1 m). Unit I contained coarsest sediments and rounded pebbles, which point to a strong fluvial influence. Here, we found the highest TOC values (17.8 wt%) and drift wood (organic remains up to 4 cm in size). The dominant mid-chains n-alkanes n-C23 and n-C25 and a high aquatic plant n-alkane proxy Paq (median: 0.65) suggest the growth of submerged/floating macrophytes. With a value of 2.2, the odd-over-even predominance (OEP) is lowest in Unit I. Unit II has a lower relative distribution of the midchain n-alkanes, which suggests the vegetation was likely emergent rather than submerged (median Paq: 0.44). This indicates the onset of Yedoma formation and low-centered polygon development. In the finer sediments of Unit III, the Paq further decreases (median: 0.32) and n-C31 becomes more important, indicating the transition to a drier, grass dominated environment. The thermokarst lake (Unit IV) formed about 8 cal ka BP, indicated by a peat layer. The OM in Unit IV is fresh (median OEP: 8.4) and has the highest n-alkane concentration (20.8 µg g-1 sediment). In this study, we show that thermokarst formation has a potential of mobilizing a large OC pool to tens of meters deep: even though the OM in the sediments below the thaw bulb is furthest degraded, still a substantial amount of OC is stored here. The study of n-alkanes is very useful in identifying OM source and degradability and will help to improve OM mobilization estimates in thawing permafrost by investigating the molecular lipid structure.