Description: This data set includes measurement during the CACOON 2019 expeditions (Fuchs et al., 2021; Strauss et al., 2021) in the Lena Delta region in Siberia. The data collection took place during two field campaigns; the first one was in March-April 2019, the second one in August 2019. Measurements were taken with a handheld SontekTM CastAway sensor with an integrated GPS. The measured data include pressure (dbar; accuracy: 0.25%), depth (m; ±0.25%), temperature (°C; ±0.05°C), conductivity (mS/cm; 0.25%±0.005 mS/cm), specific conductance (mS/cm; 0.25%±0.005 mS/cm), salinity (practical salinity scale; ±0.1), sound velocity (m/s; ±0.15 m/s), and density (kg/m3; ±0.02 kg/m3). In total, 31 depth profiles were measured from the Sardakhskaya main river channel in the Lena Delta to 80 km offshore in the Laptev Sea to specifically target the mouth area of the Sardakhskaya channel. The CTD was lowered from water surface with an additional ballast to make sure the small CTD device reached the sea (or river) bed. The data consists of two data sheets. The first one (CTD cast locations in the Lena Delta region) includes detailed data about the locations and time of CTD measurements; the second data sheet (CTD measurements in the Lena Delta region) includes the CTD data from all the measured locations.

Description: Organic carbon (OC) stored in Arctic permafrost represents one of Earth’s largest and most vulnerable terrestrial carbon pools. Amplified climate warming across the Arctic results in widespread permafrost thaw. Permafrost deposits exposed at river cliffs and coasts are particularly susceptible to thawing processes. Accelerating erosion of terrestrial permafrost along shorelines leads to increased transfer of organic matter (OM) to nearshore waters. However, the amount of terrestrial permafrost carbon and nitrogen as well as the OM quality in these deposits are still poorly quantified. Here, we characterise the sources and the quality of OM supplied to the Lena River at a rapidly eroding permafrost river shoreline cliff in the eastern part of the delta (Sobo-Sise Island). Our multi-proxy approach captures bulk elemental, molecular geochemical and carbon isotopic analyses of late Pleistocene Yedoma permafrost and Holocene cover deposits, discontinuously spanning the last ~52 ka. We show that the ancient permafrost exposed in the Sobo-Sise cliff has a high organic carbon content (mean of about 5 wt%).We found that the OM quality, which we define as the intrinsic potential to further transformation, decomposition, and mineralization, is also high as inferred by the lipid biomarker inventory. The oldest sediments stem from Marine Isotope Stage (MIS) 3 interstadial deposits (dated to 52 to 28 cal kyr BP) and is overlaid by Last Glacial MIS 2 (dated to 28 to 15 cal ka BP) and Holocene MIS 1 (dated to 7–0 cal ka BP) deposits. The relatively high average chain length (ACL) index of n-alkanes along the cliff profile indicates a predominant contribution of vascular plants to the OM composition. The elevated ratio of iso and anteiso-branched FAs relative to long chain (C ≥ 20) n-FAs in the interstadial MIS 3 and the interglacial MIS 1 deposits, suggests stronger microbial activity and consequently higher input of bacterial biomass during these climatically warmer periods. The overall high carbon preference index (CPI) and higher plant fatty acid (HPFA) values as well as high C / N ratios point to a good quality of the preserved OM and thus to a high potential of the OM for decomposition upon thaw. A decrease of HPFA values downwards along the profile probably indicates a relatively stronger OM decomposition in the oldest (MIS 3) deposits of the cliff.