Distinguishing between old and modern permafrost sources with compound-specific δ2H analysis
Authors: Vonk Jorien E. , Tesi Tommaso , Bröder Lisa , Holmstrand Henry , Hugelius Gustaf , Andersson August , Dudarev Oleg , Semiletov Igor , Gustafsson Örjan 
Published: February 2017

Abstract. Pleistocene ice complex permafrost deposits contain roughly a quarter of the organic carbon (OC) stored in permafrost terrain. When permafrost thaws, its OC is remobilized into the (aquatic) environment where it is available for degradation, transport or burial. Aquatic or coastal environments contain sedimentary reservoirs that can serve as archives of past climatic change. As permafrost thaw is increasing throughout the Arctic, these reservoirs are important locations to assess the fate of remobilized permafrost OC. We here present compound-specific deuterium (δ2H) analysis on leaf waxes as a tool to distinguish between OC released from thawing Pleistocene permafrost (Ice Complex Deposits; ICD) and from thawing Holocene permafrost (from near-surface soils). Bulk geochemistry (%OC, δ13C, %total nitrogen; TN) was analyzed as well as the concentrations and δ2H signatures of long-chain n-alkanes (C21 to C33) and mid/long-chain n-alkanoic acids (C16 to C30) extracted from both ICD-PF samples (n = 9) and modern vegetation/O-horizon (Topsoil-PF) samples (n = 9) from across the northeast Siberian Arctic. Results show that these Topsoil-PF samples have higher %OC, higher OC/TN values, and more depleted δ13C-OC values than ICD-PF samples, suggesting that these former samples trace a fresher soil and/or vegetation source. Median concentrations of high-molecular weight n-alkanes (sum of C25-C27-C29-C31) were 210 ± 350 µg/gOC (median ± IQR) for Topsoil-PF and 250 ± 81 µg/gOC for ICD-PF samples. Long-chain n-alkanoic acids (sum of C22-C24-C26-C28) were more abundant than long-chain n-alkanes, both in Topsoil-PF samples (4700 ± 3400 µg/gOC) and in ICD samples (6630 ± 3500 µg/gOC). Whereas the two investigated sources differ on the bulk geochemical level, they are, however, virtually indistinguishable when using leaf wax concentrations and ratios. However, on the molecular-isotope level, leaf wax biomarker δ2H values are statistically different between Topsoil-PF and ICD-PF. The mean δ2H value of C29 n-alkane was −246 ± 13 ‰ (mean ± stdev) for Topsoil-PF and −280 ± 12 ‰ for ICD-PF, whereas the C31 n-alkane was −247 ± 23 ‰ for Topsoil-PF and −297 ± 15 ‰ for ICD-PF. The C28 n-alkanoic acid δ2H value was −220 ± 15 ‰ for Topsoil-PF and −267 ± 16 ‰ for ICD-PF. With a dynamic isotopic range (difference between two sources) of 34 to 50 ‰, the isotopic fingerprints of individual, abundant, biomarker molecules from leaf waxes can thus serve as end-members to distinguish between these two sources. We tested this molecular δ2H tracer along with another source-distinguishing approach, dual-carbon (δ13C-δ14C) isotope composition of bulk OC, for a surface sediment transect in the Laptev Sea. Results show that general offshore patterns along the shelf-slope transect are similar, but the source apportionment between the approaches vary, which may highlight the advantages of either. The δ2H molecular approach has the advantage that it circumvents uncertainties related to a marine end-member, yet the δ13C-δ14C approach has the advantage that it represents the bulk OC fraction thereby avoiding issues related to the molecular-bulk upscaling challenge. This study indicates that the application of δ2H leaf wax values has potential to serve as a complementary quantitative measure of the source and differential fate of OC thawed out from different permafrost compartments.