Distinguishing between old and modern permafrost sources in the northeast Siberian land–shelf system with compound-specific δ²H analysis

Abstract. Pleistocene ice complex permafrost deposits contain roughly a quarter of the organic carbon (OC) stored in permafrost (PF) 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- to long-chain n-alkanoic acids (C₁₆ to C₃₀) extracted from both ICD-PF samples (n =  9) and modern vegetation and 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. 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. For example, the mean δ²H value of C₂₉ n-alkane was −246 ± 13 ‰ (mean ± SD) for topsoil PF and −280 ± 12 ‰ 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 endmembers 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. 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.