Distinguishing between Old and Modern Permafrost Sources with Compound-Specific Deuterium Analysis

Abstract:

Permafrost underlies about a quarter of the northern hemispheric landmass and stores about 1450 Pg organic carbon (OC). When permafrost thaws, its OC is remobilized into the environment where it is available for degradation, transport or burial. The fate of this OC is partly determined by the type of thaw: (i) abrupt thaw of ice-rich Pleistocene permafrost causing collapse and deep exposure, and (ii) gradual thaw of the surface active layer causing release of topsoil, modern permafrost. The relative magnitude of each thaw pathway, along with its potential impacts on the permafrost carbon feedback, are however not known.

We here present compound-specific deuterium analysis as a permafrost tracing tool to distinguish between old permafrost OC release, primarily released through abrupt thaw, and modern permafrost OC release, mostly released through gradual active layer thaw. We analyzed δ²H values of leaf waxes extracted from both Pleistocene-aged permafrost samples (n=9) and modern vegetation/O-horizon (topsoil) samples (n=9) from across northeast coastal Siberia. We analyzed both long-chain n-alkanes (chain lengths C25, C27, C29 and C31) as well as long-chain n-alkanoic acids (C22, C24, C26, C28).

Results show more depleted values for the Pleistocene samples (-264±33 and -280±6.5‰ for average n-alkanes and n-alkanoic acids, respectively) than for the topsoil samples (-227±23 and -234±13‰). The variability in distribution and δ²H signals for n-alkanoic acids and n-alkanes of topsoils is higher than the Pleistocene samples, that show more constant values, yet the isotopic distinction between the two sources is consisten. For n-alkanes, there is a clear decrease in δ²H signal with chain length (from -201±32 to -247±23‰ for topsoil, and -221±26 to -297±15‰ for Pleistocene samples). These data suggest that δ²H analysis on leaf wax lipids in environments receiving OC from permafrost thaw can be used to assess the relative input of Pleistocene versus modern permafrost material.