A 240,000-year stable carbon and nitrogen isotope record from a loess-like palaeosol sequence in the Tumara Valley, Northeast Siberia

Abstract

A 15 m loess-like palaeosol sequence located in the Tumara Valley southwest of the Verkhoyansk Mountains was investigated to reconstruct the Late Quaternary environmental history of Northeast Siberia. Total organic carbon (TOC) and total nitrogen (N) show several distinct and abrupt shifts during the last 240 ka. Both proxies have generally low values (< 0.5% and < 0.06%, respectively) in brown and weathered horizons, which indicate accelerated soil organic matter (SOM) degradation during periods of favourable climatic conditions. On the contrary, dark horizons in the permafrost profile are characterised by higher TOC and N contents (≥ 1% and ≥ 0.12%, respectively). They probably correlate with cold glacial periods, when water logging conditions and preservation of SOM were favoured due to extensive permafrost. The δ 13C values of bulk SOM range from approximately − 29‰ to − 24‰ and show distinct shifts in concert with TOC. Based on the negative correlations of δ 13C org with TOC ( R 2 = 0.49; n = 117) and C org/N ( R 2 = 0.51; n = 117), we suggest that variations of δ 13C org in the Tumara Profile are intensively controlled by SOM degradation. Additionally, also water stress and changes of the atmospheric CO 2 signal should have influenced our stable carbon isotope record. Contrariwise, δ 15N – ranging from about + 1‰ to + 6‰ – showed no significant correlations with our SOM degradation proxies TOC and C org/N. We therefore assume that processes like denitrification, N-fixation, nitrogen losses by frequent fire events and changes in the atmospheric 15N deposition contributed to an opening of the N-cycle and are thus responsible for the observed δ 15N signal of the Tumara Profile.