Characterization of terrestrial organic matter transported through the Lena River Delta (NE Siberia) to its adjacent nearshore zone using lignin phenols, δ13C and ∆14C

Abstract:

The Lena River in central Siberia is one of the major pathways translocating terrestrial organic matter (OM_terr) from its southernmost reaches near Lake Baikal to the coastal zone of the Laptev Sea and the Arctic Ocean. Permafrost soils from its vast catchment area store huge amounts of pre-aged OM, which is expected to be remobilized due to climate warming. To characterize the composition and vegetation sources of OM discharged by the Lena River, we analyzed the lignin phenol and carbon isotopic composition (δ¹³C and ∆¹⁴C) in total suspended matter (TSM) from surface waters collected in spring and summer, surface sediments from the Buor Khaya Bay along with soils from the Lena Delta. A simple linear mixing model based on the lignin phenol distributions indicates OM in TSM samples from the delta and Buor Khaya Bay surface sediments contains comparable contributions from gymnosperm sources, which are primarily from the taiga forests south of the delta, and angiosperm material typical for tundra vegetation. Considering the small area covered by tundra (~12% of total catchment), the input of tundra-derived OM input is substantial and likely to increase in a warming Arctic. Radiocarbon compositions (∆¹⁴C) of bulk OM in TSM samples varied from -55 to -391‰, i.e. ¹⁴C ages of 395 to 3920 yrs BP. Using δ¹³C compositions to estimate the fraction of phytoplankton-derived OM and assuming that this material has a modern ¹⁴C signature, we inferred the ∆¹⁴C compositions of OM_terr in TSM exported by the Lena River to range between -190 and -700‰. Such variability in the ages of OM_TERR (i.e. 1640 to 9720 ¹⁴C yrs BP) reflects the heterogeneous composition and residence time of OM in the Lena River catchment soils (Holocene to Pleistocene ages).

Lignin phenol and ∆¹⁴C compositions of surface sediments from the adjacent Buor Khaya Bay suggest that OM_TERR deposited there is older and more degraded than materials present in river particles and catchment soils. Stronger diagenetic alteration in Lena Delta TSM and marine sediments relative to soils may reflect degradation of more labile components during permafrost thawing and transport. Despite the high natural heterogeneity in catchment soils, the lignin biomarker compositions and radiocarbon ages of OM_TERR exported by the Lena River reflect catchment characteristics such as vegetation and soil type.