Deglacial records of terrigenous organic matter accumulation off the Yukon and Amur rivers based on lignin phenols and long-chain n-alkanes

Abstract

Abstract. Arctic warming and sea level change will lead to widespread permafrost thaw and subsequent mobilization. Sedimentary records of past warming events during the Last Glacial–interglacial transition can be used to study the conditions under which permafrost mobilization occurs and which changes in vegetation on land are associated with such warming. The Amur and Yukon rivers discharging into the Okhotsk and Bering seas, respectively, drain catchments that have been, or remain until today, covered by permafrost. Here we study two marine sediment cores recovered off the mouths of these rivers. We use lignin phenols as biomarkers, which are excellently suited for the reconstruction of terrestrial higher plant vegetation, and compare them with previously published lipid biomarker data. We find that in the Yukon basin, vegetation change and wetland expansion began already in the early deglaciation (ED; 14.6–19 ka). This timing is different from observed changes in the Okhotsk Sea reflecting input from the Amur basin, where wetland expansion and vegetation change occurred later in the Pre-Boreal (PB). In the two basins, angiosperm contribution and wetland extent all reached maxima during the PB, both decreasing and stabilizing after the PB. The permafrost of the Amur basin began to become remobilized in the PB. Retreat of sea ice coupled with increased sea surface temperatures in the Bering Sea during the ED might have promoted early permafrost mobilization. In modern Arctic river systems, lignin and n-alkanes are transported from land to the ocean via different pathways, i.e., surface runoff vs. erosion of deeper deposits, respectively. However, accumulation rates of lignin phenols and lipids are similar in our records, suggesting that under conditions of rapid sea level rise and shelf flooding, both types of terrestrial biomarkers are delivered by the same transport pathway. This finding suggests that the fate of terrigenous organic matter in the Arctic differs on both temporal and spatial scales.

Dynamic land-plant carbon sources in marine sediments inferred from ancient DNA

“…The transition from the glacial to the deglacial phase introduces notable changes in the marine sedimentary DNA record. Increased terrigenous organic matter sedimentation is indicated by the plant DNA share and plant DNA accumulation rate in cores off-Kamchatka and the Bering Sea, aligning with biomarker studies which show a higher input of angiosperm woody taxa during the deglacial (7) (Fig. 3 and Supplementary figures 6, 7).…”

“…3), reflecting known phases of increased terrestrial productivity and terrestrial organic matter (OM) delivery by meltwater runoff (23; 24). Furthermore, the accumulation of land plant DNA shows the same peaks as the Lignin accumulation rate of the marine sediment core SO202-18-3/6 that is located close to SO201-2-77KL in the Bering Sea (7).…”

“…This means that plant DNA can serve as a proxy for the burial of recent plant matter and such for carbon sink on human-relevant timescales. In contrast, compound-specific radiocarbon studies indicate that common biomarkers for terrigenous matter, such as leaf waxes and lignin, originate, at least partly, from a strongly pre-aged organic carbon pool, for example, from permafrost soil (7), though biomarker-specific turnover times have been recognized (38). Hence, the lateral transport of old soils or sediments (37) may be less reflected in the plant DNA signal but this requires further research.…”

“…Hitherto, even the separation of marine (4) and terrigenous (5) carbon has been a challenge for state-of-the-art biogeochemical and isotopic methods and only very rough taxonomic information has been available. For example, lignin-biomarker patterns have been used as proxies for the relative share of gymnosperms (6) and angiosperms (7); and fatty acid compound-specific isotope signals indicate the relative share and productivity of aquatic plants (8), but cannot differentiate between coastal and freshwater taxa. Furthermore, mixing models are applied to compound-specific isotope data to discern the relative contribution of C3 and C4 plants (9).…”