Deglacial variations in provenance of terrigenous organic matter and temperature in the Northwest Pacific/western Bering Sea realm inferred from biomarkers

During the last glacial termination atmospheric carbon dioxide (CO2atm) rose about 100 ppm and atmospheric radiocarbon activity (Δ14C) concurrently dropped by about ca. 400 ‰. Global warming likely triggered large-scale thawing of permafrost soils in the northern hemisphere resulting in release of 14C-depleted carbon which may have contributed to the changes in CO2atm and Δ14C1. However, the timing and duration of the thawing as well as regional differences regarding these points are poorly understood. In order to unravel the evolution of permafrost decomposition and its role within the glacial-interglacial climate change a profound understanding of the deglacial carbon-turnover and temperature development in subarctic and arctic regions are of great importance. Working with two sediment cores from the continental margin off Kamchatka Peninsula, western Bering Sea (WBS, site SO201-2-114KL) & Northwest Pacific (NW Pacific, site SO201-2-12KL) we establish Glacial to Holocene records in sea surface temperature (SST) and mean air temperature (MAT) using the TEXL86 (SST) and CBT/MBT (MAT) temperature proxies that are based on marine and terrigenous biomarkers (Glycerol dialkyl glycerol tetraethers). The hydrogen isotopic composition (δD) of plant-wax derived lipid biomarkers (long-chain n-alkanes and long-chain n-fatty acids) is another tool for reconstructing air temperature but can also provide information of the provenance of terrigenous organic matter (OM). The radiocarbon activity of the lipid biomarkers is applied to reconstruct changes in terrestrial residence times of terrigenous OM. Our records in SST and CBT/MBT-based MAT are interpreted as summer temperatures. From the Bølling/Allerød interstadial (B/A) to the present they show temperature fluctuations similar to Greenland ice core records including a warming at the onset of the B/A and a cooling during the Younger Dryas (YD) followed by a warming into the Preboreal (PB) suggesting an atmospheric coupling between N-Atlantic and N-Pacific, East Siberian/Kamchatka temperature development. However, during Heinrich Stadial 1 (HS1) where WBS SST and Kamchatka MAT cool down synchronously with Greenland temperatures, the NW-Pacific SST rises gradually and seems to be decoupled from the N-Atlantic. Since the gradual warming trend and the subsequent synchronization with Greenland ice core data during B/A is known from SST records from the Gulf of Alaska2 (GOA) surface conditions in the NW Pacific likely are controlled by the Alaskan Stream overprinting the atmospheric effect and the influence of the East Kamchatka Current. Asynchronous development of our TEXL86 record and the GOA records during the Holocene indicate that the AS weakenes over the deglaciation. For the time-span of the Last Glacial Maximum to the onset of the B/A interstadial, the CBT/MBT temperature proxy may be seasonally biased as it produces improbably high summer temperatures that level Holocene conditions. In contrast to the CBT/MBT-based temperatures the δD of n-fatty acids does not show clear stadial-interstadial fluctuations and remains on a stable level during the deglaciation instead. During the Holocene, δD increases progressively, which may have resulted from a gradual warming trend. Comparing the radiocarbon activity of the n-fatty acids to the Δ14C-signature of the atmosphere at the time of deposition the dimension of the terrestrial residence time prior to the deposition (ΔΔ14C) can be estimated. High ΔΔ14C values during deglaciation indicate that the plat-wax biomarkers are strongly pre-aged which may bias the δD-temperature record. Gradually decreasing ΔΔ14C imply declining terrestrial residence times from late glacial to late Holocene and argue for major changes in the relative contribution of weakly and strongly pre-aged OM. Possible sources for strongly pre-aged OM include permafrost decomposition and the congruent mobilization of 14C-depleted carbon but also the erosion of basal tills from the North-American or Kamchatka ice sheets that bear OM dating back to times prior to the glaciation. The n-C25/n-C25+n-C27 ratio, an indicator for the relative contribution of sphagnum spp.3, is on a stable level over most parts of the deglaciation indicating a constant composition of the vegetation and/or a constant carbon source. Between 16.5-14.6 kaBP the record shows a sharp excursions towards higher values congruent with a sharp increase in the δD of n-alkanes that is not evident in the δD of n-fatty acids. Coevally, lowered CPI-values (carbon preference index) point to a major contribution of fossil carbon at that time. Thus, our sites may be subject to a change in the carbon source. It coincides with melting events of the N-American ice sheets4 which may indicate that large amounts of OM from eastern Beringia accumulated in the WBS/NW Pacific during HS1. 1. S. A. Zimov, E. A. G.Schuur & F. S. Chapin III (2006). Permafrost and the global carbon budget.- Science, 312, p. 1612-1613 2. J. E. Vonk & Ö. Gustaffsson (2009). Calibrating n-alkanes Sphagnum proxies in sub-Arctic Scandinavia.- Organic Geochemistry, 40, p. 1085-1090. 3. S. K. Praetorius & A. C. Mix (2014). Synchronization of North Pacific and Greenland climates preceded abrupt deglacial warming.- Science, 345, p. 444-448. 4. I. L. Hendy & T. Cosma (2009).Vulnerability of the Cordilleran Ice Sheet to iceberg calving during late Quaternary rapid climate change events.- Paleoceanography, 23, p. PA210