TEX86 paleothermometer as an indication of bottom water temperature in the Yellow Sea

Abstract

The TEX86 paleothermometer was applied for reconstructing sea surface temperature (SST) from glycerol dialkyl glycerol tetraethers (GDGTs) in marine sediments. It has become clear that GDGT-producing archaea live throughout the water column, with maximum concentration usually well below the surface mixed layer of the ocean. Why the TEX86 parameter correlates well with SST remains poorly understood. Here we evaluate the fidelity of the TEX86 thermometer using surface sediments and suspended particles from the Yellow Sea (YS), a shallow marginal sea between China and the Korean peninsula. The highest concentration of GDGTs in the water column at a site (A02) from the middle of the YS occurred in the bottom layer, at 70m. This contrasts with phytoplankton lipids, which were most abundant near the surface. Consistent with the maximum abundance of GDGTs in bottom water, TEX86 values in surface sediments correlated better with mean annual bottom water temperature (BWT, R2 0.81) than with mean annual SST (R2 0.74). Moreover, TEXH86 temperature derived from a global core top calibration gave values 0.1–9.4°C colder than mean annual SST but much closer to mean annual BWT (2.7–4°C). Lastly, TEX86 and U37K′ paleotemperature values displayed distinctly different trends over the last 12kyr for sediments from the shelf between the YS and the East China Sea (ECS), consistent with the notion that TEX86 and U37K′ reflect different temperature signals. This preponderance of evidence supports the use of TEX86 as a proxy for BWT in the YS. Therefore, we propose a local calibration of TEXL86 for reconstructing mean annual BWT (TEXL86=0.03 BWT-0.94; R2 0.86, n=22, P<0.0001). The combination of alkenone (U37K′) derived SST with TEXL86 BWT yielded a quantitative reconstruction of the vertical thermal gradient in the YS, and an insight into understanding the impact of the Kuroshio Current and East Asian monsoon on the YS.