Archaeal tetraether lipids record subsurface water temperature in the South China Sea

The South China Sea (SCS) is the largest tropical marginal sea in the western Pacific Ocean. While the UK37K' proxy is known to reflect the sea surface temperature (SST) of the 0-30 m mixed layer in the SCS, little is known about the applicability of the TEX86 proxy. Here, we present results from paired analyses of TEX86H, a modified version of TEX86, and U-37(K)' from core top sediment samples from the SCS. TEX86H-based SST calculated from the global regression exhibited slightly higher values (avg. 28.7 degrees C, range 27.1-30.3 degrees C) than values from the World Ocean Atlas 1994 (avg. 27.4 degrees C, range 26.1-28.6 degrees C) and UK37K'-based temperature values (avg. 27.3 degrees C, range 26.1-29.0 degrees C). Moreover, the linear relationship between TEX86H and SST was poor (r(2), 0.44). Correlation analysis of TEX86H with annual temperature and seasonal temperature at various depths revealed that TEX86H correlated better with annual sea temperature at 30-125 m (r, 0.89) than with temperature in the 0-30 m mixed layer (r, 0.69). This suggests that TEX86H reflects a deeper and cooler subsurface temperature rather than surface or mixed layer temperature in the SCS. As a result, the global regression, which results in higher SST values, could not be applied to predict subsurface temperature in the SCS and a local calibration of TEX86H vs. 30-125 m subsurface temperature was tentatively established. Furthermore, the difference between UK37K' and TEX86H derived temperature (Delta TAlkenone-GDGT) correlated moderately (r, 0.62) with the depth of the thermocline of the 18 degrees C isotherm in the SCS, suggesting that the difference may provide a proxy for upper ocean vertical thermal gradient in this marginal sea. (C) 2012 Elsevier Ltd. All rights reserved.