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 U 37 K ′ 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 TEX 86 H , a modified version of TEX86, and U 37 K ′ from core top sediment samples from the SCS. TEX 86 H -based SST calculated from the global regression exhibited slightly higher values (avg. 28.7 °C, range 27.1–30.3 °C) than values from the World Ocean Atlas 1994 (avg. 27.4 °C, range 26.1–28.6 °C) and U 37 K ′ -based temperature values (avg. 27.3 °C, range 26.1–29.0 °C). Moreover, the linear relationship between TEX 86 H and SST was poor (r2, 0.44). Correlation analysis of TEX 86 H with annual temperature and seasonal temperature at various depths revealed that TEX 86 H 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 TEX 86 H 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 TEX 86 H vs. 30–125 m subsurface temperature was tentatively established. Furthermore, the difference between U 37 K ′ and TEX 86 H derived temperature (ΔTAlkenone–GDGT) correlated moderately (r, 0.62) with the depth of the thermocline of the 18 °C isotherm in the SCS, suggesting that the difference may provide a proxy for upper ocean vertical thermal gradient in this marginal sea.