Distribution of 3-hydroxy fatty acids in South China Sea since the last deglaciation: Applicability of 3-hydroxy fatty acid-based palaeothermometry

3-Hydroxy fatty acids (3-OH-FAs) are biomarkers for Gram -negative bacteria. Recent research has suggested that they could serve as temperature proxies in both terrestrial and marine environments, based on calibrations using modern samples. However, these proxies have been only applied in a speleothem for the Holocene and a short sediment core in the East China Sea for past 58 years, thus more evidence is needed to testify their validity. In this study, we analyzed the distribution of 3-OH-FAs in a sediment core from the northern South China Sea (SCS) spanning the past 19 thousand years (19 ka). We detected 3-OH-FAs, including normal (n-), iso (i-), and anteiso (a-) isomers, in all 56 samples with carbon numbers ranging from 10 to 18. The distribution of 3-OH-FAs in marine sediments differed from that in terrestrial sediments (soil, stalagmite, and lake sediments), with a higher fractional abundance of anteiso-FAs. Two temperature proxies, RANs and RAN13, showed a positive correlation (R-2 = 0.5; p < 0.001), with RANs-SST being closer to the actual sea surface temperature (SST). The reconstructed RANs-SST in our core exhibited a typical glacial-Holocene increase of 2-3 degrees C (20.8 +/- 2.7 degrees C vs. 24.1 +/- 1.3 degrees C). Several abrupt climate events, such as the Younger Dryas, Bolling/Allerod, and Heinrich Event 1, were also recorded in RANs-SST. Therefore, our study supports the applicability of 3-OH-FA RANs as a temperature proxy for paleoenvironmental reconstructions, at least in the last glacial periods.