Distribution and Export of Particulate 3‐Hydroxy Fatty Acids in the Northwestern Pacific Ocean: Implications for Seawater Temperature Reconstructions

Bacterial 3‐hydroxy fatty acids (3‐OH‐FAs) have emerged as promising biomarkers for reconstructing seawater temperatures. However, uncertainties regarding their production and transportation from water column to sediments have hindered accurate interpretation of temperature signals reconstructed by sedimentary 3‐OH‐FAs. Here, we present a comprehensive analysis of particulate 3‐OH‐FAs from two water column profiles and corresponding surface sediments in the Northwestern Pacific Ocean (NWP). Particulate 3‐OH‐FA concentrations ranged from 0.69 to 2.95 μg L −1 , peaking in the surface layer coinciding with elevated particulate organic carbon (POC) concentrations and higher temperatures. Seawater temperature was the primary factor controlling 3‐OH‐FA distributions in the water column, supported by its highest variable importance in projection score in the partial least squares analysis. Temperature showed a significant negative correlation with relative abundance of branched homologs, consistent with homeoviscous adaptation. An export model based on depth‐specific molecular index Br 13+17 % (proportion of branched C 13 and C 17 homologs) indicated that 3‐OH‐FAs in the study area are primarily exported from the subsurface layer (∼200 m), where production and aggregation/packaging efficiency are optimally coupled. The sedimentary 3‐OH‐FA‐based seawater temperature index RAN 13 effectively captured the variability of subsurface seawater temperature, consistent with the export model results. Overall, these findings suggest that 3‐OH‐FAs in deep NWP sediments are mainly sourced from subsurface waters, thereby highlighting their potential as a proxy for subsurface temperature reconstructions in this region.