3-Hydroxy fatty acids as proxies for seawater temperature and pH in the eastern China marginal seas

3-Hydroxy fatty acids (3-OH-FAs) have recently been proposed as proxies for terrestrial, lacustrine and marine environment reconstruction. However, their application in the latter is not yet fully understood. Here we analyzed 3-OH-FAs in surface sediments from the eastern China marginal seas (ECMS), aiming to explore the application of 3-OH-FA-based proxies in marine temperature and pH reconstruction. Results showed that both the empirical mean annual air temperature (MAAT) proxies RAN15 and RAN17 (ratio of anteiso to normal isomers for C15 and C17 homologues, respectively), and the mean annual sea surface temperature (SST) proxy RAN13 were significantly correlated with SST (with RAN15 showing the strongest correlation and highest amplitude response), indicating the applicability of 3-OH-FA-based proxies developed in terrestrial and marine settings in the ECMS. The strongest correlation of RAN15 with SST is probably attributable to the relatively higher abun-dance and larger variation range of anteiso C15 homologues in the sediments. Moreover, the above proxies were also significantly correlated with mean annual bottom water temperature (BWT) with slightly stronger corre-lation than with SST, suggesting an important contribution of bottom layer to sedimentary 3-OH-FAs. In particular, the temperature proxies showed marked seasonal variation and a stronger correlation with BWT during warm seasons. A novel temperature proxy (RAI15, the ratio of anteiso to iso C15 isomers) provided strong correlations with BWT in summer and autumn, suggesting that RAI15 is a robust potential BWT proxy for warm seasons in the ECMS. Based on the current data, the sedimentary 3-OH FAs could be a mixture of lipids both produced from the surface and the bottom water layers, and which layer sea temperature the proxies better reflect need to be further explored although a slightly stronger correlation with BWT than SST was found in the present study. In addition, the empirical soil 3-OH-FA-based pH proxies were not suitable in this study, although the influence of pH on marine 3-OH-FAs was confirmed based on a locally developed calibration. Our findings contribute to the understanding of the potential use of bacterial 3-OH-FAs as environmental proxies in temperate marginal seas.