Evaluation of environmental proxies based on long chain alkyl diols in the East China Sea
Long chain alkyl diols (LCDs) in marine environments are useful to reflect source organism, ambient temperature, upwelling and nutrient conditions. However, the distribution of LCDs in the western Pacific marginal seas has been rarely reported, where wide shallow continental shelves and huge freshwater input from many rivers occur. In this study, we analyzed LCDs in surface sediments distributed from the Changjiang River estuary (CRE) to the East China Sea shelf to evaluate their sources and associated environmental proxies. Our results showed that the fractional abundance of C32 1,15-diol (FC32 1,15-diol) was highest in coastal area close to the CRE, with FC32 1,15-diol >15% implying significant freshwater input. The C28 and C30 1,13-diols showed a similar spatial distribution to C32 1,15-diol, suggesting the long chain diol index (LDI), a sea surface temperature (SST) proxy, may be biased by freshwater derived diols. By excluding the freshwater-influenced samples (i.e., FC32 1,15-diol >15%), LDI reflected best the autumn SST, yielding minimum temperature residuals (0.2 ± 1.5 °C). The C28, C30 and C30:1 1,14-diols were abundant in the mid-depth (15–45 m) offshore environment affected slightly by the Changjiang River plume, and decreased toward both the eutrophic estuarine and oligotrophic marine environments; whereas C28:1 1,14-diol showed higher fractional abundances close to the CE, which remains elusive. The distribution pattern of 1,14-diols is similar to the reported Proboscia diatom distribution in this region, although exact sources of 1,14-diol are pending to further study. Nutrient proxies based on 1,14-diols did not correlate well with nutrient concentrations in this river-dominated marginal sea, where nutrient supply is dominated by the Changjiang River input.