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.

海洋环境中的长链烷基二醇（Long chain alkyl diols, LCDs）可用于反映其源生物、环境温度、上升流及营养盐条件。然而，西太平洋边缘海分布着广阔的浅水大陆架且有众多河流带来巨量淡水输入，但该区域内长链烷基二醇的分布特征鲜有报道。本研究对从长江口（Changjiang River estuary, CRE）至东海陆架分布的表层沉积物中的长链烷基二醇进行了分析，以评估其来源及相关环境代用指标。研究结果显示，C32 1,15-二醇的相对百分含量（FC32 1,15-diol）在紧邻长江口的近岸海域最高，当FC32 1,15-diol占比超过15%时，指示存在显著的淡水输入。C28与C30 1,13-二醇的空间分布特征与C32 1,15-二醇相似，这表明作为海表温度（sea surface temperature, SST）代用指标的长链二醇指数（long chain diol index, LDI）可能会受到淡水来源二醇的干扰。通过剔除受淡水影响的样品（即FC32 1,15-diol占比>15%的样品）后，长链二醇指数可最优反映秋季海表温度，其温度残差最小（0.2 ± 1.5 ℃）。C28、C30及C30:1 1,14-二醇在受长江冲淡水轻度影响的近海中深度（15–45 m）海域含量较高，并向富营养化河口及寡营养海洋环境方向逐渐降低；而C28:1 1,14-二醇在紧邻CE的海域相对百分含量更高，其成因目前尚不明确。尽管1,14-二醇的具体来源仍有待进一步研究，但其分布模式与该区域已报道的根状藻属（Proboscia）硅藻分布相似。在这个以河流输入为主的边缘海中，基于1,14-二醇的营养盐代用指标与营养盐浓度并未呈现显著相关性，该海域的营养盐供给主要由长江输入贡献。