Table_2_Grain size controls on long-chain diol distributions and proxy signals in marine sediments.xlsx

Long chain alkyl diols (LCDs) are lipid biomarkers that occur ubiquitously in sediments. Their abundance and distributions are increasingly used as the basis of molecular proxies for environmental parameters such as sea surface temperature (SST) via the Long chain Diol Index (LDI), and upwelling intensity and nutrient conditions (parametrized as diol indices, DI-2, and Nutrient Diol Index, NDI, respectively). Their marine producers remain the subject of debate, but in cultures, they can be found within the outer wall (algaenan) of eustigmatophytes or in Proboscia diatoms. LCDs appear to be well preserved in sediments, potentially as a result of their association with algaenan and/or minerals, but little is known of their pre-depositional histories, in particular transport dynamics. Here, 15 surface continental margin sediments as well as one high-deposition-rate sediment core (50 cm, spanning the last ~30 years) were analyzed in order to evaluate the impact of organo-mineral associations, lateral transport, and hydrodynamic sorting on sedimentary LCD signals. The abundance and distribution of LCDs in bulk sediments and corresponding grain-size fractions was determined. The highest proportion of all LCD isomers is found in the fine fraction (2 – 10 µm), which also holds the highest proportion of organic matter in relation to the other grain-size fractions. However, LCDs are also found in the other fractions (sand, coarse silt, and clay), and their concentrations are not correlated with bulk organic carbon content, indicating different preservation or transport mechanisms. LDI-SST in the bulk sediment is comparable to the mean annual SST at all sites except those influenced by upwelling and characterized by strong seasonal SST gradients. To the contrary of other biomarker-related proxies (e.g., alkenones), lateral transport does not appear to strongly affect LDI-SST in size fractions, suggesting that the intimate relationship of LCD with the algaenan may counteract the influence of hydrodynamic mineral sorting processes on related proxy signals. The difference between the fraction-weighted LCD concentration and bulk sedimentary LCD concentration indicates potential release of LCD during laboratory fractionation, suggesting degradation of algaenan or dissolution of opal frustules.

长链烷基二醇（Long chain alkyl diols, LCDs）是一类广泛分布于沉积物中的脂质生物标志物（lipid biomarkers）。其丰度与分布特征正日益被用作分子代用指标的基础，以重建多项环境参数：例如通过长链二醇指数（Long chain Diol Index, LDI）反演海表温度（sea surface temperature, SST），以及通过分别命名为二醇指数DI-2和营养盐二醇指数（Nutrient Diol Index, NDI）的指标，分别重建上升流强度与营养盐条件。
关于其海洋生源生物，目前仍存在学术争议，但在实验室培养体系中，这类物质可在真眼点藻纲（eustigmatophytes）的外壁（藻角质algaenan）或原盒形藻属（Proboscia）硅藻中被检出。LCDs在沉积物中保存状况良好，这可能与其与藻角质和/或矿物的结合作用有关，但学界对其沉积前的演化历史，尤其是搬运动力学过程，仍知之甚少。
本研究共分析了15个大陆边缘表层沉积物样品，以及1根高沉积速率沉积物岩芯（岩芯长度50 cm，覆盖过去约30年的沉积记录），旨在评估有机-矿物结合作用、侧向搬运以及水动力分选作用对沉积物中LCDs信号的影响。研究测定了全样沉积物（bulk sediment）及对应粒度组分（grain-size fractions）中LCDs的丰度与分布特征。
所有LCD同分异构体中占比最高的组分均富集于细粒级（2–10 µm），该粒级同样拥有相较于其他粒度组分最高的有机质占比。不过，LCDs也可在其他粒级（砂、粗粉砂与黏土）中被检出，且其浓度与总有机碳含量（organic carbon content）并无显著相关性，这表明LCDs存在不同的保存或搬运机制。全样沉积物中的LDI反演海表温度（LDI-SST）与所有采样点的年平均海表温度基本一致，但受上升流影响且海表温度季节梯度显著的站点除外。
与其他生物标志物代用指标（如烯酮类化合物alkenones）不同，侧向搬运似乎并未对粒度组分中的LDI-SST信号产生显著影响，这表明LCDs与藻角质的紧密结合可能抵消了水动力矿物分选作用对相关代用指标信号的干扰。粒度组分加权后的LCD浓度与全样沉积物LCD浓度之间的差值，表明在实验室粒度分离过程中可能存在LCD的释放，这暗示了藻角质的降解或硅藻硅质壳（opal frustules）的溶解。