Paleosalinity is an important environmental feature but it is difficult to evaluate. In the present study, paleosalinity was assessed during the deposition of three sets of source rocks in the western Pearl River Mouth Basin, South China Sea, utilizing four different kinds of methods, i.e., saturated hydrocarbon biomarkers, strontium abundance, non-pollen microalgae assemblies, and carbon-sulfur relationships.Results show that the second member of the Eocene Wenchang Formation (E2w2) was deposited in a freshwater environment and the Oligocene Zhuhai Formation (E3z) was deposited in a shallow marine environment. The Oligocene Enping Formation (E3e), which was believed to be deposited in a freshwater environment, was actually deposited in a brackish water environment. Mechanisms of salinity increase during the non-marine E3e deposition were mainly deep hydrothermal fluid input through the south boundary fault and episodic marine transgressions, not evaporation. The effect of salinity on organic matter content and type was investigated. Results show that salinity has no significant influence on total organic carbon (TOC) and hydrogen index (HI) of the E2w2, which was caused by the balance between freshwater algae and euryhaline algae. TOC and HI decrease with increasing salinity for samples from the E3z, which is contrary to the conventional hypothesis that marine transgressions promote source rock deposition. The decrease of TOC with carbon/sulfur ratios for samples from the E3e actually reflect the influence of thermal maturity but thermal maturity only plays the second role in HI. The effect of salinity on HI during the E3e deposition can be divided into two stages. During the first stage, the increase of salinity was mainly caused by deep hydrothermal fluid input without an oxygen level increase. HI values, therefore, remained relatively stable. During the secondary stage, the increase of salinity was mainly caused by marine transgressions which increased the oxygen level and as a consequence, HI decreased sharply with increasing salinity.This study provides a long-term salinity evolution of the western Pearl River Mouth Basin and suggests that salinity is an important factor controlling source rock deposition. In addition, this study presents an example that goes against conventional wisdom that marine transgressions promote source rock deposition in a shallow marine environment. This study also suggested that marine transgressions had already begun at the end of the early Oligocene.

中文翻译：

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南海西部珠江口盆地古盐度评价及其对烃源岩沉积的影响

古尘度是重要的环境特征，但难以评估。在本研究中，使用四种不同的方法，即饱和烃生物标志物，锶丰度，非花粉微藻组合物，对南海西部珠江口盆地三套烃源岩沉积过程中的古盐度进行了评估。结果表明，始新世文昌组第二段（E2w2）沉积在淡水环境中，渐新世珠海组（E3z）沉积在浅海环境中。渐新世恩平地层（E3e），据信沉积在淡水环境中，实际上沉积在微咸水环境中。非海洋E3e沉积过程中盐度增加的机制主要是通过南边界断层的深部热液输入和海洋的侵袭，而不是蒸发。研究了盐度对有机质含量和类型的影响。结果表明，盐度对E2w2的总有机碳（TOC）和氢指数（HI）没有显着影响，这是由淡水藻类和淡水藻类之间的平衡引起的。E3z样品的盐度增加时，TOC和HI随盐度的增加而降低，这与海洋过境促进烃源岩沉积的传统假设相反。E3e样品中TOC随碳/硫比的降低实际上反映了热成熟度的影响，但热成熟度仅在HI中起第二个作用。盐度对E3e沉积过程中HI的影响可分为两个阶段。在第一阶段，盐度的增加主要是由于深层热液的输入而不引起氧气水平的增加。因此，HI值保持相对稳定。在第二阶段，盐度的增加主要是由海侵引起的，这增加了氧气的含量，因此，HI随着盐度的增加而急剧下降。本研究为珠江口盆地西部的盐度长期变化提供了建议。盐度是控制烃源岩沉积的重要因素。此外，本研究还提供了一个与传统观点相反的例子，即海洋入侵会促进浅海环境中的烃源岩沉积。