Paleosalinity proxy: progress and challenges

Key words: 
• paleosalinity proxy /
• elemental abundance and elemental ratio /
• isotopic ratio /
• biomarker /
• geological environments and biological evolution.

Abstract: Abstract: 【Significance】Salinity is one of the fundamental physical parameters of seawater. Variations in salinity can lead to changes in seawater density, dissolved oxygen content, saturation vapor pressure, and osmotic pressure, thereby having profound implications for ecosystem diversity and ocean circulation patterns. Consequently, the reconstruction of paleosalinity is crucial for understanding geological environments and the biological evolution process. 【Progress】This study begins with an overview of the development of salinity definitions, followed by an introduction of the significance of salinity in geological environments and biological evolution, and then focuses on a critical review of various paleosalinity proxies, including their establishment processes and applicability.【Conclusions and Prospects】Current paleosalinity indicators are primarily indirect, influenced by numerous parameters beyond salinity itself, such as growth rates, temperature, light intensity, and species differences. Additionally, the empirical thresholds used to infer salinity environments exhibit regional variability, and their accuracy is further affected by the amount of available statistical data. Therefore, our research demonstrates that while current paleosalinity proxies are helpful for qualitatively assessing the depositional environments, they remain inadequate for providing quantitative salinity values. We propose that the inorganic carbonate-associated sodium (Na) or chlorine (Cl) have the potential to serve as direct, quantitative paleosalinity proxies. Such novel proxies would significantly enhance the precision of depositional environment interpretation and advance our understanding of ancient seawater salinity evolution.