Speciation patterns of elements Cl and Br in marine sediments and their significant as salinity proxies

Abstract

Salinity is a fundamental hydrochemical parameter of marine environments, yet the reconstruction of paleosalinity remains one of the most challenging issues in paleoceanographic research field. To investigate the potential of conservative elements (Cl, Br) as proxies for seawater salinity variations, this study systematically analyzed the surface sediments collected across the Eastern China marginal seas, where pronounced salinity gradients exist. Through comprehensive examination of Cl and Br speciation mechanisms in sediments, we established quantitative relationships between these halogens and seawater salinity. The results indicated that dissolved Cl and Br dominate their total sedimentary inventories, accounting for 90.27% and 86.14% respectively, while non-dissolved fractions consistently represent <15% of total content. The non-dissolved Cl appears primarily associated with terrigenous silicate minerals, showing stable concentration patterns, whereas non-dissolved Br predominantly occurs in organic matter with clear source-dependent correlations. Significantly, in sediments of the Eastern China marginal seas, dissolved Cl demonstrates an exponential correlation with bottom water salinity. Particularly in brackich water regimes (salinity <30.4 psu), both dissolved Cl and dissolved Br exhibit strong linear relationships with bottom water salinity, enabling quantitative paleosalinity reconstruction in estuarine environments. This research establishes a novel methodological framework for paleosalinity investigations and provides crucial constraints for interpreting paleo-marine environments with sedimentary geochemistry proxies.