Utility of the B/Ga salinity proxy in carbonate and marly sediments

Abstract

Elemental proxies are widely used for analyzing watermass salinity in ancient shale and siliciclastic mudstone formations, but their application to limestones and marls has not yet been systematically tested. Unlike the Sr/Ba and S/TOC proxies, the B/Ga proxy offers considerable, albeit untested, potential for paleosalinity determinations in carbonate-bearing facies. To evaluate this potential, we analyzed the concentrations of B and Ga, as well as those of Al, Ca, and other lithology-associated elements, in a large suite of modern and ancient marine and marine-influenced carbonate samples, including both sediments and fossils. Our results show that B/Ga ratios tend to be elevated in carbonate-rich material due to low concentrations of Ga, which is primarily present in the detrital siliciclastic fraction. In marly sediments, robust salinity facies determinations are generally possible for samples with Ga concentrations as low as 3 ppm, compared to the typical range of 15–20 ppm in shales. For carbonate samples with Ga concentrations <3 ppm, reliable salinity facies estimates may still be obtained using excess boron (B_xs) present in the sample, calculated as B_total – 6 × Ga. Excess boron content is typically ∼10–20 ppm for carbonates from fully marine facies. Deviations from this range may signal watermass salinities that are lower or higher than the normal marine salinity range of ∼33–38 psu (practical salinity units). This study establishes, for the first time, a robust framework for evaluating depositional salinities in ancient marine and marine-influenced carbonate and marl formations, providing a valuable tool for future paleoenvironmental research.