GEOCHEMICAL PROXIES FOR PALEOSALINITY ANALYSIS: A REVIEW

Determination of the salinity of a paleo-watermass is useful in paleoenvironmental and paleoecological studies, but the theory of application of geochemical proxies to paleosalinity reconstruction is underdeveloped. Here, we explore the use of three proxies for paleosalinity reconstruction: boron/gallium (B/Ga), strontium/barium (Sr/Ba), and organic carbon/sulfur (TOC/S) ratios. We compiled a large set of aqueous and sediment chemical data from modern watermasses, as well as sediment data for paleo-watermasses, representing a range of salinities, in order to test the robustness and accuracy of these proxies. Sediment data were limited to fine-grained siliciclastic units (muds/shales/mudstones). B/Ga ratios show similar ranges for sediments of specific salinities from both modern and ancient water bodies: <3 for freshwater facies, 3-6 for brackish facies, and >6 for marine and hypersaline facies. Sr/Ba ratios also show strong salinity-controlled variation in modern systems: <1 for freshwater facies, 1-5 for brackish facies, and >5 for marine and hypersaline facies. Although Sr/Ba is effective in discriminating salinity conditions in modern environments, ancient shales are Sr-depleted and Ba-enriched relative to modern shales, resulting in significant overlap of the Sr/Ba distributions for marine and freshwater shales, which limits their utility for paleosalinity discrimination. TOC/S can be an effective proxy for discrimination of high-sulfate marine from low-sulfate freshwater systems, with strong fidelity for modern environments. A significant number of paleolacustrine systems exhibit TOC/S ratios corresponding to modern marine environments, suggesting unrecognized marine influences on these systems. In summary, all three proxies are potentially useful for estimating paleosalinities in a semi-quantitative manner. B/Ga is the most reliable proxy owing to its systematically strong relationship to watermass salinities and to its relative immunity to burial alteration. However, we advocate that paleoenvironmental studies routinely make use of multiple geochemical proxies in order to assess the robustness of paleosalinity estimates.