Strontium and calcium in marine barite

Robust histories of seawater strontium (Sr) and calcium (Ca) concentrations are critical for understanding the long-term global carbon cycle as it is influenced by carbonate deposition, chemical weathering, and hydrothermal activity. If the quantities of Sr and Ca in marine barite (a mineral that precipitates inorganically in the water column) reflect contemporaneous seawater Sr and Ca concentrations, then the Sr/Ba and Ca/Ba ratios of ancient marine barite extracted from deep-sea sediments may provide a means for reconstructing the histories of these elements in the ocean. This work substantiates the potential utility of the Sr/Ba ratio of marine barite as a proxy of seawater Sr concentrations.; To assess the processes influencing the Sr and Ca content of marine barite, geochemical models were constructed and evaluated along with Sr/Ba and Ca/Ba ratios of samples isolated from a suite of modern sediment cores. The results preclude the use of the Ca component in marine barite for paleoreconstructions of seawater Ca. For Sr in marine barite, seawater Sr concentrations are the dominant variable in determining the extent of Sr substitution, yet because the geochemistry of the microenvironments enabling barite precipitation is ill-constrained, the Sr/Ba ratio of marine barite can only be applied as a qualitative predictor of changes in seawater Sr concentrations.; The glacial-interglacial history of Sr/Ba ratios in marine barite, and the coherency of this record with the history of sea level fluctuations, provides evidence that the Sr composition of marine barite responds to changes in seawater Sr concentrations. The observed oscillations can be explained by recrystallization of Sr-rich aragonite to calcite on exposed continental shelves, and the subsequent release and delivery of Sr to the ocean during low sea-stands. This is the first geochemical record showing that systematic changes in seawater Sr concentrations have occurred. A Cenozoic record of Sr/Ba ratios is also presented. The processes influencing this record are unclear, but interpretations may be complicated by variations in regional-scale processes not observed in the modern ocean, post-depositional processes, and insufficient sample resolution.