Abstract
Seawater Mg:Ca and Sr:Ca ratios are biogeochemical parameters reflecting the Earth-ocean atmosphere dynamic exchange of elements. The ratios dependence on the physical and chemical environment, facilitates their use as tools in marine sciences. It has been ~130 years since the last global survey. Here, we present a new measured single-laboratory global dataset, combined with previous data, to test the assumption of limited seawater Mg:Ca and Sr:Ca variability across marine environments. There is high variability in open ocean upwelling and polar regions, shelves, neritic, and river-influenced areas, where Mg:Ca and Sr:Ca range ~4.40-6.40 mmol:mol and ~6.95-9.80 mmol:mol, respectively. Open-ocean seawater Mg:Ca is semi-conservative (~4.90 to 5.30 mol:mol), while Sr:Ca is more variable and non-conservative (~7.70 to 8.80 mmol:mol); both ratios are non-conservative in coastal seas. Further, the Ca, Mg and Sr elemental fluxes are connected to total alkalinity (TA) deviations from IAPSO standard values. Because there is significant seawater Mg:Ca and Sr:Ca variability across marine environments we cannot absolutely assume that fossil archives secular changes using taxa-specific proxies reflect true global seawater chemistry, but taxa- and process specific ecosystem variations, reflecting regional conditions. This variability could reconcile secular seawater Mg:Ca and Sr:Ca ratios reconstructions using different taxa, techniques, and processes, by assuming an error of 1-1.50 mol:mol, and 1-1.90 mmol:mol, respectively. The modern ratios variability is similar to the reconstructed rise over 20 million years (Neogene period), nurturing the question of semi non-conservative behavior of Ca, Mg, and Sr over modern Earth geological history with an overlooked environmental effect.