The chemical composition of the ocean is determined by rivers, submarine hot springs, and ocean sediments that add or remove elements to seawater.
Throughout the oceans, the more abundant elements have near constant
ratios to salinity (a measure of total dissolved salts). Thus, records
of their past concentrations in sea-water should tell us how active
these sources and sinks were over long time scales. However, reliable archives of past seawater chemistry have been difficult to find (1). On page 1114 of this issue, Coggon et al. address this problem by measuring magnesium/calcium and strontium/calcium ratios in calcium carbonate (calcite) veins recovered from ocean crust buried under sediments (2). Their Mg/Ca record for the past 180 million years agrees with previous work (1), but the Sr/Ca record does not (3). The results have implications not only for seawater chemistry but also for climate change.