Tim K. Lowenstein

Secular variations in the major ion chemistry and isotopic composition of seawater on multimillion-year time scales are well documented, but the causes of these changes are debated. Fluid inclusions in marine halite indicate that the Li concentration in seawater [Li + ] SW declined sevenfold over the past 150 million years (Ma) from ~184 μmol/kg H 2 O at 150 Ma ago to 27 μmol/kg H 2 O today. Modeling of the lithium geochemical cycle shows that the decrease in [Li + ] SW was controlled chiefly by long-term decreases in ocean crust production rates and mid-ocean ridge and ridge flank hydrothermal fluxes without requiring changes in continental weathering fluxes. The decrease in [Li + ] SW parallels the 150 Ma increase in seawater Mg 2+ /Ca 2+ and 87 Sr/ 86 Sr, and the change from calcite to aragonite seas, KCl to MgSO 4 evaporites, and greenhouse to icehouse climates, all of which point to the importance of plate tectonic activity in regulating the composition of Earth’s hydrosphere and atmosphere.