Seafloor hydrothermal systems control long-term changes in seawater [Li+]: Evidence from fluid inclusions

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 similar to 184 mu mol/kg H2O at 150 Ma ago to 27 mu mol/kg H2O 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 Mg2+/Ca2+ and Sr-87/Sr-86, and the change from calcite to aragonite seas, KCl to MgSO4 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.