ABSTRACT
The surficial cycling of Mg is coupled with the global carbon cycle, a predominant control of Earth's climate. However, how Earth's surficial Mg cycle evolved with time has been elusive. Magnesium isotope signatures of seawater (δ26Mgsw) track the surficial Mg cycle, which could provide crucial information on the carbon cycle in Earth's history. Here, we present a reconstruction of δ26Mgsw evolution over the past 2 billion years using marine halite fluid inclusions and sedimentary dolostones. The data show that δ26Mgsw decreased, with fluctuations, by about 1.4 from the Paleoproterozoic to the present time. Mass balance calculations based on this δ26Mgsw record reveal a long-term decline in net dolostone burial (NDB) over the past 2 billion years, due to the decrease in dolomitization in the oceans and the increase in dolostone weathering on the continents. This underlines a previously underappreciated connection between the weathering-burial cycle of dolostone and the Earth's climate on geologic timescales.
ABSTRACT
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 H2O at 150 Ma ago to 27 µmol/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 87Sr/86Sr, 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.
