The evolution of Earth’s surficial Mg cycle over the past 2 billion years

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 (δ²⁶Mg_sw) track the surficial Mg cycle, which could provide crucial information on the carbon cycle in Earth’s history. Here, we present a reconstruction of δ²⁶Mg_sw evolution over the past 2 billion years using marine halite fluid inclusions and sedimentary dolostones. The data show that δ²⁶Mg_sw decreased, with fluctuations, by about 1.4%0 from the Paleoproterozoic to the present time. Mass balance calculations based on this δ²⁶Mg_sw 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.