EVOLUTION OF EARTH’S SURFICIAL MG CYCLES IN THE PAST 2 BILLION YEARS: A TALE FROM DOLOSTONES (Invited Presentation)

The surficial cycling of Mg is directly coupled with the global cycle of carbon, a predominant control of Earth’s climate. How global surficial Mg cycle has changed through geological history, however, has been elusive. Magnesium isotope signatures of seawater (δ²⁶Mg_sw) track the balance between silicates and carbonates as sources and sinks of Mg in the oceans. Here, we reconstructed the δ²⁶Mg_sw record for the ancient oceans using marine halite fluid inclusions and sedimentary dolostones. The two independent archives yielded consistent evolutionary trends of δ²⁶Mg_sw for the past 450 million years, and the dolostone records extend the δ²⁶Mg_sw curve to 2 billion years ago. The data show that δ²⁶Mg_sw decreased, with fluctuations, by over 1‰ from the late Paleoproterozoic to the present time, and the periods of low δ²⁶Mg_sw coincided with ice ages in the Phanerozoic. Mass balance calculations based on this record reveal that the proportion of Mg sequestered as dolomite in the total Mg outflux from the oceans was significantly greater than the proportion of Mg sourced from carbonate in the total Mg delivered to the oceans in the Proterozoic, but since then, the gap between the two decreased to a modern value of around zero. This points to a secular decay in the net CO₂ sequestration efficiency by surficial Mg cycles in the past 2 billion years, which was caused by long-term changes in dolostone weathering and precipitation intensities. Our work underlines the indispensable role of dolostones in regulating Earth’s climate on geologic time scales.