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

Abstract The surficial cycling of Mg is directly coupled with the global carbon cycle, a predominant control of Earth’s climate. However, how Earth’s surficial Mg cycle evolved with time had been elusive. Magnesium isotope signatures of seawater (δ 26 Mg 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 δ 26 Mg sw evolution over the last 2 billion years using marine halite fluid inclusions and sedimentary dolostones. The two independent archives yield consistent evolutionary trends of δ 26 Mg sw for the past 430 million years, and the dolostone records extend the δ 26 Mg sw curve to 2 billion years ago. Modeling results of the net CO 2 sequestration efficiency (E Mg−CO2 ) by the surficial Mg cycle based on the δ 26 Mg sw record reveal a secular decline E Mg−CO2 during the past 2 billion years, with the periods of low E Mg−CO2 coinciding with ice ages in the Phanerozoic. Our work underlines a previously under-appreciated, but indispensable role of dolostones in regulating Earth’s climate on geologic time scales.