The Cenozoic Seawater Conundrum: New constraints from Mg isotopes in island dolostones.

The Mg/Ca ratio of seawater increased from ∼1.5 to its present value of 5.4 over the Neogene, indicating a fundamental imbalance in the Mg–Ca cycle during the late Cenozoic. The imbalance in Mg–Ca cycles since the late Cenozoic, however, is contrasted by an arguable constancy in the Mg isotope compositions of seawater (δ 26 Mg sw) reconstructed from several low-resolution carbonate records. Addressing such Cenozoic Seawater Conundrum requires reliable seawater Mg isotopic records. The Xisha Islands in the South China Sea host large Cenozoic dolostone sequences that record seawater chemistry over the past 23 Myr. This study reports the Mg isotopic and trace elemental compositions of island dolostones in the well-studied Xike-1 drill core from the Xisha Islands. Petrographic, C–O–Sr isotopic, and paleomagnetic data from the drill core samples collectively support a "fluid-buffered", syn-depositional origin for the dolostones. The dolostones from the Xike-1 core have a narrow range of δ 26 Mg (− 2.83 ‰ ± 0.12 ‰), confirming that δ 26 Mg sw have been stable around the modern value (−0.83‰) since the late Cenozoic. This record provides a new opportunity to better understand the Mg–Ca cycle during the late Cenozoic. We ran numerical models of global seawater Mg–Ca contents and δ 26 Mg sw with the most recently published flux and isotope fractionation data. The results show that the contrast between the constancy of δ 26 Mg sw values and the dramatic increase in seawater Mg/Ca ratios cannot be explained by a single mechanism (i.e., a decrease in the rate of phyllosilicate formation on the seafloor alone). Instead, processes involving different Mg fluxes need to be coupled to explain the observed trends in Mg content and isotopic compositions of Cenozoic seawater. The solution to the Cenozoic Seawater Conundrum includes simultaneous decreases in the rates of both clay formation and dolomitization on the seafloor, or an increase in the rate of both continental weathering together with decreased seafloor clay formation. Our study underlines the sensitivity of the Mg cycle to the interactions between climate, continental weathering, and seafloor processes during the late Cenozoic. • Seawater δ 26 Mg sw values can be reconstructed from tropical island dolostones. • Record of Xisha Island dolostones shows a stable δ 26 Mg sw for the past 23 million years. • A sharp contrast between stable δ 26 Mg and rising Mg/Ca of seawater in the late Cenozoic. • Changes in dolomitization intensity canceled the Mg isotope effect of changes in clay formation intensity. [ABSTRACT FROM AUTHOR]