Testing micrite as an ancient seawater Mg isotopic composition archive: A case study of Upper Paleozoic carbonate

Abstract The marine magnesium (Mg) cycle is the nexus bridging continent, ocean, and Earth's interior. River water is the only major Mg source, while carbonate precipitation and hydrothermal reaction are the two major processes removing seawater Mg from the ocean inventory. Reconstruction of the deep time marine Mg cycle can be approached with Mg isotopic composition of seawater (δ26Mgsw), because variation of δ26Mgsw may reflect the change of riverine input, carbonate precipitation or intensity of hydrothermal reaction. As the most common component of marine carbonate rocks, micrite or lime mud has been regarded as the premium material that records ancient seawater geochemical compositions and accordingly has been widely used in paleoenvironment studies. However, it is unclear whether micrite can be applied to reconstruct ancient δ26Mgsw. Because of the long residence time of seawater Mg, if micrite archives δ26Mgsw, it is predicted that δ26Mgmicrite should show limited temporal and spatial variation in a short (