Magnesium isotopes as a proxy to decode the effects of microbial metabolisms on the formation of stromatolitic carbonate

The growth and morphology of stromatolitic carbonates, i.e., lithified microbial mats, are tightly linked to complex microbial activities. However, the role of microbial metabolism in carbonate nucleation and formation during stromatolite growth remains controversial. Magnesium isotopes are a promising tool to trace carbonate formation processes due to significant isotopic fractionations associated with carbonate precipitation. This study reports Mg isotope compositions of Holocene, ca. 4000 years old stromatolites from the Lagoa Salgada lagoon (Brazil) with different morphologic features. Petrological and C-S isotopic evidence indicates microbial metabolisms varied during the formation of morphologically different stromatolites. However, the direction and magnitude of Mg isotope fractionation between Mg-calcite and ambient fluid are relatively constant despite the changes of microbial activities in ecosystems and consistent with the typical Mg isotope behavior during abiotic calcite precipitation. As a result, we propose that microbial metabolism do not influence the dehydration pathway of aqueous Mg²⁺ into calcite lattices. Our study corroborates earlier findings indicating that metabolism-related fractionation of Mg isotopes is limited or even absent during stromatolite growth. This finding also promotes understanding of Mg isotopic records in widespread Precambrian stromatolites for reconstructing seawater chemistry in deep time.