DOES MICRITE (LIME MUD) ARCHIVE SEAWATER MG ISOTOPIC COMPOSITION?

In the marine magnesium cycles, riverine input serves as the major source, while carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Thus, the Mg cycle would provide the key information about the interactions between continent system, ocean system, and Earth’s interior in Earth’s history. Reconstruction of marine Mg cycle can be approached with seawater Mg isotopic composition (d²⁶Mg_sw), because hydrothermal reactions involve with negligible or slightly negative fractionation (0.7‰) in d²⁶Mg, while carbonate precipitation preferentially scavenge ²⁴Mg (with isotopic fractionation up to 5‰). Seawater chemistry might be recorded in marine carbonate. However, it is unclear with carbonate component can be used to reconstruct d²⁶Mg_sw. As the most common component of carbonate, micrite has been widely used to reconstruct marine biogeochemical cycles (e.g. C and S cycles). However, micrite derives from multiple sources (micritization of carbonate, direct precipitation from seawater/porewater, and fragmentation of calcified algae), and carbonate precipitation (biogenic or abiotic) displays a wide range of fractionation in Mg isotopes, it is reasonable to speculate that marine micrite may not archive d²⁶Mg_sw. To test this hypothesis, we measured d²⁶Mg of micrite (d²⁶Mg_micrite) recognized in late Paleozoic limestone from South China. Based on the high resolution conodont biostratigraphic framework, d²⁶Mg_micrite shows significant spatial (i.e. between different sections) and temporal (i.e. in stratigraphy) heterogeneities. The modeling results indicate that such d²⁶Mg_micrite variations cannot be explained by the fluctuation of d²⁶Mg_sw values. Thus, our study indicates that micrite cannot be used to reconstruct d²⁶Mg_sw.

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Keyword: micrite, seawater Mg isotopic composition