Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of ²⁴Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of ²⁶Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ²⁶Mg_sw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ²⁶Mg_sw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ²⁶Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in ²⁴Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ²⁶Mg_sw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ²⁶Mg_sw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ²⁶Mg of the Huangjin limestone makes it difficult to reconstruct δ²⁶Mg_sw using bulk carbonate/calcareous sediments. Finally, δ²⁶Mg_sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.