Abstract
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 (δ26Mgsw), 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 δ26Mgsw. 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 δ26Mgsw. 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 δ26Mgsw. 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 δ26Mgsw using bulk carbonate/calcareous sediments. Finally, δ26Mgsw 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.
Highlights
Secular variation of carbonate mineralogy in the Phanerozoic ocean is observed in both non-skeletal and skeletal carbonate (Stanley and Hardie 1998, 1999; Stanley et al 2010)
The marine Mg cycle could be reflected by seawater Mg isotopic composition (d26Mgsw), which might be recorded in marine carbonate
The four components had distinct d26Mg: (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
Summary
Secular variation of carbonate mineralogy in the Phanerozoic ocean is observed in both non-skeletal and skeletal carbonate (Stanley and Hardie 1998, 1999; Stanley et al 2010). Oscillation in the mineralogy of marine carbonate is attributed to fluctuation of the seawater Mg/Ca ratio. Acta Geochim (2018) 37(1): ridges (MOR), which determines the intensity of hydrothermal reactions (Stanley and Hardie 1999). In the high-temperature hydrothermal systems along MOR, oceanic basalt is converted to greenstone by sequestration of seawater Mg2? Into seawater (Elderfield and Schultz 1996); low-temperature alteration of basalt in ridge flanks removes seawater Mg (Higgins and Schrag 2015). A high spreading rate of MOR corresponds with low seawater Mg/Ca, and vice versa (Stanley and Hardie 1999). Recent research suggests that low-temperature and high-temperature hydrothermal reactions account for comparable amounts of the Mg sink in the modern ocean, while carbonate precipitation leads to about 20%–25% of the total (Higgins and Schrag 2015)
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