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Evaporite sequences as archives for Mg isotope compositions of seawater - Evidence from a Tethys marginal shelf basin in the Anisian
Chemical Geology ( IF 3.6 ) Pub Date : 2024-08-22 , DOI: 10.1016/j.chemgeo.2024.122346 Zhongya Hu , Weiqiang Li , Simon V. Hohl , Patrick Meister , Shouye Yang , Bolin Zhang , Zhiguang Xia , Chuan Liu
Chemical Geology ( IF 3.6 ) Pub Date : 2024-08-22 , DOI: 10.1016/j.chemgeo.2024.122346 Zhongya Hu , Weiqiang Li , Simon V. Hohl , Patrick Meister , Shouye Yang , Bolin Zhang , Zhiguang Xia , Chuan Liu
Evaporites have recently been suggested as a potential archive for recording the Mg isotope compositions (δMg) of coeval seawater. However, episodic dolomitization during the deposition of massive evaporites could cause considerable Mg removal and isotopic fractionation. To constrain the hydrological changes and influence of dolomitization on ambient brine δMg, we present petrographic and mineralogical features, as well as Mg-C-O-Sr isotope data extracted from carbonate phases of a middle Triassic (ca. 247 Myr) marine anhydrite-dolostone sequence from a drill core in eastern China. The drilled lithologies are characterized by massive dolostone layers in the lower part, followed by an upward decline in dolomite contents accompanied by a rise in anhydrite. Multiple lines of evidence consistently point to a -depositional origin for the dolostones in a marginal basin of the Tethys Ocean and a lack of diagenetic alteration since deposition. We reconstructed the dynamic changes of δMg values of basin waters based on Mg isotope compositions in dolomite leachates. According to our findings, the δMg values of the basin waters were remarkably high (about 0.38 ± 0.05‰) at the onset of evaporation, indicating a significant Mg sink of the massive dolomitization. The δMg of brine in the basin then changed towards the value of coeval seawater (about −0.32 ± 0.05‰) starting with the deposition of evaporites. Concurrently, Sr/Sr ratios of dolomites shift from radiogenic values towards contemporaneous seawater composition. Our results demonstrate that the evaporite basin was not strictly restricted, and water exchange with the open ocean never ceased. Modeling calculation reveals that, even when the seawater exchange rate is far below the average ocean circulation, δMg of brine in the basin will reach the value of open ocean within 1 Myr, completely removing the influence of early dolomitization. We suggested that massive marine evaporite sequences have the potential to record the long-term evolution of seawater Mg isotopes.
更新日期：2024-08-22
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