The Effects Of Multiple Diagenesis On Carbonate Mg Isotopes And Application On Reconstruction Of Paleo-Marine Environments Changes

In this study we selected several typical marine carbonate sections that precipitated in shallow carbonate platform.Magnesium isotope analysis was complemented by analyses of Sr-C-O isotopic compositions,major and trace element concentrations,and petrographic and mineralogical features.Our study indicated Mg isotope compositions of the porewater were homogeneous during massive dolomitization and dolomite Mg isotopes remained in equilibrium with seawater.Additionally,the ?26Mg values of the altered dolostone do not show correlations with diagenetic and hydrothermal signals,demonstrating that dolomite Mg isotopes are insensitive to postdepositional alteration.Given these facts,we propose that dolomite can be an archive of past seawater Mg isotope signals.And we analyzed Mg isotope compositions of dolomite precipitated in Permian-Trassic transition period in Austria.The resulted revealed that ?26Mgdolomite shifted remarkably around the extinction interval,and anti-correlated with the global perturbations in carbonate ?13C during that period,reflecting that radical changes in major cations of seawater in the Tethys ocean during the Permian-Triassic transition,which exacerbated the crisis for skeletal marine organisms.Dolomitization is a widely-occurred diagenetic process after calcite deposits in Paleozoic and Mesozoic.The massive dolostone that distributed in geological history mainly formed via dolomitization process in semi-closed lagoon,observations on modern dolomitization processes in disparate sedimentary and diagenetic settings have shown that evolutionary patterns of porewater chemistry vary in different types of early diagenetic systems due to changes in hydrologic conditions,resulting in different stratigraphic variations in Mg isotope compositions in dolomite.However,the large widely distributed dolostones of the Mesozoic-Paleozoic formed via massive dolomitization,for which there is no genuine analog in modern marine environments,more geological investigations are needed to better characterize Mg isotope behaviors in natural massive dolomitization.In this study,according to detailed petrographic classification,a systematic Mg isotope dataset from various dolomite and corresponding host rock at Tremadocian in Tarim Basin was analyzed together with analyses of C-O isotopic compositions and REE concentrations in bulk dolostone.Multiple lines of evidence consistently confirmed remarkable hydrothermal activities occurred in massive dolostone layers,however,dolomite that underwent multiple hydrothermal alterations showed similar ?26Mg values with corresponding primary dolostone,which demonstrated the robustness of dolomite Mg isotopes against diagenetic alterations.All dolostone samples,from various sedimentary settings in carbonate platform,exhibited homogeneous Mg isotopic compositions,suggesting limited impact of sedimentology distinctions on magnesium isotope ratios,and no systematic variations could be detected along depth profile in all sections.These results suggested uniform Mg isotopes behaviors and equilibrium fractionations during dolomite formation in natural massive dolomitization.Calcite is another important content in marine carbonate.Besides pure dolostones,the studies on carbonate Mg isotpe variation in burial diagenetic alteration are still limited.Excellent outcrops of Zhoucunchong Formation,early Anisian of Middle Triassic(?247Ma)in age,are exposed as quarry mining sites at Yixing,Geshan,in Lower Yangtze Block of Southeast China.Locally,the section has experienced a burial history that is well-documented,and the samples in the section contain carbonates with variable proportions of calcite and dolomite,these features make the section an ideal natural laboratory for investigation of the behavior of Mg isotopes in calcite during dolomitization and burial diagenesis.Here we report our investigation on the Mg isotope systematics of a dolomitized Middle Triassic Geshan carbonate section in eastern China.Magnesium isotope analysis was complemented by analyses of Sr-C-O isotopic compositions,major and trace element concentrations,and petrographic and mineralogical features.Multiple lines of evidence consistently indicated that post-depositional diagenesis of carbonate minerals occurred to the carbonate rocks.Magnesium isotope compositions of the carbonate rocks closely follow a mixing trend between a high ?26Mg dolomite end member and a low ?26Mg calcite end member,irrespective of sample positions in the section and calcite/dolomite ratio in the samples.The calculation results of a two-end member mixing model indicated that both calcite and dolomite had been re-equilibrated during burial metamorphism,and based on isotope mass balance,the system was buffered by dolomite in the section.Therefore,burial metamorphism may reset Mg isotope signature of calcite,and Mg isotope compositions in calcite should be dealt with caution in studies of carbonate rocks with thermal history.By contrast,Mg isotopes of dolomite are less prone to post-depositional resetting due to a number of properties including high Mg abundance and high thermodynamic stability,and Mg isotopes in dolomite may be a more robust recorder for original carbonate precipitates.The Permian-Triassic transition witnessed the largest mass extinction event in Earth's history,but its cause-effect links remain unsolved.Here we address this issue using Mg isotope compositions of syndepositional dolomite from the Gartnerkofel core in Carnic Alps of Austria.?26Mgdolomitle shifted remarkably around the extinction interval,and anti-correlated with the global perturbations in carbonate ?13C during that period.The results suggest that Mg isotope compositions of seawater fluctuated by 0.4‰ within?750 kyr,at a very rapid rate despite that Mg is a major cation in seawater.The Mg isotope records reflect radical changes in major cations of seawater in the ocean during the Permian-Triassic transition,which exacerbated the crisis for skeletal marine organisms.Modelings reveal that the fast drifts in seawater Mg isotope compositions required extremely short resident time for Mg in the ocean,which resulted from a combined effect of dolomitization and transient periods of carbonate platform restriction.