The Disturibution Correlation Of Elements In Carbonate As Proxies For Depositional Redox Condition

Redox condition of depositional environment is a vital issue in palaeoenvironment research,as it links directly to the atmospheric oxygen level.The shallow water carbonate rocks are widely distributed on the Earth's surface;extracting redox-related data from such rocks is crucial for tracing atmospheric oxygen level during the critical period of the Earth's evolution.Some proxies,such as I/Ca,Ce,S isotopes,and the ratio of highly reactive Fe to total Fe,have been built in tracing depositional redox condition.However,it is still necessary either to improve the existing proxies,or develop new proxies,as the existing proxies have their own deficiencies.A consensus has been reached that a global mass extinction event was happened during the middle Permian(~260 Ma before present).Some studies realize that shallow water anoxia was widespread during that period,which thus is considered to be the potential culprit on mass extinction.This study collected carbonate samples of the middle Permian Maokou Formation from the profile which is located in the western Kunming.The mass extinction layer was confirmed through evidence of microfossils,carbon isotope and magnetic susceptibility.The distributions of trace elements in foraminifer‘s tests were analyzed through synchrotron radiation-based X-ray fluorescence spectroscopy(?XRF),and the contents of trace elements in the samples were determined by LA-ICPMS.A geochemical proxy on depositional redox condition was built through analyzing the correlations of elemental distribution and content.The results showed that for samples from non-mass extinction layer,the distributions of As and Fe,and Ce and Mn are identical in the foraminifer‘s tests.Raman results showed that the distributions of As and Ce are not constrained by organic matters and sulfides.While in the mass-extinction layer,such correlations disappear,and the distribution of As is constrained by organic matter.LA-ICPMS results showed that the contents of Ce and Mo decrease from the non-mass extinction layer to the mass-extinction layer,while that for V and Cr increase.Ce is positively correlated to Cr,Mn and Mo in the non-extinction layer.Such correlations disappear in the mass-extinction layer.The results of this study verified the inference that the deposition environment changed from oxic to anoxic in the middle Permian.In non-mass extinction period,oxic depositional condition led to the stability of Fe and Mn(hydr)oxides,which in turn constrained the geochemical behaviors of As?V?Cr?Mo.While in the mass extinction period,Fe and Mn(hydr)oxides were reductively dissolved,and the trace elements were not correlated.This study considers that the distribution correlations of As and Fe,and Ce and Mn is efficient proxy for tracing depositional redox condition,where positive correlation and non-correlation indicate oxic and anoxic depositional conditions,respectively.This proxy is an important supplement to the traditional Fe speciation proxy,and has implication in tracing atmospheric oxygen level during the critical period of the Earth‘s evolution.