鄂尔多斯盆地Toarcian大洋缺氧事件时期菱铁矿的成因及其地质意义OA

[Objective]The widespread siderites in the marine strata of the Toarcian Oceanic Anoxic Event(T-OAE)are believed to have resulted from a low sulfate concentration and increased terrestrial iron supply.Similar large amounts of siderite have also been reported in paleolakes in the same period.However,the mechanism of lacus-trine siderite development and its connection with the T-OAE remain unexplored,but is crucial to a better compre-hension of lacustrine biogeochemical cycling in the early Toarcian.[Methods]Mineralogical and carbon isotope geo-chemical analyses were performed on siderite samples from the lacustrine Anya section of the Ordos paleolake.The sources of iron and carbon,as well as the sedimentary environment,were investigated by integrating major and trace element data,which allowed exploration of the formation mechanism of siderites during the T-OAE period of climatic change.[Results]The coexistence of pyrites and siderites was observed by scanning electron microscopy(SEM).The value of inorganic carbon isotopes(δ13Ccarb)varies from-11.25‰ to+16.32‰,and the organic carbon isotope(δ13Corg)value ranges between-32.03‰ and+14.33‰.Light rare earth elements(REEs)are enriched and heavy REEs are depleted,with Eu anomalies ranging from 0.69 to 0.85.[Conclusions]The iron in the Anya siderites is derived from terrestrial inputs;the carbon included HCO3-produced by methanogenesis,as well as carbon released by organic matter decomposition and aerobic methane oxidation,either bacterially in the suboxic zone or archaeally below the oxic-anoxic interface immediately after methane production.The pore fluids during siderite formation were a suboxic to anoxic environment.During the T-OAE,intensified continental weathering and increased surface runoff transported abundant Fe2+ions into basin lakes whose widespread anoxia,abundant organic matter and low sulfate concentrations created favorable conditions for extensive siderite deposition.