摘要
【目的】目前鄂尔多斯盆地安崖剖面早侏罗世Toarcian大洋缺氧事件(Toarcian Oceanic Anoxic Event,T-OAE;~183 Ma)的研究主要聚焦于古气候条件、沉积环境演化和生物多样性变化等方面,对于T-OAE时期湖泊硫循环的特征尚不明确。因此,有必要进一步明确该事件期间湖泊硫循环机制。【方法】对安崖剖面泥岩和黑色页岩样品进行矿物学和同位素地球化学分析。同时,利用扫描电镜对黄铁矿形态进行鉴定,结合黄铁矿硫同位素(δ^(34)S_(py))、主微量元素,建立T-OAE时期硫同位素的组成特征,并对其成因进行探讨。【结果】安崖剖面样品中黄铁矿形态主要为四面体和八面体晶型集合形成的草莓状黄铁矿,样品的δ^(34)S_(py)表现出正值,介于2.7‰~14.1‰,平均值为8.3‰。根据δ^(34)S_(py)和总有机碳(TOC)含量的变化,并结合地球化学指标(δ^(13)C、TS、C_(org)/P、(La/Yb)_(N)),将湖泊沉积环境演化分为四个阶段(富有机质阶段Ⅰ、Ⅱ,贫有机质阶段Ⅰ、Ⅱ)。【结论】鄂尔多斯盆地安崖剖面TOAE时期,样品的δ^(34)S_(py)值与大气降水和地表径流的硫同位素组成相吻合,表明湖泊水体硫的来源主要为大气降水和地表径流。样品的硫同位素组成主要受湖泊底水的氧化还原环境与有机质含量/类型的共同影响。安崖剖面T-OAE时期硫循环受到局部/区域环境控制,而该时期全球气候变暖及水文循环加剧等为安崖剖面的硫循环提供了这种局部/区域条件。
[Objective]At present,research on the Early Jurassic Toarcian Oceanic Anoxic Event(T-OAE;~183 Ma)in the Ordos Basin primarily emphasizes paleoclimate conditions,the evolution of the sedimentary environment,and biodiversity changes.However,the mechanisms governing sulfur cycling in lakes during the T-OAE remain un⁃clear.Consequently,further examination of the sulfur cycling mechanism is essential.[Methods]Twenty-eight sam⁃ples were selected from the Anya section.Mineralogy and isotope geochemistry analyses were performed on mudstone and black shale samples from the Anya section.The mechanisms of sulfur isotope fractionation during the T-OAE were investigated using major and trace elements,as well as pyrite sulfur isotopes(δ^(34)S_(py)).To avoid interference from marcasite and siderite,mineral morphology and energy spectra were examined using a scanning electron microscope(SEM).[Results]The pyrite in the samples from the Anya section predominantly were tetrahedral and octahedral crystals,forming framboidal pyrite.The mineral surface exhibits a smooth texture without the presence of platy marca⁃site.The coexistence of radial siderite with pyrite is also observable.The δ^(34)S_(py) values of these samples consistently display positive values,ranging from 2.7‰to 14.1‰,with an average of 8.3‰.By analyzing the fluctuations in δ^(34)S_(py) and total organic carbon(TOC),along with the use of geochemical indicators such as δ^(13)C,total sulfur(TS),C_(org)/P,and(La/Yb)_(N),we delineated the evolution of the lacustrine sedimentary environment during the T-OAE into four dis⁃tinct stages,comprising periods characterized by high(Stages Ⅰ and Ⅱ)and low organic matter(Stages Ⅰ and Ⅱ).[Conclusions]The δ^(34)S_(py) values during the T-OAE in the Anya section of the Ordos Basin align with those of atmo⁃spheric precipitation and surface runoff,suggesting that sulfate in the water mass serves as the primary source of sul⁃fur in the lake.Atmospheric precipitation and surface runoff mainly consist of dissolved sulfate ions exhibiting lacus⁃trine sulfate isotope compositions between 0 and 10%.However,the samples from the Anya section include black shale,mudstone,sandstone,and other low sulfur source rocks,likely resulting in smaller sulfur isotope composi⁃tions within the lakes.It is improbable that the sulfur isotope composition of pyrite is inherited from lacustrine sulfate.The sulfur isotopes in the pyrite of the samples are primarily influenced by the redox conditions of the lake bottom water and the organic matter content/type within the sediment.When the lake bottom water is oxygen-rich and the or⁃ganic matter content is low,dissolved oxygen infiltrates into sediments,activating anaerobic oxidants such as NO_(2)^(-) and Mn^(2+),while promoting H_(2)S reoxidation.This process leads to a positive δ^(34)S_(py) shift,as described by the Rayleigh fractionation model.In contrast,when the lake bottom water is oxygen-deficient and the organic matter content high,sulfate-reducing bacteria become more active,promoting the microbial sulfate reduction(MSR)reaction,which preferentially incorporates 32S,which results in a negative δ^(34)S_(py) shift.During the T-OAE,the lake redox conditions and climate underwent significant changes,exerting a substantial influence on the form,content,and fractionation of sulfur within lake.The sulfur cycle during the T-OAE in the Anya section is predominantly dictated by local and regional sedimentary environments,although it is also affected by global warming and intensified hydrological circula⁃tion during the T-OAE.
作者
陈杨
金鑫
郎咸国
李滨兵
CHEN Yang;JIN Xin;LANG XianGuo;LI BinBing(Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu 610059,China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,China)
出处
《沉积学报》
北大核心
2025年第6期1952-1967,共16页
Acta Sedimentologica Sinica
基金
成都理工大学沉积地质研究院创新人才培养基金项目。
