Abstract
Marine redox conditions and their dynamic changes were a major factor that controlled the formation of black shale and caused the late Ordovician marine extinction in the Upper Yangtze Basin (South China). However, the spatiotemporal variation and potential controlling factors of marine redox conditions in this area remain unclear. We analyzed whole-rock geochemistry and pyrite sulfur isotopes (δ34Spy) of 47 shale samples from the Late Katian to Rhuddanian in a shelf-to-slope (Qianjiang Shaba section and Wc-1 well) region of northeastern Upper Yangtze Basin, and reconstructed water column redox conditions during the Late Ordovician–Early Silurian Transition. The geochemical characteristics of shale, including the ratio of elements, discriminant function and ternary diagram location in the study area suggest a passive continental margin sedimentary environment, wherein the terrigenous detritus is mainly derived from felsic igneous rocks in the upper crust, showing characteristics of near-source deposition. The redox indices (Fe speciation, Corg/P, UEF, and MoEF) showed that the development of anoxic water, especially euxinia, has obvious spatiotemporal heterogeneity. Under conditions of high availability of active organic carbon and limited sulfate supply, high active Fe input and strong biological irrigation in the shallow water area may effectively remove H2S produced by microbial sulfate reduction, conducive to the prevalence of ferruginous water columns. However, for this deep water area, the rapid accumulation rate of organic matter, decrease in dissolved Fe (caused by upwelling in the open sea), and seawater stratification (caused by the rising of sea level) promoted the development of a euxinic water column. This inference is supported by the covariant relationship between organic carbon accumulation rate, chemical index of alteration, Co × Mn, and δ34Spy. Our study highlights the potential control effects of sea level change, continental weathering and upwelling on the development of euxinic water columns.
Highlights
All geochemical data of the Wc-1 well and Shaba section are included in the supplementary material
The rare earth elements (REEs) distribution pattern standardized by chondritenormalization was similar to that of UCC and post-Archean average shale (PAAS), suggesting that the sedimentary component of black shale was controlled by upper crustal source rock and showed near-source deposits (Figure 4D)
Based on a high-resolution geochemical analysis of Shaba Section and Wc-1 well, combined with published data from two other sections, our study explored the redox conditions of water in the northeastern part of the Upper Yangtze Basin during the Ordovician to the Early Silurian (O–S) transition period
Summary
The paleo-ocean during the transition from the Late Ordovician to the Early Silurian (O–S) in the Upper Yangtze Basin, South China, has been widely studied with respect to the sedimentary environment (Wang et al, 2016; Li et al, 2017; Liang et al, 2017; Wu et al, 2019; Wang et al, 2021), redox conditions (Li et al, 2019; Liu et al, 2019; Wang et al, 2019), and sea level change (Wang et al, 2017; Jin et al, 2020) During this period, a set of organic-rich black shale was deposited, including the Wufeng Formation (WF, Katian—Hirnantian) and Longmaxi Formation (LMX, Hirnantian—Rhuddanian), which have become the main targets for shale gas exploration in China (Zou et al, 2016; Zou et al, 2019). Because of the lack of highresolution geochemical records, especially the lack of research evidence on paleo-ocean topography and external ocean current communication in the continental margin area, the controlling factors of redox conditions in the Upper Yangtze Basin water column need to be explored further
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