Environmental perturbations during the latest Permian: Evidence from organic carbon and mercury isotopes of a coal-bearing section in Yunnan Province, southwestern China
Introduction
As a geologic material composed of organic and mineral matter, coal can provide vital information on the regional geologic evolution, including sedimentation, climate, volcanic activity, and tectonic regime (Dai et al., 2018a, 2020; Finkelman et al., 2019; Spiro et al., 2019). Late Permian is a significant geologic period for both coal formation and volcanic activity in South China (Bagherpour et al., 2019; Dai et al., 2017a; Shen et al., 2013; Zhou et al., 2000). The end-Permian mass extinction (EPME) is the most severe biological crisis in Phanerozoic (Erwin, 1994; Shen et al., 2011). Before the mass extinction, ecological resilience is likely diminished by some volcanism and associated degassing events in many parts of the world, which has not culminated to lead immediate mass extinction (Shen et al., 2012a, Shen et al., 2012b, Shen et al., 2018). These environmental perturbations are manifested by, in particular, different degrees of negative carbon isotope excursions recorded in the latest Permian rock (mainly marine) successions of South China and elsewhere (Bagherpour et al., 2019; Shen et al., 2011, Shen et al., 2018). Another robust proxy of volcanic activity is a mercury enrichment anomaly in sediments (Pyle and Mather, 2003; Selin, 2009; Shen et al., 2019b). The anomalous mercury enrichment has been reported in several marine Permian-Triassic boundary (PTB) sections (Shen et al., 2019a; Wang et al., 2018b) and a few terrestrial PTB sections in South China (Chu et al., 2020; Shen et al., 2019b; Zhang et al., 2016a). In addition, some PTB sections measured for mercury isotope ratios also showed distinct mercury isotope signatures, particularly mass-independent fractionation values (Δ199Hg, see definition in method section), from those pre- and post- PTB sections (Shen et al., 2019a, Shen et al., 2019b; Wang et al., 2019). Δ199Hg in PTB sections either approach to zero values or show slightly positive values, suggesting large amounts of volcanic Hg inputs followed by different degrees of photochemical reduction in the atmosphere (Shen et al., 2019a). Although there have been extensive studies on EPME, few data have been reported on environmental perturbations before EPME, which diminished the ecological resilience of communities and reduced ecological functions (Shen et al., 2018).
In this work, the Late Permian coals from a 120-m-thick coal-bearing section (LY907 drill-hole, Lianying Coalfield) were analyzed for organic carbon isotopes and elemental geochemistry, with an emphasis on the LY2 coal seam that contains some distinct anomalies of organic carbon isotopes and Hg contents. In LY2 seam, the detailed geochemical, mineralogical data, and Hg isotope compositions have been used to elucidate the regional geological evolution and events (e.g., volcanic activities) during peat deposition, as well as diagenetic processes during coalification. This study also focused on the origin of tonstein partings and host (roof and floor) rocks in the coal-bearing sequence. Tonstein is volcanic in origin (Bohor and Triplehorn, 1993; Dai et al., 2017a), and the geochemical and mineralogical characteristics of tonsteins can reflect the composition and geodynamic setting of magmatism associated with the formation of coal basins (Dai et al., 2017a; Spears, 2012).
Section snippets
Geological setting
The Lianying Coalfield is located in the eastern Yunnan Province inside the intermediate zone of Emeishan Large Igneous Province (ELIP; Fig. 1A). The coal-bearing strata in this coalfield are the upper and middle Xuanwei Formation of Late Permian age. These strata are overlain by coal-free terrigenous rocks of the Late Permian and the Early Triassic Kayitou Formation. The Permian-Triassic boundary is conventionally positioned inside the Kayitou Formation above the youngest coal seam of Xuanwei
Sampling and analytical methods
The whole coal-bearing section was from the LY907 drill-hole, spanning over 120 m and covering 14 coal seams in total. The coal seams were named in ascending order, from the uppermost seam (LY1) to the lowermost seam (LY15). In the LY907 drill-hole, coal seam LY5 was missing because no peat was deposited during the corresponding time (Fig. 2). A total of 101 samples were collected from the Late Permian sequences of the Lianying Coalfield, including 52 coals, 17 carbonaceous mudstones, 4
Organic carbon isotope compositions
δ13Corg values of the Lianying coals vary from −25.26‰ to −22.96‰, with an average of −24.14‰ (Supplementary Table 1). An apparent negative shift of δ13Corg (−25.09‰, average value) appears in the LY2 seam, with a minimum value of −25.26‰ in coal sample LY2-3 (Fig. 2A, Supplementary Table 1). δ13Corg values of the coals, carbonaceous mudstone, and intra-seam parting samples of LY2 seam vary from −24.62‰ to −25.99‰, with an average of −25.11‰ and obvious negative shifts in two tonstein partings
Origin of δ13Corg, Eu and Hg anomalies
The negative δ13Corg excursions show near-synchronous onsets with the negative Eu anomalies and the elevated Hg concentrations within the LY2 seam (Fig. 2B). Several factors might cause carbon isotope excursions including CO2 release from volcanic eruption (Shen et al., 2012a), methane release from coal or organic-rich shales influenced by contact metamorphism (mafic intrusions or flood basalts) (Svensen et al., 2009), and reorganization of the carbon cycle with major sedimentary burial of
Conclusions
Two negative δ13Corg excursions are near-synchronous with the negative Eu anomalies and the elevated Hg concentrations and specific Hg isotope compositions in the tonstein samples of LY2 seam in LY907 coal-bearing section from the Lianying Coalfield, Yunnan Province, South China. These C and Hg anomalies and associated mineralogical compositions and elemental geochemical signatures manifested by Al2O3/TiO2, Zr/TiO2, and Nb/Yb ratios are probably caused by the subduction and/or collision-related
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (Nos. 41902166 and 91962220), the 111 Project (No. B17042), and Youth Innovation Team Development Plan of Universities in Shandong Province (No. 01010410105). We are grateful to the two anonymous reviewers for their valuable comments on the manuscript.
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