Responses of oceanic chemistry to climatic perturbations during the Ordovician-Silurian transition: Implications for geochemical proxies and organic accumulations
Introduction
The deposition of organic-rich black shales in geologic intervals has been considered an archive that records information on the carbon cycle, oceanic anoxic events, and mass extinctions (Jenkyns, 2003, 2010). These organic-rich sediments are also of an important economic benefit as hydrocarbon source rocks and reservoirs of shale gas (Armstrong et al., 2009; Zou et al., 2010; Gambacorta et al., 2016). However, despite their significant roles in environmental reconstruction and in oil-gas exploration, their origin is still highly controversial (Rimmer, 2004; Tyson, 2005; Mort et al., 2007).
Two widely adopted models, the “productivity model” and the “preservation model”, have been proposed to explain the formation of organic-rich sediments. The former suggests that the high export of organic matter from surface water plays an important role in organic matter accumulation (Murphy et al., 2000; Sageman and Lyons, 2003), while the latter stresses that the anoxic water conditions caused by basin restriction are the main factor controlling organic matter accumulation (Arthur and Sageman, 1994; Mort et al., 2007). Recent efforts have added more factors to the process of organic matter accumulation: the sedimentary rate (Bohacs et al., 2005; Yang et al., 2020), the dilution effect of terrestrial input (Sageman and Lyons, 2003; Sageman et al., 2014) and the protection of organic matter by clay minerals (Kennedy et al., 2002; Kennedy and Wagner, 2011; Blattmann et al., 2019). The comprehensive multifactor model has been used to interpret the organic matter accumulated in marine or lacustrine sediments (Ma et al., 2016; Yan et al., 2018; Yang et al., 2020; Qiao et al., 2020; Zhang et al., 2020). However, it is worth noting that although the different factors may have various weights in a given depositional environment, the reduced water conditions seem to be a common feature, regardless of whether they are caused by water column stratification or a high surface productivity.
The Ordovician-Silurian transition (OST) is a critical geologic interval that witnesses a series of significant environmental and biological perturbations, such as mass extinction (Sepkoski, 1996; Chen et al., 2004), Hirnantian glaciation (Trotter et al., 2008; Finnegan et al., 2011), tectonic movements (Chen et al., 2014; Li et al., 2018) and volcanic eruptions (Su et al., 2009; Huff, 2016). Accompanying these important geologic events, organic-rich sediments were widely deposited at regional and/or global scales (Armstrong et al., 2009; Melchin et al., 2013). Two sets of source rocks, named the Wufeng and (lower) Longmaxi Formations, were deposited on the upper Yangtze Platform during the OST. They have been considered the most favorable horizons for shale gas exploration and development in China (Hao et al., 2013; Zou et al., 2015; Hu et al., 2017). Many efforts have been made to investigate the origin of these organic-rich sediments. Most authors have proposed that the accumulation of organic matter in sediments was dependent on both anoxic water columns and proper productivity (Yan et al., 2015, 2019, 2019; Li et al., 2017; Liu et al., 2019; Wang et al., 2019; Zhang et al., 2019; Yang et al., 2020). However, some authors have also suggested that organic matter accumulation only relies on preservation conditions, regardless of primary productivity (Zeng et al., 2015; Chen et al., 2016). Lu et al. (2019) further suggested that the significance of the primary productivity in organic matter accumulation increased from the innermost Yangtze Platform to the outer Yangtze Platform. Furthermore, views on the terrestrial input's effect on organic matter accumulation are also divided into two camps. Some studies have suggested that terrestrial inputs severely impact organic matter accumulation (Li et al., 2017; Wang et al., 2019), while others have noted that the impact of terrestrial inputs on organic matter accumulation is limited (Yan et al., 2015; Liu et al., 2019). The above contradictory explanations imply strong local controls on organic matter accumulation and, thus, are necessary to further investigate sedimentary conditions in more locations. To date, compared with the numerous studies in the Longmaxi Formation, much less attention has been given to the Wufeng Formation, which was deposited under fluctuating climatic conditions, inhibiting the clear picture of organic matter accumulation on the Yangtze Platform during the OST.
In this study, we measured the high-resolution major and trace elements, as well as the total sulfur contents (TS) of the Datianba section located southeast of the upper Yangtze Platform. This section has suffered a series of geochemical analyses in Liu et al. (2016) and Liu et al. (2020), including total organic carbon (TOC), carbon, nitrogen and sulfur isotopes and iron speciation. By combining these published data, we aim to (1) compare the redox reconstruction based on trace elements and on Fe speciation; (2) investigate the climatic change and its possible influence on oceanic chemistry; and (3) discuss the mechanism(s) for organic matter accumulation on the Yangtze Platform during the OST.
Section snippets
Geologic setting
During the OST, the equatorial South China Carton was a separated plate, but was still connected to the northwestern margin of Gondwana (Fig. 1A; Metcalfe, 1994; Cocks and Torsvik, 2002). It was composed of the Cathaysia block in the southeast and the Yangtze block in the northeast, which may have been separated by an immature intracontinental rift (e.g., Xiang-Gui basin) (Song et al., 2020). During the Middle-Late Ordovician, the Yangtze Platform was covered by a broad epeiric sea that
Analytical methods
The TS and major and trace element contents for the Datianba section were original in this study. They were measured on the same samples that have been used in a previous C–Fe–S study (Liu et al., 2016). All samples were pulverized to powders of >200 mesh after removing any potential weathering surfaces and veins. All analytical work in this study was performed at the Guangzhou Institute of Geochemistry, Chinese Academy of Science (CAS).
The TS contents were measured using a LECO CS-344
Results
All geochemical data are listed in Table S1, and the key geochemical results are summarized in Fig. 4, Fig. 5.
Paleo-redox proxies
The use of RSTEs to reconstruct paleo-redox conditions is mainly based on the distinct solubility of special elements in response to different redox potentials (Algeo and Maynard, 2004; Tribovillard et al., 2006; Algeo and Li, 2020). In oxic facies, all RSTEs exist as conservation and soluble forms in water columns (Tribovillard et al., 2006). Accompanying the deterioration of water redox conditions, vanadium (V) can be reduced from V(V) to V (Ⅳ) under suboxic conditions, and further to V (Ⅲ)
Implications for oceanic evolution and organic matter accumulation
Our new and integrated geochemical data provide an opportunity to survey the covariations among climatic changes, oceanic evolutions, and organic matter accumulations. In the pre-Hirnantian, strong local upwelling may bring about sufficient nutrients from deep water to surface water, sustaining long-term high productivity. The high productivity, in turn, promoted the formation of euxinic water columns. This is supported by the strong positive correlation between the TOC and pyrite contents (
Conclusions
To further investigate the covariation of climate and palo-ocean and to consider the main controls of organic accumulation on the Yangtze Platform during the OST, we analyzed the major and trace elements contents as well as the total sulfur content of the Datianba section, which has been studied for C–Fe–S geochemistry. Compared with the Fe speciation results, our trace element proxies showed obvious suboxic conditions in the Guanyinqiao Bed. We suggested that the anoxic signals provided by Fe
Declaration of competing interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
We thank Qing Chen and Zhongyang Chen for their help in the field. This study was supported by the NSFC (grant No. 41802122) to LY. HYC acknowledges support from the NSFC (grant No. 42002123), and HTZ acknowledges support from the NSFC (grant No. 41802126). We thank all the reviewers for their constructive suggestions.
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