Development of a high-productivity and anoxic-euxinic condition during the late Guadalupian in the Lower Yangtze region: Implications for the mid-Capitanian extinction event

Significant disruption of global biogeochemical cycles occurred in the late Guadalupian, coincident with a great biotic crisis known as “end-Guadalupian mass extinction” (Jin et al., 1994; Stanley and Yang, 1994) and other synchronous geological events such as eruption of the Emeishan large igneous province (ELIP) (e.g., Wignall et al., 2009a). As a prelude to the biological transition from the Paleozoic to the Mesozoic, this event has been considered the real start of the Paleozoic-Mesozoic transition (Isozaki, 2009b). Although the original onset timing of the event was constrained to the end-Guadalupian (i.e., end-Capitanian; Isozaki, 2009a, Isozaki, 2009b; Retallack et al., 2006; Wang and Sugiyama, 2000; Yang et al., 2004), accumulating evidence suggests that the event may have begun in the mid-Capitanian (Bond et al., 2010a, Bond et al., 2010b, Bond et al., 2015; De la Horra et al., 2012; Wignall et al., 2009a, Wignall et al., 2012), which is in accordance with the initial eruption of the ELIP (Jinogondolella altudaensis-J. prexuanhanensis conodonts zone) (Bond et al., 2010b; Wignall et al., 2009a) and significantly before the G-LB. Based on the paleontological stratigraphy and zircon chronostratigraphy of the Middle Permian in the Chaohu area, Zhang et al. (2018) suggested that a significant biotic crisis could have occurred in the Lower Yangtze region of South China during the mid-Capitanian.

In addition to the great regression and consequent shallow-marine habitat loss (Bond and Wignall, 2009; Jin et al., 1994; Wignall et al., 2009b; Wei et al., 2017), the flood basalt eruptions and explosive volcanism of the ELIP (Ali et al., 2002; Bond et al., 2010b; Wignall et al., 2009a; Zhou et al., 2002) and the Kamura event (Isozaki et al., 2007a, Isozaki et al., 2011; Isozaki, 2009b), the development of widespread marine anoxia during the late Guadalupian was believed to impact benthic communities and contribute to the simultaneous extinction of several fossil groups (e.g., Bond et al., 2015; Isozaki, 1997; Saitoh et al., 2013a, Saitoh et al., 2013b; Saitoh et al., 2014; Zhang et al., 2015; Wei et al., 2016). Oxygen deficiency was already widespread in the pre-extinction mid-Capitanian oceans such as Panthalassa (e.g., Isozaki, 1997, Isozaki, 2009b), eastern Paleotethys in South China (e.g., Lai et al., 2008; Saitoh et al., 2013a, Saitoh et al., 2013b; Saitoh et al., 2014; Wei et al., 2016) and Spitsbergen (e.g., Bond et al., 2015) with possible sulfidic conditions developing at mid-water column depths. However, the mechanisms that formed, expanded and sustained the oceanic anoxia are still debated. Two competing hypotheses for the development of widespread oceanic anoxia (e.g., Georgiev et al., 2015) are presented: the more sluggish oceanic circulation that resulted from the global warming associated with the eruption of the Large Igneous Province, and the increased marine primary productivity related to enhanced upwelling that led to intensified heterotrophic respiration and oxygen consumption in the ocean. The two competing hypotheses clearly need to be tested further.

Here, we present an analysis of the redox conditions and marine productivity during the early-middle Capitanian of the Lower Yangtze region, South China. In this study, we utilize redox-sensitive trace elements (RSTEs), C-S-Fe systematics and morphology of pyrite as proxies for marine redox conditions, and total organic carbon (TOC), total phosphorus (TP), biolimiting elements, radiolarian abundance and biogenic SiO₂ as proxies for marine productivity. These analyses provide new insights into changes in paleo-environments and their underlying controls.

Paleogeography and stratigraphy

During the Guadalupian, the South China craton was in the eastern part of the Paleotethys Ocean (Fig. 1A) just north of the paleo-equator (Wang and Jin, 2000), with intense upwelling developed along its western coast (Kametaka et al., 2005; Yao et al., 2015). A large northeast-southwest trending carbonate platform known as the Yangtze Platform was developed and surrounded by deep-water basins (Fig. 1B). The widespread chert-mudstone rhythmic sequence of the Gufeng Formation was deposited from

Samples and methods

A total of 46 samples, including black chert (n = 15), siliceous mudstone (n = 24) and grey shale (n = 7), were collected from the Upper Gufeng Formation and the Lower Yinping Formation of the Hexian core (Table S1, Fig. 2) drilled by the Anhui Provincial Bureau of Coal Geology. All of the samples were fresh, unweathered rocks and pulverized to 200 mesh in an agate mortar and pestle for geochemical analysis. Both TOC and TS abundances of all samples were measured by the Elementar® Vario MACRO

Lithology

The radiolarian chert is black in colour and composed mainly of chalcedonic quartz, radiolaria and their debris (Fig. 3A, B). Slightly oriented along the chert micro-layes, radiolarian filled with chalcedonic quartz and other fine-grained material are uniform in size with a diameter of 50–100 μm and diverse in shape with round or oval as dominant (Fig. 3A–C). A bird's-eye structure showing a geopetal fabric (Fig. 3A) and rhombic cavities (Fig. 3B) is also observed in the radiolarian. Although

Age constraints

The identified Ruzhencevispongus uralicus-Follicucullus scholasticus assemblage zone (Fig. 4) from the LCMM chert, which can be compared with the upper Gufeng Formation of the Anmenkou section of the Chaohu area (Kametaka et al., 2009) and Maocaojie section of Hubei Province (Shi et al., 2016), indicate an early-middle Capitanian age. Abrupt lithological changes from deep water chert-mudstone facies to shallow shale-siltstone facies during the transition of the Gufeng and Yinping Formations

Conclusions

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant nos. U1663202, 41372127, 41690131 and 41172139) and the Science and Technology program of Land and Resources of Anhui Province (Grant nos. 2015-K-4).