Geochemical evidence from bio-apatite for multiple oceanic anoxic events during Permian–Triassic transition and the link with end-Permian extinction and recovery

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Abstract

A detailed, 20 myr redox history of Permian to Triassic oceans (Changhsingian to Carnian stages) has been constructed using Ce-anomaly (ΩCe) and Th/U ratios from conodont albid crown apatite material. The results show that the well-established phenomenon of intense ocean anoxia (coincident with the end-Permian mass extinction) is faithfully recorded in conodont ΩCe and Th/U data. Extending this conodont redox record shows that end-Permian anoxia persisted possibly into the earliest Dienerian Stage and that two intense oceanic anoxic events also occurred later in the Early Triassic (earliest Smithian–earliest Spathian, and middle Spathian), followed by a weaker manifestation of anoxia in the Anisian Stage, seen in ΩCe data. Marine benthic radiation, following the end-Permian mass extinction, began after the Smithian–earliest Spathian anoxic event suggesting a suppression of evolution prior to this due to these inimical conditions. The failure of the middle Spathian anoxic event to retard the evolutionary rebound implies shallow shelf seas remained well ventilated at this time even if the oceans did not. Other attributes of the Early Triassic record also closely coincide with redox fluctuations: phases of anoxia intensification saw the proliferation of microbial carbonates and major negative carbon isotope swings that can be attributed to chemocline shallowing causing alkalinity pulses and enrichment in light, remineralised carbon and/or indicate a trigger meachnaims related to increased fluxes of light C from Siberian volcanic sources.

Highlights

► Shows the value of ΩCe and Th/U data from conodont crown tissue as a reliable proxy for ocean redox. ► First geochemical study to look at end-Permian anoxia in an extended temporal context. ► Reveals 3 oceanic anoxic events in latest Permian–Early Triassic interval. ► Shows the delayed Early Triassic radiation is not simply related to ocean anoxia.

Introduction

The long-delayed and highly variable recovery rates of marine invertebrates after the end-Permian mass extinction is one of the most intriguing facets in life's history. Although, nektonic groups, such as ammonoids and conodonts recovered rapidly (Brayard et al., 2009, Stanley, 2009), the majority of benthic groups did not re-diversify until either the Spathian or Anisian stages more than 5 million years later (Payne et al., 2011, Song et al., 2011). Early Triassic seafloors were characterised by the proliferation of anachronistic Precambrian-like facies, especially in equatorial carbonate locations where flat-pebble conglomerates and microbial reefs developed (Pruss et al., 2006, Wignall and Twitchett, 1999). Ocean anoxia has been widely implicated in the end-Permian extinction, and it is suggested that prolongation of these conditions hindered radiation of seafloor communities (Hallam, 1991, Isozaki, 1997). This hypothesis may provide an explanation for the uniquely long Triassic recovery interval and it is investigated here using independent geochemical redox proxies derived from Triassic bio-apatite from equatorial carbonate localities in South China. Recent studies in this region have revealed the detailed timing of the recovery (Song et al., 2011), allowing potential cause-and-effect to be directly compared.
The Permian–Triassic (P–Tr) redox history of Panthalassa Ocean reveals major fluctuations in the intensity of anoxia after the onset of ocean euxinicity at the end of the Permian as shown by pyrite petrography (Wignall et al., 2010). However, comparable studies of shelf seas have only focused on the P–Tr boundary interval (Algeo et al., 2008, Cao et al., 2009, Fenton et al., 2007, Grice et al., 2005, Liao et al., 2010, Riccardi et al., 2006, Wignall and Hallam, 1992, Wignall and Twitchett, 2002). In this study we have examined the 20 million year record of redox conditions spanning the Late Permian to Late Triassic based on conodont apatite geochemical data from three Permian and Triassic shallow-water sections in South China, (i.e. Meishan, Qingyan, Guandao, Fig. 1).
In comparisons to other sources of bio-apatite, such as ichthyoliths, conodont albid crown has been shown to provide a record of water column cerium anomalies (ΩCe) and thorium/uranium ratios (Th/U), little affected by diagenetic issues (Trotter and Eggins, 2006, Wright et al., 1987). Both ΩCe and the ratios of Th/U provide valuable indices for estimating past redox conditions in seawater. In oxic conditions, soluble Ce3+ is oxidized to precipitates of CeO2, whilst other lanthanides are trivalent and unaffected by redox states (Liu et al., 1988). This results in negative ΩCe values in oxic oceans whilst anoxic oceans have zero or positive values (de Baar et al., 1988: Wright et al., 1987). Uranium has two different redox states: in oxic conditions U6+ is stable and highly soluble but it converts to the insoluble U4+ in anoxic waters whilst the solubility of Th is unaffected by redox changes. This results in an increase of Th/U ratios in anoxic facies (Wignall and Myers, 1988). If the extent of ocean anoxia becomes substantial, as suggested for the Early Triassic (Brennecka et al., 2011), then the ocean U reservoir will become depleted (Ehrenberg et al., 2008) leading to an increase in Th/U ratios.

Section snippets

Geological setting and studied sections

During the P–Tr transition, the South China Block was located near the equator in the eastern Palaeotethys Ocean (Fig. 1A). The intensively studied Meishan section crops out in the county of Changxing, about 300 km west of Shanghai city, eastern China (Fig. 1B). The Global Stratotype Section and Point (GSSP) of the P–Tr boundary is placed at the base of Bed 27c (Yin et al., 2001). Abundant conodont species in the Changhsingian and Griesbachian stages enable correlation with sections elsewhere in

Materials and methods

A total of 196 bulk samples (about 2–5 kg) ranging from Upper Permian to Upper Triassic were collected from South China. Among them, 36 samples are from the Meishan section spanning Changhsingian to lower Griesbachian; 47 samples are from Qingyan section spanning Griesbachian to middle Anisian; 113 samples are from Guandao section spanning Griesbachian to Carnian (Fig. 2). For the isolation of single conodonts, limestone samples were crushed into 1 cm3 size fragments and reacted with 10% acetic

Cerium anomalies (ΩCe)

Conodont ΩCe of the studied interval ranges from −0.75 to +0.19 (Table 1; Fig. 2A). Both original data and 5-point moving average data show that most of conodont ΩCe from Changhsingian and Anisian–Carnian stages are less than −0.1 while the Early Triassic values are generally greater than −0.1 (Fig. 2). Changhsingian ΩCe fluctuates slightly between −0.3 and −0.1 except for several high values in beds 11, 15, and 21 at the Meishan section (Fig. 2A). A rapid rise of ΩCe from −0.2 to 0 happens

Discussion

At the Meishan section, both ΩCe and Th/U curves climb rapidly at bed 25 (Fig. 2), suggesting the rapid onset of anoxia coincident with the mass extinction level (Jin et al., 2000), thus reinforcing the link between these two phenomena highlighted in many previous studies (e.g., Wignall and Hallam, 1992, Wignall and Twitchett, 1996, Isozaki, 1997, Wignall and Twitchett, 2002, Grice et al., 2005). This change of redox state, indicated by conodont apatite redox indices is corroborated by evidence

Conclusions

The onset of anoxia at the end-Permian is well documented from diverse redox indices and is further corroborated here using ΩCe and Th/U data from conodont apatite albid crowns. Ocean anoxia caused the loss of Ce anomaly (ΩCe values rise to around zero) and an increase in Th/U ratios due to the depletion of the U reservoir in an ocean with widespread anoxic facies (Ehrenberg et al. 2008). We have extended our record to encompass the entire interval from the Late Permian (Changhsingian Stage) to

Acknowledgements

Research was supported by “973 Program” (2011CB808800), National Natural Science Foundation of China (40830212, 40921062, 41172312), and “111 Project” (B08030). This paper is a contribution to the IGCP 572 “Permian-Triassic Ecosystems”. We thank Tom Algeo, Adam Woods and an anonymous reviewer for their comments on the manuscript.

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