Multiple sulfur isotope records at the end-Guadalupian (Permian) at Chaotian, China: Implications for a role of bioturbation in the Phanerozoic sulfur cycle

Highlights

• Quadruple sulfur isotope stratigraphy at the end-Guadalupian at Chaotian, China.

• Sulfur mixing in the sediments promoted by bioturbation.

• New role of bioturbation in the sulfur cycle in the Phanerozoic oceans.

Abstract

A recent study on quadruple sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) of sedimentary pyrite suggested that the end-Guadalupian extinction was caused by shoaling of the sulfidic deep-water. This scenario is based on the assumption that sulfur isotopic compositions of pyrite from hosting sediments were controlled by benthos activities, thus by the redox conditions of the sedimentary environments. Nonetheless, the relationship between the sulfur isotope records and redox conditions, reconstructed from litho- and bio-facies, are poorly known. In order to examine the effect of bioturbation in sediments, quadruple sulfur isotopic compositions of sedimentary pyrite from the end-Guadalupian succession in Chaotian, South China, were analyzed. Black mudstones of deep-water facies immediately below the extinction horizon have consistently high Δ³³S values of ca. +0.079‰, clearly suggesting a sulfate reduction in the anoxic water column. Our new data are consistent with the emergence of a sulfidic deep-water mass prior to the end-Guadalupian extinction; the upwelling of the toxic deep-water may have contributed to the extinction. In contrast, shallow-marine bioclastic limestones with burrows deposited under oxic conditions have negative Δ³³S values. This anomalous isotopic signal indicates the mixing of two distinct types of pyrite; one generated during the sulfate reduction in an open system and the other in a closed system. We interpret that bioturbation supplied sulfate in the sediments and promoted sulfate reduction and in-situ sulfide precipitation within the sediments. The negative Δ³³S values of oxic sediments in Chaotian are inconsistent with the previous model and demonstrate that the sedimentary sulfur cycle associated with bioturbation was more complicated than previously thought. Our study also implies that, more generally, the role of bioturbation in increasing seawater sulfate concentration in the Phanerozoic may have been overestimated in the previous studies, because bioturbation may have enhanced sulfide burial or sulfur output from the oceans.

Introduction

The end-Paleozoic mass extinction was one of the largest biodiversity crises in the Phanerozoic (e.g., Erwin, 2006, Alroy, 2010) and had two phases: the biodiversity decline at the end-Guadalupian (ca. 260 Ma) and the abrupt extinction at the latest Permian (ca. 252 Ma) (Jin et al., 1994, Stanley and Yang, 1994). Around the Guadalupian-Lopingian (Late Permian) boundary (G-LB), several global-scale geologic phenomena occurred, such as the eruption of the Emeishan flood basalts in South China (Chung and Jahn, 1995, Zhou et al., 2002), the onset of prolonged deep-sea anoxia (Isozaki, 1997), a substantial global sea-level fall (Jin et al., 1994, Haq and Schutter, 2008, Kofukuda et al., 2014), the ‘Kamura’ cooling event (Isozaki et al., 2007, Isozaki et al., 2011), and authigenic carbonate precipitation (Grotzinger and Knoll, 1995, Saitoh et al., 2015). Many researchers have considered the Emeishan volcanism as the leading candidate for the cause of the end-Guadalupian extinction (e.g., Wignall et al., 2009). However, the causal link between the global environmental changes and the extinction at the end-Guadalupian remains a topic of discussion (e.g., Clapham et al., 2009, Bond et al., 2010, Jost et al., 2014). Recently, several studies reported anoxic/sulfidic conditions in the oceans along the continental margins on a global scale at the end-Guadalupian (Schoepfer et al., 2012, Schoepfer et al., 2013, Saitoh et al., 2013a, Saitoh et al., 2013b, Yan et al., 2013, Zhang et al., 2015, Shi et al., 2016). The upwelling of the anoxic/sulfidic deep-waters may have contributed to the end-Guadalupian biodiversity decline (Saitoh et al., 2014a).

Measurements of all four stable sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) in geologic records are useful for understanding the evolutionary history of the ocean/atmosphere system (e.g., Farquhar et al., 2000, Johnston, 2011). Coupled with photochemical experiments, this method has shed light on the characteristic sulfur cycle in the Archean atmosphere (e.g., Ono et al., 2003, Ueno et al., 2008, Ueno et al., 2009, Ueno et al., 2015). Moreover, the quadruple sulfur isotopic analysis of geologic records and products of microbial incubation experiments is useful for detecting the biogeochemical processes in the oceans from the Proterozoic to the present (e.g., Farquhar et al., 2003, Johnston et al., 2005, Aoyama et al., 2014). Shen et al. (2011) first applied the analysis of quadruple sulfur isotopes to the end-Paleozoic mass extinction event. They analyzed carbonate rocks across the Permian-Triassic boundary (P-TB) at the Meishan section, the Global Stratotype Section and Point (GSSP) for the P-TB, in South China and suggested that the episodic shoaling of anoxic deep-water caused the latest Permian extinction. Recently, Zhang et al. (2015) analyzed the quadruple sulfur isotopic compositions of carbonates across the G-LB at two sections in South China, including the Penglaitan section in Guangxi, the GSSP for the G-LB, and the EF section in west Texas, USA. Based on the results, Zhang et al. (2015) used the upwelling scenario proposed by Shen et al. (2011) to the end-Guadalupian case and suggested that the shoaling of sulfidic deep-waters contributed to the end-Guadalupian extinction.

The common and critical isotopic signal in Shen et al., 2011, Zhang et al., 2015 are negative Δ³³S (={(³³S/³²S)_sample/(³³S/³²S)_reference − [(³⁴S/³²S)_sample/(³⁴S/³²S)_reference]^0.515}) values of pyrites in the analyzed rocks. This anomalous evidence indicates the mixing of ³⁴S-enriched and ³⁴S-depleted sulfur (Ono et al., 2006). Both Shen et al., 2011, Zhang et al., 2015 interpreted that the negative Δ³³S values of pyrites, and thus the mixing of sulfur from two different sources, recorded the shutdown of bioturbation in the sediments caused by shoaling of toxic (anoxic/sulfidic) deep-waters. According to their model, negative Δ³³S values would only be recognized in anoxic sediments with no bioturbation. However, they did not examine the correlation between the quadruple sulfur isotope records and the redox conditions of the sedimentary environments, which were reconstructed by litho- and bio-facies characteristics, including ichnofabrics, of the analyzed rocks. Thus, the shoaling scenario at the end-Guadalupian by Zhang et al. (2015) has not been fully validated.

The roles of bioturbation in the oceanic geochemical cycles in the Phanerozoic were recently emphasized (e.g., Boyle et al., 2014; Tarhan et al., 2015). In particular, Canfield and Farquhar (2009) pointed out a role of bioturbation in the oceanic sulfur cycle in the past. According to their model, bioturbation promotes sulfur recycling from the sediments to the oceans because the benthic fauna “dig out” sediments and supply oxygen deep into the sediments to promote the oxidation of once buried sulfide. Hence, bioturbation likely contributed to the increase in seawater sulfate concentration throughout the Phanerozoic, but its role in the oceanic sulfur cycle has not been evaluated in detail.

The present study analyzed quadruple sulfur isotope records of marine carbonates of shelf/slope facies across the G-LB at Chaotian in northern Sichuan, South China. This article discusses the sulfur cycle in the end-Guadalupian oceans at Chaotian, focusing on the redox conditions of the sedimentary environments and benthos activity, and examines the main role of bioturbation in the oceanic sulfur cycle in the Phanerozoic.