A sulfate control on marine mid-depth euxinia on the early Cambrian (ca. 529–521 Ma) Yangtze platform, South China
Introduction
The biological innovations in Earth history are intimately associated with the evolution of the ocean redox chemistry (Anbar and Knoll, 2002, Knoll, 1992). Lines of evidence show that the rise of the atmospheric oxygen level may have facilitated the oxygenation of the deep oceans in the late Neoproterozoic and ultimately the radiation of eukaryotes culminating in the emergence of early animals (Canfield et al., 2007, Fike et al., 2006, McFadden et al., 2008, Sahoo et al., 2012). The significant bio-radiation, known as the ‘Cambrian Explosion’, occurred in stages of the early Cambrian (Brasier, 1992, Knoll and Carroll, 1999, Marshall, 2006, Zhu, 2010, Zhu et al., 2007a), implying further oxygenation of the deep ocean. However, oceanic anoxia and even euxinia (anoxic with free H2S) occurred in the early Cambrian, particularly in South China, as suggested by elemental (e.g., Kimura and Watanabe, 2001, Pi et al., 2013, Schröder and Grotzinger, 2007, Zhou and Jiang, 2009) and isotopic (e.g., Wille et al., 2008, Xu et al., 2012) evidence and iron chemistry (e.g., Canfield et al., 2008, Och et al., 2013, Wang et al., 2012b). The widespread occurrence of ocean euxinia poses apparent conflict with the great increase in abundance and diversity of metazoans during the early Cambrian.
Recent geochemical investigations showed that the Precambrian oceans may have been highly stratified, and that mid-depth euxinic waters may have dynamically developed along continental margins, sandwiched by oxic surface waters and ferruginous deep waters (e.g., Li et al., 2010, Li et al., 2012, Poulton et al., 2010, Reinhard et al., 2009; reviewed in Lyons et al., 2012, Poulton and Canfield, 2011). In particular, a spatiotemporal geochemical investigation of the Ediacaran Doushantuo Formation showed that this highly stratified ocean redox structure existed in the late Neoproterozoic of South China and may have significant implications on the Ediacaran fossil records (Li et al., 2010). The emerging picture for the Precambrian – in particular for the late Neoproterozoic – ocean chemistry indicates that the redox conditions could be highly heterogeneous in early Earth's oceans, and such ocean redox heterogeneity may continue toward the early Cambrian (Jin et al., 2014). However, the details on the early Cambrian ocean redox structure remain ambiguous.
Early Cambrian black shale-rich strata are widespread across the shelf-to-basin transects of the Yangtze platform in South China. Previous geochemical studies on the basal Cambrian organic-rich shales suggested widespread anoxic/euxinic conditions in the basin (e.g., Och et al., 2013, Pi et al., 2013, Wang et al., 2012b, Wille et al., 2008). In order to explore the early Cambrian ocean redox structure and its possible causes, we carried out a high-resolution Fe–Mo–S–C chemostratigraphic investigation on the black shale-rich strata from a shelf margin section in Yangjiaping, northwestern Hunan province, near the slope environment of the Yangtze platform. This section is critical for identifying the marginal euxinic wedge, if it existed, in a continental shelf-slope transition. In combination with previously published Fe–S–C–Mo data in other three water-depth-different sections, this paper aims to explore spatiotemporal variability of marine redox chemistry on the early Cambrian Yangtze platform, South China and its geochemical controlling mechanisms in order to better understand the environmental backdrop for the evolution of early animals in the early Cambrian.
Section snippets
Lithostratigraphy and paleogeography
The Yangtze platform developed as a rift margin along the southeastern side of the Yangtze Block during the breakup of the Rodinia supercontinent at ∼820 Ma (Wang and Li, 2003) and subsequently evolved into a passive continental margin when the rifting ceased in the late Cryogenian (Jiang et al., 2003). The Neoproterozoic to Cambrian sediments were widely deposited on the platform along a paleo-water depth gradient from the northwest to southeast (Wang and Li, 2003, Zhu et al., 2007b).
The
Sampling and methods
A total of 45 samples were collected from the freshest exposures of the Yangjiaping section in March 2007. The sampling interval spans the black shale sequence (∼24 m) and overlying silty shales (∼56 m) until a ∼30 m covered interval which was overlain by the trilobite-bearing horizon reported by Yin et al. (1999) (Fig. 1C). All samples were carefully crushed into powders for analyses of TOC, Fe species, isotopic composition of pyrite (δ34Spy), and elemental concentrations such as Fe and Mo.
Results
The key geochemical data collected in this study from the Yangjiaping section (summarized below) and those compiled from literatures for the Shatan, Songtao and Longbizui sections are listed in Table S1 and plotted stratigraphically in Fig. 2, Fig. 3. The TOC contents are high (average 10.7%) and variable (from 7.2% to 15.6%) in the black shales, but low (average 1.3%) and stable (from 0.7% to 2.4%) in the overlying silty shales. The δ34Spy values stratigraphically increase from 1.7‰ to 11.5‰
Marine redox conditions in shelf margin area
Ocean redox conditions can be better evaluated by proxies of Fe speciation (Li et al., 2010, Lyons and Severmann, 2006, Planavsky et al., 2011, Poulton and Canfield, 2005, Raiswell and Canfield, 1998) when aided by Mo abundance data (Scott and Lyons, 2012, Scott et al., 2008). In modern and ancient sediments deposited beneath anoxic waters, FeHR/FeT usually exceeds 0.38 (Poulton and Raiswell, 2002, Raiswell and Canfield, 1998, Raiswell et al., 2001). However, thermal alteration on ancient
Summary and conclusions
In order to explore the spatiotemporal redox conditions on the early Cambrian Yangtze platform, South China, we carried out a preliminary ocean redox chemistry analysis based on the newly collected Fe–Mo–S–C systematic data from the Stage 2 black shale-rich strata in the shelf margin Yangjiaping section as well as three previously reported Shatan (inner-shelf), Songtao (slope) and Longbizui (basinal). The main findings can be summarized as below:
- (1)
By correlation roughly within a two-stage
Acknowledgements
We thank S. Bates, B. Gill, W. Gilhooly, C. Reinhard, N. Planavsky, J. Owens, Q.R. Zhang, H. Li, X.D. Jin for laboratory and field assistances. We are also grateful to Ganqing Jiang, Timothy Lyons, Gordon Love, Tais Dahl and two anonymous reviewers, and Editor Guochun Zhao for their valuable comments or discussions. This research was supported by the Chinese 973 Program (2013CB955704), the National Natural Science Foundation of China (41172030) to C.L. and (40603021 and 41072145) to L.F.
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C.L. and L.F. equally contributed to this work.