Ediacaran δ13C chemostratigraphy of South China
Introduction
It has been nearly twenty years since the early attempts in δ13C chemostratigraphic correlation of Neoproterozoic carbonate successions (Knoll et al., 1986, Magaritz et al., 1986, Tucker, 1986). Significant progress has been made in the past two decades to sharpen the δ13C chemostratigraphic tool. Previous compilations of Neoproterozoic δ13C curves (Kaufman and Knoll, 1995, Kaufman et al., 1997, Shields, 1999, Walter et al., 2000, Halverson et al., 2005) show major shifts, at least partly reflecting extraordinary climate change in the Neoproterozoic (Hoffman et al., 1998). However, a global δ13C curve for the Neoproterozoic is far from consensus. Published Neoproterozoic δ13C curves show major inconsistencies, many of which stem from the lack of reliable and independent time markers (e.g., index fossils and radiometric dates) (Melezhik et al., 2001). Yet, Phanerozoic wisdom does suggest that seawater geochemical signature can be preserved and used in chemostratigraphic correlation, if diagenetic alteration and local variation are carefully assessed (Veizer et al., 1999). Therefore, there is a compelling need to reexamine Neoproterozoic δ13C data of each sedimentary basin in light of new age constraints, in order to improve the usefulness of Neoproterozoic δ13C chemostratigraphy.
In this paper, we present new and review published δ13C data of the Ediacaran System, equivalent to the redefined Sinian System (China Commission on Stratigraphy, 2001), in South China. The choice of the Ediacaran System in South China as the subject of our study is guided by several factors. First, the ratification of the base of the Nuccaleena Formation at Enorama Creek, South Australia, as the GSSP of the Ediacaran System (Knoll et al., 2004) makes it our future task to subdivide and correlate the Ediacaran System using bio-, chemo-, and magnetostratigraphic tools. Second, the Ediacaran System in South China is bounded by distinct lithologies of the underlying Nantuo diamictite and overlying phosphorites and cherts that contain basal Cambrian shelly fossils and acritarchs (Yao et al., 2005). The lithologies of the Ediacaran Doushantuo and Dengying formations can also be easily distinguished. Thus, a first-order lithostratigraphic control is in place. Third, the Ediacaran System in South China contains some of the best preserved fossil assemblages, including abundant and diverse acritarchs, algae, lichen-like fossils, micrometazoans, Ediacaran macrofossils, and Cloudina-like tubular fossils (Sun, 1986, Bengtson and Zhao, 1992, Zhang et al., 1998, Xiao et al., 1998, Yuan et al., 1999, Xiao et al., 2000, Xiao et al., 2002, Hua et al., 2003, Xiao et al., 2004b, Xiao, 2004b, Hua et al., 2005, Yuan et al., 2005, Xiao et al., 2005, Hagadorn et al., 2006). Thus, a biostratigraphic framework is available. In addition, recent progress in basin development (Wang and Li, 2003), sequence stratigraphy (Jiang et al., 2003b), and geochronometry (Barfod et al., 2002, Chen et al., 2004, Zhou et al., 2004, Condon et al., 2005, Yin et al., 2005, Chu et al., 2005, Zhang et al., 2005) of the Ediacaran System in South China also provides independent tools to evaluate δ13C chemostratigraphic signatures. Finally, there have been numerous studies of Ediacaran δ13C chemostratigraphy in South China (Lambert et al., 1987, Brasier et al., 1990, Liu et al., 1992, Zhou, 1997, Zhou et al., 1997, Yang et al., 1999, Li et al., 1999, Shen and Schidlowski, 2000, Zhou et al., 2001b, Huang and Buick, 2002, Shen, 2002, Wang et al., 2002a, Wang et al., 2002b, Chu et al., 2003, Zhang et al., 2003, Jiang et al., 2003a, Macouin et al., 2004, Feng et al., 2004, Condon et al., 2005, Zhang et al., 2005), making it possible to evaluate spatial and environmental variations. The purpose of this paper is to examine δ13C curves of multiple Ediacaran sections in South China using independent litho-, bio-, and (wherever possible) sequence stratigraphic frameworks. We then construct a composite δ13C curve to determine whether there are any discernable chemostratigraphic patterns beyond diagenetic and spatial variations. A composite curve helps us to visualize the big picture of δ13C evolution in South China, because some sections are incomplete and many include intervals of lithologies that are inappropriate for δ13C analysis. The ultimate goal of this endeavor is to assess the possibility of interbasinal correlation of Ediacaran successions using a combination of bio-, physico-, and chemostratigraphic tools, in order to subdivide and correlate the newly ratified Ediacaran System.
Section snippets
Geological setting
The Ediacaran System in South China was deposited in a passive continental margin on the Yangtze Block. The continental margin in early Ediacaran can be divided into three basins (Cao et al., 1989, Wang and Li, 2003). These are (1) the Sichuan–Guizhou–Yunnan (SGY) shallow-water platform in the west; (2) the Hunan–Guangxi (HG) basin in the middle; and (3) the Southern Anhui–Northern Zhejiang (SANZ) basin in the east (Fig.1). Several smaller and isolated shallow-water platforms, such as Western
Methods
We collected Doushantuo cap carbonates at several sections in South China (Zhou et al., 2004), as well as the complete Ediacaran System at Chaoyang (Fig. 4). Diagenetic alteration was evaluated based on petrographic observation as well as δ13C–δ18O crossplot (Fig. 4). Sample preparation and analysis followed procedures described by Kaufman and Knoll (1995). CO2 was extracted using standard offline techniques, with samples reacting with concentrated H3PO4 at 50 °C for 12 h. Carbon and oxygen
Caveats
The reliability of the composite δ13C curve critically depends on the synchroneity of selected stratigraphic markers (deglaciation, sequence boundaries, lithostratigraphy, and fossils). An ultimate test of their synchroneity requires precise radiometric dating from all sections. However, at the time scale of our interest, we believe that these anchor points provide reasonable time markers to align Ediacaran δ13C profiles in South China. A more serious concern relates to missing stratigraphy in
Ediacaran δ13C profile at Chaoyang (Fig. 4)
The features of the Chaoyang δ13C profile broadly agree with those from the Yangtze Gorges area (Fig. 2). Chemostratigraphic features shared between the Chaoyang and Yangtze Gorges profiles include 1) a negative δ13C excursion at the basal Doushantuo cap carbonate; 2) a positive excursion (up to 5.1‰ at Chaoyang) in the lower Doushantuo Formation; 3) negative values (nadir at − 4.6‰ at Chaoyang) near the uppermost Doushantuo Formation; 4) a pronounced positive excursion in the lower Dengying
Discussion
It is tempting to carry out a similar exercise beyond South China to construct a global δ13C curve for the Ediacaran System. To do so, we need to have independent chronostratigraphic anchor points to avoid circularity. Unfortunately, unlike in South China, Ediacaran successions elsewhere in the world typically do not have a combination of radiometric dates, acanthomorphic acritarchs, Ediacaran macrofossils, multicellular algae, micrometazoans, carbonaceous algae, trace fossils, and continual
Conclusions
Ediacaran successions in shallow-water platform facies of South China appear to show consistent secular variations in carbonate δ13C. A composite δ13C curve, constructed using independent chronostratigraphic calibrators, clearly shows that secular δ13C trends stand out among spatial/environmental variations and diagenetic overprints. Eight secular features can be identified in the composite δ13C curve of the Ediacaran System in South China. The robustness of these isotopic features indicates
Acknowledgments
We thank W. Sun, Y. Xue, and X. Yuan for field assistance and intellectual guidance. X. Chen analyzed carbon and oxygen isotope ratios. G. Jiang, A. J. Kaufman, K. A. McFadden, and an anonymous reviewer provided useful comments. This work was supported by the Chinese Academy of Sciences (KZCX3-SW-141), the National Natural Science Foundation of China (40372006), the Ministry of Science and Technology of China (2006CB806401 and 2003CB716805), and the US National Science Foundation (EAR-0354807
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