Ediacaran δ13C chemostratigraphy of South China

doi:10.1016/j.chemgeo.2006.06.021

Chemical Geology

Volume 237, Issues 1–2, 15 February 2007, Pages 89-108

https://doi.org/10.1016/j.chemgeo.2006.06.021 Get rights and content

Abstract

We report new δ¹³C chemostratigraphic data of the Ediacaran System in South China. Using litho- and biostratigraphic markers as independent calibrators, we combine new and previously published δ¹³C data to construct a composite δ¹³C profile for the Ediacaran System in South China. The composite curve shows, in chronostratigraphic order, 1) a negative δ¹³C excursion (EN1) in the Doushantuo cap carbonate that overlies the Ghaub-age Nantuo diamictite; 2) a pronounced positive δ¹³C excursion (EP1) in the lower Doushantuo Formation where Doushantuo-type acanthomorphic acritarchs, red algae, and animal embryos first appear; 3) a mild negative δ¹³C excursion (EN2) in the middle Doushantuo Formation; 4) a positive δ¹³C excursion (EP2) in the upper Doushantuo Formation where Doushantuo-type acanthomorphic acritarchs, micrometazoans, and stem group florideophyte red algae become abundant and diverse; 5) a pronounced negative δ¹³C excursion (EN3) near the uppermost Doushantuo Formation; 6) a strongly positive δ¹³C excursion (up to + 6.3‰ PDB; EP3) in the lower Dengying Formation; 7) a stable δ¹³C plateau (ca. + 2.5‰ PDB; EI) in the middle and upper Dengying Formation where abundant vendotaenid algae, macroscopic Ediacaran fossils, biomineralized animal tubes, and mostly horizontal animal traces occur; and 8) a − 10‰ negative δ¹³C excursion (EN4) immediately below the Ediacaran–Cambrian boundary. The general pattern of this composite δ¹³C curve is robust, despite a considerable amount of variation among the δ¹³C profiles of different sections. This composite δ¹³C curve provides a first-order chemostratigraphic framework for the subdivision and correlation of the Ediacaran System in 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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Ediacaran δ13C chemostratigraphy of South China

Abstract

Introduction

Section snippets

Geological setting

Methods

Caveats

Ediacaran δ13C profile at Chaoyang (Fig. 4)

Discussion

Conclusions

Acknowledgments

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Ediacaran δ¹³C chemostratigraphy of South China

Ediacaran δ¹³C profile at Chaoyang (Fig. 4)

δ13C_carb and Ce_anom excursions in the post-glacial Neoproterozoic and Early Cambrian interval in Guizhou, South China