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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Earth and Planetary Science Letters 317-318 (2012) 96–110 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl The origin of decoupled carbonate and organic carbon isotope signatures in the early Cambrian (ca. 542–520 Ma) Yangtze platform Ganqing Jiang a,⁎, Xinqiang Wang b, Xiaoying Shi b, Shuhai Xiao c, Shihong Zhang b, Jin Dong d a Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA b State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing, 100083, China c Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA d Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China article info abstract Article history: The early Cambrian (ca. 542–520 Ma) strata in South China record two prominent negative carbonate carbon iso- Received 14 April 2011 13 tope (δ Ccarb)excursionsofearlyNemakit–Daldynian (N–D) and early Tommotian ages. Across each of these Received in revised form 12 September 2011 13 13 excursions, carbonate and organic carbon isotopes (δ Ccarb and δ Corg) are strongly decoupled. Regional corre- Accepted 18 November 2011 13 lation across a shelf-to-basin transect shows lateral heterogeneity of δ Corg during the early-middle N–Dbut Available online xxxx 13 more homogenized δ Corg values across the basin during the late N–D and Tommotian. The temporal and lateral δ13 δ13 δ13 – δ13 Editor: P. DeMenocal variations in Corg suggest that decoupled Ccarb and Corg across the N D Ccarb excursion were possibly caused by diagenetic alteration of organic matter and/or amplification of detrital organic carbon isotope signa- δ13 δ13 – Keywords: ture in low-TOC carbonates. In contrast, decoupled Ccarb and Corg of the upper N D and Tommotian were Ediacaran likely resulted from chemoautotrophic–methanotrophic biomass contribution to TOC in organic-rich black 13 13 early Cambrian shale and carbonates. The decoupled δ Ccarb–δ Corg pattern from the lower N–D strata (ca. 542 Ma) shows organic carbon isotopes striking similarities with those from the basal (ca. 635 Ma) and upper (ca. 551 Ma) Doushantuo Formation. In ocean anoxia 13 13 all three cases, decoupled δ Ccarb–δ Corg are seen in organic-poor carbonates (TOC≤ 0.1‰)andcoupled Yangtze platform 13 13 13 δ Ccarb–δ Corg occur in organic-rich black shale and carbonates at the end of the negative δ Ccarb excur- South China 13 13 sion. These similarities suggest that the shift from decoupled to coupled δ Ccarb–δ Corg has no causal link with the terminal oxidation of a large oceanic DOC reservoir. Given the pervasive anoxia/euxinia in Ediacaran– early Cambrian oceans, local DOC-rich environments may have been common, but a large oceanic DOC reservoir capable of buffering the δ13C of marine organic matter requires independent evidence. © 2011 Elsevier B.V. All rights reserved. 1. Introduction for millions of years from the onset of the Cryogenian glaciations (ca. 720 Ma; Swanson-Hysell et al., 2010) to the latest Ediacaran Period 13 Decoupled carbonate and organic carbon isotopes (δ Ccarb and (b560 Ma; Fike et al., 2006; McFadden et al., 2008; Rothman et 13 δ Corg) from some late Neoproterozoic successions have been inter- al., 2003). These uncertainties question the casual link between 13 13 preted as evidence for the existence of a large oceanic dissolved organic δ Ccarb–δ Corg decoupling and an inferred large oceanic DOC reser- 13 13 carbon (DOC) reservoir capable of buffering carbon isotopes of organic voir, and require tests on the validity of taking decoupled δ Ccarb–δ Corg matter (Fike et al., 2006; McFadden et al., 2008; Rothman et al., 2003; from single sections as globally consistent isotope signature. Swanson-Hysell et al., 2010). However, existing isotope record from Similar to the late Neoproterozoic successions, the early Cambrian 13 roughly time-equivalent Ediacaran sections in Oman (Fike et al., (ca. 540–520 Ma) strata record large δ Ccarb excursions (e.g., Brasier 2006), South China (McFadden et al., 2008), northwest Canada et al., 1990, 1994, 1996; Ishikawa et al., 2008; Kouchinsky et al., 2007; 13 (Kaufman et al., 1997), and Australia (Calver, 2000)showlargevaria- Maloof et al., 2005, 2010a, 2010b), but existing δ Corg data show a 13 13 tions in absolute δ Corg values from −23‰ to −37‰, which is inconsis- rather complex picture. Earlier studies reported a negative δ Corg ex- 13 13 tent with an isotopically homogenized ocean. Coupled δ Ccarb–δ Corg cursion at the Precambrian–Cambrian transition (Banerjee et al., data from the entire Cryogenian–Ediacaran interval in northwest Canada 1997; Kimura and Watanabe, 2001; Knoll et al., 1995; Shen and 13 13 (Kaufman et al., 1997) and at the Cryogenian–Ediacaran transition in Schidlowski, 2000) and a weak co-varying δ Ccarb–δ Corg trend of South China (Jiang et al., 2010) also contradict with the isotopic buffering Tommotian age (Goldberg et al., 2007; Guo et al., 2007a). Most of from a long-lasting DOC reservoir that has been inferred to have lasted these data, however, were obtained from organic-rich shales with 13 13 only a few paired δ Ccarb–δ Corg analyses. More recent studies of the carbonate-rich succession in Morocco showed that decoupled δ13 δ13 ⁎ Corresponding author. Tel.: +1 702 895 2708; fax: +1 702 895 4064. Ccarb and Corg are common in early Cambrian successions 13 E-mail address: [email protected] (G. Jiang). (Maloof et al., 2010b), but detailed δ Corg profiles across the 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.11.018 Author's personal copy G. Jiang et al. / Earth and Planetary Science Letters 317-318 (2012) 96–110 97 13 individual δ Ccarb excursions and their spatial variability are not the base of the Yanjiahe Formation. The presence of small shelly fossils yet available. To better understand the temporal and spatial vari- Anabarites trisulcantus and Protohertzina unguliformis of Assemblage 13 13 ability of δ Ccarb–δ Corg patterns, in this study we present paired Zone 1 (SS1; Qian et al., 2001; Steiner et al., 2007) in the lower Yanjiahe 13 13 δ Ccarb–δ Corg data of the early Cambrian (ca. 542–520 Ma) strata FormationsuggestsanageofNemakit–Daldynian or lower-middle from the Yangtze platform in South China and compare the early Cam- Meishucunian. The uppermost Yanjiahe Formation and lower Shuijingtuo 13 13 brian δ Ccarb–δ Corg patterns with those from the Ediacaran strata in Formation contain typical small shelly fossil Aldanella yanjiahensis of As- the same region. semblage Zone 3 (SS3; Steiner et al., 2007), which is of Tommotian or upper Meishucunian age. The first appearance of trilobites in the lower 2. Paleogeography and stratigraphy to middle Shuijingtuo Formation markedthebaseoftheAtdabanianor Qiongzhusian (Qian et al., 2001; Steiner et al., 2007; Zhu et al., 2003). 2.1. Paleogeography Other microfossils including organic-walled microfossils, macroscopic carbonaceous compressions, and sponge spicules (e.g., Guo et al., 2008, The Ediacaran–Cambrian Yangtze platform developed over a late 2010; Zhao et al., 1988) have also been reported, but they have limited Neoproterozoic rifted continental margin that is inferred to have initiat- biostratigraphic significance. ed at the southeastern side of the Yangtze block (Fig. 1A) at ca. 830–820 Ma (Wang and Li, 2003; Zhao et al., 2011). A passive continen- 2.3. Regional stratigraphic correlation tal margin setting has been inferred for the Ediacaran–Cambrian strata on the basis of stratigraphic pattern, thickness, and the lack of signifi- Regional correlation of the early Cambrian (ca. 542–520 Ma) strata cant igneous activity (Jiang et al., 2003, 2011; Wang and Li, 2003). in South China is mainly based on small shelly fossils and distinctive During the latest Ediacaran and earliest Cambrian (lower-middle marker beds including phosphorite-rich and highly metalliferous Meishucunian or Nemakit–Daldynian), the Yangtze platform was black shales (Fig. 1C). Small Shelly Fossil Assemblage Zone 1 (SS1) characterized by predominately carbonate deposits in the northwest represented by A. trisulcantus and P. unguliformis and Assemblage and mudstone/shale deposits in the southeast (Fig. 1B; Steiner et al., Zone 3 (SS3) represented by Watsonella crosbyi are widespread in 2007; Wang, 1985; Zhu et al., 2007). A ‘transitional zone’ character- shelffaciesoftheYangtzeplatform(Steiner et al., 2007; Zhu et al., ized by interbedded shale and carbonate deposits is located in north- 2003). Traditionally their appearance was taken as the base of the N– eastern Guizhou, northern Hunan and Hubei provinces (Fig. 1B), D and Tommotian stages, respectively, but integrated biostratigraphy representing outer shelf environments of the Yangtze platform. At and carbon isotope chemostratigraphy suggested that the base of the the end of the middle Meishucunian (after small shelly fossil assem- N–D stage (i.e., the Precambrian–Cambrian boundary) would be lower blage 3), the shelf regions of the Yangtze platform was subaerially ex- than SS1, most likely coincident with the regional stratigraphic discon- posed (Steiner et al., 2007; Zhu et al., 2003), resulting in partial tinuity at the top of the Dengying Formation (Fig.