Precambrian Research 224 (2013) 169–183 Contents lists available at SciVerse ScienceDirect Precambrian Research journa l h omepa g e: www.elsevier.com/locate/precamres Isotopic composition of organic and inorganic carbon from the Mesoproterozoic Jixian Group, North China: Implications for biological and oceanic evolution Hua Guo a,b , Yuansheng Du a,∗ , Linda C. Kah b , Junhua Huang a , Chaoyong Hu a , Hu Huang a , Wenchao Yu a a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan, Hubei 430074, China b Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States a r t i c l e i n f o Article history: Received 4 July 2012 Received in revised form 25 September 2012 Accepted 25 September 2012 Available online xxx Keywords: Mesoproterozoic North China Carbon isotopes Organic carbon Ocean oxygenation a b s t r a c t Analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of Earth surface environments, particularly in the latter portion of the Proterozoic Eon. Earlier Mesoproterozoic successions, however, have received comparatively little attention due to the relatively subdued nature of carbon isotope variation. In this study, we present high-resolution isotopic profiles from three sections in the Yanshan Basin, North China craton that, combined, comprise the entirety of the early Mesoproterozoic (1600–1400 Ma, Calymmian period) Jixian Group. High-resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con- strain both the pattern and origin of isotopic variation in the Mesoproterozoic. Marine carbonate rocks of the Jixian Group show generally muted isotopic variation with average values near 0‰, consistent with previous observations from the early Mesoproterozoic. Data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long-term decrease in pCO 2 and, conse- quently, in the isotopic buffering capacity of marine dissolved inorganic carbon (DIC). By contrast, the isotopic composition of marine organic matter suggests facies-dependent differences in carbon cycling. Organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom- position in shallow-water environments, and increased remineralization by anaerobic heterotrophs in deeper-water environments. Correlation between organic carbon composition and depositional environ- ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Combined isotopic records of organic and inorganic carbon can provide critical insight into the behavior of the global carbon cycle and have been used extensively to investigate the continu- ally evolving relationship between biology and ocean-atmosphere chemistry (Bartley and Kah, 2004). Paired isotopes of carbon and organic carbon been used, for instance, to infer some of Earth’s ear- liest biological metabolisms (Schidlowski, 2001; Ueno et al., 2001, 2004) and, more recently, to infer the global marine redox state in both the Early and Late Proterozoic (Rothman et al., 2003; Kump et al., 2011; Johnston et al., 2012; Och and Shields-Zhou, 2012). Previous efforts to constrain the behavior of the oceanic car- bon cycle focused primarily at the two ends of the Proterozoic Eon. Strongly positive carbon isotope signatures preserved in marine carbonate rocks (Karhu and Holland, 1996; Halverson et al., 2005; Melezhik et al., 2005) are interpreted as resulting from ∗ Corresponding author. Tel.: +86 18986127299. E-mail addresses: duyuansheng126@126.com (Y. Du), lckah@utk.edu (L.C. Kah). increased organic carbon burial (Des Marais et al., 1992; Hayes and Waldbauer, 2006; Holland, 2006) that, in turn, may have resulted in global-scale ocean oxygenation. By contrast, the Mesoproterozoic (1.6–1.0 Ga) has received substantially less attention. Relatively subdued secular variation recorded in carbon-isotope composition of marine carbonate (Kah et al., 1999, 2012; Kah and Bartley, 2011) has been assumed to reflect a combination of geologic and ecologic stasis (Buick et al., 1995; Brasier and Lindsay, 1998). A growing body of evidence, however, suggests that the Mesoproterozoic Era may represent a critical interval in terms of evolution of the Earth’s ocean-atmosphere system (Kah and Bartley, 2011, and references therein). For instance, a relatively abrupt increase in both the iso- topic composition and isotopic variability of marine carbonate (Kah et al., 1999, 2012; Frank et al., 2003; Bartley et al., 2007), which may reflect a global increase in oxygenation, co-occurs with both increased marine sulfate concentrations and the first appearance of widespread bedded marine gypsum (Whelan et al., 1990; Kah et al., 2001), as well as diversification within both prokaryotic and eukaryotic clades (Butterfield, 2000; Johnston et al., 2005; Knoll et al., 2006). Take together, these observations suggest that the availability of oxygen in Earth’s surface environments had, by the 0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2012.09.023