Isotopic Composition of Organic and Inorganic Carbon from the Mesoproterozoic
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Precambrian Research 224 (2013) 169–183 Contents lists available at SciVerse ScienceDirect Precambrian Research journa l homepage: 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 a,b a,∗ b a a a a Hua Guo , Yuansheng Du , Linda C. Kah , Junhua Huang , Chaoyong Hu , Hu Huang , Wenchao Yu 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 a b s t r a c t Article history: Analyses of marine carbon isotope profiles have provided much of our current understanding of the Received 4 July 2012 evolution of Earth surface environments, particularly in the latter portion of the Proterozoic Eon. Earlier Received in revised form Mesoproterozoic successions, however, have received comparatively little attention due to the relatively 25 September 2012 subdued nature of carbon isotope variation. In this study, we present high-resolution isotopic profiles Accepted 25 September 2012 from three sections in the Yanshan Basin, North China craton that, combined, comprise the entirety of the Available online xxx 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- Keywords: Mesoproterozoic 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 North China Carbon isotopes previous observations from the early Mesoproterozoic. Data furthermore record an increase in isotopic Organic carbon variation through the succession that is interpreted to reflect a long-term decrease in pCO2 and, conse- Ocean oxygenation 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 increased organic carbon burial (Des Marais et al., 1992; Hayes and Waldbauer, 2006; Holland, 2006) that, in turn, may have resulted in Combined isotopic records of organic and inorganic carbon global-scale ocean oxygenation. By contrast, the Mesoproterozoic can provide critical insight into the behavior of the global carbon (1.6–1.0 Ga) has received substantially less attention. Relatively cycle and have been used extensively to investigate the continu- subdued secular variation recorded in carbon-isotope composition ally evolving relationship between biology and ocean-atmosphere of marine carbonate (Kah et al., 1999, 2012; Kah and Bartley, 2011) chemistry (Bartley and Kah, 2004). Paired isotopes of carbon and has been assumed to reflect a combination of geologic and ecologic organic carbon been used, for instance, to infer some of Earth’s ear- stasis (Buick et al., 1995; Brasier and Lindsay, 1998). A growing liest biological metabolisms (Schidlowski, 2001; Ueno et al., 2001, body of evidence, however, suggests that the Mesoproterozoic Era 2004) and, more recently, to infer the global marine redox state in may represent a critical interval in terms of evolution of the Earth’s both the Early and Late Proterozoic (Rothman et al., 2003; Kump ocean-atmosphere system (Kah and Bartley, 2011, and references et al., 2011; Johnston et al., 2012; Och and Shields-Zhou, 2012). therein). For instance, a relatively abrupt increase in both the iso- Previous efforts to constrain the behavior of the oceanic car- topic composition and isotopic variability of marine carbonate (Kah bon cycle focused primarily at the two ends of the Proterozoic et al., 1999, 2012; Frank et al., 2003; Bartley et al., 2007), which Eon. Strongly positive carbon isotope signatures preserved in may reflect a global increase in oxygenation, co-occurs with both marine carbonate rocks (Karhu and Holland, 1996; Halverson et al., increased marine sulfate concentrations and the first appearance 2005; Melezhik et al., 2005) are interpreted as resulting from 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 Corresponding author. Tel.: +86 18986127299. E-mail addresses: duyuansheng126@126.com (Y. Du), lckah@utk.edu (L.C. Kah). 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.2012.09.023 170 H. Guo et al. / Precambrian Research 224 (2013) 169–183 Fig. 1. Simplified paleogeographic maps during this time period at the North China Platform, which is modified from Wang et al. (1995) and Chu et al. (2007). The investigated sections are marked as numbers 1–3. mid-Mesoproterozoic, reached important geochemical and biologi- 2.2. Sedimentary strata of the Yanshan Basin cal thresholds. Yet despite evidence for increased oxygenation (Kah and Bartley, 2011, and references therein), marine environments Sedimentary strata of the Yanshan Basin are represented appear to have remained relatively oxygen-deficient (Kah et al., by unmetamorphosed and relatively undeformed successions of 2004, 2012), with broad regions of the seafloor overlain by anoxic, the Changcheng (1800–1600 Ma), Jixian (1600–1400 Ma), Xis- either sulfidic or ferruginous, waters (Brocks et al., 2005; Planavsky han (1400–1200 Ma), and Qingbaikou groups (1000–800 Ma) et al., 2011; Blumenberg et al., 2012). (Qiao et al., 2007; Sun et al., 2012; see Chen et al., 1980 for In this study, we concentrate on the early Mesoproteorozic antecedent stratigraphic division). The Changcheng system consists (Calymmian period; 1600–1400 Ma). We present high-resolution of four formations (Changzhougou, Chuanlingguo, Tuanshanzi, and carbon isotope data for carbonate and organic matter from the Jix- Dahongyu) that unconformably overlie Archean and Paleoprotero- ian Group, Yanshan Basin, North China. The dataset presented here zoic basement and represent initial deposition with Yanshan rift currently represents the highest resolution chemostratigraphic basin. Strata consist of >1000 m of alluvial to fluvial conglomerate dataset from the early Mesoproterozoic and, as such, permits and sandstone that fine and deepen upward into a >2000 m-thick unprecedented exploration of the isotopic patterns and origin of succession of marine sandstone, silt, shale, and carbonate (Li et al., isotopic variation in the early Mesoproteorozic carbon cycle. 2003a,b). Uppermost units of the Changcheng Group notably con- tain high-potassium volcanic interbeds that are associated with continued, regional extension (Lu et al., 2002, 2008). 2. Geological setting and age The overlying Jixian Group consists of five formations (Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang, and 2.1. Regional geological setting Tieling), dominated by marine carbonate deposition, that represent marine transgression and development of a stable cratonic plat- The North China craton, which refers to the Chinese part of the form across the Yanshan Basin (Ying et al., 2011). Jixian Group strata Sino-Korea Platform, is a triangular-shaped region with an area of unconformably overlie mixed siliciclastic and carbonate strata of 2 approximately 1 500 000 km , which covers most of North China the Changcheng Group, although the boundary appears, in places, (Fig. 1, Zhao et al., 2001). The North China craton consists of variably to be conformable (Chen et al., 1980; Song, 1991; Lu et al., 1996; exposed gneiss, granite, and amphibolite, as well as shist, marble Zhao, 1997). The majority of Jixian strata consist of finely lami- and iron formation (Zhao, 2001; Kusky and Li, 2003; Zhao et al., nated to stromatolitic dolomite, argillaceous dolomite, and chert 2005) that represent a long history of accretion beginning in the deposited under a range of intertidal to supratidal environments. Archean and ending in the late Paleoproterozoic (Zhao et al., 2005; The Xiamaling Formation, composed mainly of organic-rich Li et al., 2011a,b, 2012; Nutman et al., 2011; Peng et al., 2011; Wan shale, is the only formation in the newly established Xishan et al., 2011; Liu et al., 2012a,b; Lü et al., 2012; Peng et al., 2012). Group (Qiao et al., 2007). The Xiamaling Formation unconformably Proterozoic carbonate strata are well preserved across the North underlies the two formations (Changlongshan and Jingeryu) that China craton, where they unconformably overlie basement litholo- comprise the Qingbaikou Group and mark the return of domi- gies. The most extensive accumulations occur within the Yanshan nantly siliciclastic deposition prior to regional