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Precambrian Research 224 (2013) 169– 183

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Precambrian Research

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sotopic composition of organic and inorganic carbon from the Mesoproterozoicixian Group, North China: Implications for biological and oceanic evolution

ua Guoa,b, Yuansheng Dua,∗, Linda C. Kahb, Junhua Huanga, Chaoyong Hua, Hu Huanga, Wenchao Yua

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan, Hubei 430074, ChinaDepartment of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States

r t i c l e i n f o

rticle history:eceived 4 July 2012eceived in revised form5 September 2012ccepted 25 September 2012vailable online xxx

eywords:esoproterozoicorth Chinaarbon isotopesrganic carboncean oxygenation

a b s t r a c t

Analyses of marine carbon isotope profiles have provided much of our current understanding of theevolution of Earth surface environments, particularly in the latter portion of the Proterozoic Eon. EarlierMesoproterozoic successions, however, have received comparatively little attention due to the relativelysubdued nature of carbon isotope variation. In this study, we present high-resolution isotopic profilesfrom three sections in the Yanshan Basin, North China craton that, combined, comprise the entirety of theearly Mesoproterozoic (1600–1400 Ma, Calymmian period) Jixian Group. High-resolution profiles of bothcarbonate 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 ofthe Jixian Group show generally muted isotopic variation with average values near 0‰, consistent withprevious observations from the early Mesoproterozoic. Data furthermore record an increase in isotopicvariation through the succession that is interpreted to reflect a long-term decrease in pCO2 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 indeeper-water environments. Correlation between organic carbon composition and depositional environ-ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under lowoxygen conditions and dynamically maintained stratification of marine waters.
. Introduction

Combined isotopic records of organic and inorganic carbonan provide critical insight into the behavior of the global carbonycle and have been used extensively to investigate the continu-lly evolving relationship between biology and ocean-atmospherehemistry (Bartley and Kah, 2004). Paired isotopes of carbon andrganic carbon been used, for instance, to infer some of Earth’s ear-iest biological metabolisms (Schidlowski, 2001; Ueno et al., 2001,004) and, more recently, to infer the global marine redox state inoth the Early and Late Proterozoic (Rothman et al., 2003; Kumpt al., 2011; Johnston et al., 2012; Och and Shields-Zhou, 2012).

Previous efforts to constrain the behavior of the oceanic car-on cycle focused primarily at the two ends of the Proterozoic
on. Strongly positive carbon isotope signatures preserved inarine carbonate rocks (Karhu and Holland, 1996; Halverson et al.,
005; Melezhik et al., 2005) are interpreted as resulting from

∗ Corresponding author. Tel.: +86 18986127299.E-mail addresses: [email protected] (Y. Du), [email protected] (L.C. Kah).

301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2012.09.023

© 2012 Elsevier B.V. All rights reserved.

increased organic carbon burial (Des Marais et al., 1992; Hayes andWaldbauer, 2006; Holland, 2006) that, in turn, may have resulted inglobal-scale ocean oxygenation. By contrast, the Mesoproterozoic(1.6–1.0 Ga) has received substantially less attention. Relativelysubdued secular variation recorded in carbon-isotope compositionof marine carbonate (Kah et al., 1999, 2012; Kah and Bartley, 2011)has been assumed to reflect a combination of geologic and ecologicstasis (Buick et al., 1995; Brasier and Lindsay, 1998). A growingbody of evidence, however, suggests that the Mesoproterozoic Eramay represent a critical interval in terms of evolution of the Earth’socean-atmosphere system (Kah and Bartley, 2011, and referencestherein). For instance, a relatively abrupt increase in both the iso-topic composition and isotopic variability of marine carbonate (Kahet al., 1999, 2012; Frank et al., 2003; Bartley et al., 2007), whichmay reflect a global increase in oxygenation, co-occurs with bothincreased marine sulfate concentrations and the first appearanceof widespread bedded marine gypsum (Whelan et al., 1990; Kah
et al., 2001), as well as diversification within both prokaryotic andeukaryotic clades (Butterfield, 2000; Johnston et al., 2005; Knollet al., 2006). Take together, these observations suggest that theavailability of oxygen in Earth’s surface environments had, by the
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170 H. Guo et al. / Precambrian Research 224 (2013) 169– 183

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ig. 1. Simplified paleogeographic maps during this time period at the North China Pections are marked as numbers 1–3.

id-Mesoproterozoic, reached important geochemical and biologi-al thresholds. Yet despite evidence for increased oxygenation (Kahnd Bartley, 2011, and references therein), marine environmentsppear to have remained relatively oxygen-deficient (Kah et al.,004, 2012), with broad regions of the seafloor overlain by anoxic,ither sulfidic or ferruginous, waters (Brocks et al., 2005; Planavskyt al., 2011; Blumenberg et al., 2012).

In this study, we concentrate on the early MesoproteorozicCalymmian period; 1600–1400 Ma). We present high-resolutionarbon isotope data for carbonate and organic matter from the Jix-an Group, Yanshan Basin, North China. The dataset presented hereurrently represents the highest resolution chemostratigraphicataset from the early Mesoproterozoic and, as such, permitsnprecedented exploration of the isotopic patterns and origin of

sotopic variation in the early Mesoproteorozic carbon cycle.

. Geological setting and age

.1. Regional geological setting

The North China craton, which refers to the Chinese part of theino-Korea Platform, is a triangular-shaped region with an area ofpproximately 1 500 000 km2, which covers most of North ChinaFig. 1, Zhao et al., 2001). The North China craton consists of variablyxposed gneiss, granite, and amphibolite, as well as shist, marblend iron formation (Zhao, 2001; Kusky and Li, 2003; Zhao et al.,005) that represent a long history of accretion beginning in therchean and ending in the late Paleoproterozoic (Zhao et al., 2005;i et al., 2011a,b, 2012; Nutman et al., 2011; Peng et al., 2011; Want al., 2011; Liu et al., 2012a,b; Lü et al., 2012; Peng et al., 2012).

Proterozoic carbonate strata are well preserved across the Northhina craton, where they unconformably overlie basement litholo-ies. The most extensive accumulations occur within the Yanshanasin (Xiao et al., 1997; Li et al., 2003a,b) in the northern part of theraton. The Yanshan Basin represents a continental rift basin thateveloped at the margin of the eastern block of the North Chinaraton at approximately 1.8 Ga (Lu et al., 2002, 2008; Hou et al.,006). Late Paleoproterozoic extension has been recorded across
he North China craton (Lu et al., 2008, and references therein) ands potentially associated with the breakup of the supercontinentolumbia (Rogers and Santosh, 2002; Zhao et al., 2002, 2003, 2004,011).
m, which is modified from Wang et al. (1995) and Chu et al. (2007). The investigated

2.2. Sedimentary strata of the Yanshan Basin

Sedimentary strata of the Yanshan Basin are representedby unmetamorphosed and relatively undeformed successions ofthe Changcheng (1800–1600 Ma), Jixian (1600–1400 Ma), Xis-han (1400–1200 Ma), and Qingbaikou groups (1000–800 Ma)(Qiao et al., 2007; Sun et al., 2012; see Chen et al., 1980 forantecedent stratigraphic division). The Changcheng system consistsof four formations (Changzhougou, Chuanlingguo, Tuanshanzi, andDahongyu) that unconformably overlie Archean and Paleoprotero-zoic basement and represent initial deposition with Yanshan riftbasin. Strata consist of >1000 m of alluvial to fluvial conglomerateand sandstone that fine and deepen upward into a >2000 m-thicksuccession of marine sandstone, silt, shale, and carbonate (Li et al.,2003a,b). Uppermost units of the Changcheng Group notably con-tain high-potassium volcanic interbeds that are associated withcontinued, regional extension (Lu et al., 2002, 2008).

The overlying Jixian Group consists of five formations(Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang, andTieling), dominated by marine carbonate deposition, that representmarine transgression and development of a stable cratonic plat-form across the Yanshan Basin (Ying et al., 2011). Jixian Group strataunconformably overlie mixed siliciclastic and carbonate strata ofthe Changcheng Group, although the boundary appears, in places,to be conformable (Chen et al., 1980; Song, 1991; Lu et al., 1996;Zhao, 1997). The majority of Jixian strata consist of finely lami-nated to stromatolitic dolomite, argillaceous dolomite, and chertdeposited under a range of intertidal to supratidal environments.

The Xiamaling Formation, composed mainly of organic-richshale, is the only formation in the newly established XishanGroup (Qiao et al., 2007). The Xiamaling Formation unconformablyunderlies the two formations (Changlongshan and Jingeryu) thatcomprise the Qingbaikou Group and mark the return of domi-nantly siliciclastic deposition prior to regional uplift of YanshanBasin strata.

This study focuses on carbonate strata of the Jixian Group, whichincludes, in an ascending order, the Gaoyuzhuang, Yangzhuang,Wumishan, Hongshuizhuang, and Tieling formations (Fig. 2).Together, these five formations represent near continuous marine
deposition within the Yanshan Basin. All samples were col-lected near the type Jixian section (Fig. 1), with samples ofthe Gaoyuzhuang and Yanghzuang formations collected from thePingquan section, samples of the Wumishan and Tieling formations

H. Guo et al. / Precambrian Resea

Fig. 2. Stratigraphic log of the early Mesoproterozoic succession, with geochrono-la

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and glauconite (Zhong, 1977; Li, 1993; Wang et al., 1995), as

ogical constraints. Geochronological data are from: Li et al. (2010), Su et al. (2010)nd Qiao et al. (2007).

ollected from the Lingyuan section, and samples of the Hong-huizhuang Formation collected from the Huailai section.

.2.1. Gaoyuzhuang FormationThe Gaoyuzhuang Formation comprises the lower 947 m of the

ixian Group at the Pingquan section (Fig. 2). From the base, theaoyuzhuang Formation can be divided into four members, includ-

ng (1) cherty, stromatolitic dolomicrite interbedded with sandyo shaley dolomicrite; (2) thinly bedded clayey—and often Mn-ich—dolomicrite; (3) organic-rich, clayey dolomicrite with chertyoncretions; and (4) massive dolomicrite with cherty concretions.enerally finely crystalline, micritic carbonate, planar bedding,nd evidence for elevated concentrations of organic matter andanganese (Fig. 3) suggest deposition of the Gaoyuzhuang Forma-

ion largely within subtidal, oxygen-limited shelf environments;
lthough some stromatolitic, cherty intervals, particularly in thepper Gaoyuzhuan formation (cf. Seong-Joo and Golubic, 1999)ay represent periods of shallower-water deposition.
rch 224 (2013) 169– 183 171

2.2.2. Yangzhuang FormationAt the Pingquan section, the overlying Yangzhuang Formation

is relatively thin (only 16.7 m), and composed predominantly ofalternating red and white sandy to silty dolomicrite. An increasein more coarsely grained siliciclastic components, as well as thepresence of wave ripples and mud cracks, suggest deposition withinpredominantly intertidal to supratidal environments.

2.2.3. Wumishan FormationThe Wumishan Formation, which is nearly 2785 m thick in

the Lingyuan section, is the thickest unit within the Jixian Group.Four members were subdivided in this formation and the litholo-gies for each member are briefly described as: (1) argillaceous,cherty dolomicrite; (2) thick-bedded stromatolitic, and bitumi-nous dolostones with minor chert; (3) rhythmically layered siltand dolomicrite, with minor cherty and oolitic layers; (4) chertycalcitic to dolomitic carbonate intercalated with bituminous andstromatolitic dolostone. The Wumishan Formation is particularlynotable for its great thickness, abundant stromatolites (including awide variety of microdigitate structures with precipitate textures;cf. Shi et al., 2008; Mei et al., 2010), intraclastic conglomerate, fos-siliferous chert, and general paucity of terrigenous material, and isinterpreted as representing deposition predominantly in peritidal,epicontinental environments (Kuang et al., 2012).

2.2.4. Hongshuizhuang and Tieling formationThe Hongshuizhuang and Tieling formation represent the

uppermost carbonate deposition of the Jixian Group. These unitsare relatively thin (88 m in the Hualai section and 91 m inthe Lingyuan section, respectively) and conformably overlie theWumishan Formation. These units consist of finely crystalline,thinly bedded, muddy dolostone interbedded with varicolored,sometimes pyritic, shale (in the Hongshuizhuang Formation) andstromatolitic dolostone (in the Tieling Formation). Combined, theseunits suggest deposition in predominantly nearshore, peritidaldepositional environments. Mn-bearing SEDEX mineralization inthe lower, shaley member of the Tieling Formation (Lu et al., 1996)suggests that shaley lithologies at this stratigraphic position mayhave acted as a primary conduit for postdepositional, hydrothermalfluid flow.

2.3. Geochronological age constraints

The age of Changcheng Group carbonate strata is well con-strained within the Yanshan Basin and has been reviewed in detailin previous publications (Xiao et al., 1997; Li et al., 2003a,b; Chuet al., 2007). Deposition of the underlying Changcheng Group,within the Yanshan Basin, is constrained to have initiated by ca.1800 Ma, which is the youngest age of detrital zircons from twosandstone samples in the lowermost Changcheng Group (Wanet al., 2003). A maximum age of approximately 1800 Ma is also con-sistent with ages of 1769 ± 2.5 Ma for mafic intrusions that occurwithin basal sedimentary strata of the Changcheng Group (Li et al.,2000). Additional ages of 1683 ± 67 Ma (U–Pbzircon, Li et al., 1995)and 1625 ± 6 Ma (U–Pbzircon, Lu and Li, 1991) obtained from extru-sive trachytes and trachyandesites of the Tuanshanzi and Dahongyuformations, respectively, provide constraint on the upper age ofdeposition of the Changcheng Group and, consequently, constrainton the lower limit of sedimentation within the Jixian Group.

The age of the Jixian Group is less well constrained. Tradi-tionally, the Jixian Group has been assumed to be approximately1400–1100 Ma based on 40Ar/39Ar ages obtained from chert

well as a single Pb–Pb model age of 1434 ± 50 for the middleGaoyuzhuang Formation (Chung, 1977). Recently, however, Li et al.(2010) reported new ages of 1559 ± 12 Ma (U–Pb SHRIMP) and


Fig. 3. Major facies of the Jixian Group. The peritidal facies (supertidal and intertidal) are dominated by a wide variety of shallow water depositional marks, such asr congi e coni tional

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ipples, tabular cross-bedding, wave microbial mats, ministromatolites, intraclastics dominated by column stromatolites, oncolites, cross-bedding structures (H–J). Thn this stratigraphic succession, the storm-dominated carbonates are typical deposirea is dominated by thin and horizontal bedding limestones (N–O).

560 ± 5 Ma (U–Pb LA-MC-ICPMS) from a single volcanic tuff inhe upper Gaoyuzhuang Formation (Yanqing Country, Beijing), sug-esting a notably older age for initiation of Jixian Group deposition.imilarly, Su et al. (2010) reported an age of 1437 ± 21 Ma (U–Pb

lometrate, teepee structures, mud cracks (A–G); The shallow subtidal facies realmcretions and muddy dolostones are often present at the deep subtidal facies (K–L);facies marks in the shallow shelf (M), and the deep shelf facies realm in the studied

SHRIMP) from the upper part of the Tieling Formation in thePingquan region. This new date, along with ages of 1368 ± 12 Ma,1370 ± 11 Ma, and 1366 ± 9 Ma (U–Pb SHRIMP, Gao et al., 2007,2008a,b, 2009) from a series of closely-spaced tuffs within the

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nconformably overlying Xiamaling Formation, constrains theeposition of the Jixian Group to between approximately 1600 and400 Ma, or wholly within the early Mesoproterozic (Calymmianeriod) (Qiao et al., 2007).

. Materials and analytical methods

.1. Sample collection and petrographic analysis

A total of 623 carbonate samples were collected from measuredections of Jixian Group carbonates. This sample set comprises59 samples from the Gaoyuzhuang Formation, 13 samples fromhe Yanghzuang Formation, 311 samples from the Wumishan For-

ation, 4 samples from the Hongshuizhuang Formation, and 36amples from the Tieling Formation. Weathered surfaces and largeeins were removed either during field collection or secondarilyn the laboratory. Thin sections of samples were then evaluated byptical petrography to catalog the range of carbonate fabrics withinach sample. Petrographic analyses provides a first-order assess-ent of depositional and/or diagenetic heterogeneity (cf. Kaufman

t al., 1991; Kah et al., 1999; Frank et al., 2003; Bartley et al., 2007)hat can be used in conjunction with isotopic and elemental anal-ses to determine the potential for retaining little altered isotopicompositions.

.2. Isotopic analysis of carbonate rocks

Whole-rock samples were crushed to less than 150 �m foreochemical analysis. Samples for carbon and oxygen isotope anal-sis were reacted with 100% H3PO4 under vacuum for 24 h at5 ◦C for limestone and at 50 ◦C for dolomite. Evolved CO2 washen purified and its C- and O-isotope composition measuredn a Finnigan MAT 251 IRMS (isotope ratio mass spectrometer)t the State Key Laboratory of Geological Processes and Min-ral Resources, China University of Geosciences (Wuhan). Isotopicompositions are reported in standard delta notation, where ıin ‰) = [(Rsample − Rstandard)/Rstandard]*1000, relative to the VPDBVienna Pee Dee Belemnite) standard. Precision is better than 0.1‰or �13C and �18O, based on multiple analyses of laboratory stan-ards.

.3. Isotopic analysis of organic carbon

An additional 252 samples were chosen from throughout theixian Group for isotopic analysis of organic carbon. Samples forrganic carbon analysis were treated with 10% HCl to remove car-onate, rinsed with distilled water, and freeze-dried at −40 ◦Cvernight. The isotopic composition of organic carbon was thenetermined using a Thermo Finnigan MAT 253 IRMS (isotope ratioass spectrometer) at the State Key Laboratory of Geological Pro-

esses and Mineral Resources, China University of GeosciencesWuhan). Isotopic compositions are reported in standard deltaotation relative to the VPDB (Vienna Pee Dee Belemnite) standard.recision is better than 0.2‰, based on multiple analyses of labo-atory standards.

.4. Analysis of major and minor elements

In addition to isotopic analyses, samples from the Gaoyuzhuangormation were also analyzed for major (Ca, Mg) and minor (Mn,r) element concentrations. Samples were dissolved in concen-
rated hydrochloric acid, and the solutions analyzed on an IRISntrepidIIXSP ICP-AES (inductively coupled plasma-atomic emis-ion spectrometry) at the State Key Laboratory of Biogeology andnvironmental Geology, China University of Geosciences (Wuhan).
rch 224 (2013) 169– 183 173

Analytical uncertainty for both major and minor elements averages±5 wt‰.

4. Results and interpretations

4.1. Evaluation of diagenesis

4.1.1. Petrographic preservation of Jixian Group microfaciesBecause of the complex post-depositional history of most sed-

imentary carbonate rocks, post-depositional alteration must beconsidered prior to the interpretation of isotopic compositions. Pet-rographic analyses provide a first-order assessment of depositionaland/or diagenetic heterogeneity that can be used in conjunc-tion with isotopic and elemental analyses to identify both themost chemically altered samples—i.e., those that are least likelyto preserve meaningful carbon isotopic signatures; and the leastchemically altered samples—i.e., those that are most likely to pre-serve near-primary isotopic compositions (Kaufman et al., 1991;Kah et al., 1999; Frank et al., 2003; Bartley et al., 2007).

Within the Jixian Group most samples record little evidence ofextensive post-depositional recrystallization (Fig. 4). The major-ity of sampled material—particularly within the GaoyuzhuangFormation—consists of calcitic to dolomitic micrite and finemicrospar. The finely crystalline nature of these components, andthe general absence of more coarsely crystalline phases suggestthat diagenetic stabilization occurred within penecontemporane-ous marine fluids. Other primary microfacies within these samplesinclude micrite, coated grains, intraclasts, clotted microbial struc-tures, and stromatolitic and oncolitic laminae.

Coated grains and ooids of the Jixian Group are typically spher-ical or ellipsoidal in shape, and preserve a radial-concentric cortexthat surrounds a nucleus composed of micritic intraclasts or sin-gle quartz grains (Fig. 4A and B). Cortex microfabics commonlyshow clear, well-preserved, acicular crystals arranged radially tothe nucleus and separated by thin, micritic bands. Calcareousto dolomitic micritic intraclasts (Fig. 4C and D) are commonlyfound associated with coated grains in peritidal environments.As with the coated grains, intraclasts are typically finely crys-talline and show evidence for only minimal post-depositionalrecrystallization. Recrystallization most commonly occurs as par-tial recrystallization to a slightly more coarsely crystalline phase.In these cases, interlocked, anhedral crystal fabrics suggest neo-morphic recrystallization during early diagenesis. Preservation ofdiverse, fine-scale structures in intraclasts and within the cortexof coated grains and ooids suggest preservation of primary deposi-tional textures (Wright, 1990) and indicate only limited influence ofpost-depositional fluids, even in otherwise coarse-grained, quartz-bearing lithologies.

Equally diverse microfabrics occur within the microbially lam-inated facies of the Jixian Group. Finely crystalline clots withinmore coarsely crystalline, microsparitic matrix are common withinstromatolitic and oncolitic lithologies (Fig. 4E and F). Such clotsoccur commonly as a result of in situ cyanobacterial calcificationor micrite precipitation during microbial decomposition (Wright,1990; Turner et al., 1993; Riding, 2006; Kah and Riding, 2007).We attribute similar formation mechanisms to small peloids thatoccur in association with microbial fabrics. As with intraclasticand ooilitic fabrics, the diverse and fine-scale fabric preservationobserved in microbial fabrics suggests generally restricted influ-ence of later diagenetic fluids.

By contrast to these primary depositional microfabics,
some samples within the Jixian Group show clear indicationof recrystallization during post-depositional fluid interactions.These samples are typically characterized by coarse, euhe-dral crystalline phases that show little evidence of primary


Fig. 4. Petrographic preservation of the Jixian Group carbonate microfacies. (A) Coated ooids with intraclastic nuclei and well-preserved acicular cortices. Samples from theGaoyuzhuang Formation; (B) Aragonite ooid show large radial ‘club’ structures and concentric layers in the outer part of the cortex, Gaoyuzhuang Formation; (C) Intraclastsmostly contain complex structures, interpreted as representing re-cementation of pre-existing individual intraclast grains, Gaoyuzhuang Formation; (D) Intraclasts withv ures,

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arious sharps and sizes, Gaoyuzhuang Formation; (E) Well-preserved clotted structormation; (F) Oncoids, spherical or ellipsoidal in shape, with less regular concentric lormation and (H) coarse-grained structures representing more significant degree

epositional textures (Fig. 4H). Dark brown cores to coarselyrystalline—often dolomitic—phases may represent partial
etention of primary depositional components, but substan-ial geochemical remobilization cannot be ruled out. We interprethese phases as having undergone recrystallization in the presencef post-depositional fluids.
in which sparitic cement contains small peloids with indistinct margins, Wumishantions, Wumishan Formation; (G) well preserved fine-grained structures, Wumishanenesis, Gaoyuzhuang Formation.

4.1.2. Examination of isotopic and elemental trendsThe potential effects of diagenesis on the preservation of marine

carbon isotope records can also be evaluated by examination ofcombined elemental and isotopic trends (Figs. 5 and 6; Table S1).In particular, numerous studies of Paleozoic and Proterozoic carbo-nates have shown that carbon isotope composition is less sensitive

H. Guo et al. / Precambrian Resea

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ig. 6. Geochemical data from the Gaoyuzhuang Formation. (A) Cross plot of �18O versus Mlimestone and dolomitic limestone). (B and C) Sr and Mn concentrations of Gaoyuzhuan>200 ppm) occur predominately in less altered limestones (black squares, B), inferred oroncentrations (black squares, C) that suggests carbonate precipitation in the presenceormation samples.

rch 224 (2013) 169– 183 175

to diagenetic alteration than either oxygen isotope or trace element(Mn, Sr) composition, as long as diagenetic fluids are relativelycarbon-poor (Banner and Hanson, 1990; Kaufman et al., 1991;Banner and Kaufman, 1994).

Carbon and oxygen isotope values of Jixian Group samples showa range of values between approximately −2‰ and +2‰ for carbonand −4‰ and −11‰ for oxygen. No clear co-variation betweencarbon and oxygen isotope composition is observed, suggestingthat post-depositional fluid flow and associated organic diagenesislikely was not a strong influence on isotopic compositions (Veizer,1983; Marshall, 1992). Furthermore, the majority of petrographi-cally well-preserved samples record oxygen isotope compositionsbetween −6 and −9‰. Oxygen isotopes are generally considereda sensitive indicator of diagenetic processes, even at low degreesof fluid-rock interaction (Banner and Hanson, 1990). Although thepotential for secular change in the oxygen isotope compositionof past marine systems remains controversial (see Brand, 2004;Kasting et al., 2006; Jaffrés et al., 2007, for review), oxygen iso-tope values recorded here are similar to those recorded by mostpetrographically well-preserved, non-evaporitic Proterozoic andearly Paleozoic carbonate rocks (Frank and Lyons, 2000; Kah, 2000;
Bartley et al., 2007; Thompson and Kah, 2012; Kah et al., 2012),and it has been proposed that such values record early diagenetic
Sr (ppm)

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g/Ca ratios suggests minor later diagenesis has affected the two end member phasesg Formation samples, plotted against �18O vaules. The elevated Sr concentrationsiginal mineralogy is aragonite, some well-preserved samples display elevated Mn

of Mn-rich waters. (D) cross-plot of Mn and Sr concentrations of Gaoyuzhuang

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tabilization in isotopically light marine waters (Veizer and Hoefs,976; Veizer et al., 1992).

If oxygen isotopic compositions between approximately −6 and9‰ are interpreted to reflect little altered marine isotopic compo-

itions, our data indicate that at least a portion of the Gaoyuzhuangnd Wumisham formation samples with more positive oxygen iso-opic compositions may reflect moderately evaporitic depositionalonditions, which is consistent with microfabics and microfos-il assemblages determined in previous studies (Seong-Joo andolubic, 1999). By constrast, nearly all samples from the upper-ost Tieling Formation preserve oxygen isotope compositions

etween −9 and −19‰. Such negative isotopic values, combinedith evidence for SEDEX mineralization within this unit, suggest

he potential for substantial overprinting of primary isotopic valuesuring post-depositional, and potentially hydrothermal, fluid-rock

nteraction.Elemental analyses of nearly 200 samples from the

aoyuzhuang Formation permit a more detailed evaluationf post-depositional diagenesis. We first consider effects ofarbonate mineralogy on isotopic and elemental composition.aoyuzhuang Formation carbonates show variation in their degreef dolomitization, with Mg/Ca ranging from 0.0 to 0.4 (Fig. 6A).s with previous studies (Bartley et al., 2007), isotopically light18O associated with post-depositional alteration is recorded

n both calcitic (Mg/Ca < 0.2) and more dolomitic (Mg/Ca > 0.2)amples, although these samples comprise only 20% of the totalample pool. Among samples inferred to be “less altered” (i.e.,18O > −9‰), those with extensive, fabric-retentive dolomitizationhow slightly more positive oxygen isotope values, suggestinghe potential for either preservation of originally heavier isotopicalues, or dolomitization enhanced via evaporative fluid flow (cf.ah, 2000; Bartley et al., 2007; Kah et al., 2012).

In addition to Mg/Ca ratios, which are most commonly associ-ted with post-depositional dolomitization of primary phases, tracelement content can also be used as a powerful indicator of post-epositional recrystallization under a range of fluid compositionsBrand and Veizer, 1980; Banner and Hanson, 1990). Strontiumoncentrations in carbonate rocks of the Gaoyuzhuang Formationre typically <200 ppm (Fig. 6B). Strontium is typically elevated inarine environments, but is rapidly lost during recrystallization,

ven at minimal degrees of water rock interaction. Empirically,trontium concentrations near 200 ppm are common for petro-raphically well-preserved Proterozoic limestone (Kaufman andnoll, 1995). Within the Gaoyuzhuang Formation, the lowest con-entrations (<100 ppm Sr) occur primarily in dolomitized phases,hich likely reflect the preference for Sr to reside in the Ca site

f the calcite lattice (Pierson, 1981; Carpenter et al., 1991). Sev-ral samples, however, record strontium concentrations as high as000 ppm (Fig. 6B). These samples may reflect (1) minimal non-arine post-depositional diagenesis, (2) elevated concentration of

trontium from a primary aragonite depositional phase, or (3) pre-ipitation from evaporative marine fluids with elevated strontiumoncentrations (cf. Kah et al., 2012; Gilleaudeau and Kah, in press).amples with elevated strontium concentrations, however, havexygen isotope compositions within the range of inferred openarine samples, suggesting that elevated strontium reflects marine

tabilization of an originally aragonitic phase, with only a minoregree of post-depositional recrystallization.

Similarly, manganese shows an interesting pattern of enrich-ent within Gaoyuzhuang Formation carbonates (Fig. 6C and D).anganese concentrations are generally <100 ppm, which is inter-

reted to reflect only restricted influence of post-depositional
uid flow. Approximately 25% of samples that are inferred toeflect little-altered, open-marine, calcitic deposition (i.e., �18Oalues between −6 and −9‰ and Mg/Ca < 0.2) also contain man-anese concentrations between 400 and 1200 ppm. Because of its
rch 224 (2013) 169– 183

propensity to record the effect of reducing fluids during burialdiagenesis, Mn concentration (as well as Mn/Sr ratios) have beenregarded as sensitive indicators of diagenetic alteration (Veizer,1983; Derry et al., 1992; Lindsay and Brasier, 2000), with a Mn/Sr > 2regarded as having suffered substantial diagenetic alteration (Derryet al., 1992, 1994; Kaufman and Knoll, 1995; Montanez et al.,1996). Most Mn/Sr ratios in the Gaoyuzhuang Formation and sev-eral other formations within the Jixian Group (Li et al., 1999, 2009;Kuang et al., 2009) are typically less than 1, which conforms withprevious inferences of minimal post-depositional alteration. Sam-ples with elevated Mn, however, commonly have Mn/Sr between4 and 12. We suggest, however, that these elevated values mayreflect primary compositional differences. A growing number ofstudies suggest that redox-sensitive elements such as Mn (Bartleyet al., 2007; Thompson and Kah, 2012; Kah et al., 2012) and Iron(Gilleaudeau and Kah, in press) can also reflect low oxygen condi-tions of ancient seawater and the relative mobility of reduced ionsin microbially dominated environments (cf. Davison, 1982).

4.2. Chemostratigraphic results

As discussed above, petrographic and geochemical data sug-gest that Gaoyuzhuang Formation carbonates are considered to begenerally well preserved and likely to retain little-altered carbonisotope compositions. The carbon isotope stratigraphy of both car-bonate (Fig. 7) and organic matter (Fig. 8) in the Yanshan Basin,North China, are listed in Table S1.

The chemostratigraphic patterns derived from marine car-bonates of the Jixian Group are broadly similar to patternsobserved in other Mesoproteorozic carbonate successions olderthan ∼1300 Ma. Carbon isotope compositions through the Jixiansuccession range from −2.6‰ to +1.8‰ (with an average value of−0.2‰), and are arranged in a series of alternating positive and neg-ative excursions. The 946 m thick Gaoyuzhuang Formation recordsthe lowest degree of isotopic variability. With the exception ofan anomalous 50-m thick interval, carbon isotope compositionsof the Gaoyuzhuang Formation average −0.3‰ (s.d. = 0.6‰) andrange between −1‰ and +1‰. Stability of these isotopic values con-trasts sharply with those preserved within the anomalous interval,during which carbon isotope compositions drop to nearly −2.5‰.A return to more muted isotopic compositions in the 20 m-thickYangzhuang Formation then give way to increased isotopic vari-ability in the >2600 m-thick Wumishan Formation, with averagevalues of −0.1‰ (s.d. = 0.8‰), and a total range from −1.8‰ to+1.8‰. Basal Wumishan strata record isotopic values near 0‰,which then fall below −1.5‰ before rising rapidly to carbon iso-tope values near +2‰, and then falling once again to values below−1.5‰. Above this, a second positive excursion reaches valuesnear +1‰ before falling to values near −1.5‰ and rising a finaltime through the uppermost Wumishan Formation to reach val-ues near +2.0‰. These moderately positive values are retainedthrough the 25 m thick Hongshuizhuang Formation, before record-ing a precipitous drop to values ≤2.5‰ in the 90 m thick TielingFormation.

A chemostratigraphic profile constructed from the carbon iso-tope composition of organic matter, however, shows distinctdifferences from that of marine carbonate, particularly between theGaoyuzhuang and Wumishan formations (Fig. 8). Because marineorganic carbon is largely derived from the direct autrophic fixationof marine DIC (dissolved inorganic carbon), the isotopic compo-sition of marine organic carbon typically co-varies with that ofmarine carbonate. Such covariance is observed through the entire
Wumishan Formation and the upper portion of the GaoyuzhuangFormation, where the isotopic composition of organic carbonranges from −21.6‰ to −30.4‰, with most samples having arange from −26‰ to −30‰. Such values yield an average isotopic

H. Guo et al. / Precambrian Research 224 (2013) 169– 183 177

Fig. 7. C isotope chemostratigraphy correlation of the comprehensive sections in this study with Jixian and Ming Tombs sections in Yanshan Basin. Detailed description ofJ a,b), r

fwior(fotWi

5

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ca

ixian and Ming Tombs sections can be found in Chu et al. (2007) and Li et al. (2003

ractionation (�13C = �13Ccarb − �13Corg) of 28.0‰ (s.d. = 2.0‰),hich is consistent with derivation of organic matter from predom-

nantly autotrophic communities. Organic carbon compositionsf the lower 750 m of the Gaoyuzhuang Formation, however,ange from −26.3‰ to −34.4‰, with values averaging −31.1‰s.d. = −1.5‰). Such isotopic values, which yield average isotopicractionations of ∼31.0‰, suggest that the isotopic compositionf organic matter in the lower Gaoyuzhuang Formation—in con-rast to that of the upper Gaoyuzhuang Formation and the entire

umishan Formation—may reflect a degree of decoupling from thesotopic composition of marine carbonate.

. Discussion

.1. Yanshan Basin as a Mesoproterozoic reference section

Carbon isotope stratigraphy has been widely applied to theomparison and division of various geological times and bound-ry events, and the pattern of secular variation in the isotopic

espectively.

composition of marine carbonate is now well established for muchof the last billion years of Earth history (e.g., Gale et al., 1993; Hilland Walter, 2000; Shen and Schidlowski, 2000; Krull et al., 2004;Halverson et al., 2005, 2010; Bergström et al., 2008; Thompsonet al., 2012). Although carbon isotope profiles are comparativelyrare from Mesoproterozoic and older successions, a growing num-ber of studies suggest a distinctive pattern of isotopic variationthrough the Mesoproterozoic (cf. Kah et al., 1999, 2001, 2012;Bartley et al., 2007). Specifically, carbon isotope records from theearly Mesoproterozoic (pre-1300 Ma) show relatively subdued iso-topic signatures that oscillate around an average value near 0‰,and rarely reach values more negative or positive than approxi-mately −1.5‰ or +1.5‰, respectively (Buick et al., 1995; Knoll et al.,1995; Frank et al., 1997; Xiao et al., 1997; Lindsay and Brasier, 2000;Lindasy and Brasier, 2004; Bartley et al., 2001; Chu et al., 2007; Kah
et al., 2007). Sparse data sets from rocks deposited between 1300and 1200 Ma suggest an increase in both the range and variability ofcarbon isotope data (Frank et al., 2003; Bartley et al., 2007; Chiglinoet al., 2010), which leads to distinct isotopic plateaus near +4‰ and


arbon

ntK

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Fig. 8. The carbon isotopic compositions of organic carbon (�13Corg), c

egative isotopic excursions to ≤2.5‰ that characterize sedimen-ary successions of the later Mesoproterozoic (Whelan et al., 1990;noll et al., 1995; Kah et al., 1999, 2001, 2012; Bartley et al., 2001).

Thick, carbonate-dominated sedimentary successions of theanshan Basin, North China, offer an ideal opportunity to exploreecular variation in marine carbon isotopes during the earlyesoproterozoic (from ∼1600 to 1400 Ma). Compared to earlyesoproterozoic strata elsewhere, strata within the Yanshan Basin

ave superb age constraints, including: (1) U–Pbzircon ages of683 ± 67 Ma and 1625 ± 6 Ma for extrusive volcanics within thenderlying Tuanshanzi and Dahongyu formations (Lu and Li, 1991;i et al., 1995; Lu et al., 2008); (2) U–Pbzircon ages of 1368 ± 12 Ma,370 ± 11 Ma, and 1366 ± 9 Ma for volcanic rocks within the over-

ying Xiamaling Formation (Gao et al., 2007, 2008a, 2008b, 2009);3) U–Pbzircon ages of 1559 ± 12 Ma and 1560 ± 5 Ma for severalolcanic horizons within the upper part of the Gaoyuzhuang For-ation (Li et al., 2010) and ages of 1437 Ma from the upper part of

he Tieling Formation (Su et al., 2010).In this study, we present the highest resolution carbon isotope

urve, to date, of any Mesoproterozoic succession. More than 623ata points reveal values within a narrow range from −2‰ to +2‰,

ate (�13Ccarb) in the early Mesoproterozoic Jixian Group, North China.

which are considered typical for the early Mesoproterozoic. Carbonisotope data here oscillate in a repeated succession of positive andnegative excursions around values near 0‰ (Fig. 7). Data presentedhere also highlight the importance of high-resolution sampling.Earlier investigations of Jixian Group strata from the Yanshan Basinwere constructed from stratigraphic sections measured at Jixian(Chu et al., 2007) and near the Ming Tombs (Li et al., 2003a,b).Although each of these successions show strong isotopic similarityto the data presented here, only the highest resolution samplingreveals, with clarity, the oscillatory nature of marine carbon iso-topes (Fig. 7). We therefore suggest that the carbon isotope valuespresented here have the potential to serve as a reference for theentire early Mesoproterozoic.

Interestingly, these data suggest a more complex and protractedevolution of the Mesoproterozoic carbon isotope record than earliersuggested (see above, Kah et al., 1999, 2012; Kah and Bartley, 2011).Whereas our data support the observation of an increase in both the
average isotopic composition and an increase in the magnitude ofisotopic excursions in the Mesoproterozoic (Bartley and Kah, 2004),the data also suggest that these transitions may have occurredgradually through the Mesoproterozoic. In particular, data from

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he Gaoyuzhuang Formation, which record—with the exceptionf a single, 50 m-thick interval—only minimal isotopic variationfrom −1‰ to +1‰), are quite similar to the generally isotopically

onotonous signatures from youngest Paleoproterozoic to oldestesoproterozoic (∼1575 Ma) strata from the McArthur and Mount

sa basins in Australia (Lindsay and Brasier, 2000). In turn, the dis-inct shift to more variable isotopic signatures (from −1.8‰ to1.8‰) typified by in the Wumishan Formation are strikingly sim-lar to isotopic signatures from ∼1400 ma strata from the Helenaormation, Belt Supergroup (Frank et al., 1997), as well as thoseecorded in approximately 1500–1400 Ma strata from the Kama-elaya trough, Russia (Kah et al., 2007). This increase in preserved

sotopic variation appears to continue into Mesoproterozoic strataeposited between approximately 1300 and 1200 Ma, where suc-essions preserve isotopic variation that commonly see values thatange from −2‰ to greater than +2.5‰ (Frank et al., 2003; Bartleyt al., 2007).

.2. Origin of carbon isotope variation

The long-held view of Mesoproterozoic carbon isotopetability—that the Mesoproterozoic represented a protracted inter-al of environmental stability (Brasier and Lindsay, 1998)—is notell supported by the growing body of evidence that suggests that

he Mesoproterozoic may reflect a critical time in the evolutionf Earth surface environments from the standpoint of global tec-onic reorganization, changes in both oceanic composition (Kaht al., 2001; Bartley and Kah, 2004) and oceanic redox (Shen et al.,002; Anbar and Knoll, 2002; Arnold et al., 2004; Scott et al., 2008;lanavsky et al., 2011), evolution within both prokaryotic (Johnstont al., 2005) and eukaryotic clades (Javaux et al., 2004; Knoll et al.,006), and the advent of eukarotic multicellularity (Butterfield,000). Rather, a more plausible explanation of apparent isotopictability might lie within the accumulated data for environmentalhange (cf. Bartley and Kah, 2004; Kah and Bartley, 2011). In thiscenario elevated marine carbonate saturation would have resultedn a system wherein marine carbon isotope change was buffered byhe composition of marine DIC. Meanwhile, gradual oxygenationf Mesoproterozoic biosphere would result in both a decreaseduffering capability and an increase in biotic influence, ultimatelyesulting in a greater isotopic variability.

Under such conditions, short-term isotopic variability preservedn Mesoproterozoic successions is likely to result from organic car-on production and burial, marine stratification and overturn, or

n situ organic carbon remineralization (Bartley and Kah, 2004).n addition, low-oxygen conditions of the Mesoproterozoic wouldave promoted geochemical stratification of the water column,ith broad regions of even shallow marine seafloor overlain by

noxic, sulfidic, or possibly ferruginous waters (Brocks et al., 2005;lanavsky et al., 2011; Kah et al., 2009; Kah et al., 2012; Blumenbergt al., 2012; Gilleaudeau and Kah, in press). Under such condi-ions, oxidation of organic byproducts (either dissolved organicarbon, Rothman et al., 2003; or oxidation of microbially producedethane, Conrad et al., 2007) may result in the transient influx

f isotopically light organic carbon to the marine reservoir. Theegree of organic mineralization may also be affected by both waterepth (i.e., a dynamically maintained carbon isotope depth profile;ah et al., 2007) or restricted mixing between open marine andhallow-water epicratonic environments (Gilleaudeau and Kah, inress).

During the early Mesoproteorozic Era, the Yanshan Basin wasominated by stable, epicontinental deposition. Based on field and
etrographic observation, strata of the lower Jixian Group (hereypified by the Gaoyuzhang Formation) record the deepest watereposition, with deposition from quiet water, deeper subtidalnvironments, to shallow subtidal (and perhaps intertidal; cf.
rch 224 (2013) 169– 183 179

Seong-Joo and Golubic, 1999) environments. By constrast, strataof the upper Jixian Group (here typified by the Wumishan For-mation) record deposition primarily within peritidal (intertidal tosupratidal) environments. Excursions within the Jixian system iso-topic profile (Fig. 7) show no clear relationship to changes in waterdepth, and therefore suggest that isotopic variation does not simplyreflect a marine isotopic gradient.

5.3. Organic carbon production and remineralization

Although isotopic excursions within the Jixian Group do notappear to be related directly to water depth and a vertical gra-dient in the isotopic composition of marine carbon that mightbe expected under low-oxygen conditions, they may also poten-tially reflect the in situ remineralization of organic matter, whichwill ultimately affect organic carbon burial rates. Using carbon-ate and organic carbon samples, we can calculate both the totalfractionation (�13C) and the fraction of organic carbon burial(forg), at steady state, necessary to produce the observed isotopicexcursions (see Table S1; Kump and Arthur, 1999). In order toproduce observed excursions, organic carbon burial rates wouldneed to fluctuate between approximately 0.15 and 0.25. Suchmoderate differences in organic carbon burial could easily reflectprocesses either intrinsic to or extrinsic to the Yanshan Basin. Ifintrinsic to the basin, these values would suggest small changesin the burial and/or decomposition of organic matter under arange of peritidal to subtidal conditions. Such changes could easilybe achieved under low-oxygen conditions that have been shownto affect both the composition and behavior of benthic micro-bial mats (Bartley and Kah, 2004; Brocks et al., 2005; Planavskyet al., 2011; Blumenberg et al., 2012; Gilleaudeau and Kah, inpress).

Low oxygen (and likely stratified) oceanic conditions can beinferred from the differences in isotopic behavior of organicbetween the Wumishan and Gaoyuzhuang formations. As dis-cussed in numerous publications (Rothman et al., 2003; Coetzeeet al., 2006; Conrad et al., 2007; Baker, 2010; Jiang et al., 2012), thecarbon isotope composition of bulk organic matter can be influ-enced by many factors, including the metabolic behavior of thestanding organic community, potential input from marine versusterrestrial sources of organic matter, degradation of particulateorganic carbon below the sediment water interface and reminer-alization of dissolved organic carbon or other carbon byproductswithin the water column. Organic matter that derives predomi-nantly from the autotrophic fixation of marine DIC typically recordsfractionation of 28–30‰ (Hayes et al., 1999; Kump and Arthur,1999). By contrast, isotopically lighter values typically reflectorganic carbon derived from a combination of heterotrophic andsecondary chemoautotrophic activity, along with remineralizationof carbon byproducts. Heterotrophic bacteria, for instance, uti-lize 13C-depleted organic matter as their primary carbon source,and express their own kinetic isotopic fractionation, resulting inthe formation of organic byproducts (typically CO2) that can bedepleted by several permil with respect to the original, autotrophiccarbon source (Hollander and Smith, 2001). In another example,under sulfate-depleted conditions undergoing active fermenta-tion, heterotrophic (acetotrophic) methanogenesis has been shownto produce biogenic methane with 13C depletions of up to 10‰over primary, autotrophic organic carbon (Conrad et al., 2010). Ifremineralized in situ and added into the overall pool of organicmatter, this contribution from anaerobic heterotrophs results inmore negative 13Corg and higher �13C values (Hollander and
Smith, 2001). Organic matter derived, in part, from the sedi-mentation of anaerobic microbial biomass, in fact, often records�13C values ≥32‰ (Hayes et al., 1999; Teranes and Bernasconi,2005; Li et al., 2011b; Jiang et al., 2012). By contrast, more

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xtreme 13C-depletion (15–50‰ depletion over the primary car-on source) typically records a pronounced contribution fromecondary chemoautotrophic activity (such as hydrogenitrophicethanogenesis) and its subsequent remineralization (Conrad

t al., 2010).Isotopic depletion of the standing biomass resulting from a

ontribution from heterotrophic and secondary chemoautotrophicacterial consortia contrasts sharply with the isotopic compositionf organic matter derived, in part, from terrigenous components,hich often record more positive �13Corg and, consequently, lower�13C values. In this latter example, isotopically heavy terrigenousaterial most commonly derived from higher plants that have sub-

tantially different mechanisms of carbon cycling. Because sucherrigenous materials are believe to have been absent in the early

esoproterozoic (Kennedy et al., 2006; Battison and Brasier, 2012;enrick et al., 2012), isotopically heavy organic matter is more

ikely to reflect a combination of low organic carbon content andxtensive in situ degradation of primary organic material (cf. Kaht al., 1999) or, alternatively the post-depositional, thermal decom-osition of organic matter (Strauss et al., 1992; Derry, 2010; Tocquét al., 2005).

As noted above (Section 4.2, Fig. 8), organic carbon signaturesithin the Jixian Group can be divided into three discrete intervals:

sotopically lighter organic carbon in the lower part (lower 750 m)f the Gaoyuzhuang Formation with values averaging −31.1‰with values as low as −34.4‰); Organic carbon values averaging28.0‰ in the upper 200 m of the Gaoyuzhuang Formation and

he entire Wumishan Formation; and highly variable values (from22‰ to −34‰) within uppermost Tieling Formation.

These data suggest that the majority of the Jixian Group, rep-esented by extensive, peritidal deposits of the Wumishan andpper Gaoyuzhuang formations, reflects a microbial communityominated by autotrophic organisms, and an environment whichrohibited extensive heterotrophic remineralization of organicatter and, instead, facilitated the oxidative decomposition of ben-

hic microbial material. Such values are therefore consistent witheritidal, and presumably well-oxygenated conditions inferred forhe depositional environment, and are consistent with biomarkerata suggesting a high concentration of organic matter from pho-osynthetic bacteria/algae (Li et al., 2003a,b). By constrast, muchf the lower Gaoyuzhuang Formation shows substantially lightersotopic compositions for organic carbon, which suggests a sub-tantial contribution from a non-photosynthetic origin (Jiang etl., 2012). Such observation is consistent with biomarker datarom the Gaoyuzhuang Formation, which shows a clear bimodal-ty in n-alkanes that are interpreted to reflect a combinationf biodegradation and the contribution of non-photosyntheticacteria to the biomass (Li et al., 2003a,b), as well as biomarkerata from similarly-aged strata in the McArthur basin that pre-erves evidence of anoxic conditions potentially within the photicone (Brocks et al., 2005). Li et al. (2003a,b) also note that aery similar distribution of biomarkers occurs in thin, subtidaltrata of the Hongshizhuang Formation, which caps the Wumis-an Formation. Here, we suggest that the combination of isotopicnd biomarker data likely reflects the development of anoxicottom waters in deeper-water environments represented byhe Gaoyuzhuang and Hongshizhuang formations, which favorednhanced heterotrophic remineralization of benthic microbialats.By contrast, the Tieling Formation records a dramatic and sys-

ematic change in the isotopic composition of both inorganic andrganic carbon (Fig. 8) that corresponds to strata that also show
ubstantial ore-grade mineralization. We therefore suggest thatsotopic compositions of Tieling strata likely reflect the degradationf organic carbon during post-depositional, hydrothermal alter-tion.
rch 224 (2013) 169– 183

5.4. Implications for ocean-atmosphere evolution

In summary, we suggest that differences in isotopic com-position of inorganic and organic carbon samples between theGaoyuzhuang and Wumishan formations most likely reflect acombination of buffering of the marine system to DIC and differ-ential carbon cycling within benthic microbial mats that reflecta depth-dependent stratification within the overlying water col-umn. Despite relatively substantial differences in �13C, the degreeof organic carbon burial (forg) calculated for these formationsremained within a fairly narrow range, which suggests that theincreased isotopic variation observed in marine carbonate betweenthese two successions must have been driven, instead by a differ-ence in the buffering capacity of the marine DIC system (cf. Bartleyand Kah, 2004). We suggest that a decreased buffering capacitymay have been driven by a long-term decrease in pCO2 throughthe late Paleoproterozoic, originally suggested by Li et al. (2003a,b),and continuing through the Gaoyuzhuang Formation until perhaps1500 Ma.

Differences in the isotopic composition of organic carbon, how-ever, more likely reflect differences in carbon cycling within benthicmicrobial mats, wherein well-oxygenated shallow marine waterare characterized by benthic mats dominated by autotrophic car-bon fixation and the abiogenic oxidation of this organic matter.In this case, organic decomposition and remineralization resultedin organic matter with isotopic compositions similar to that of theoriginal autotrophic biomass. By contrast, sub-oxic to anoxic deepermarine waters are characterized by benthic mats that experience agreater degree of decomposition via anaerobic heterotrophs, whichresulted in isotopic depletion of the carbon byproducts. In situremineralization of isotopically light DOC therefore resulted ina greater contribution of isotopically light carbon to the organiccarbon pool. This distribution of chemical characteristics impliesunique chemical conditions for the early Mesoproterozoic, in whichanoxic (either sulfidic or ferruginous) conditions (Shen et al., 2002;Brocks et al., 2005; Planavsky et al., 2011; Gilleaudeau and Kah,in press) likely occurred close to oxygenated, well-mixed surface-oceans. These results are consistent with evidence suggestingthe protracted oxygenation of Proterozoic surface environmentsand suggest that a dynamic stratification of the ocean that wasmaintained primarily by wave-mixing at the ocean-atmosphereinterface, and in which anoxia would be sustained directly beneaththe zone of active wave mixing.

6. Conclusions

Here we report high-resolution chemostratigraphic data frominorganic and organic carbon samples within the Jixian Group,North China Platform. Strata of the Jixian Group are wellconstrained to represent nearly continuous deposition fromapproximately 1600 to 1400 Ma. Data presented here representthe most continuous, high-resolution dataset from the early Meso-proterozoic, and can be used as a critical reference section forchemostratigraphic comparison.

Marine carbonate rocks of the Jixian Group show generallymuted isotopic compositions with average values near 0‰; isotopicvariation around this average, however, increases from approxi-mately ±1‰ in the lowermost Gaoyuzhuang Formation to ±1.8‰ inthe Wumishan Formation. This increased isotopic variation appearsto be part of a global trend of increasing isotopic variation thatspans from the Paleoproterozoic through the Neoproterozoic (cf.
Bartley and Kah, 2004) and is consistent with a long-term decreasein pCO2 and the decreased buffering capacity of marine DIC. Inthis scenario, variation in marine carbon isotope composition waslikely driven by transient changes in organic matter production and

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ecomposition. The isotopic composition of marine organic mat-er, however, suggests more dramatic differences in carbon cyclingetween different lithologic units. In particular, the Gaoyuzhuangormation records isotopic compositions of organic matter thatndicate a substantial remineralization of organic carbon by het-rotrophic (and likely anaerobic) components of the benthicicrobial community. By constrast, the upper Gaoyuzhuang andumishan Formations record isotopic compositions of organicatter consistent with organic matter produced primarily through

utotrophic carbon fixation and remineralized through oxida-ive decomposition. These differences in carbon cycling correlateith depositional environment and therefore most likely reflect

enerally low oxygen conditions and a dynamically maintainedtratification of marine waters.

cknowledgements

We thank Xiaoying Shi, Shucheng Xie, Qinglai Feng (China Uni-ersity of Geosciences), who joined fieldwork of the Gaoyuzhuangormation and participated in discussions of the field sections..S. students Jun Cao and Lidan Lei in CUG are thanked for their

elp in running samples on the MAT 253. Two anonymous review-rs provided thoughtful reviews that greatly improved the qualityf the manuscript. This research was supported by the Nationalasic Research Program of China (Grant no. 2011CB808800), the111 Project” (Grant no. B08030), and the University of Tennessee

alker Professorship (to LCK).

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.precamres.2012.09.023.

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