Precambrian Research 137 (2005) 243–251

C-isotopic stratification in a Neoproterozoic postglacial ocean

Yanan Shena,∗, Tonggang Zhangb, Xuelei Chub

a Centre GEOTOP, Universit´e du Quebeca Montreal, C.P. 8888, Succursale Centre-Ville, Montr´eal, Qc., Canada H3C 3P8b Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China

Accepted 1 March 2005

Abstract

We report C-isotopic data from the cap carbonate overlying the terminal Proterozoic Nantuo diamictites in south China.The C-isotopic measurements reveal a large C-isotopic gradient of∼3‰ along paleoenvironmental transect from shelf to deepbasinal sedimentary facies. The C-isotopic stratification may attribute to the effect of biological pumping in a postglacial ocean.Our results confirm that the sea-level rise was rapid and that atmospheric CO2 was significantly high immediately followingdeglaciation.© 2005 Elsevier B.V. All rights reserved.

Keywords:Neoproterozoic; Cap carbonate; 13C/12C; Stratification; Ocean chemistry

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. Introduction

It is widely accepted today that glacial deposits ofeoproterozoic age (1000–544 Ma) occur globally and

epeatedly (Hambrey and Harland, 1981; Kaufman etl., 1997; Evans, 2000and references therein) and theost extensive of these Neoproterozoic glacial deposits

ormed during the Marinoan glaciation of∼600 Ma inge (Knoll and Walter, 1992; Knoll, 2000). Immedi-tely above many of these tillites are distinctive “caparbonate” beds, which are usually several meters thicknd typically consist of dolostone and limestone (e.g.,arbonne et al., 1994; Kennedy, 1996; Walter et al.,000; James et al., 2001). These cap carbonates are

∗ Corresponding author.E-mail address:shen.yanan@uqam.ca (Y. Shen).

distributed globally and exhibit unique sedimentstructures and textures such as megaripples, peand seafloor-encrusting cements and they are sicantly depleted in13C with typical C-isotopic valuof 0∼ −5‰ (Knoll et al., 1986; Kennedy et al., 199Hoffman et al., 1998; Halverson et al., 2002, 2004).

Three models have proposed to explain the unusedimentary structure of cap carbonate such astal fans and vertical tube-like structures as welthe 13C-depleted isotopic signature.Grotzinger andKnoll (1995) attributed the precipitation of cap cbonate and large negative C-isotopic excursions towelling processes during deglaciation. Accordingthe upwelling model, the ocean was physically stfied during glaciation, and, as a result, the dissoinorganic carbon in the surface waters becameriched in13C as light carbon was exported to the d

301-9268/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.precamres.2005.03.004

244 Y. Shen et al. / Precambrian Research 137 (2005) 243–251

ocean. Thus, the C-isotopic signature of deep watersbecame significantly enriched12C. When turnover be-gan during deglaciation, alkalinity-laden deep waterwith an extraordinarily light C-isotopic compositionmixed with surface water, and, consequently, the capcarbonates with extremely lightδ13C values precipi-tated (Grotzinger and Knoll, 1995).

The second model is called “Neoproterozoic snow-ball Earth”.Hoffman et al. (1998)proposed a snowballEarth hypothesis that was elaborated fromKirschvink(1992). The snowball Earth model hypotheses that theEarth was covered by ice for millions of years and there-fore the hydrological cycle and continental weatheringwere essentially shut down during the Neoproterozoicglaciations. The continuous buildup of CO2 in the at-mosphere for millions of years from volcanic and meta-morphic outgassing is argued to have melted the iceand eventually triggered deglaciations (Hoffman et al.,1998). According to the snowball Earth hypothesis, therapid continental weathering promoted by high levelsof atmospheric CO2 during deglaciation delivered largequantities of dissolved inorganic carbon to the oceans,and subsequently led to the precipitation of cap carbon-ate.

In contrast,Kennedy et al. (2001)and Jiang etal. (2003b)argued that the Neoproterozoic cap car-bonates may have resulted from postglacial sea-levelrise and they attributed the large negative C-isotopicexcursion associated with cap carbonates to destabi-lization of methane hydrate formed during the Neo-p d ins lightC of−h atecc tion,m shelfc me1

eo-p pica ly onti so-t es-t istryb

2. Geological setting and sampling

Neoproterozoic successions in south China containmultiple diamictites and many of them can be differ-entiated and correlated on the basis of regional geolog-ical observations (e.g.,Liu, 1991). Among them, themost widely distributed diamictite across south Chinais the Nantuo tillite. Biostratigraphy, chemostratigra-phy, geochronology, as well as paleomagnetic dataprovide strong evidence that the Nantuo glaciation islikely of the Mariano age (∼600 Ma) (Liu, 1991; Bar-fod et al., 2002; Shen and Schidlowski, 2000; Shen,2002; Macouina et al., 2004). The Nantuo tillite isoverlain by the Doushantuo Formation, which con-sists mostly of carbonates and black shales, as well asphosphate deposits. Sedimentary facies and paleogeog-raphy of the Doushantuo Formation have been exten-sively studied by Chinese geologists over the last fewdecades (e.g.,Tang et al., 1982, 1998; Liu, 1991; Xueet al., 1993; Jiang et al., 2003a). Well-reconstructeddifferent sedimentary facies along paleoenvironmentalgradient from the carbonate platform to deep basinalfacies are perfectly preserved in south China (Fig. 1),and thus provide an excellent opportunity to investi-gate the two dimensional structure of the postglacialocean.

Like coeval Neoproterozoic cap carbonates world-wide, the Nantuo cap dolostone overlies glaciogenic di-amictite without evidence of reworking or hiatus. Thelithological contact between the cap carbonates ando en-t eepw natei con-f theD ions,t lacks han1 natesa helff andu ck-b e ear-l ;C apc reef elf-s

roterozoic glaciation. Biogenic methane foundeeafloor clathrates is characterized by extreme-isotopic compositions with an average value60‰ (e.g., Schidlowski et al., 1983), and mayave played significant role in regulating past climhanges (e.g.,Dickens, 2003; Pavlov et al., 2003). Be-ause of its extremely depleted isotopic composiethane released from permafrost on continental

an result in carbonates precipitation with extre3C-depletion.

In order to test different hypotheses about the Nroterozoic earth history, we performed C-isotonalyses of cap carbonates deposited immediate

he top of the Nantuo tillite of Marinoan age (∼600 Ma)n south China. More specifically, we integrated iopic study with analysis of sedimentary facies to invigate isotopic change and postglacial ocean chemoth in time and space.

verlying sedimentary rocks varies among sedimary facies. In the siliciclastic successions of the dater basinal environment, the Nantuo cap carbo

s composed of thin-bedded dolostone (1.7–4.5 m)ormably overlain by organic-rich black shales ofoushantuo Formation. In the shelf-slope success

he cap dolostones are overlain by organic-rich bhales with variable thickness (but usually more t0 m) that pass upward into a sequence of carbond phosphorite. In the carbonate platform and s

acies, the Nantuo cap consists of fine-laminatedsually pinkish dolostone. It is overlain by the thiedded carbonates as well as phosphates where th

iest animal fossils were discovered (Xiao et al., 1998hen et al., 2004). We studied seven sections of carbonates overlying the Nantuo tillite including throm carbonate platform-shelf facies, two from shlope, and two from basinal facies.

Y. Shen et al. / Precambrian Research 137 (2005) 243–251 245

Fig. 1. Distribution of sedimentary facies of the Doushantuo Formation across south China (not on scale).

3. Results and discussion

3.1. C-isotope record of the Nantuo capcarbonates

δ13C andδ18O data for the Nantuo cap carbonateare plotted inTable 1andFigs. 2–4. In two measuredplatform-shelf sections,δ13C begin negative values of−2.5 and−3.8‰ and remain stable through the strati-graphic sections. However, in measured section 2,δ13Cvalues exhibit some oscillation at the base, though mostof isotopic compositions are around−3‰ (Fig. 2).In two shelf-slope sections,δ13C are approximately−4‰ at the base and gradually decreased by about 1‰through the cap dolostone (Fig. 3). A similar decreaseof ∼1‰ in δ13C value is observed in two basinal sec-tions (Fig. 4). However, basinal cap dolostones displaymuch lighterδ13C values, down to−9‰, which are notobserved in platform-shelf or shelf-slope facies.

In general, the C-isotopic values of the Nantuo capcarbonates decrease stratigraphically for most of thesections we studied (Figs. 2–4), consistent with iso-topic records from many Marinoan cap carbonates ex-amined elsewhere (Kaufman et al., 1997; Kennedy etal., 1998; Halverson et al., 2002, 2004; Hoffman andSchrag, 2002; Nogueira et al., 2003; Porter et al., 2004).The oscillation of C-isotopic composition observed inplatform-shelf facies may reflect mixing of different

proportion of a light carbon reservoir from deeper wa-ters by upwelling processes during deglaciation. Inaddition, our results reveal distinctive C-isotopic dif-ferences of∼3‰ in average between shallow watershelf facies and deep basinal facies (Figs. 2–4). Thereare several potential explanations for the significant C-isotopic difference among different facies of the Nan-tuo cap carbonates. We will explore different interpre-tations and discuss the implications of C-isotopic strat-ification for postglacial environments.

3.2. Possible interpretations for C-isotopicstratification

C-isotopic compositions are often well preservedin Proterozoic carbonates because diagenetic recrystal-lization of carbonates occurs in a system with a low wa-ter/rock ratio for carbon and therefore they record majorchanges of ocean chemistry (e.g.,Buick et al., 1995;Kaufman and Knoll, 1995and references therein).It is generally accepted that Neoproterozoic cap car-bonates were formed by active CaCO3 precipitationthat was lithified synchronous with deposition, whichprovided favorable conditions for preservation of pri-mary isotopic signature (Kaufman et al., 1997; Hoff-man et al., 1998; James et al., 2001). Meteoric fluidscould alter isotopic compositions of carbonates (Veizer,1983; Banner and Hansen, 1990), but the Nantuo cap

246 Y. Shen et al. / Precambrian Research 137 (2005) 243–251

Table 1Carbon and oxygen isotopic compositions of the Nantuo capcarbonates

Section Depth (m) δ13C δ18O

Platform-shelf section 1

4.66 −3.5 −7.24.36 −3.8 −8.33.96 −3.6 −7.53.66 −3.6 −7.13.16 −3.3 −7.92.76 −3.4 −7.51.96 −2.7 −7.11.95 −3.2 −9.21.35 −2.8 −7.20.85 −4.1 −9.20.35 −3.5 −7.80.05 −3.8 −8.4

Platform-shelf section 2

1.5 −3 −6.41 0.4 0.10.9 −2.9 −60.5 −3 −6.80.2 −1.5 −4.10.03 −2.4 −7.3

Platform-shelf section 3

2.55 −2.5 −4.81.55 −2 −4.41.25 −2.5 −4.90.85 −2.2 −5.30.6 −2.3 −3.50.2 −2.2 −3.1

Shelf-slope section 1

2.8 −4.6 −82.6 −4.7 −8.22.4 −4.5 −8.92.2 −3.6 −5.91.9 −4.1 −7.21.6 −3.9 −7.31.3 −3.5 −8.31 −3.4 −8.50.8 −3 −100.6 −2.8 −8.50.45 −3.1 −100.3 −3.2 −9.60.11 −4.2 −8.90.01 −3.9 −7

Section shelf-slope section 2

2.4 −5.2 −9.41.5 −4.2 −8.61.2 −4.1 −91 −3.8 −7.20.8 −4.5 −7.40.42 −4.3 −9.60.26 −3.6 −90.03 −3.8 −9.3

Table 1 (Continued)

Section Depth (m) δ13C δ18O

Basinal section1

4.5 −9.7 −8.94 −10 −8.93 −8.9 −8.52 −8.9 −8.51 −9.2 −8.70.1 −8.3 −10

Basinal section 2

2.15 −7.2 −9.12.05 −7.3 −9.31.9 −6.9 −81.7 −7.1 −8.71.45 −6.3 −8.31.25 −5.9 −8.71 −5.9 −9.30.75 −7.1 −8.70.55 −6.4 −8.50.45 −6.7 −8.20.35 −5.9 −7.30.2 −6.2 −7.8

Fig. 2. C-isotopic chemostratigraphy of the Nantuo cap carbonatesfrom platform-shelf sedimentary facies.

Y. Shen et al. / Precambrian Research 137 (2005) 243–251 247

Fig. 3. C-isotopic chemostratigraphy of the Nantuo cap carbonatesfrom shelf-slope sedimentary facies.

carbonates from shelf-slope as well as basinal faciesare bounded above by a flooding surface with little ev-idence for exposure. Therefore, it is unlikely that thecap carbonates and their C-isotopic compositions weresignificantly altered by post-depositional processes.

Secondly, a crossplot ofδ13C andδ18O, a widelyapplied indicator for evaluation of meteoric alterationshows little evidence for a linear correlation betweenδ13C andδ18O (Fig. 5) and therefore provides little sup-port for significant C-isotopic modification by meteoricdiagenesis. Mn/Sr ratios of the Nantuo cap carbonateswere not measured to evaluate meteoric diagenesis, be-cause the Nantuo cap carbonates are significantly en-riched in Mn, presumably originating from weatheringproducts of underlying Mn-rich sediments.

Thirdly, the strong reproducibility of C-isotopicchemostratigraphy and the distinctive trend towards

Fig. 4. C-isotopic chemostratigraphy of the Nantuo cap carbonatesfrom basinal sedimentary facies.

more negative isotopic values for the Nantuo capcarbonate, as well as their consistency with isotopicrecords elsewhere suggests that the cap carbonateswere not subjected to extensive alteration by meteoric

Fig. 5. Cross plot betweenδ13C andδ18O for the Nantuo cap car-bonates.

248 Y. Shen et al. / Precambrian Research 137 (2005) 243–251

fluids after deposition. Therefore, stratigraphic andgeochemical characteristics exclude the possibility thatprimaryδ13C values of the Nantuo cap carbonates weresignificantly reset during meteoric diagenesis.

A second possible interpretation of the isotopicstratification is that the Nantuo cap carbonate precipi-tation was not synchronous in shallow shelf and deepbasinal facies. Under this scenario, the cap carbonate inshallow-water facies would only correlate to the lowerpart of the deep basinal sections. Marine dissolutionbelow some critical oceanic compensation depth couldleave no isotopic signature. However, had the cap car-bonates been dissolved, it would have been the cap inbasinward facies rather than those in shelfward facies,in contrast to the isotopic record. Also, similarly de-creasing trends of C-isotope of∼1‰ among differentfacies of the Nantuo cap carbonates are inconsistentwith an incomplete isotopic record (Figs. 2–4). There-fore, diachroneity of the cap carbonate precipitationis not adequate to explain the C-isotopic difference be-tween the shallow- and deep-water facies of the Nantuocap carbonate.

The third, and preferred interpretation of theC-isotopic data presented in this study is that the C-isotopic difference between the shallow- and deep-water facies represents a near primary isotopic gradientassociated with water depth, and record unusual envi-ronmental conditions in the aftermath of a Neoprotero-zoic glaciation.

3p

nceb acea ,a icalp ofb keo rdt -gW n-w omei -l opicd ched∼ n-

tained because of the sluggish vertical circulation of thebasin. In contrast, remineralization of organic matterin deep water environments would release light carbonthat could be transported to the photic zone via up-welling and would homogenize the carbon chemistryof the ocean (Kump, 1991). Therefore, theδ13C valueof deep waters and the isotopic gradient may be de-termined by biogeochemical processes within the deepocean (Kump, 1991).

The C-isotopic gradients between platform and basi-nal facies of the Nantuo cap carbonates suggest an in-tense biological pump operated by the proliferation ofmicrobial life in surface waters of a postglacial ocean.High pCO2 in the atmosphere immediately after theNeoproterozoic deglaciation (Grotzinger and Knoll,1995; Hoffman et al., 1998) would have facilitated thesequestration of CO2 via the biological pump and pro-duced enrichment of13C in surface waters consistentwith the observed isotopic signature in the Nantuo capof platform-shelf facies. In this way, biological pumpmight have presented a lower partial pressure of CO2and thus lowered the CO2 content in the atmosphere.

As discussed above, the isotopic consequence ofthe biological pump could be homogenized by verticaloceanic circulation. However, the rapid rise of the post-glacial sea level (Kennedy, 1996; Hoffman et al., 1998;Jiang et al., 2003b) would have reduced the oceanic cir-culation and provided favorable conditions to maintainisotopic stratification. Therefore, the C-isotopic strati-fication reported in this study may reflect extreme phys-i dr ero-z

3p

-t icalp oc geo-c eC atesf liza-t ro-t t,t callym cing

.3. Implications of isotopic stratification forostglacial environmental changes

In the today’s world oceans, the isotopic differeetween dissolved inorganic carbon (DIC) in surfnd deep waters in the O2-minimum zone is about 2‰nd this difference results from the effect of biologump (Kroopnick, 1985). The isotopic consequenceiological pumping may result from preferential uptaf 12C by primary producers followed by downwa

ransport and remineralization of the13C-depleted oranic matter in deep waters (Broecker and Peng, 1982).ith greater primary productivity increasing doward flux of organic matter, the surface waters bec

ncreasingly enriched in13C. For example, in the isoated anoxic basin of modern Black Sea, the C-isotifference between surface and deep waters rea7‰ (Deuser, 1970). This isotopic gradient is mai

cal controls such as highpCO2 in the atmosphere anapid sea-level rise immediately after the Neoprotoic deglaciation.

.4. A word about methane cycling in aostglacial ocean

Although the enrichment of13C in the shallow waer Nantuo caps may record the effect of biologump, the significant lightδ13C values of the Nantuap carbonates may reflect methane-influenced biohemical process.Jiang et al. (2003b)argue that th-isotopic compositions of the Nantuo cap carbon

rom south China may have resulted from destabiion of methane hydrate formed during the Neoperozoic glaciation. In the O2-depleted environmenhe methane could have been oxidized anaerobiediated by consortia of archaea and sulfate-redu

Y. Shen et al. / Precambrian Research 137 (2005) 243–251 249

bacteria (Boetius et al., 2000) and thus facilitated pre-cipitation of cap carbonates according to the followingequation:

CH4 + SO42− + Ca2+ → CaCO3 + H2S + H2O

(1)

The C-isotopic compositions of biogenic methaneare extraordinarily light with an average value of−60‰ (e.g.,Schidlowski et al., 1983). Therefore, theanaerobic oxidation of methane would have resultedin extremely light C-isotope values, similar to some ofthose observed in the Nantuo cap carbonates. Amongother sources, methane could originate from methano-genesis in an unusual postglacial ocean, particularlyunder low sulfate concentrations.

During the glaciation the riverine input of sulfate,the primary source of sulfate for oceans, would havebeen greatly decreased. However, biological sulfate re-duction in a glacial ocean can be significant becausesulfate-reducers can thrive across a wide range of eco-logical conditions, from extremely cold habitats to ac-tive hydrothermal systems (e.g.,Shen and Buick, 2004and references therein). In a glacial ocean with limitedsulfate inputs, biological sulfate reduction would havesignificantly drawn down the sulfate concentration. Inconsequence, the postglacial oceans would have beenlow in sulfate as shown by S-isotopic data of trace sul-fate in cap carbonates (Hurtgen et al., 2002). At lowsulfate concentrations, methanogenesis could have outc omed

C

A on,m -i andr fromt

4

iths gra-d inals t ofb o-

bial life in photic zone of the postglacial ocean likelydrove the strong biological pump. Consistent with theprevious hypotheses, our isotopic results suggest thatatmosphericpCO2 was extremely high immediately af-ter the deglaciation and that sea-level rise was quiterapid. Though our results do not provide clear answerabout global distribution of isotopic stratification in theNeoproterozoic postglacial oceans, the data presentedin this study allow us to understand ocean chemistryand physical conditions in a more detail. Clearly, theemerging approach of isotopic investigation in spaceand time (Shen et al., 2003; Hotinski et al., 2004) willcontinue to improve our knowledge about evolution ofEarth’s surface environment.

Note added in proof

While our paper was waiting for publishing,Zhouet al. (2004)reported a 1–2‰ difference between shal-low and deep water facies of the Nantuo cap carbonates.

Acknowledgements

This study was supported by Canada Research ChairProgram, Natural Sciences and Engineering ResearchCouncil of Canada, as well as National Natural Sci-ence Foundation of China. We thank Jon Payne andtwo reviewers for constructive comments.

R

B s iso-ctionchim.

B ,ts on212.

B del,2000.obic

B ress,

B om-mall

fects

C ffins,ian

ompeted over biological sulfate reduction and becominate biogeochemical process in the system:

O2 + 4H2 → CH4 + 2H2O (2)

s such, in concert with biological sulfate reductiethane cycling (Eqs.(1) and (2)) could have mod

fied the carbon reservoir in a postglacial oceanesulted in unusual isotopic records such as thosehe Nantuo cap carbonates in south China.

. Conclusions

By integration of C-isotopic measurements wedimentary facies, we recognized a large isotopicient from the carbonate platform-shelf to deep basettings that may result dominantly from the effeciological pumping. If so, the proliferation of micr

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