Precambrian Research 137 (2005) 243–251
C-isotopic stratification in a Neoproterozoic postglacial ocean Yanan Shen a,∗ , Tonggang Zhang b , Xuelei Chu b a
Centre GEOTOP, Universit´e du Qu´ebec a` Montr´eal, C.P. 8888, Succursale Centre-Ville, Montr´eal, Qc., Canada H3C 3P8 b 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 deep basinal 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 following deglaciation. © 2005 Elsevier B.V. All rights reserved. Keywords: Neoproterozoic; Cap carbonate; 13C/12C; Stratification; Ocean chemistry
1. Introduction It is widely accepted today that glacial deposits of Neoproterozoic age (1000–544 Ma) occur globally and repeatedly (Hambrey and Harland, 1981; Kaufman et al., 1997; Evans, 2000 and references therein) and the most extensive of these Neoproterozoic glacial deposits formed during the Marinoan glaciation of ∼600 Ma in age (Knoll and Walter, 1992; Knoll, 2000). Immediately above many of these tillites are distinctive “cap carbonate” beds, which are usually several meters thick and typically consist of dolostone and limestone (e.g., Narbonne et al., 1994; Kennedy, 1996; Walter et al., 2000; James et al., 2001). These cap carbonates are ∗
Corresponding author. E-mail address:
[email protected] (Y. Shen).
0301-9268/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2005.03.004
distributed globally and exhibit unique sedimentary structures and textures such as megaripples, peloids and seafloor-encrusting cements and they are significantly depleted in 13 C with typical C-isotopic value of 0 ∼ −5‰ (Knoll et al., 1986; Kennedy et al., 1998; Hoffman et al., 1998; Halverson et al., 2002, 2004). Three models have proposed to explain the unusual sedimentary structure of cap carbonate such as crystal fans and vertical tube-like structures as well as the 13 C-depleted isotopic signature. Grotzinger and Knoll (1995) attributed the precipitation of cap carbonate and large negative C-isotopic excursions to upwelling processes during deglaciation. According to the upwelling model, the ocean was physically stratified during glaciation, and, as a result, the dissolved inorganic carbon in the surface waters became enriched in 13 C as light carbon was exported to the deep
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ocean. Thus, the C-isotopic signature of deep waters became significantly enriched 12 C. When turnover began during deglaciation, alkalinity-laden deep water with an extraordinarily light C-isotopic composition mixed with surface water, and, consequently, the cap carbonates with extremely light δ13 C values precipitated (Grotzinger and Knoll, 1995). The second model is called “Neoproterozoic snowball Earth”. Hoffman et al. (1998) proposed a snowball Earth hypothesis that was elaborated from Kirschvink (1992). The snowball Earth model hypotheses that the Earth was covered by ice for millions of years and therefore the hydrological cycle and continental weathering were essentially shut down during the Neoproterozoic glaciations. The continuous buildup of CO2 in the atmosphere for millions of years from volcanic and metamorphic outgassing is argued to have melted the ice and eventually triggered deglaciations (Hoffman et al., 1998). According to the snowball Earth hypothesis, the rapid continental weathering promoted by high levels of atmospheric CO2 during deglaciation delivered large quantities of dissolved inorganic carbon to the oceans, and subsequently led to the precipitation of cap carbonate. In contrast, Kennedy et al. (2001) and Jiang et al. (2003b) argued that the Neoproterozoic cap carbonates may have resulted from postglacial sea-level rise and they attributed the large negative C-isotopic excursion associated with cap carbonates to destabilization of methane hydrate formed during the Neoproterozoic glaciation. Biogenic methane founded in seafloor clathrates is characterized by extreme light C-isotopic compositions with an average value of −60‰ (e.g., Schidlowski et al., 1983), and may have played significant role in regulating past climate changes (e.g., Dickens, 2003; Pavlov et al., 2003). Because of its extremely depleted isotopic composition, methane released from permafrost on continental shelf can result in carbonates precipitation with extreme 13 C-depletion. In order to test different hypotheses about the Neoproterozoic earth history, we performed C-isotopic analyses of cap carbonates deposited immediately on the top of the Nantuo tillite of Marinoan age (∼600 Ma) in south China. More specifically, we integrated isotopic study with analysis of sedimentary facies to investigate isotopic change and postglacial ocean chemistry both in time and space.
2. Geological setting and sampling Neoproterozoic successions in south China contain multiple diamictites and many of them can be differentiated and correlated on the basis of regional geological observations (e.g., Liu, 1991). Among them, the most widely distributed diamictite across south China is the Nantuo tillite. Biostratigraphy, chemostratigraphy, geochronology, as well as paleomagnetic data provide strong evidence that the Nantuo glaciation is likely of the Mariano age (∼600 Ma) (Liu, 1991; Barfod et al., 2002; Shen and Schidlowski, 2000; Shen, 2002; Macouina et al., 2004). The Nantuo tillite is overlain by the Doushantuo Formation, which consists mostly of carbonates and black shales, as well as phosphate deposits. Sedimentary facies and paleogeography of the Doushantuo Formation have been extensively studied by Chinese geologists over the last few decades (e.g., Tang et al., 1982, 1998; Liu, 1991; Xue et al., 1993; Jiang et al., 2003a). Well-reconstructed different sedimentary facies along paleoenvironmental gradient from the carbonate platform to deep basinal facies are perfectly preserved in south China (Fig. 1), and thus provide an excellent opportunity to investigate the two dimensional structure of the postglacial ocean. Like coeval Neoproterozoic cap carbonates worldwide, the Nantuo cap dolostone overlies glaciogenic diamictite without evidence of reworking or hiatus. The lithological contact between the cap carbonates and overlying sedimentary rocks varies among sedimentary facies. In the siliciclastic successions of the deep water basinal environment, the Nantuo cap carbonate is composed of thin-bedded dolostone (1.7–4.5 m) conformably overlain by organic-rich black shales of the Doushantuo Formation. In the shelf-slope successions, the cap dolostones are overlain by organic-rich black shales with variable thickness (but usually more than 10 m) that pass upward into a sequence of carbonates and phosphorite. In the carbonate platform and shelf facies, the Nantuo cap consists of fine-laminated and usually pinkish dolostone. It is overlain by the thickbedded carbonates as well as phosphates where the earliest animal fossils were discovered (Xiao et al., 1998; Chen et al., 2004). We studied seven sections of cap carbonates overlying the Nantuo tillite including three from carbonate platform-shelf facies, two from shelfslope, and two from basinal facies.
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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 cap carbonates δ13 C and δ18 O data for the Nantuo cap carbonate are plotted in Table 1 and Figs. 2–4. In two measured platform-shelf sections, δ13 C begin negative values of −2.5 and −3.8‰ and remain stable through the stratigraphic sections. However, in measured section 2, δ13 C values exhibit some oscillation at the base, though most of isotopic compositions are around −3‰ (Fig. 2). In two shelf-slope sections, δ13 C are approximately −4‰ at the base and gradually decreased by about 1‰ through the cap dolostone (Fig. 3). A similar decrease of ∼1‰ in δ13 C value is observed in two basinal sections (Fig. 4). However, basinal cap dolostones display much lighter δ13 C values, down to −9‰, which are not observed in platform-shelf or shelf-slope facies. In general, the C-isotopic values of the Nantuo cap carbonates decrease stratigraphically for most of the sections we studied (Figs. 2–4), consistent with isotopic records from many Marinoan cap carbonates examined elsewhere (Kaufman et al., 1997; Kennedy et al., 1998; Halverson et al., 2002, 2004; Hoffman and Schrag, 2002; Nogueira et al., 2003; Porter et al., 2004). The oscillation of C-isotopic composition observed in platform-shelf facies may reflect mixing of different
proportion of a light carbon reservoir from deeper waters by upwelling processes during deglaciation. In addition, our results reveal distinctive C-isotopic differences of ∼3‰ in average between shallow water shelf facies and deep basinal facies (Figs. 2–4). There are several potential explanations for the significant Cisotopic difference among different facies of the Nantuo cap carbonates. We will explore different interpretations and discuss the implications of C-isotopic stratification for postglacial environments. 3.2. Possible interpretations for C-isotopic stratification C-isotopic compositions are often well preserved in Proterozoic carbonates because diagenetic recrystallization of carbonates occurs in a system with a low water/rock ratio for carbon and therefore they record major changes of ocean chemistry (e.g., Buick et al., 1995; Kaufman and Knoll, 1995 and references therein). It is generally accepted that Neoproterozoic cap carbonates were formed by active CaCO3 precipitation that was lithified synchronous with deposition, which provided favorable conditions for preservation of primary isotopic signature (Kaufman et al., 1997; Hoffman et al., 1998; James et al., 2001). Meteoric fluids could alter isotopic compositions of carbonates (Veizer, 1983; Banner and Hansen, 1990), but the Nantuo cap
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Table 1 Carbon and oxygen isotopic compositions of the Nantuo cap carbonates δ13 C
δ18 O
4.66 4.36 3.96 3.66 3.16 2.76 1.96 1.95 1.35 0.85 0.35 0.05
−3.5 −3.8 −3.6 −3.6 −3.3 −3.4 −2.7 −3.2 −2.8 −4.1 −3.5 −3.8
−7.2 −8.3 −7.5 −7.1 −7.9 −7.5 −7.1 −9.2 −7.2 −9.2 −7.8 −8.4
Platform-shelf section 2
1.5 1 0.9 0.5 0.2 0.03
−3 0.4 −2.9 −3 −1.5 −2.4
−6.4 0.1 −6 −6.8 −4.1 −7.3
Platform-shelf section 3
2.55 1.55 1.25 0.85 0.6 0.2
−2.5 −2 −2.5 −2.2 −2.3 −2.2
−4.8 −4.4 −4.9 −5.3 −3.5 −3.1
Shelf-slope section 1
2.8 2.6 2.4 2.2 1.9 1.6 1.3 1 0.8 0.6 0.45 0.3 0.11 0.01
−4.6 −4.7 −4.5 −3.6 −4.1 −3.9 −3.5 −3.4 −3 −2.8 −3.1 −3.2 −4.2 −3.9
−8 −8.2 −8.9 −5.9 −7.2 −7.3 −8.3 −8.5 −10 −8.5 −10 −9.6 −8.9 −7
Section shelf-slope section 2
2.4 1.5 1.2 1 0.8 0.42 0.26 0.03
−5.2 −4.2 −4.1 −3.8 −4.5 −4.3 −3.6 −3.8
−9.4 −8.6 −9 −7.2 −7.4 −9.6 −9 −9.3
Section
Platform-shelf section 1
Depth (m)
Table 1 (Continued ) δ13 C
δ18 O
Basinal section1
4.5 4 3 2 1 0.1
−9.7 −10 −8.9 −8.9 −9.2 −8.3
−8.9 −8.9 −8.5 −8.5 −8.7 −10
Basinal section 2
2.15 2.05 1.9 1.7 1.45 1.25 1 0.75 0.55 0.45 0.35 0.2
−7.2 −7.3 −6.9 −7.1 −6.3 −5.9 −5.9 −7.1 −6.4 −6.7 −5.9 −6.2
−9.1 −9.3 −8 −8.7 −8.3 −8.7 −9.3 −8.7 −8.5 −8.2 −7.3 −7.8
Section
Depth (m)
Fig. 2. C-isotopic chemostratigraphy of the Nantuo cap carbonates from platform-shelf sedimentary facies.
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Fig. 3. C-isotopic chemostratigraphy of the Nantuo cap carbonates from shelf-slope sedimentary facies.
carbonates from shelf-slope as well as basinal facies are bounded above by a flooding surface with little evidence for exposure. Therefore, it is unlikely that the cap carbonates and their C-isotopic compositions were significantly altered by post-depositional processes. Secondly, a crossplot of δ13 C and δ18 O, a widely applied indicator for evaluation of meteoric alteration shows little evidence for a linear correlation between δ13 C and δ18 O (Fig. 5) and therefore provides little support for significant C-isotopic modification by meteoric diagenesis. Mn/Sr ratios of the Nantuo cap carbonates were not measured to evaluate meteoric diagenesis, because the Nantuo cap carbonates are significantly enriched in Mn, presumably originating from weathering products of underlying Mn-rich sediments. Thirdly, the strong reproducibility of C-isotopic chemostratigraphy and the distinctive trend towards
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Fig. 4. C-isotopic chemostratigraphy of the Nantuo cap carbonates from basinal sedimentary facies.
more negative isotopic values for the Nantuo cap carbonate, as well as their consistency with isotopic records elsewhere suggests that the cap carbonates were not subjected to extensive alteration by meteoric
Fig. 5. Cross plot between δ13 C and δ18 O for the Nantuo cap carbonates.
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fluids after deposition. Therefore, stratigraphic and geochemical characteristics exclude the possibility that primary δ13 C values of the Nantuo cap carbonates were significantly reset during meteoric diagenesis. A second possible interpretation of the isotopic stratification is that the Nantuo cap carbonate precipitation was not synchronous in shallow shelf and deep basinal facies. Under this scenario, the cap carbonate in shallow-water facies would only correlate to the lower part of the deep basinal sections. Marine dissolution below some critical oceanic compensation depth could leave no isotopic signature. However, had the cap carbonates been dissolved, it would have been the cap in basinward facies rather than those in shelfward facies, in contrast to the isotopic record. Also, similarly decreasing trends of C-isotope of ∼1‰ among different facies of the Nantuo cap carbonates are inconsistent with an incomplete isotopic record (Figs. 2–4). Therefore, diachroneity of the cap carbonate precipitation is not adequate to explain the C-isotopic difference between the shallow- and deep-water facies of the Nantuo cap carbonate. The third, and preferred interpretation of the C-isotopic data presented in this study is that the Cisotopic difference between the shallow- and deepwater facies represents a near primary isotopic gradient associated with water depth, and record unusual environmental conditions in the aftermath of a Neoproterozoic glaciation. 3.3. Implications of isotopic stratification for postglacial environmental changes In the today’s world oceans, the isotopic difference between dissolved inorganic carbon (DIC) in surface and deep waters in the O2 -minimum zone is about 2‰, and this difference results from the effect of biological pump (Kroopnick, 1985). The isotopic consequence of biological pumping may result from preferential uptake of 12 C by primary producers followed by downward transport and remineralization of the 13 C-depleted organic matter in deep waters (Broecker and Peng, 1982). With greater primary productivity increasing downward flux of organic matter, the surface waters become increasingly enriched in 13 C. For example, in the isolated anoxic basin of modern Black Sea, the C-isotopic difference between surface and deep waters reached ∼7‰ (Deuser, 1970). This isotopic gradient is main-
tained because of the sluggish vertical circulation of the basin. In contrast, remineralization of organic matter in deep water environments would release light carbon that could be transported to the photic zone via upwelling and would homogenize the carbon chemistry of the ocean (Kump, 1991). Therefore, the δ13 C value of deep waters and the isotopic gradient may be determined by biogeochemical processes within the deep ocean (Kump, 1991). The C-isotopic gradients between platform and basinal facies of the Nantuo cap carbonates suggest an intense biological pump operated by the proliferation of microbial life in surface waters of a postglacial ocean. High pCO2 in the atmosphere immediately after the Neoproterozoic deglaciation (Grotzinger and Knoll, 1995; Hoffman et al., 1998) would have facilitated the sequestration of CO2 via the biological pump and produced enrichment of 13 C in surface waters consistent with the observed isotopic signature in the Nantuo cap of platform-shelf facies. In this way, biological pump might have presented a lower partial pressure of CO2 and thus lowered the CO2 content in the atmosphere. As discussed above, the isotopic consequence of the biological pump could be homogenized by vertical oceanic circulation. However, the rapid rise of the postglacial sea level (Kennedy, 1996; Hoffman et al., 1998; Jiang et al., 2003b) would have reduced the oceanic circulation and provided favorable conditions to maintain isotopic stratification. Therefore, the C-isotopic stratification reported in this study may reflect extreme physical controls such as high pCO2 in the atmosphere and rapid sea-level rise immediately after the Neoproterozoic deglaciation. 3.4. A word about methane cycling in a postglacial ocean Although the enrichment of 13 C in the shallow water Nantuo caps may record the effect of biological pump, the significant light δ13 C values of the Nantuo cap carbonates may reflect methane-influenced biogeochemical process. Jiang et al. (2003b) argue that the C-isotopic compositions of the Nantuo cap carbonates from south China may have resulted from destabilization of methane hydrate formed during the Neoproterozoic glaciation. In the O2 -depleted environment, the methane could have been oxidized anaerobically mediated by consortia of archaea and sulfate-reducing
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bacteria (Boetius et al., 2000) and thus facilitated precipitation of cap carbonates according to the following equation: CH4 + SO4 2− + Ca2+ → CaCO3 + H2 S + H2 O (1) The C-isotopic compositions of biogenic methane are extraordinarily light with an average value of −60‰ (e.g., Schidlowski et al., 1983). Therefore, the anaerobic oxidation of methane would have resulted in extremely light C-isotope values, similar to some of those observed in the Nantuo cap carbonates. Among other sources, methane could originate from methanogenesis in an unusual postglacial ocean, particularly under low sulfate concentrations. During the glaciation the riverine input of sulfate, the primary source of sulfate for oceans, would have been greatly decreased. However, biological sulfate reduction in a glacial ocean can be significant because sulfate-reducers can thrive across a wide range of ecological conditions, from extremely cold habitats to active hydrothermal systems (e.g., Shen and Buick, 2004 and references therein). In a glacial ocean with limited sulfate inputs, biological sulfate reduction would have significantly drawn down the sulfate concentration. In consequence, the postglacial oceans would have been low in sulfate as shown by S-isotopic data of trace sulfate in cap carbonates (Hurtgen et al., 2002). At low sulfate concentrations, methanogenesis could have out competed over biological sulfate reduction and become dominate biogeochemical process in the system: CO2 + 4H2 → CH4 + 2H2 O
(2)
As such, in concert with biological sulfate reduction, methane cycling (Eqs. (1) and (2)) could have modified the carbon reservoir in a postglacial ocean and resulted in unusual isotopic records such as those from the Nantuo cap carbonates in south China.
4. Conclusions By integration of C-isotopic measurements with sedimentary facies, we recognized a large isotopic gradient from the carbonate platform-shelf to deep basinal settings that may result dominantly from the effect of biological pumping. If so, the proliferation of micro-
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bial life in photic zone of the postglacial ocean likely drove the strong biological pump. Consistent with the previous hypotheses, our isotopic results suggest that atmospheric pCO2 was extremely high immediately after the deglaciation and that sea-level rise was quite rapid. Though our results do not provide clear answer about global distribution of isotopic stratification in the Neoproterozoic postglacial oceans, the data presented in this study allow us to understand ocean chemistry and physical conditions in a more detail. Clearly, the emerging approach of isotopic investigation in space and time (Shen et al., 2003; Hotinski et al., 2004) will continue to improve our knowledge about evolution of Earth’s surface environment. Note added in proof While our paper was waiting for publishing, Zhou et al. (2004) reported a 1–2‰ difference between shallow and deep water facies of the Nantuo cap carbonates.
Acknowledgements This study was supported by Canada Research Chair Program, Natural Sciences and Engineering Research Council of Canada, as well as National Natural Science Foundation of China. We thank Jon Payne and two reviewers for constructive comments.
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