Autoplay

quinta-feira, 1 de março de 2012

Uncovering the Neoproterozoic carbon cycle

Nature
(2012)
doi:10.1038/nature10854
Received
Accepted
Published online
Interpretations of major climatic and biological events in Earth history are, in large part, derived from the stable carbon isotope records of carbonate rocks and sedimentary organic matter1, 2. Neoproterozoic carbonate records contain unusual and large negative isotopic anomalies within long periods (10–100 million years) characterized by δ13C in carbonate (δ13Ccarb) enriched to more than +5 per mil. Classically, δ13Ccarb is interpreted as a metric of the relative fraction of carbon buried as organic matter in marine sediments2, 3, 4, which can be linked to oxygen accumulation through the stoichiometry of primary production3, 5. If a change in the isotopic composition of marine dissolved inorganic carbon is responsible for these excursions, it is expected that records of δ13Ccarb and δ13C in organic carbon (δ13Corg) will covary, offset by the fractionation imparted by primary production5. The documentation of several Neoproterozoic δ13Ccarb excursions that are decoupled from δ13Corg, however, indicates that other mechanisms6, 7, 8 may account for these excursions. Here we present δ13C data from Mongolia, northwest Canada and Namibia that capture multiple large-amplitude (over 10 per mil) negative carbon isotope anomalies, and use these data in a new quantitative mixing model to examine the behaviour of the Neoproterozoic carbon cycle. We find that carbonate and organic carbon isotope data from Mongolia and Canada are tightly coupled through multiple δ13Ccarb excursions, quantitatively ruling out previously suggested alternative explanations, such as diagenesis7, 8 or the presence and terminal oxidation of a large marine dissolved organic carbon reservoir6. Our data from Namibia, which do not record isotopic covariance, can be explained by simple mixing with a detrital flux of organic matter. We thus interpret δ13Ccarb anomalies as recording a primary perturbation to the surface carbon cycle. This interpretation requires the revisiting of models linking drastic isotope excursions to deep ocean oxygenation and the opening of environments capable of supporting animals9, 10, 11.

Figures at a glance

Main

There are two leading hypotheses for the documented large-amplitude12 Neoproterozoic carbon isotope anomalies. The first proposal is that the Neoproterozoic deep ocean carried a massive dissolved organic carbon (DOC) reservoir6. This model stems from the observation of unlinked changes in δ13Ccarb and δ13Corg, with the postulated large DOC pool allowing for δ13Corg records to be buffered against isotopic change. Another view of these large carbon isotopic excursions is represented by a set of hypotheses arguing for the infidelity of δ13Ccarb records. In this view, secondary alteration by meteoric waters or burial diagenesis is invoked to satisfy carbon isotope decoupling7, 8. Given such disparate proposals for explaining the Neoproterozoic carbon isotope record, and the implications for surface environments, climate, oxygen and animals, we aim to revisit the behaviour of the marine carbon cycle and test the validity of the above proposals with a new data set and model.
We sampled stratigraphic sections spanning the mid-Neoproterozoic Cryogenian glacial intermission (<717 to >635 million years ago) at high resolution in Mongolia, northern Namibia and northwest Canada (see Supplementary Information). In Mongolia, limestone of the Tayshir member (of the Tsagaan Oloom Formation) is bracketed by the Maikhan Ul and Khongoryn diamictites13 (Fig. 1). The Tayshir carbon isotope anomaly13 is recorded in stratigraphic sections throughout the basin, commences with a distinct flooding surface, and is developed predominantly in limestone micrite with relatively low primary porosity. The Tayshir anomaly can either be correlated to moderately negative values in the Gruis Formation of northern Namibia14 and Bonahaven Dolomite of the British–Irish Caledonides15, or to the Trezona anomaly in Australia16 and Namibia17, 18. The Trezona anomaly was also sampled through the Ombaatjie Formation in northern Namibia (see Supplementary Information), which directly underlies an erosion surface related to the Ghaub glaciation of 635.5±1.2 million years ago19. We further sampled the Cryogenian record from between the glaciations in the Mackenzie Mountains of northwest Canada (see Supplementary Information), where mixed carbonate and siliciclastic rocks of the Twitya and Keele formations are bounded by diamictites of the Sturtian-age Rapitan Group20 (716.47±0.24 million years old) and Marinoan-age (~635million years old) Ice Brook Formation. These strata preserve the post-Sturtian Rasthof anomaly in the Twitya Formation and an anomaly at the top of the Keele Formation that can be correlated to the Trezona anomaly21. Although there are alternative published correlations for particular anomalies described above, these correlations are not central to our argument.
Figure 1: Chemostratigraphic sections through the Tayshir member in the Uliastay and Tsagaan Gols (river valleys), Mongolia13.
Chemostratigraphic sections through the Tayshir member in the Uliastay and Tsagaan Gols (river valleys), Mongolia.
Red and grey symbols represent δ13Ccarb data from paired and unpaired samples, respectively (paired n = 235). The 2-s.d. errors are smaller than the symbols. The stratigraphic section to the right was measured 35km to the northeast, a distance shortened by the Cambrian Altaid orogeny13. Age constraints on the Dzabkhan volcanics are from ref. 31. K, Khongoryn.
We present two δ13Ccarb and δ13Corg records through the Tayshir member of Mongolia (Fig. 1). The overall total organic carbon (TOC) contents broadly follow siliciclastic content and are highest at the base of the more distal section (Uliastay Gol), but remain at about 0.08% throughout the succession with no apparent facies dependence (see Supplementary Information). In the Uliastay Gol section (n = 171 pairs), an over 10‰ negative δ13Ccarb anomaly (the Tayshir) at a depth of around 150m preserves an extremely tight coupling with δ13Corg through the entire excursion. Isotopic covariance is observed at even finer stratigraphic scales, such as the inflection recorded at around 50m depth. Consistent with this, a parallel section 35km to the east also preserves a tight isotopic coupling and similar net isotopic offset, or εTOC = δ13Ccarbδ13Corg (Fig. 1; see Supplementary Information).
In northwest Canada, the Cryogenian Twitya and Keele formations provide a comparison to glacial intermission records from Mongolia, and preserve the Rasthof and a latest-Cryogenian isotope excursion (Fig. 2). The post-Sturtian Twitya Formation preserves strong isotopic covariance between carbonate and organic carbon, and an εTOC value that closely approximates what is extracted from Mongolia (see Supplementary Information). The pre-Marinoan Keele Formation also appears to preserve carbon isotope covariance, but records a more variable εTOC, ranging from values similar to those observed in Mongolia to that recorded in the Pleistocene epoch (see Supplementary Information)5. In contrast to these records of tight covariance is the Trezona anomaly in Namibia (a TOC-poor dolomudstone, dolograinstone and dolomicrobialaminite; see Supplementary Information), in which δ13Ccarb and δ13Corg have little relationship to one another. This later observation is consistent with much of the published Neoproterozoic data6, 10, 11, 22. Our data set thus reflects both the convention (isotopic decoupling) as well as strong evidence of isotopic covariance.
Figure 2: A chemostratigraphic section through the late Cryogenian period of northwest Canada.
A chemostratigraphic section through the late Cryogenian period of northwest Canada.
As in Fig. 1, red circles represent carbonate carbon and blue denotes paired organic carbon (n = 73). Isotope scales are equal for carbonate and organic carbon, and the 2-s.d. errors are smaller than the data points. Stratigraphic keys are as in Fig. 1, as are the TOC (wt%) calibrations. Abbreviations correspond to the following: Ra, Rapitan Group; T, Twitya; I, Ice Brook. Carbonate data in the Keele and Twitya formations is from ref. 21 and overall trends are consistent with ref. 4.
We used these data to evaluate the late-Cryogenian carbon cycle. The classic interpretation of isotopic covariance suggests that biomass is synthesized from a relatively well-mixed marine dissolved inorganic carbon (DIC) reservoir and results in an εTOC value that reflects a kinetic fractionation associated with carbon fixation2. This simple depiction of the carbon cycle cleanly explains records from Mongolia and northwest Canada (Figs 1 and 2). Our finding contrasts with models requiring a large DOC pool, which preclude covariance between δ13Ccarb and δ13Corg. That is, data from Mongolia and northwest Canada are quantitatively inconsistent with a large DOC model (Figs 1 and 2). It has also been argued that burial diagenesis8 or meteoric alteration7 could reset the primary δ13Ccarb, calling into question the robustness of carbonate records and their use as a pre-fossil correlation tool. Rather than targeting the fidelity of δ13Corg, these studies emphasize the tight covariance between δ13Ccarb and δ18Ocarb through certain negative δ13Ccarb excursions as evidence for alteration (see Supplementary Information). However, the observation of a tight coupling between δ13Ccarb and δ13Corg through several Neoproterozoic anomalies (Figs 1 and 2) rules out alteration as the primary cause of all large Neoproterozoic negative δ13Ccarb excursions (see Supplementary Information). The δ18Ocarb in these rocks has almost certainly been reset, given fluid-rock exchange and the high concentration of oxygen in water, but explaining how exchange can reset δ13Ccarb given the enormous mass of carbon in carbonate sequences is more challenging8. The correlation of δ13Ccarb and δ18Ocarb is intriguing and may in fact have an environmental/diagenetic explanation (see Supplementary Information), but requires further investigation. Importantly, however, diagenesis is not responsible for the excursions through which δ13C covaries (Figs 1 and 2).
A complete understanding of the Neoproterozoic carbon cycle requires not only the interpretation of carbon isotope covariance (Figs 1 and 2)2, 4, but also an explanation for the data that underpinned these alternative hypotheses. We propose that much of the existing δ13Corg from Neoproterozoic sedimentary rocks reflects contamination with a secondary source of organic carbon, possibly from the erosion of organic-rich shale on land or the migration of hydrocarbons within the basin. The isotopic contribution from exogenous sources would be most evident when TOC values are low, consistent with a majority of the published data10, 22. This hypothesis does not require that all samples with low TOC carry anomalous δ13Corg, but notes the greater susceptibility of these samples2, 4, 23 and challenges their universal inclusion in environmental interpretations, especially when δ13Corg remains constant through intervals when δ13Ccarb is variable.
To explore the hypothesis that these data are produced from both an exogenous component and a primary source, we present a model for two-component mixing (Fig. 3a, b). Here, each component carries an organic carbon flux (monitored using TOC) and a distinct δ13Corg, so the product of mixing does not require a linear relationship between components (see Supplementary Information). One component in the model is organic carbon from primary producers (A in Fig. 3a), with a high mass fraction and an isotopic composition offset from dissolved inorganic carbon by εTOC. The second component (B in Fig. 3a) is exogenous, and could be terrigenous (detrital) inputs, secondary hydrocarbons, or contamination at some point on the rock’s life history (with an isotopic composition in the range of typical organic matter). Post-depositional processes, such as heterotrophic remineralization, will also alter mixed contributions (Fig. 3b). When then compared to δ13Ccarb (Fig. 3c, d), this simple mixing model fully describes Cryogenian and Ediacaran records without invoking alteration7, 8 or unfamiliar ocean chemistry6, 10, 11, 22. Our model does not require developing and oxidizing an ocean where DOC overwhelms DIC multiple times throughout the Neoproterozoic era, or driving extensive global alteration of carbonate platform sediments. This does not preclude contributions from DOC24, but the observation of isotopic covariance at the scales observed requires DOC to be significantly subordinate to DIC24. Therefore, we find our model more plausible from a physical and chemical oceanographic standpoint, because it requires only established, well-understood processes (that is, weathering, a circulating ocean and remineralization). It follows from these conclusions that δ13Ccarb may be a more faithful indicator of environmental change, and δ13Corg should be viewed critically, especially when TOC is low and invariant.
Figure 3: A two-component mixing model built to address variability of δ13C with changes in TOC.
A two-component mixing model built to address variability of [dgr]13C with changes in TOC.
Analytical precision for TOC measurements is 0.05wt % (1 s.d.). a, Where the mass fractions of each component are equal, traditional linear mixing is predicted. As one component increases in relative concentration, the mixing array becomes hyperbolic. The ratios listed on the figure denote the relative contribution of A and B. b, An example of the zone of possible solutions (red-shaded region) given contributions from a 1:10 mixing, as well as remineralization and post-deposition processes. These processes can alter samples to lower TOC, with modest changes in δ13C. In c and d the data are binned by age. c, Interglacial records from Mongolia and northwest Canada, with additional data from the Ombaatjie Formation (this study; see Supplementary Information; n = 79) and Trezona Formation22. d, Published Shuram data from Oman and China10, 11. Although the vertical axis is different in d, this model approach cleanly explains the excursion.
Given that δ13Ccarb may well faithfully preserve primary environmental information, we revisit the reading of this record. The conventional interpretation of the carbon cycle and isotope records uses a simplification of the full carbon flux equation for isotopic mass balance, where δ13Cvol = [δ13Corg×forg] + [δ13Ccarb×(1forg)], and forg is the relative fraction of carbon buried as organic matter. In the full carbon flux equation, δ13C inputs reflect a weighted average of volcanism (carbon entrained during subduction and mantle outgassing) and carbonate and organic carbon weathering25, 26, and the sinks include both carbonate and organic matter burial. So if we consider the full carbon flux equation, other ways of driving δ13C variations become apparent. Each of the above fluxes can change in magnitude and isotopic composition, and each carry specific consequences for climate and oxidant budgets. Also, the Neoproterozoic δ13C excursions occur in tens to hundreds of metres of shallow-water strata (third-order sequence tracts), implying a characteristic timescale of 105 to 106 years (ref. 27). It is challenging to produce such large (about 10‰) isotopic excursions, which in certain cases plunge beyond the canonical mantle value, on these timescales without the input of an enormous mass of low-δ13C carbon, presumably in the form of either organic matter or methane28. This requisite light carbon load may, in part, reflect the mass-balance counterpart to enriched background Cryogenian δ13Ccarb values. Such an injection of carbon would also significantly raise atmospheric or and consume existing oxidant reservoirs29, which is difficult to reconcile with the close stratigraphic association of at least some of the excursions and glacial deposits26. Another possibility is that the oxidation of reduced carbon is accomplished with SO42− or Fe oxides as the electron acceptor30, rather than O2, which may allow for some stabilization of through the effect on alkalinity24. If the duration of these events is found to be longer, then carbon mass input arguments should be modified accordingly, but the relevant reservoirs interacting with the surface carbon cycle remain unchanged.
The data and analysis presented here illustrates that δ13Ccarb preserves a faithful snapshot of the Neoproterozoic surface carbon cycle. This suggests that there is an environmental explanation for the enriched δ13Ccarb background values, and that at least some of the 105-to-106-year negative δ13Ccarb excursions reflect primary features of the surface carbon cycle. These anomalies thus require the episodic input of isotopically light carbon, presumably methane or organic matter, as well as the cyclic consumption and production of electron acceptors such as sulphate and O2. Our data do not eliminate diagenetic explanations for particular excursions throughout the world, but the covariation evident during many of the excursions does suggest a common explanation for all of them. Whether these excursions are driven by the recycling of stored carbon in the form of methane hydrate or ancient organic matter in sediments, or by the dynamics of nutrient-dependent productivity in the ocean remains uncertain. Consistent with both is the idea is that these excursions simply reflect a series of failed transitions from the more reducing and biologically simple Proterozoic world towards the more oxidizing and biologically complex Phanerozoic Earth on which we now live.

Methods

All isotope data are presented relative to the Vienna PeeDee Belemnite (VPDB) standard and calculated according to standard delta notation and reported in units of ‰. Carbonate carbon and oxygen isotope measurements made as one individual measurement, with the organic carbon isotopic composition measured separately, but on the same hand sample. Carbonate carbon isotopes were prepared by cutting hand samples to expose a fresh surface followed by micro-drilling 0.01g. A small aliquot of this powder was analysed on a VG Optima run in dual inlet mode, abutted to a common acid bath preparation device. Samples were acidified in H3PO4 at 90°C. The evolved CO2 was cyro-focused and analysed against an in-house standard reference gas. The reproducibility of this measurement was better than 0.1‰ (one standard deviation). Organic carbon analyses were performed on large (about 10g) samples to accommodate low TOC values. Samples were decalcified with concentrated HCl for 48h, buffered back to a neutral pH (>pH5), filtered, and dried. The mass of insoluble residue was taken as siliciclastic content. Homogenized residues were analysed on a Carlo Erba Elemental Analyzer attached to a ThermoFinnigan Delta V configured in continuous flow mode. Samples and standards were bracketed such that our >350 unique organic carbon analyses (each run in duplicate) were associated with 370 internal standards. These standards have known organic carbon contents and isotope values, and were used to calibrate TOC contents and isotopic compositions. Estimates of error are reported in the text.
The model presented in the manuscript is a multi-component mixing scenario, herein defined by a hyperbolic relationship. The equation represents a style of two-component mixing, where:

with the subscripts M, A and B representing the mixture, end-member A and end-member B. There are two weighting factors fA and (1fA), which are fractional terms seen in the equation describing linear mixing:

where X is mass. In solving this, we can choose the end-member compositions—XA, (δ13CTOC)A, XB and (δ13CTOC)B—and allow fA to vary from 0 to 1 (this is a relative flux term). This allows us to solve for XM (from equation (2) and then (δ13CTOC)M from equation (1)). For application in Fig. 3, we can include the δ13Ccarb on the same sample.

Author information

Affiliations

  1. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA

    • D. T. Johnston,
    • F. A. Macdonald,
    • B. C. Gill,
    • P. F. Hoffman &
    • D. P. Schrag
  2. School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada

    • P. F. Hoffman

Contributions

This project was conceived by D.T.J., F.A.M. and D.P.S. Field work was conducted by F.A.M. and P.F.H. Carbonate carbon analyses were performed by F.A.M. Organic carbon analyses and modelling were carried out by D.T.J. and B.C.G. The paper was written by all authors.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Nenhum comentário:

Postar um comentário

Observação: somente um membro deste blog pode postar um comentário.