Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

Abstract

Many negative δ¹³ C excursions in marine carbonates from the geological record are interpreted to record significant biogeochemical events in early Earth history. The assumption that no post-depositional processes can simultaneously alter carbonate and organic δ¹³ C values towards more negative values is the cornerstone of this approach. However, the effects of post-depositional alteration on the relationship between carbonate and organic δ¹³ C values have not been directly evaluated. Here we present paired carbonate and organic δ¹³ C records that exhibit a coupled negative excursion resulting from multiple periods of meteoric alteration of the carbonate δ¹³ C record, and consequent contributions of isotopically negative terrestrial organic matter to the sedimentary record. The possibility that carbonate and organic δ¹³ C records can be simultaneously shifted towards lower δ¹³ C values during periods of subaerial exposure may necessitate the reappraisal of some of the δ¹³ C anomalies associated with noteworthy biogeochemical events throughout Earth history.

ARTICLE

Received 19 Dec 2013 | Accepted 14 Jul 2014 | Published 19 Aug 2014

DOI: 10.1038/ncomms5672

Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

Amanda M. Oehlert1 & Peter K. Swart1

Many negative d13C excursions in marine carbonates from the geological record are interpreted to record signicant biogeochemical events in early Earth history. The assumption that no post-depositional processes can simultaneously alter carbonate and organic d13C values

towards more negative values is the cornerstone of this approach. However, the effects of post-depositional alteration on the relationship between carbonate and organic d13C values

have not been directly evaluated. Here we present paired carbonate and organic d13C records that exhibit a coupled negative excursion resulting from multiple periods of meteoric alteration of the carbonate d13C record, and consequent contributions of isotopically negative terrestrial organic matter to the sedimentary record. The possibility that carbonate and organic d13C records can be simultaneously shifted towards lower d13C values during periods of subaerial exposure may necessitate the reappraisal of some of the d13C anomalies associated with noteworthy biogeochemical events throughout Earth history.

1 Department of Marine Geosciences, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA. Correspondence and requests for materials should be addressed to A.M.O. (email: mailto:[email protected]

Web End [email protected] ).

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Signicant events in Earth history are often associated with major changes in the carbon isotopic composition of marine carbonates (d13Ccarb) and co-occurring sedimentary organic

matter (d13Corg). Globally correlatable excursions in marine d13Ccarb records are often thought to be related to global changes in the carbon cycle, such as those induced by snowball Earth events in the Neoproterozoic13, the oxygenation of Earths atmosphere47, the evolution of Ediacaran metazoans8,9, as well as marine and terrestrial extinction episodes1014. A common approach used to establish whether the variations in a d13Ccarb record reect changes in the isotopic composition of the ancient dissolved inorganic carbon pool is to assess the covariation between coeval carbonate and sedimentary organic carbon isotope records1522. Classically, covariant d13Ccarb and d13Corg records are interpreted as evidence that both the carbonate and organic matter were originally produced in the surface waters of the ocean, and that they have retained their original d13C composition10,1925, while decoupled d13Ccarb and d13Corg records have been interpreted as evidence for diagenetic alteration16,19,20,26, the Rothman ocean model27, or that local syn-sedimentary processes have made the d13Corg record noisy8.

The application of covariance between d13Ccarb and d13Corg is

based upon the theoretical assumption that y.no secondary processes are known (or for that matter, conceivable) which always shift the isotopic composition of carbonate and organic carbon in the same direction at the same ratey20.

This assumption has been widely used to establish the original nature of Precambrian and Palaeozoic d13Ccarb records derived from shallow platform and marginal marine carbonates10,15,16,19,20,22. Consequently, the analysis of coeval d13Ccarb and d13Corg values have been a fundamental approach in

studies of Precambrian and Palaeozoic carbon cycling, because it is thought to distinguish geologically meaningful records of signicant biogeochemical changes in Earth history from those records that have been altered by diagenesis.

Since shallow marine deposits may have been periodically subaerially exposed during sea-level oscillations, it is important to address the possibility of diagenetic alteration. This is particularly important since freshwater alteration has been shown to generate negative d13Ccarb excursions that are similar in magnitude to those observed in early Earth history28. Although a variety of other diagnostic tools have been employed to assess the degree of alteration, including trace element ratios29,30, cathodoluminescence31,32, as well as the relationship between d13Ccarb and d18Ocarb records3335, the covariance between

d13Ccarb and d13Corg records is thought to prove that the

system is rock buffered and that the records have retained their initial d13C values10,15,16,19,20,22. Remarkably, however, the effects of diagenesis on the relationship between carbonate and organic d13C records have never been directly investigated.

Here we evaluate the effects of post-depositional alteration on paired d13Ccarb and d13Corg values from a core that has

unequivocally been altered by freshwater and marine diagenetic processes (Clino, Fig. 1). More than 470 paired d13C measurements were conducted on this B700-m core that was drilled into the margin of the Great Bahama Bank36. During the late-Pleistocene, multiple sea-level oscillations exposed the upper 120 m of the platform to the inuence of meteoric waters. Ten subaerial exposure surfaces have been identied in the top 100 m of the core, each of which is proposed to have been related to a Pleistocene glacial period37,38. Evidence of both meteoric and marine diagenesis has been recorded within this 5.3 Ma record of marginal and shallow marine carbonates3941, including the development of caliche crusts, blocky spar cements, large-scale dissolution and soil development37,38,40. Evidence of marine burial diagenesis includes non-depositional surfaces in the core42,

which in some cases are associated with dolomites containing negative d13Ccarb signatures43. Paired d13Ccarb and d13Corg analyses in these altered sediments are strongly covariant throughout the length of the core, particularly in the Plio Pleistocene section of the record. These results demonstrate how post-depositional processes, linked in time by subaerial exposure, have shifted the isotopic composition of carbonate and organic carbon in the same direction at the same time.

ResultsBulk geochemical relationships from Clino. The Clino core has previously been separated into three diagenetic zones based upon petrographic characteristics and d13Ccarb and d18Ocarb values39.

Although the bulk d13Ccarb and d18Ocarb (Fig. 2a, r2 0.44,

Po0.05, n 465) and d13Ccarb and d13Corg values (Fig. 2b,

r2 0.59, Po0.05, n 465) show statistically signicant positive

correlations, the relationships are variable within each of the different diagenetic environments (Fig. 3a,b). The d13Ccarb and d13Corg data (Supplementary Table 1) are considered within this framework.

Geochemical relationships within each diagenetic zone. The uppermost portion of Clino (0100 m), corresponding to the vadose and freshwater phreatic zones35, is characterized by large variations in the d13Ccarb record and rather constant, but negative d18Ocarb values. Throughout this interval there are abundant subaerial exposures (Fig. 4), which have more negative d13Ccarb values (but constant and negative d18Ocarb) and high

concentrations of trace metals such as Fe and Mn37. The

d13Ccarb and d18Ocarb values are not statistically signicantly

correlated (Fig. 3a), the concentration of total organic carbon (TOC) is o0.1% (Fig. 4) and there is no statistically signicant correlation between the d13Ccarb and d13Corg within the meteoric

zone (r2 0.21, P40.05, n 53, Fig. 3b). Between 100 and 200 m,

there is a transition from negative to positive d13Ccarb and d18Ocarb values (the mixing zone35) (Fig. 4). This portion of the core exhibits a very strong positive correlation between the d13Ccarb and d18Ocarb values (Fig. 3a), as well as between the

26

Miami

FLORIDA

Clino

Andros Island

Straits of Florida

CAY SAL BANK

GREAT BAHAMA BANK

Nicholas Channel

N

CUBA

25

24

23

80 79 78

Figure 1 | Location of Clino core. Clino was drilled on the platform top of the Great Bahama Bank, and the water depth at the time of drilling was7.6 m (ref. 36).

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a b

5

3

1

1

3

5

5 Lithology

Skeletal-peloidal

Reefal

Peloidal

Skeletal

[afii9829]18 O carb

[afii9829]13 C org

4 2 0 2 4 6 4 2 0 2 4 6

[afii9829]13Ccarb

[afii9829]13Ccarb

Figure 2 | Relationship between isotope records from the whole core subdivided by lithology. (a) Correlation between d13Ccarb and d18Ocarb values

from the entire length of the core Clino (r2 0.44, Po0.05, n 465) subdivided by published lithological assignments37,42. (b) Relationship between

d13Ccarb and d13Corg values from the entire length of the core Clino (r2 0.59, Po0.05, n 465) subdivided by published lithological assignments37,42.

a b

5

3

1

1

3

5

Meteoric

Marine burial

Mixing

[afii9829]18 O carb

[afii9829]13 C org

5

10

15

20

25

30

4 2 0 2 4 6 4 2 0 2 4 6

[afii9829]13Ccarb

[afii9829]13Ccarb

Figure 3 | Relationship between isotope records subdivided by the diagenetic zone. (a) Correlations between d13Ccarb and d18Ocarb records from the

diagenetic zones dened by Melim et al. (ref. 39): meteoric (blue circles, r2 0.01, P40.05, n 53), mixing (purple circles, r2 0.81, Po0.05, n 58) and

marine burial (orange circles, r2 0.22, Po0.05, n 354). (b) Relationship between d13Ccarb and d13Corg values subdivided by diagenetic zones: meteoric

(blue circles, r2 0.21, P40.05, n 53), mixing (purple circles, r2 0.87, Po0.05, n 58) and marine burial (orange circles, r2 0.06, P40.05, n 354).

d13Ccarb and d13Corg values (r2 0.87, Po0.05, n 58, Fig. 3b).

Below the mixing zone there is a region in which there are relatively positive d13Ccarb and d18Ocarb values, an area

interpreted as having been affected only by marine diagenesis39. The marine burial zone shows no statistically signicant relationship between the d13Ccarb and d18Ocarb (Fig. 3a) or

between d13Ccarb and d13Corg values (r2 0.06, P40.05, n 354,

Fig. 3b). The concentration of TOC increases through the mixing zone and is an order of magnitude higher in the marine burial zone reaching values up to 1.2% (Fig. 4).

DiscussionThe zone of meteoric alteration in Clino has the lowest d13Ccarb ( 2 to 2%) and d13Corg ( 29 to 17%) values, and no

statistically signicant covariance between d13Ccarb and d13Corg

(Fig. 3b). Both the d13Ccarb and d13Corg values are signicantly

lower than those reported for modern shallow marine sediments from Great Bahama Bank, which average 4.5 and 12%,

respectively44,45. We suggest that these low d13Ccarb values arise from the oxidation of organic matter, which imparts a low d13C value to the dissolved inorganic carbon, along with cementation, mineralogical stabilization and recrystallization of the carbonate33,39. Concurrently, d13Corg values became more negative as the labile marine organic matter was oxidized, and additional organic material was contributed from terrestrial C3 plants and freshwater algae, which colonized the newly exposed platform top. Multiple subaerial exposures of the platform top during the Pleistocene have superimposed the effects of such diagenetic processes on the carbonates and organic matter

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Dunham Classification

% TOC

Carbonate [afii9829]13C

m w p g b

0 0.5 1.0

3 1

1

3

20

120

220

320

420

520

620

LEGEND

Pleist. Pliocene Miocene

Geochemical records

Lithology

Surfaces

Erosional contact/ marine hardground

Diagenetic zones

Meteoric Mixing Marine burial

%TOC Organic 13C

Carbonate 13C

Carbonate 18O

Peloidal

Skeletal

Skeletal-peloidal

Reefal

Depth (mbmp)

Subaerial exposure

Marine hardground

0 50

% Dolomite

25

15

5

6

4 0

2 2

Organic [afii9829]13C

18O

Figure 4 | Geochemical records and lithostratigraphy of Clino. Total organic carbon (TOC) content, carbonate d13C values, organic d13C values and carbonate d18O values (n 465 for each record) produced by this study from the Neogene carbonates in the core Clino. The data used to construct the

simplied stratigraphic column presented in this gure were obtained from Kenter et al. (ref. 42) and Kievman (ref. 37). Diagenetic zones (meteoric, mixing and marine burial) were dened by published interpretations of both petrographic40 and isotopic constraints35. The record of percent dolomite was determined by X-ray diffractometry39.

preserved in Clino, and as a result, the records currently observed are the cumulative product of these post-depositional changes.

Although preferential degradation of labile organic compounds can cause the d13Corg value of the residual organic carbon to become more positive46, in the majority of cases degradation has been shown to produce residual organic carbon with more negative d13Corg values4749. However, these processes can only produce changes of up to 45% in the d13Corg record46,4851, and since the lowest d13Corg value of sedimentary organic matter from the platform top is 17% (ref. 45), diagenetic reactions alone

could not have produced the d13Corg values of 29% observed

in the upper 200 m of Clino. Consequently, a source of organic carbon with a d13Corg value lower than 22% is required to

produce the d13Corg values observed in the top 100 m of the core. Such a source is likely to be terrestrial C3 plant matter, such as mangroves and freshwater algae, which have d13Corg values ranging from 20 to 32% (ref. 52). Evidence of terrestrial

plant contribution is provided by root casts observed in the subaerial exposure surfaces37 (Supplementary Figs 1 and 2). In addition, terrestrial organic matter is known to be preferentially preserved through time, especially in oxidizing settings where marine organic compounds have been found to be degraded twice as fast as terrestrial soil-derived organic compounds53. We suggest that these post-depositional processes may account for the low concentration of organic carbon (o0.2%), the negative d13Corg values (Fig. 2) and the increase in the proportion of low-magnesium calcite40 in the section of the core affected by meteoric diagenesis.

The highest correlation between the d13Ccarb and d13Corg

values (Fig. 3b) is observed between 100 and 200 mbmp, in the section of the core associated with a strong correlation between d13Ccarb and d18Ocarb. The strong positive correlation between

d13Ccarb and d13Corg records in the mixing zone (Fig. 3b) can be attributed to a gradient of post-depositional changes. The sediments and organic matter preserved closer to 100 mbmp exhibit low d13Ccarb and d13Corg values, because they have been

repeatedly affected by freshwater diagenetic reactions and post-depositional contributions of terrestrial organic matter, as previously described. In contrast, the d13Ccarb and d13Corg values

in the section of the core closer to 200 mbmp are comparatively more positive, and similar to those observed both on the modern platform top45 as well as those preserved in the marine burial zone (Fig. 4), suggesting that this section of the core has experienced fewer episodes of alteration and lower contributions, if any, of terrestrial organic carbon.

The d13Ccarb and d13Corg records in the marine burial

diagenetic zone probably represent the least altered values within the entire core. The absence of covariance between d13Ccarb and d13Corg records, and the range of d13Ccarb and d13Corg values in the marine burial diagenetic zone (Fig. 3b) are similar to unaltered Pleistocene periplatform sediments deposited on the slope of the Great Bahama Bank45. Throughout the marine burial zone, there are minor variations in d13Ccarb and d13Corg that

represent subtle changes in the source of the sediments through time, as well as diagenetic processes. An example of the inuence that a change in source can have on both the d13Ccarb and d13Corg

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values is the synchronous change towards more positive values observed at 367 mbmp. At this depth, the background sediment type changes from a mixed peloidal-skeletal packstone with signicant contributions from pelagic foraminifera, to a peloid-dominated chalky wackestone to packstone almost entirely devoid of planktic foraminifera42. The synchronous positive shifts in the d13Ccarb and d13Corg records are consistent with

increased off-bank shedding as the platform prograded towards the Straits of Florida during the Pliocene54,55. Off-bank shedding would have contributed increasingly higher proportions of platform-derived carbonates and organic matter, which are characterized by relatively higher d13Ccarb and d13Corg values45.

Minor uctuations in the d13Ccarb and d13Corg records occur at

marine hardgrounds (Fig. 4)42, and are likely associated with the oxidation of marine organic matter and the precipitation of dolomite below non-depositional surfaces43 within the marine burial diagenetic zone.

This data set clearly demonstrates how two post-depositional changes linked in time by periods of subaerial exposure, the diagenetic alteration of the carbonate and the post-depositional contribution of terrestrial organic carbon, can produce negative excursions with highly covariant d13Ccarb and d13Corg records.

The excursion observed in the Neogene is similar in magnitude to those observed in Palaeozoic and Precambrian deposits. Whether or not those ancient deposits were exposed to the same degree of freshwater alteration as Clino is still a matter of debate28. In many cases, negative d13Ccarb excursions have been interpreted to be pristine records of global carbon cycling15,1921,56,57, because sedimentological evidence of subaerial exposure was not observed26,58. However, subaerial exposure surfaces can be cryptic in the rock record, and other workers have interpreted the same geochemical changes to be diagenetic in origin28,59. If the latter is true, and multiple sources of organic carbon contributed to the sedimentary organic matter preserved in the deposit, as was recently shown to be the case for the Ediacaran Shuram Formation in Oman60, then the model presented here could conceivably explain covarying trends in paired d13Ccarb and d13Corg records from the ancient geological record. Although higher level terrestrial plants were not present until the late Palaeozoic, the presence of terrestrial life in earlier time periods, including photosynthetic cyanobacteria, fungi and algae6166, supports the possibility that ancient sedimentary organic carbon could have been composed of mixtures of marine and terrestrial organic carbon, in a situation analogous to the model of subaerial exposure proposed for the Neogene. In fact, the range in d13Corg values of Precambrian sedimentary organic matter is the largest for any time period in Earth history67. Although the organisms were different in the ancient geological record, processes similar to those described here could have occurred.

In contradiction to the assumption that coupled negative excursions in d13Ccarb and d13Corg values can only be produced

by changes in the global carbon cycle, these results suggest that post-depositional processes can play an inuential role in generating covariant d13Ccarb and d13Corg values. Consequently,

interpretations of strongly correlated d13Ccarb and d13Corg values

from the ancient geological record should reconsider the inuence that similar post-depositional processes may have in generating some of the coupled negative excursions associated with noteworthy biogeochemical events in early Earth history.

Methods

Sampling strategy. Clino was sampled at roughly 1.5 m intervals to obtain at least 50 samples per diagenetic zone (meteoric zone, n 53, mixing zone, n 58 and

marine burial diagenetic zone, n 354). However, major sedimentological features

such as subaerial exposure surfaces and hardgrounds were avoided to preserve limited core material. Such features had been sampled in previous studies3941,

which accounts for the larger ranges in d13Ccarb reported in those studies. For each paired carbon isotope data point, roughly a gram of bulk sediment was powdered and homogenized to provide subsamples for carbonate and organic carbon isotope analysis.

Carbonate d13C and d18O measurements. Carbonate d13C and d18O values were analysed via dissolution in phosphoric acid using the common acid bath method68. The CO2 gas produced by the reaction of phosphoric acid and carbonate was analysed on a Finnigan MAT 251 (Thermo Fisher Scientic, Bremen, Germany). In each run of 24 samples, four standards were processed at the start of the run and two at the end, followed by a measurement of the zero enrichment. Data were then corrected for any fractionation in the reference gas during the run and for the usual isobaric interferences modied for a triple collector mass spectrometer. Data are reported relative to the Vienna Pee Dee Belemnite (VPDB) scale, dened for carbonates by the d13C value of NBS-19 (1.95% versus Pee Dee Belemnite (PDB)69). The error for these analyses is o0.1% as indicated by replicate analyses of internal standards.

Organic d13C and TOC measurements. Co-occurring sedimentary organic material was separated via dissolution in 10% HCl acid overnight, followed by subsequent vacuum ltration onto glass microber lters (Whatman GF/C). The insoluble residue (IR) on the lter was allowed to dry for at least 48 h, or until a constant dry weight was achieved. The weights of the insoluble material were quantied by subtracting the weight of the empty lter from the weight of the dried insoluble material and lter after ltration. Samples of the insoluble material were scraped off of the lters, weighed and packed into tin capsules and loaded into a Costech ECS 4010 (Costech Analytical Technologies Inc., Valencia, CA, USA), where they were combusted. The resulting CO2 gas transferred for isotopic measurement to a continuous ow isotope-ratio mass spectrometer (Delta V Advantage, Thermo Fisher Scientic). For every run of 36 samples, 12 internal standards were analysed to calibrate the machine and to assess the precision of the measurements. An analytic blank and 6 internal standards preceded the rst sample analysis, and two standards were run for every 10 samples analysed. The reproducibility of d13C values is 0.1% as indicated by the s.d. of replicate analyses of internal standards of glycine (n 54, d13C value 31.8% VPDB).

All d13Corg data are reported relative to the VPDB scale, dened for organic carbon as the d13C value of graphite (USGS24) 16.05% versus VPDB70.

To calculate weight percent carbon in the IR, a calibration line was established that related the peak area measured by the Delta V Advantage (Thermo Fisher Scientic) to the known weight of carbon in the internal standard, glycine. The weights of the standards were chosen to bracket the expected range of organic carbon in the samples. The s.d. of these analyses is 0.4% based upon repeated analyses of glycine (n 54). Delta V Advantage peak area measurements for each

sample was transformed to mg of organic carbon in the insoluble residue using the equation of the calibration line. Organic carbon concentration in the insoluble residue in mg was converted to TOC by the following equation:

TOC ((Org C in IR (mg) total IR weight (mg))/initial weight of the

sediment (mg)) 100

Statistical analyses. Pearsons regression analysis was used to determine the relationship between isotope records. The r2, P and n values are listed in the main text for each analysis conducted.

References

1. Hoffman, P. F., Kaufman, A. J., Halverson, G. P. & Schrag, D. P. A Neoproterozoic snowball Earth. Science 281, 13421346 (1998).

2. Halverson, G. P. et al. A major perturbation of the carbon cycle before the Ghaub glaciation (Neoproterozoic) in Namibia: prelude to snowball Earth? Geochem. Geophys. Geosyst. 3, 124 (2002).

3. Kaufman, A. J. & Knoll, A. H. Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications. Precambr. Res. 73, 2749 (1995).

4. Shields-Zhou, G. & Och, L. The case for a Neoproterozoic oxygenation event: geochemical evidence and biological consequences. GSA Today 21, 411 (2011).

5. Och, L. M. & Shields-Zhou, G. A. The Neoproterozoic oxygenation event: environmental perturbations and biogeochemical cycling. Earth Sci. Rev. 110, 2657 (2012).

6. Sahoo, S. K. et al. Ocean oxygenation in the wake of the Marinoan glaciation. Nature 489, 546549 (2012).

7. Halverson, G. P., Wade, B. P., Hurtgen, M. T. & Barovich, K. M. Neoproterozoic chemostratigraphy. Precambr. Res. 182, 337350 (2010).

8. Maloof, A. C. et al. The earliest Cambrian record of animals and ocean geochemical change. Geol. Soc. Am. Bull. 122, 17311774 (2010).

9. Johnston, D. T. et al. Late Ediacaran redox stability and metazoan evolution. Earth Planet. Sci. Lett. 335-336, 2535 (2012).

NATURE COMMUNICATIONS | 5:4672 | DOI: 10.1038/ncomms5672 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 5

& 2014 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5672

10. Korte, C. & Kozur, H. W. Carbon-isotope stratigraphy across the Permian Triassic boundary: a review. J. Asian Earth Sci. 39, 215235 (2010).

11. Berner, R. A. Examination of hypotheses for the Permo-Triassic boundary extinction by carbon cycle modeling. Proc. Natl Acad. Sci. USA 99, 41724177 (2002).

12. Payne, J. L. & Kump, L. R. Evidence for recurrent Early Triassic massive volcanism from quantitative interpretation of carbon isotope uctuations. Earth Planet. Sci. Lett. 256, 264277 (2007).

13. Galli, M. T., Jadoul, F., Bernasconi, S. M. & Weissert, H. Anomalies in global carbon cycling and extinction at the Triassic/Jurassic boundary: evidence from a marine C-isotope record. Palaeogeogr. Palaeoclimatol. Palaeoecol. 216, 203214 (2005).

14. Luo, G. et al. Isotopic evidence for an anomalously low oceanic sulfate concentration following end-Permian mass extinction. Earth Planet. Sci. Lett. 300, 101111 (2010).

15. Johnston, D. T., Macdonald, F. A., Gill, B. C., Hoffman, P. F. & Schrag, D. P. Uncovering the Neoproterozoic carbon cycle. Nature 483, 320324 (2012).16. Jiang, G. et al. The origin of decoupled carbonate and organic carbon isotope signatures in the early Cambrian (ca. 542520Ma) Yangtze platform. Earth Planet. Sci. Lett. 317-318, 96110 (2012).

17. Cramer, B. D. & Saltzman, M. R. Early Silurian paired d13Ccarb and d13Corg

analyses from the Midcontinent of North America: implications for paleoceanography and paleoclimate. Palaeogeogr. Palaeoclimatol. Palaeoecol. 256, 195203 (2007).18. Young, S. A., Saltzman, M. R., Bergstrom, S. M., Leslie, S. A. & Xu, C. Paired d13Ccarb and d13Corg records of upper Ordovician (SandbianKatian)

carbonates in North America and China: Implications for paleoceanographic change. Palaeogeogr. Palaeoclimatol. Palaeoecol. 270, 166178 (2008).19. Meyer, K. M., Yu, M., Lehrmann, D., van de Schootbrugge, B. & Payne, J. L. Constraints on early Triassic carbon cycle dynamics from paired organic and inorganic carbon isotope records. Earth Planet. Sci. Lett. 361, 429435 (2013).

20. Knoll, A. H., Hayes, J. M., Kaufman, A. J., Swett, K. & Lambert, I. B. Secular variation in carbon isotope ratios from upper Proterozoic successions of Svalbard and East Greenland. Nature 321, 832838 (1986).

21. Swanson-Hysell, N. L. et al. Cryogenian glaciation and the onset of carbon-isotope decoupling. Science 328, 608611 (2010).

22. Krull, E. S. et al. Stable carbon isotope stratigraphy across the Permian Triassic boundary in shallow marine carbonate platforms, Nanpanjiang Basin, South China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 204, 297315 (2004).

23. Ader, M. et al. A multilayered water column in the Ediacaran Yangtze platform? Insights from carbonate and organic matter paired d13C. Earth

Planet. Sci. Lett. 288, 213227 (2009).24. Werne, J. P. & Hollander, D. J. Balancing supply and demand: controls on carbon isotope fractionation in the Cariaco Basin (Venezuela) Younger Dryas to present. Mar. Chem. 92, 275293 (2004).

25. LaPorte, D. F. et al. Local and global perspectives on carbon and nitrogen cycling during the Hirnantian glaciation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 276, 182195 (2009).

26. Grotzinger, J. P., Fike, D. A. & Fischer, W. W. Enigmatic origin of the largest-known carbon isotope excursion in Earths history. Nat. Geosci. 4, 285292 (2011).

27. Rothman, D., Hayes, J. & Summons, R. Dynamics of the Neoproterozoic carbon cycle. Proc. Natl Acad. Sci. USA 100, 81248129 (2003).

28. Swart, P. K. & Kennedy, M. J. Does the global stratigraphic reproducibility of d13C in Neoproterozoic carbonates require a marine origin? A Pliocene-

Pleistocene comparison. Geology 40, 8790 (2011).29. Brand, U. & Veizer, J. Chemical diagenesis of a multicomponent carbonate system-1: trace elements. J. Sed. Petrol. 50, 12191236 (1980).

30. Banner, J. L. & Hanson, G. N. Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis. Geochim. Cosmochim. Acta 54, 31233137 (1990).

31. Frank, T. D., Lohmann, K. C. & Meyers, W. J. Chronostratigraphic signicance of cathodoluminescence zoning in syntaxial cement: Mississippian Lake Valley Formation, New Mexico. Sed. Geol. 105, 2950 (1996).

32. Meyers, W. J. Carbonate cement stratigraphy of the Lake Valley Formation (Mississippian) Sacramento Mountains, New Mexico. J. Sed. Petrol. 44, 837861 (1974).

33. Gross, M. G. Variations in the 18O/16O and 13C/12C ratios of diagenetically altered limestones in the Bermuda islands. J. Geol. 72, 172193 (1964).

34. Marshall, J. D. Climatic and oceanographic isotopic signals from the carbonate rock record and their preservation. Geol. Mag. 129, 143160 (1992).

35. Allan, J. R. & Matthews, R. K. Isotope signatures associated with early meteoric diagenesis. Sedimentology 29, 797817 (1982).

36. Ginsburg, R. N. in Subsurface Geology of a Prograding Carbonate Platform Margin, Great Bahama Bank: Results of the Bahamas Drilling Project Vol. 70 (ed. Ginsburg, R. N.) 61100 (SEPM Special Publication, 2001).

37. Kievman, C. M. Sea-level effects on carbonate platform evolution: Plio-Pleistocene, Northwestern Great Bahama Bank (PhD thesis, University Miami 1245, 1996).

38. Kievman, C. M. Match between late Pleistocene Great Bahama Bank and deep-sea oxygen isotope records of sea level. Geology 26, 635638 (1998).

39. Melim, L. A., Swart, P. K. & Maliva, R. G. in Subsurface Geology of a Prograding Carbonate Platform Margin, Great Bahama Bank: Results of the Bahamas Drilling Project Vol. 70 (ed. Ginsburg, R. N.) 61100 (SEPM Special Publication, 2001).

40. Melim, L. A., Swart, P. K. & Maliva, R. G. Meteoric-like fabrics forming in marine waters; implications for use of petrography to identify diagenetic environments. Geology 23, 755758 (1995).

41. Melim, L. A., Westphal, H., Swart, P. K., Eberli, G. P. & Munnecke, A. Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas. Mar. Geol. 185, 2753 (2002).

42. Kenter, J. A. M., Ginsburg, R. N. & Troelstra, S. R. in Subsurface Geology of a Prograding Carbonate Platform Margin, Great Bahama Bank: Results of the Bahamas Drilling Project Vol. 70 (ed. Ginsburg, R. N.) 61100 (SEPM Special Publication, 2001).

43. Swart, P. K. & Melim, L. A. The origin of dolomites in Tertiary sediments from the margin of Great Bahama Bank. J. Sed. Res. 70, 738748 (2000).

44. Swart, P., Reijmer, J. & Otto, R. in Perspectives in Carbonate Geology: A Tribute to the Career of Robert Nathan Ginsburg, IAS Special Publication. (edsSwart, P. K., Eberli, G. P. & McKenzie, J. A.) 4760 (Wiley-Blackwell, 2009).

45. Oehlert, A. M. et al. The stable carbon isotopic composition of organic material in platform derived sediments: implications for reconstructing the global carbon cycle. Sedimentology 59, 319335 (2012).

46. Hatch, J. R. & Leventhal, J. S. Early diagenetic partial oxidation of organic matter and suldes in the Middle Pennsylvanian (Desmoinesian) Excell Shale Member of the Fort Scott Limestone and equivalents, northern Midcontinent region, USA. Chem. Geol. 134, 215235 (1997).

47. De Lange, G. et al. in Carbon Cycling in the Glacial Ocean: Constraints on the Oceans Role in Global Change Vol. 17 (eds Zahn, R., Pederson, T. F., Kaminski,M. A. & Labeyrie, L.) 225258 (Springer, 1994).48. Prahl, F. G., De Lange, G. J., Scholten, S. & Cowie, G. L. A case of post-depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the Madeira Abyssal Plain. Org. Geochem. 27, 141152 (1997).

49. Bttcher, M. E., Oelschlager, B., Hopner, H. J., Brumsack, H. J. & Rullkotter, J. Sulfate reduction related to the early diagenetic degradation of organic matter and black spot formation in tidal sandats of the German Wadden Sea (southern North Sea): stable isotope (13C, 34S, 18O) and other geochemical results. Org. Geochem. 29, 15171530 (1998).

50. Freudenthal, T., Wagner, T., Wenzhoffer, F., Zabel, M. & Wefer, G. Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes. Geochim. Cosmochim. Acta 65, 17951808 (2001).

51. Lehmann, M. F., Bernasconi, S. M., Barbieri, A. & McKenzie, J. A. Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis. Geochim. Cosmochim. Acta 66, 35733584 (2002).

52. Lamb, A. L., Wilson, G. P. & Leng, M. J. A review of coastal palaeoclimate and relative sea-level reconstructions using d13C and C/N ratios in organic material.

Earth Sci. Rev. 75, 2957 (2006).53. Huguet, C. et al. Selective preservation of soil organic matter in oxidized marine sediments (Madeira Abyssal Plain). Geochim. Cosmochim. Acta 72, 60616068 (2008).

54. Eberli, G. P. in Proceedings of the Ocean Drilling Program, Scientic Results Vol. 166 (eds Swart, P. K., Eberli, G. P., Malone, M. J. & Sarg, J. F.) 167177 (Ocean Drilling Program, 2000).

55. Schlager, W., Reijmer, J. J. G. & Droxler, A. Highstand shedding of carbonate platforms. J. Sed. Res. B64, 270281 (1994).

56. Jiang, G. et al. Organic carbon isotope constraints on the dissolved organic carbon (DOC) reservoir at the CryogenianEdiacaran transition. Earth Planet. Sci. Lett. 299, 159168 (2010).

57. Sansjofre, P. et al. A carbon isotope challenge to the snowball Earth. Nature 478, 9396 (2011).

58. Jiang, G., Christie-Blick, N., Kaufman, A. J., Banerjee, D. M. & Rai, V. Sequence stratigraphy of the Neoproterozoic infra Krol formation and Krol Group, lesser Himalaya, India. J. Sed. Res. 72, 524542 (2002).

59. Derry, L. A. On the signicance of d13C correlations in ancient sediments. Earth Planet. Sci. Lett. 296, 497501 (2010).

60. Lee, C. et al. Carbon isotopes and lipid biomarkers from organic-rich facies of the Shuram Formation, Sultanate of Oman. Geobiology 11, 406419 (2013).

61. Horodyski, R. J. & Knauth, L. P. Life on land in the Precambrian. Science 263, 494498 (1994).

62. Prave, A. R. Life on land in the Proterozoic: evidence from the Torridonian rocks of northwest Scotland. Geology 30, 811814 (2002).

6 NATURE COMMUNICATIONS | 5:4672 | DOI: 10.1038/ncomms5672 | http://www.nature.com/naturecommunications

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5672 ARTICLE

63. Buttereld, N. J. Probable Proterozoic fungi. Paleobiology 31, 165182 (2005).64. Heckman, D. S. et al. Molecular evidence for the early colonization of land by fungi and plants. Science 293, 11291133 (2001).

65. Knoll, A. H. Learning to tell Neoproterozoic time. Precambr. Res. 100, 320 (2000).

66. Beraldi-Campesi, H. Early life on land and the rst terrestrial ecosystems. Ecol. Proc. 2, 117 (2013).

67. Schidlowski, M., Hayes, J. M. & Kaplan, I. R. in Earths Earliest Biosphere. (ed. Schopf, J.) 149186 (Princeton University Press, 1983).

68. Swart, P. K., Burns, S. J. & Leder, J. J. Fractionation of the stable isotopes of oxygen and carbon in carbon dioxide during the reaction of calcite with phosphoric acid as a function of temperature and technique. Chem. Geol. 86, 8996 (1991).

69. Friedman, I., ONeil, J. & Cebula, G. Two new carbonate stable isotope standards. Geostandard Newslett. 6, 1112 (1982).

70. Coplen, T. B. et al. New guidelines for d13C measurements. Anal. Chem. 78, 24392441 (2006).

Acknowledgements

The collection of the Clino core was made possible by a NSF grant OCE 8917295 to P.K.S. and R.N. Ginsburg. This project was supported by NSF grant OCE 0825577 to P.K.S., the Stable Isotope Laboratory at RSMAS and the sponsors of the Center for

Carbonate Research in the Comparative Sedimentology Laboratory at the University of Miami. We would like to thank the members of the Bahamas Drilling project for the collection of the core, C. Kaiser, D. Hardisty and R. Wdowinski for laboratory assistance,L. Peterson for scanning the core, as well as L. Kump and G.J. Mackenzie for early reviews of this manuscript.

Author contributions

A.M.O. and P.K.S. each contributed in developing the project, sampling the core, analysing samples and writing the text.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

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Competing nancial interests: The authors declare no competing nancial interests.

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How to cite this article: Oehlert, A. M. and Swart, P. K. Interpreting carbonate and organic carbon isotope covariance in the sedimentary record. Nat. Commun. 5:4672 doi: 10.1038/ncomms5672 (2014).

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