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Abstract

Figures and Tables

Supplementary Figures and Tables

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Introduction

Earth’s O₂–rich atmosphere, unique among known planets, has played an essential role in evolving feedbacks between life and environment. Atmospheric O₂ was extremely low in the Archean Eon (>2.5 Ga), and while multiple lines of evidence suggest that Earth’s oxygenation was protracted (Kah et al., 2004

Kah, L.C., Lyons, T.W., Frank, T.D. (2004) Low marine sulphate and protracted oxygenation of the proterozoic biosphere. Nature 431, 834-838.

; Kah and Bartley, 2011

Kah, L.C., Bartley, J.K. (2011) Protracted oxygenation of the Proterozoic biosphere. International Geology Review 53, 1424-1442.

; Lyons et al., 2014

Lyons, T.W., Reinhard, C.T., Planavsky, N.J. (2014) The rise of oxygen in Earth's early ocean and atmosphere. Nature 506, 307-315.

; Planavsky et al., 2014

Planavsky, N.J., Reinhard, C.T., Wang, X., Thomson, D., McGoldrick, P., Rainbird, R.H., Johnson, T., Fischer, W.W., Lyons, T.W. (2014) Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635-638.

), pO₂ may have risen abruptly at two different points in time: first during the “Great Oxygenation Event” (GOE) at ~2.4 Ga (Canfield, 2005

Canfield, D.E. (2005) The early history of atmospheric oxygen: Homage to Robert A. Garrels. Annual Review of Earth and Planetary Sciences 33, 1-36.

; Holland, 2006

Holland, H.D. (2006) The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B: Biological Sciences 361, 903-915.

; Guo et al., 2009

Guo, Q.J., Strauss, H., Kaufman, A.J., Schroder, S., Gutzmer, J., Wing, B., Baker, M.A., Bekker, A., Jin, Q.S., Kim, S.T., Farquhar, J. (2009) Reconstructing Earth's surface oxidation across the Archean-Proterozoic transition. Geology 37, 399-402.

; Farquhar et al., 2011

Farquhar, J., Zerkle, A., Bekker, A. (2011) Geological constraints on the origin of oxygenic photosynthesis. Photosynthesis Research 107, 11-36.

), when atmospheric O₂ rose from <0.001 % to an intermediate value commonly estimated as 1 to 10 % of the current level (Farquhar et al., 2000

Farquhar, J., Bao, H.M., Thiemens, M. (2000) Atmospheric influence of Earth's earliest sulfur cycle. Science 289, 756-758.

; Pavlov and Kasting, 2002

Pavlov, A.A., Kasting, J.F. (2002) Mass-independent fractionation of sulfur isotopes in Archean sediments: Strong evidence for an anoxic Archean atmosphere. Astrobiology 2, 27-41.

), and again during a “Neoproterozoic Oxygenation Event” (NOE) at ~800 to 542 million years ago (Canfield and Teske, 1996

Canfield, D.E., Teske, A. (1996) Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies. Nature 382, 127-132.

; Fike et al., 2006

Fike, D.A., Grotzinger, J.P., Pratt, L.M., Summons, R.E. (2006) Oxidation of the Ediacaran Ocean. Nature 444, 744-747.

; Frei et al., 2009

Frei, R., Gaucher, C., Poulton, S.W., Canfield, D.E. (2009) Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes. Nature 461, 250-U125.

; Och and Shields-Zhou, 2012

Och, L.M., Shields-Zhou, G.A. (2012) The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling. Earth-Science Reviews 110, 26-57.

). The latter transition may well have continued into the Phanerozoic Eon, eventually resulting in near-present O₂ (Berner, 2006

Berner, R.A. (2006) GEOCARBSULF: A combined model for Phanerozoic atmospheric O₂ and CO₂. Geochimica et Cosmochimica Acta 70, 5653-5664.

; Dahl et al., 2010

Dahl, T.W., Hammarlund, E.U., Anbar, A.D., Bond, D.P.G., Gill, B.C., Gordon, G.W., Knoll, A.H., Nielsen, A.T., Schovsbo, N.H., Canfield, D.E. (2010) Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish. Proceedings of the National Academy of Sciences of the United States of America 107, 17911-17915.

; Sperling et al., 2015

Sperling, E.A., Wolock, C.J., Morgan, A.S., Gill, B.C., Kunzmann, M., Halverson, G.P., Macdonald, F.A., Knoll, A.H., Johnston, D.T. (2015) Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation. Nature 523, 451-454.

).

Redox-sensitive major and trace elements in iron formations and black shales deposited beneath euxinic waters have been developed as proxies to reconstruct palaeoenvironmental history in deep time (Scott et al., 2008

Scott, C., Lyons, T.W., Bekker, A., Shen, Y., Poulton, S.W., Chu, X., Anbar, A.D. (2008) Tracing the stepwise oxygenation of the Proterozoic ocean. Nature 452, 456-U5.

; Konhauser et al., 2009

Konhauser, K.O., Pecoits, E., Lalonde, S.V., Papineau, D., Nisbet, E.G., Barley, M.E., Arndt, N.T., Zahnle, K., Kamber, B.S. (2009) Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event. Nature 458, 750-753.

; Sahoo et al., 2012

Sahoo, S.K., Planavsky, N.J., Kendall, B., Wang, X., Shi, X., Scott, C., Anbar, A.D., Lyons, T.W., Jiang, G. (2012) Ocean oxygenation in the wake of the Marinoan glaciation. Nature 489, 546-549.

). The paucity of these facies in many Proterozoic successions, however, limits the continuity of current reconstructions of Earth’s oxygenation. Here we provide evidence for the hypothesis that carbonate-based redox proxies can provide an independent estimate of past pO₂, expanding the palaeoredox record in time and space (Hardisty et al., 2014

Hardisty, D.S., Lu, Z., Planavsky, N.J., Bekker, A., Philippot, P., Zhou, X., Lyons, T.W. (2014) An iodine record of Paleoproterozoic surface ocean oxygenation. Geology doi: 10.1130/G35439.1.

). Limestone and penecontemporaneous dolomites that retain depositional signatures well (Wilson et al., 2010

Wilson, J.P., Fischer, W.W., Johnston, D.T., Knoll, A.H., Grotzinger, J.P., Walter, M.R., McNaughton, N.J., Simon, M., Abelson, J., Schrag, D.P., Summons, R., Allwood, A., Andres, M., Gammon, C., Garvin, J., Rashby, S., Schweizer, M., Watters, W.A. (2010) Geobiology of the late Paleoproterozoic Duck Creek Formation, Western Australia. Precambrian Research 179, 135-149.

) are abundant in the geologic record, typically recording shallow marine environments that would have been in open communication with the overlying atmosphere. Palaeoenvironmental research on carbonate rocks commonly focuses on individual stratigraphic successions; here we adopt a complementary strategy, analysing a large suite of Phanerozoic, Proterozoic, and Archean samples that enables us to make statistical statements (Sperling et al., 2015

Sperling, E.A., Wolock, C.J., Morgan, A.S., Gill, B.C., Kunzmann, M., Halverson, G.P., Macdonald, F.A., Knoll, A.H., Johnston, D.T. (2015) Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation. Nature 523, 451-454.

) about Zn/Fe in the global surface ocean through geologic time. More importantly, we develop a new tool to provide quantitative constraints on atmospheric pO₂ through Earth history.

In the modern ocean, zinc input from hydrothermal ridge systems (~4.4 x 10⁹ mol yr^-1) is an order of magnitude greater than riverine fluxes (~3.4 x 10⁸ mol yr^-1; Robbins et al., 2013

Robbins, L.J., Lalonde, S.V., Saito, M.A., Planavsky, N.J., Mloszewska, A.M., Pecoits, E., Scott, C., Dupont, C.L., Kappler, A., Konhauser, K.O. (2013) Authigenic iron oxide proxies for marine zinc over geological time and implications for eukaryotic metallome evolution. Geobiology 11, 295-306.

). As an essential nutrient in many phytoplankton enzymes, especially those of eukaryotes (Williams and da Silva, 1996

Williams, R.J.P., da Silva, J.J.R.F. (1996) The natural selection of the chemical elements. Great Britian, Bath Press Ltd.

), zinc plays an important role in marine primary production, and for this reason, Zn is depleted in surface waters relative to the deep sea (Morel and Price, 2003

Morel, F.M.M., Price, N.M. (2003) The Biogeochemical Cycles of Trace Metals in the Oceans. Science 300, 944-947.

). Zn concentrations in euxinic black shale and iron formations (Robbins et al., 2013

Robbins, L.J., Lalonde, S.V., Saito, M.A., Planavsky, N.J., Mloszewska, A.M., Pecoits, E., Scott, C., Dupont, C.L., Kappler, A., Konhauser, K.O. (2013) Authigenic iron oxide proxies for marine zinc over geological time and implications for eukaryotic metallome evolution. Geobiology 11, 295-306.

; Scott et al., 2013

Scott, C., Planavsky, N.J., Dupont, C.L., Kendall, B., Gill, B.C., Robbins, L.J., Husband, K.F., Arnold, G.L., Wing, B.A., Poulton, S.W., Bekker, A., Anbar, A.D., Konhauser, K.O., Lyons, T.W. (2013) Bioavailability of zinc in marine systems through time. Nature Geoscience 6, 125-128.

), however, suggest that the bioavailability of Zn has not changed dramatically through Earth history. The Fe budget is similar to that of Zn, wherein hydrothermal input dominates over riverine fluxes by a factor of ~9 (Wheat et al., 2002

Wheat, C.G., Mottl, M.J., Rudnicki, M. (2002) Trace element and REE composition of a low-temperature ridge-flank hydrothermal spring. Geochimica et Cosmochimica Acta 66, 3693-3705.

). Under sulphidic conditions, dissolved Zn²⁺ and Fe²⁺ behave similarly and are rapidly precipitated as sulphides (Morse and Luther III, 1999

Morse, J.W., Luther III, G.W. (1999) Chemical influences on trace metal-sulfide interactions in anoxic sediments. Geochimica et Cosmochimica Acta 63, 3373-3378.

). In addition, because both Fe and Zn behave as incompatible elements during mantle partial melting, Zn/Fe has been developed as a tracer of mantle redox, revealing that the oxygen fugacity of the upper mantle has remained relatively constant through Earth history (Lee et al., 2010

Lee, C.T.A., Luffi, P., Le Roux, V., Dasgupta, R., Albarede, F., Leeman, W.P. (2010) The redox state of arc mantle using Zn/Fe systematics. Nature 468, 681-685.

). In the following discussion, we assume that Zn/Fe in hydrothermal inputs into the ocean have not changed significantly through time. We recognise, however, that a number of factors could limit this assumption, and consider these below.

Zn/Fe in the sedimentary record thus has the potential to document Earth surface redox evolution if we consider the following assumptions: 1) Zn and Fe budgets in the oceans are dominated by hydrothermal inputs and are therefore not significantly influenced by secular evolution of continental inputs; 2) Fe²⁺ and Zn²⁺ have similar solubility in the oceans; 3) the partition coefficient of Zn/Fe ratios into carbonates has remained the same through time; and 4) when Fe²⁺ is oxidised to Fe³⁺, it precipitates from seawater and thus is not incorporated into carbonate; zinc, however, remains divalent as Zn²⁺.

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Methods

Major, trace, and rare earth elements (REE) concentrations were determined with a Thermo Scientific^® iCAP-Q ICP-MS (Inductively Coupled Plasma – Mass Spectrometry) at the Carnegie Institution of Washington. Approximately 5 to 10 mg of micro-drilled sample powders were weighed and dissolved in 2 ml distilled 0.4 M HNO₃ and reacted for 12 hours. The resulting solutions were centrifuged for 5 minutes at ~6000 rps and 1 ml of the supernatant was pipetted and diluted with distilled 4 ml 0.4 M HNO₃ for elemental analysis. Calibration curves were created using multi-elemental standards with different dilutions made from pure element solutions (Alfa Aesar®). Both standard and sample solutions were doped with 4 ppb In to correct for instrumental drift. Precision of the analyses was determined by repeated analyses of an in-house carbonate standard, and was typically better than 5 % (2σ) for major elements, and better than 10 % (2σ) for most trace elements including REE. Accuracy of the analyses was determined by replicates of an international coral standard (JCp-1), as shown in Figure S-1.

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Results and Discussion

Here, we report Zn/Fe molar ratios in marine carbonate rock through Earth history (Fig. 1) and provide a quantification of atmospheric O₂ evolution since the Mesoarchean Era. Samples (n = 1700) come from our analyses (n = 300), as well as a literature compilation (see SI-1 Table S-1). In all carbonate samples, the potential for diagenetic alteration is of concern. To evaluate the degree of sample alteration, we selected specimens with known sedimentological and stratigraphic context and investigated their petrography and elemental and isotope geochemistry. Samples used in this study were primarily composed of fine-grained limestone and penecontemporaenous, fabric-retentive dolostone, including both micrites and stromatolites. We micro-sampled carbonate specimens from polished billets to avoid weathering alteration, secondary veins/precipitation, and areas with visible non-carbonate phases. In addition to geological and petrographic criteria, we further selected samples based on primary isotopic and trace element patterns (see SI-2).

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Even if we carefully select the most primary samples, we cannot ignore diagenetic influences on the elemental composition of sampled carbonates, as this can contribute to local variation of Zn/Fe (Fig. 1). Both Zn and Fe partition coefficients (K_d) from fluid to carbonates increase with increasing diagenesis as shown by earlier work of Brand and Veizer (1980)

Brand, U., Veizer, J. (1980) Chemical diagenesis of a multicomponent carbonate system; 1, Trace elements. Journal of Sedimentary Research 50, 1219-1236.

. Also, K_d (Fe) increases faster compared to K_d (Zn), from 1 to 20 and from 5.2 to 5.5 for Fe and Zn, respectively. According to this work, diagenesis will cause a decrease in Zn/Fe ratios by incorporating more Fe than Zn in carbonates. We acknowledge that all of the carbonates examined here have undergone some degree of burial diagenesis, and this will be reflected in the variance of Zn/Fe within individual time intervals. Also, local primary production differences may contribute to Zn/Fe variability of different formations from the same interval. In the modern oxidised shallow ocean, particulate Fe sourced from eroding continents remains biogeochemically labile and may be cycled back to a dissolved phase during diagenesis in reducing continental margin sediments (Raiswell et al., 2006

Raiswell, R., Tranter, M., Benning, L.G., Siegert, M., De’ath, R., Huybrechts, P., Payne, T. (2006) Contributions from glacially derived sediment to the global iron (oxyhydr)oxide cycle: Implications for iron delivery to the oceans. Geochimica et Cosmochimica Acta 70, 2765-2780.

). Therefore, there is also a potentially large and variable source of reactive Fe to shallow marine settings that is decoupled from the Zn flux, which likely causes Zn/Fe ratios to be lower and therefore could contribute to the variations observed in Zn/Fe data. In addition, theoretical calculations suggest that kinetic effects on trace element partitioning in carbonate may contribute to Zn/Fe variability in samples from the same locality (Watson, 2004

Watson, E.B. (2004) A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals. Geochimica et Cosmochimica Acta 68, 1473-1488.

; DePaolo, 2011

DePaolo, D.J. (2011) Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochimica et Cosmochimica Acta 75, 1039-1056.

). Importantly, however, these influences should not result in systematic variations that would contribute to observed first-order secular changes. We plot all carbonate samples based on their lithology in Figure S-2; this shows that there is no systematic difference between limestone and dolomite samples through time - not unexpected, as many Proterozoic dolomites formed penecontemporaneously and preserve geochemical signatures as well as coeval limestones that underwent neomorphism during burial.

We observe a distinct trend of increasing Zn/Fe through time (Fig. 1), especially around the GOE and NOE. Our Palaeoproterozoic data are also consistent with earlier suggestions that pO₂ may have risen substantially during the GOE and then declined again to persistent Proterozoic values (Lyons et al., 2014

Lyons, T.W., Reinhard, C.T., Planavsky, N.J. (2014) The rise of oxygen in Earth's early ocean and atmosphere. Nature 506, 307-315.

). Employing three statistically complementary approaches (see details in SI-3), carbonate Zn/Fe could follow “step” or “smooth” fits through Earth’s history (Figs. S-3, S-4 and S-5), where we prefer the “step” approach with lognormal distributions (see Figs. 2 and S-3). Using lognormal distributions to estimate Zn/Fe through time, we can provide quantitative constraints on Earth’s atmospheric O₂ evolution, as follows.

From the chemical reaction of Fe oxidisation from Fe²⁺ to Fe³⁺:

4Fe²⁺ (aq) + O₂ (aq) + 10H₂O (aq) = 4Fe(OH)₃ (s) + 8H⁺ (aq),

where K is the equilibrium constant and is activity. In this equation, we assume that when Fe²⁺ oxidises to Fe³⁺ and is precipitated from the aqueous system as iron hydroxide, and only Fe²⁺ gets incorporated into carbonates. We are aware that secular variations in seawater sulphate might modulate hydrothermal iron fluxes through time via the formation of iron sulphides (Kump and Seyfried, 2005), we do not know the extent to which Zn abundances might similarly be buffered and so do not consider this in our first-order model. Assuming O₂ equilibrium between atmosphere and surface ocean on hundred million year time scales, we can write the equation using atmospheric oxygen fugacity,            , as

if we assume the Zn concentrations in seawater and partitioning of Zn/Fe from seawater to carbonate minerals are constant over Earth history. Therefore, we can write the equation normalised to Zn²⁺ as

in which superscripts P and M indicate the past and modern parameters. Assuming pH and K are constant (see SI-4), we can simplify the relationship between Fe/Zn ratios and            , as

where             is the oxygen fugacity in the past (any time in Earth’s history),             is the oxygen fugacity in modern time, and

provides Zn/Fe ratios in past carbonate normalised to modern values. If we assume that atmospheric O₂ is in equilibrium with the shallow marine environment, and that we know the current atmospheric pO₂ (0.21) and the modern seawater Zn/Fe ratios as reflected in Zn/Fe ratios of marine carbonates, we can use Zn/Fe to calculate fO₂ (also expressed as pO₂) at any given time of Earth history (Fig. 3). This pO₂ curve provides a more continuous coverage of atmospheric O₂ levels compared to compilations derived from multiple geochemical tracers, such as mass-independent S isotopes and palaeosol records (Rye and Holland, 1998

Rye, R., Holland, H.D. (1998) Paleosols and the evolution of atmospheric oxygen: A critical review. American Journal of Science 298, 621-672.

; Catling and Claire, 2005

Catling, D.C., Claire, M.W. (2005) How Earth's atmosphere evolved to an oxic state: A status report. Earth and Planetary Science Letters 237, 1-20.

).

Full size image | Download in Powerpoint

Full size image | Download in Powerpoint

The log pO₂ curve in Figure 3 reproduces what we think we know about oxygen history: estimated pO₂ is extremely low in the Archean and reaches modern levels only in the mid-Palaeozoic Era. Moreover, the estimates match our current understanding (Lyons et al., 2014

Lyons, T.W., Reinhard, C.T., Planavsky, N.J. (2014) The rise of oxygen in Earth's early ocean and atmosphere. Nature 506, 307-315.

) of a general two-step increase of atmospheric O₂ around the GOE and the NOE. Importantly, our study provides an estimate of the upper and lower bounds on pO₂ in the mid-Proterozoic atmosphere, with a preferred value between 0.1 and 1 % PAL. This value is substantially lower than traditional estimates based on palaeosol work (Canfield, 1998

Canfield, D.E. (1998) A new model for Proterozoic ocean chemistry. Nature 396, 450-453.

; Rye and Holland, 1998

Rye, R., Holland, H.D. (1998) Paleosols and the evolution of atmospheric oxygen: A critical review. American Journal of Science 298, 621-672.

), but consistent with recent estimates based on an independent tracer, a kinetic model for Cr-Mn oxidation and Cr isotopes in ironstones (Planavsky et al., 2014

Planavsky, N.J., Reinhard, C.T., Wang, X., Thomson, D., McGoldrick, P., Rainbird, R.H., Johnson, T., Fischer, W.W., Lyons, T.W. (2014) Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals. Science 346, 635-638.

). We conducted sensitivity tests of temperature and pH variations on our pO₂ estimates and found that the influence of temperature is negligible. pH, however, could potentially lower pO₂ estimates, especially for earlier samples when pCO₂ was high (see SI-4 for details); thus our estimates of Proterozoic pO₂ should be considered conservative and may overestimate past oxygen levels. There are hints of biologically interesting structure in the Neoproterozoic and Cambrian records, but at present our sample numbers and bin sizes are too small to address this in detail. As more carbonate data become available for key transitional time periods such as those around GOE and NOE, potentially complex secular patterns of redox change may become clearer. Further investigations on well-constrained modern and Phanerozoic marine carbonates are currently underway to evaluate with more quantitative rigour the potential effects of diagenesis, mineralogy, and ocean depth gradient distributions on the proxy proposed here.

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Conclusion

In summary, we have demonstrated the potential for using divalent cations in carbonates as sensitive proxies for the evolution of Earth’s near surface environment. Because many marine carbonate rocks were deposited in shallow marine environments, in direct contact with the atmosphere, elemental ratios are likely to reflect equilibrium atmospheric conditions extending back to the Archean Eon and including time intervals poorly represented by other lithologies. Although further work will be needed to fully validate this promising palaeoredox proxy, carbonate-based redox proxies show great potential to expand the palaeoredox record and to provide self-consistent and quantitative constraints on atmospheric O₂ through Earth’s history.

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Acknowledgements

We are grateful to T. Mock for assistance with the Q-ICP-MS analyses, and M. Horan for help in the clean lab. We are grateful to Mahrnaz Siahy and Axel Hofmann for providing the Pongola samples. We are grateful to M. Van Kranendonk for sampling help in Western Australia and M. Evans, J. Hao, T. Lyons, D. Sverjensky and J. Veizer for discussions. The Alfred P. Sloan Foundation, the Deep Carbon Observatory, the National Science Foundation, the NASA Astrobiology Institute, and the Carnegie Institution of Washington provided financial support to RMH and X-ML. AHK thanks the NASA Astrobiology Institute.

Editor: Eric H. Oelkers

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Author Contributions

X-ML and RMH designed the project with inputs from all authors. X-ML performed the chemical analyses. X-ML wrote the manuscript with inputs from all authors. LK, AHK, HC, AJK, and RMH provided samples.

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References

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Supplementary Information

SI-1: Table S-1, Figures S-1 and S-2

Geologic Unit	Sample #	Age (Ma)	Zn/Fe*104	Reference
Strelley Pool Fm, Warrawoona Group, Pilbara, Western Australia	BH2	3400	0.9	Hazen (unpublished)
Strelley Pool Fm, Warrawoona Group, Pilbara, Western Australia	BH3	3400	17.6	Hazen (unpublished)
Strelley Pool Fm, Warrawoona Group, Pilbara, Western Australia	BH7	3400	5.3	Hazen (unpublished)
Strelley Pool Fm, Warrawoona Group, Pilbara, Western Australia	BH10	3400	6.5	Hazen (unpublished)
Calcite filling in Mt. Ada basalt	BH18	3470	1.5	Hazen (unpublished)
Pongola Fm, Sourth Africa	Po1	2950	16.1	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po2	2950	45.9	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po3	2950	4.5	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po4	2950	6.4	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po5	2950	5.9	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po6	2950	7.8	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po7	2950	5.8	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po8	2950	4.0	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po9	2950	3.8	Beukes and Lowe (1989)
Pongola Fm, Sourth Africa	Po10	2950	4.8	Beukes and Lowe (1989)
Tumbiana Fm, Fortesue Group, Western Australia	BH11	2700	5.8	Hazen (unpublished)
Tumbiana Fm, Fortesue Group, Western Australia	BH12	2700	24.6	Hazen (unpublished)
Tumbiana Fm, Fortesue Group, Western Australia	BH13	2700	8.9	Hazen (unpublished)
Tumbiana Fm, Fortesue Group, Western Australia	BH14	2700	8.0	Hazen (unpublished)
Tumbiana Fm, Fortesue Group, Western Australia	BH15	2700	2.8	Hazen (unpublished)
Tumbiana Fm, Fortesue Group, Western Australia	BH16	2700	3.4	Hazen (unpublished)
Kazput Fm, Turee Creek, Australia	AN94	2350	7.6	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN95	2350	8.4	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN96	2350	9.9	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN97	2350	7.7	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN98	2350	8.2	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN99	2350	5.2	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN100	2350	7.8	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN101	2350	10.4	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN102	2350	11.5	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN103	2350	13.1	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN104	2350	23.0	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	AN105	2350	23.5	Knoll et al. (unpublished)
Kazput Fm, Turee Creek, Australia	BH19	2350	8.5	Hazen (unpublished)
Duck Creek, Auatralia	AN80	1835	53.0	Wilson et al. (2010)
Duck Creek, Auatralia	AN82	1835	8.0	Wilson et al. (2010)
Duck Creek, Auatralia	AN83	1835	36.6	Wilson et al. (2010)
Duck Creek, Auatralia	AN84	1835	53.0	Wilson et al. (2010)
Duck Creek, Auatralia	AN85	1835	26.5	Wilson et al. (2010)
Duck Creek, Auatralia	AN86	1835	16.2	Wilson et al. (2010)
Duck Creek, Auatralia	AN87	1835	27.6	Wilson et al. (2010)
Duck Creek, Auatralia	AN88	1835	6.2	Wilson et al. (2010)
Vempalle Fm, Cuddapah Basin, India	V-SC/1	1750	8.5	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-SC/2	1750	4.8	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-SC/3	1750	5.3	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-SC/5	1750	10.6	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-SC/6-1	1750	6.8	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-P2/1	1750	5.5	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-P8/1	1750	12.2	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-P10/1	1750	7.5	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-P11/1	1750	6.9	Chakrabarti et al. (2014)
Vempalle Fm, Cuddapah Basin, India	V-P12/1	1750	6.1	Chakrabarti et al. (2014)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C133-3252.3	1350	17.8	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C133-3253.3	1350	12.8	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C133-3038	1350	9.5	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C133-2767.5	1350	27.7	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C133-3114.5	1350	20.1	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C203-3852A	1350	24.4	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C203-2459.8	1350	10.0	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C203-2459.6-1	1350	18.1	Kah et al. (2007)
Kyrpy Group, East Eurapean Platform, Southern Urals, Russia	C203-2353	1350	20.9	Kah et al. (2007)
Sulky Fm, Dismal Lakes, Canada	DL1-364-1	1300	13.9	Kah et al. (2006)
Sulky Fm, Dismal Lakes, Canada	DL1-306-1	1300	28.0	Kah et al. (2006)
Sulky Fm, Dismal Lakes, Canada	DL1-332	1300	22.4	Kah et al. (2006)
Sulky Fm, Dismal Lakes, Canada	SL16-1-1	1300	2.7	Kah et al. (2006)
Sulky Fm, Dismal Lakes, Canada	SL17-10-1	1300	4.5	Kah et al. (2006)
Avzyan Fm, Southern Urals, Russia	M1(AZ)-39	1150	3.0	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	M1(AZ)-47	1150	31.7	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	RV(AZ)-15	1150	8.8	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	RV(AZ)-33	1150	13.8	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	KT(AZ)-49.5	1150	17.6	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	KT(AZ)-131.6	1150	3.0	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	KT(AZ)-236-1	1150	7.0	Bartley et al. (2007)
Avzyan Fm, Southern Urals, Russia	KT(AZ)-373.5	1150	8.0	Bartley et al. (2007)
El Mreiti,Atar Group, West Africa	F4-10-1	1100	42.6	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-19-1	1100	17.7	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-50-1	1100	13.9	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-53-1	1100	16.4	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-90-1	1100	17.8	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-95-1	1100	11.9	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-98-1	1100	17.7	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-99-1	1100	39.6	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-102-1	1100	20.0	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-104-1	1100	8.6	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-106-1	1100	11.4	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-107-1	1100	13.0	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-108-1	1100	6.8	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-109-1	1100	6.1	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-113-1	1100	13.1	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-114-1	1100	7.2	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-115-1	1100	7.8	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-116-1	1100	7.8	Gilleaudeau and Kah (2013)
El Mreiti,Atar Group, West Africa	F4-117-1	1100	7.9	Gilleaudeau and Kah (2013)
Atar Group, West Africa	ATS-31	1100	14.8	Kah et al. (2012)
Atar Group, West Africa	ATS-53	1100	92.9	Kah et al. (2012)
Atar Group, West Africa	ATS-61	1100	16.5	Kah et al. (2012)
Atar Group, West Africa	ATS-5-1	1100	13.8	Kah et al. (2012)
Atar Group, West Africa	ATS-119	1100	11.6	Kah et al. (2012)
Atar Group, West Africa	ATS-154	1100	12.8	Kah et al. (2012)
Atar Group, West Africa	ATD-17	1100	18.4	Kah et al. (2012)
Atar Group, West Africa	ATD-61	1100	18.2	Kah et al. (2012)
Atar Group, West Africa	ATL-51-1	1100	9.3	Kah et al. (2012)
Atar Group, West Africa	ATL-58	1100	66.2	Kah et al. (2012)
Atar Group, West Africa	ATL-68	1100	4.9	Kah et al. (2012)
Atar Group, West Africa	ATL-105	1100	19.3	Kah et al. (2012)
Atar Group, West Africa	ATL-110-1	1100	6.3	Kah et al. (2012)
Atar Group, West Africa	ATD-27.5	1100	12.3	Kah et al. (2012)
Atar Group, West Africa	ATD-45	1100	18.4	Kah et al. (2012)
Atar Group, West Africa	R1-Δ-1	1100	16.4	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-7	1100	25.5	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-13	1100	14.9	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-25	1100	12.7	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-29	1100	12.0	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-30	1100	10.3	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-31	1100	5.8	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-38	1100	12.2	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-39	1100	6.5	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-41	1100	9.6	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-42	1100	6.7	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-48	1100	8.1	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-54-1	1100	9.6	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-55	1100	7.1	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-58	1100	7.6	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-60	1100	4.7	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-61	1100	4.6	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-65	1100	9.9	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-68-1	1100	18.3	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-72	1100	6.6	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-76	1100	12.4	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-79-1	1100	8.3	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-81-1	1100	5.2	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-85	1100	6.0	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-89	1100	16.8	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-90	1100	4.4	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-97	1100	3.1	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-98	1100	9.2	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-100	1100	3.6	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-103	1100	3.1	Manning-Berg and Kah (in prep)
Atar Group, West Africa	R1-Δ-106	1100	2.2	Manning-Berg and Kah (in prep)
Chattisgarh, India	SRJ-2I	1000	25.9	Bickford et al. (2011)
Chattisgarh, India	SRJ-3I	1000	22.8	Bickford et al. (2011)
Chattisgarh, India	SRJ-4I	1000	13.0	Bickford et al. (2011)
Chattisgarh, India	SRJ-6I	1000	20.6	Bickford et al. (2011)
Chattisgarh, India	SRJ-8I	1000	37.8	Bickford et al. (2011)
Chattisgarh, India	SRJ-9I	1000	85.2	Bickford et al. (2011)
Chattisgarh, India	TML-3I	1000	7.7	Bickford et al. (2011)
Chattisgarh, India	TML-6I	1000	17.8	Bickford et al. (2011)
Chattisgarh, India	TML-7I	1000	14.9	Bickford et al. (2011)
Sukhaya Tunguska Formation, Russia	AN59	950	29.5	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN60	950	46.1	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN61	950	25.7	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN62	950	76.4	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN63	950	110.5	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN64	950	17.0	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN66	950	28.1	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN67	950	20.0	Sergeev et al. (1997)
Sukhaya Tunguska Formation, Russia	AN68	950	20.2	Sergeev et al. (1997)
Akademikerbeen Group, Spitsbergen	AN1	775	29.2	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN2	775	40.3	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN3	775	17.3	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN4	775	6.2	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN5	775	63.2	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN6	775	65.0	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN7	775	27.6	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN8	775	74.3	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN9	775	95.4	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN10	775	59.4	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN12	775	71.5	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN13	775	15.8	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN14	775	49.4	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN16	775	15.1	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN18	775	17.6	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN19	775	9.4	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN20	775	34.4	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN21	775	276.2	Knoll and Swett (1990)
Akademikerbeen Group, Spitsbergen	AN22	775	34.6	Knoll and Swett (1990)
Limestone-Dolomite Series, East Greenland	AN24	775	126.1	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN25	775	59.3	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN26	775	45.7	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN28	775	5.3	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN29	775	61.2	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN30	775	307.6	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN31	775	310.7	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN32	775	38.3	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN33	775	33.3	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN34	775	15.5	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN35	775	8.3	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN36	775	102.9	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN37	775	67.0	Knoll et al. (1986)
Limestone-Dolomite Series, East Greenland	AN38	775	41.4	Knoll et al. (1986)
Shaler Group, Arctic Canada	AN41	775	5.6	Jones et al. (2010)
Shaler Group, Arctic Canada	AN42	775	17.3	Jones et al. (2010)
Shaler Group, Arctic Canada	AN43	775	45.7	Jones et al. (2010)
Shaler Group, Arctic Canada	AN44	775	7.0	Jones et al. (2010)
Shaler Group, Arctic Canada	AN45	775	154.8	Jones et al. (2010)
Shaler Group, Arctic Canada	AN46	775	6.2	Jones et al. (2010)
Shaler Group, Arctic Canada	AN47	775	40.3	Jones et al. (2010)
Shaler Group, Arctic Canada	AN48	775	14.8	Jones et al. (2010)
Shaler Group, Arctic Canada	AN49	775	116.9	Jones et al. (2010)
Shaler Group, Arctic Canada	AN50	775	8.4	Jones et al. (2010)
Shaler Group, Arctic Canada	AN51	775	10.9	Jones et al. (2010)
Shaler Group, Arctic Canada	AN52	775	57.5	Jones et al. (2010)
Shaler Group, Arctic Canada	AN53	775	6.9	Jones et al. (2010)
Shaler Group, Arctic Canada	AN54	775	6.0	Jones et al. (2010)
Shaler Group, Arctic Canada	AN55	775	12.8	Jones et al. (2010)
Shaler Group, Arctic Canada	AN56	775	17.8	Jones et al. (2010)
Lagoa Do Jacare Formation, Brazil	KM7-14.0.0	650	38.1	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-01.0	650	21.9	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-02.0	650	49.0	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-03.0	650	17.6	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-04.0	650	12.5	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-05.0	650	22.0	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-06.0	650	38.9	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-07.0	650	54.1	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-08.0	650	32.8	Misi et al. (2007)
Lagoa Do Jacare Formation, Brazil	KM7-14-09.0	650	23.8	Misi et al. (2007)
Huttenburg Formation, Namibia	S86A-971.2	650	105.2	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-976.0	650	77.5	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-977.0	650	120.9	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-980.8	650	53.5	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-985.1	650	147.1	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-987.8	650	28.4	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-988.2	650	41.8	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1033.8	650	424.1	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1060.2	650	156.2	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1077.1	650	57.4	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1144.8	650	52.8	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1145.1	650	47.7	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1148.4	650	55.6	Kaufman et al. (2009)
Huttenburg Formation, Namibia	S86A-1213.2	650	51.1	Kaufman et al. (2009)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-7	600	61.0	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-21	600	62.5	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-26	600	30.7	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-34	600	37.5	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-38	600	19.8	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-38b	600	40.1	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-39	600	38.7	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-46	600	24.9	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-49	600	59.0	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-51	600	19.8	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	Dhaiqa-54	600	24.9	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	M1-with fossil	600	62.2	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	N-2-3	600	113.4	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	N-2-11	600	73.1	Miller et al. (2008)
Dhaiqa Formation, NW Arabian shield, Saudi Arabia	N-2-16	600	68.9	Miller et al. (2008)
Yangjiaping, Doushantuo Formation, South China	YD-01	551	12.9	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-02	551	47.1	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-03	551	11.7	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-04	551	25.1	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-05	551	71.0	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-06	551	13.9	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-07	551	28.4	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-08	551	6.6	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-09	551	6.2	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-10	551	26.9	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-11	551	50.4	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-12	551	25.1	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-13	551	23.7	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-14	551	55.0	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-15	551	115.4	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-16	551	221.5	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-17	551	253.8	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-18	551	195.9	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-19	551	101.2	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-20	551	14.5	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-21	551	10.9	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-22	551	29.1	Cui et al. (2015)
Yangjiaping, Doushantuo Formation, South China	YD-23	551	40.1	Cui et al. (2015)
Orthoceras limestone, Ohio	BH21	465	27.5	Hazen (unpublished)
Bryozoan limestone, Ohio	BH24	465	7.2	Hazen (unpublished)
Branch Hill limestone, Ohio	BH25	465	10.9	Hazen (unpublished)
Jersey Shore Station, Pennsylvania	BH26	465	7.0	Hazen (unpublished)
Madison County, Kentucky	BH31	465	14.3	Hazen (unpublished)
Ripley, Ohio	BH36	465	8.3	Hazen (unpublished)
Washington, Kentucky	BH37	465	11.4	Hazen (unpublished)
Bryozoan limestone, Ohio	BH33	465	3.5	Hazen (unpublished)
Coburn Fm, Pennsylvania	BH41	465	10.9	Hazen (unpublished)
La Silla Fm, Argentina	AF-23	464	22.9	Thompson and Kah (2012)
La Silla Fm, Argentina	AF-30	464	12.4	Thompson and Kah (2012)
La Silla Fm, Argentina	SJF08-14	464	16.1	Thompson and Kah (2012)
La Silla Fm, Argentina	LFG-27	464	40.5	Thompson and Kah (2012)
La Silla Fm, Argentina	LFG-55	464	17.6	Thompson and Kah (2012)
La Silla Fm, Argentina	LS-01	464	29.7	Thompson and Kah (2012)
La Silla Fm, Argentina	SJC-116	464	42.1	Thompson and Kah (2012)
West Newfoundland	TH-1	464	15.3	Thompson and Kah (2012)
West Newfoundland	TH-18	464	41.7	Thompson and Kah (2012)
Clay's Ferry Fm, Kentucky	BH49	457	7.1	Hazen (unpublished)
Cincinnati, Ohio	BH32	451	6.5	Hazen (unpublished)
Richmond, Indiana	BH27	451	7.3	Hazen (unpublished)
Ludlow Fm, Silurian	BH28	432	30.3	Hazen (unpublished)
Limestone Quarry, Dickensonville, Virginia	BH29	406	7.7	Hazen (unpublished)
John Boyd Thatchen State Park, New York	BH30	406	12.8	Hazen (unpublished)
Blue Stone limestone Quarry, New York	BH34	406	19.0	Hazen (unpublished)
Isle La Motte, Vermont	BH38	406	21.6	Hazen (unpublished)
Isle La Motte, Vermont	BH39	406	12.2	Hazen (unpublished)
Isle La Motte, Vermont	BH40	406	22.9	Hazen (unpublished)
Hamilton Group, New York	BH22	388	3.6	Hazen (unpublished)
Plainville Quarry, Ohio	BH44	388	17.4	Hazen (unpublished)
Limestone Quarry, Ohio	BH52	388	4.2	Hazen (unpublished)
Coral limestone, Michigan	BH53	388	21.0	Hazen (unpublished)
Madision limestone, Garrett Co., Maryland	BH23	341	31.7	Hazen (unpublished)
Mammoth Cave National Park, Kentucky	BH47	341	190.6	Hazen (unpublished)
Sheep Mountain	BH51	341	53.6	Hazen (unpublished)
Lydstep Haven coral, Wales	BH35	329	70.4	Hazen (unpublished)
Tenby, Wales	BH43	329	69.0	Hazen (unpublished)
Cheddar Gorge, England	BH46	329	65.5	Hazen (unpublished)
Everett Quarry, Missouri	BH45	311	15.4	Hazen (unpublished)
Shark Bay, Australia	BH20	0	28.4	Hazen (unpublished)

*See references in SI-5.

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SI-2: Data Filtering

Diagenetic alteration can be problematic when interpreting the chemical composition of carbonate rocks; therefore, we carefully screened carbonate specimens for a range of diagenetic effects and hydrothermal alteration by combining geologic, petrographic, and element and isotope geochemical analyses. For the published literature, we compiled data only for samples that are considered to reflect primary depositional environments. For our own analyses, we selected samples from within known sedimentological and stratigraphic context, most of which are from previously investigated rock units. Petrographic analysis was used to select sample areas preserving original sedimentary fabrics, which suggests the least interaction with diagenetic fluids, and samples were microdrilled from these localities. To identify possible affects of diagenesis, we screened the samples based on the classic geochemical tracers of late diagenesis, including major and minor elements (e.g., Fe, Mn, Sr) as well as C and O isotopic signatures. We did not fix the selection criteria; rather, we adopted the selection criteria used by individual researchers for the different localities and published in their original descriptions. Readers can refer to the original papers that describe the samples in the reference lists below (see SI-5). In addition, we studied REE patterns and Eu anomalies to avoid samples with significant hydrothermal alternation (e.g., Frimmel, 2009

Frimmel, H.E. (2009) Trace element distribution in Neoproterozoic carbonates as palaeoenvironmental indicator. Chemical Geology 258, 338-353.

). Both Zn and Fe partition coefficients (K_d) from fluid to carbonates increase with increasing diagenesis as shown by earlier work of Brand and Veizer (1980)

Brand, U., Veizer, J. (1980) Chemical diagenesis of a multicomponent carbonate system; 1, Trace elements. Journal of Sedimentary Research 50, 1219-1236.

. Also, (K_d) (Fe) increases faster compared to (K_d) (Zn), from 1 to 20 and from 5.2 to 5.5 for Fe and Zn, respectively. So, diagenesis will cause a decrease in Zn/Fe ratios by incorporating more Fe than Zn in carbonates. Even though, every carbonates we studied today have been through diagenesis. That is why we are adopting a statistic treatment of our global data.


SI-3: Statistical Analysis of Data

Temporal data sets are subject to biases associated with sampling: recent geological eras are commonly represented by more samples, and some formations are represented by multiple analyses. Since we do not know exactly how the effect of diagenesis, local primary production, mineralogy and kinetic effect influence the Zn/Fe values, we therefore adopted three discrete approaches to evaluate Zn/Fe data statistically through time aiming to investigate the population behaviour. First, we divided the entire sample population into eight bins of different duration to make sure each bin has statistically meaningful sample numbers (where n > 50, expect for one bin with n = 38). The bins were chosen to reflect current hypotheses for pO₂ evolution through time; specifically, we chose a bin boundary at 800 Ma to reflect the recent hypothesis of Planavsky et al. (2014) concerning mid-Neoproterozoic oxygenation, and we broke out Cryogenian, Ediacaran and Lower Palaeozoic bins in an attempt to illuminate existing hypothesis concerning Ediacaran-Cambrian oxygen increase. Each bin contains samples from at least two different geological formations. The choice of the bin breaking points is based on convenience and what is already known about pO₂ evolution through Earth history. For example, the first bin is from 3.5–2.5 Ga, which is the pre-GOE time, where we expect low pO₂ and we observe low Zn/Fe and little variation of Zn/Fe. We divide the following bin into 2.5-2.0 Ga and 2.0-1.5 Ga, to have statistically meaningful number of samples in each bin. Then we have three different formations from ~800 Ma with high Zn/Fe compared to pre-1.0 Ga samples and therefore we make a bin from 1.5-0.8 Ga. For the following time periods, we divide bins into several geological meaningful groups, as 800-635 Ma (Cryogenian), 635-541 Ma (Ediacaran), 541-300 Ma (earlier Palaeozoic), and 300-0 Ma (later Palaeozoic, Mesozoic and Cenozoic) to make self-consistent and statistically meaningful sample subsets.

We then performed a box-whisker plot for all data (Fig. S-3), where median, 50 %, and outliers (outside of 3 sigma of the population) of Zn/Fe values were calculated for each bin, which are shown with red lines (orange lines in Fig. 2), blue boxes, and red crosses, respectively in Fig. S-3. In a second approach, we divided data into the same ten bins as in the previous approach, but we plotted histograms for each bin (Fig. S-4). Data in nearly all of the bins follows a lognormal distribution, which permits calculation of means and standard deviations for each of the 10 bins (shown with blue lines in Fig. 2). In a third approach, we calculated average composition for each geological formation, which reduces the influence from sampling bias. For example, some localities/formations are represented by 20 samples, whereas others incorporate 10 or fewer. The formation averages are shown in Figure 2 and a polynomial fit is calculated and displayed in Figure S-5.

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SI-4: Assumptions, Deviation of Equations and Sensitivity Tests

Fe oxidisation a chemical reaction can be written as:

4Fe²⁺ (aq) + O₂ (aq) + 10H₂O (aq) = 4Fe(OH)₃ (s) + 8H⁺ (aq)

where K is the equilibrium constant and is activity. In this equation, we assume that when Fe²⁺ oxidises to Fe³⁺ and is precipitated from the aqueous system as iron hydroxide [Fe(OH)₃], only Fe²⁺ is incorporated into carbonates. Assuming equilibrium between atmosphere and surface ocean on hundred million year time scales (the smallest bin size is around 100 million years, Bin 6: 635-541 Ma), we replace             in Equation S-1 with oxygen fugacity in the atmosphere,            , as

Then we can reorganise Equation S-2 to

if we assume the Zn concentrations in seawater and partitioning of Zn/Fe from seawater to carbonate minerals are constant over Earth’s history. Therefore, we can write the equation normalised to Zn²⁺ as

in which superscripts P and M indicate the past and modern parameters. Assuming pH and K are constant, we can simplify the relationship between Fe/Zn ratios and            , as

where             is the oxygen fugacity in the past (any time in Earth’s history),             is the oxygen fugacity in modern time, and

provides Zn/Fe ratios in past carbonate normalised to the modern values. If we assume that atmospheric O₂ is in equilibrium with the shallow marine environment, and that we know the current atmospheric pO₂ (0.21) and the modern seawater Zn/Fe ratios as reflected in Zn/Fe ratios of marine carbonates, we can therefore calculate fO₂ at any given time of Earth’s history if we know the Zn/Fe ratio.

Discussion of Assumptions

This approach requires several critical assumptions, one of which is that we assume the pH of the ocean has not changed significantly through Earth’s history—an assumption supported by some studies (Grotzinger and Kasting, 1993

Grotzinger, J.P., Kasting, J.F. (1993) New constraints on precambrian ocean composition. Journal of Geology 101, 235-243.

; Sumner and Grotzinger, 2004

Sumner, D.Y., Grotzinger, J.P. (2004) Implications for Neoarchaean ocean chemistry from primary carbonate mineralogy of the Campbellrand-Malmani Platform, South Africa. Sedimentology 51, 1273-1299.

). However, we do recognise the possibly large influence of pH change on the quantitative constraint of atmospheric O₂. We also assume a constant Zn concentration in seawater through time. Although Zn concentrations, theoretically may have responded to changes in enzymatic use during biosphere evolution, there is no evidence for change in Zn bioavailability change through time (Robbins et al., 2013

Robbins, L.J., Lalonde, S.V., Saito, M.A., Planavsky, N.J., Mloszewska, A.M., Pecoits, E., Scott, C., Dupont, C.L., Kappler, A., Konhauser, K.O. (2013) Authigenic iron oxide proxies for marine zinc over geological time and implications for eukaryotic metallome evolution. Geobiology 11, 295-306.

; Scott et al., 2013

Scott, C., Planavsky, N.J., Dupont, C.L., Kendall, B., Gill, B.C., Robbins, L.J., Husband, K.F., Arnold, G.L., Wing, B.A., Poulton, S.W., Bekker, A., Anbar, A.D., Konhauser, K.O., Lyons, T.W. (2013) Bioavailability of zinc in marine systems through time. Nature Geoscience 6, 125-128.

). However, Zn concentrations in seawater may drop during the Mid-Proterozoic due to some extent of euxinia (~1~10 % of modern seafloor area; Reinhard et al., 2013

Reinhard, C.T., Planavsky, N.J., Robbins, L.J., Partin, C.A., Gill, B.C., Lalonde, S.V., Bekker, A., Konhauser, K.O., Lyons, T.W. (2013) Proterozoic ocean redox and biogeochemical stasis. Proceedings of the National Academy of Sciences of the United States of America 110, 5357-5362.

). These two important factors (pH and Zn content) may contribute in various degrees to the accuracy of atmospheric O₂ estimation in our model. However, we do not have a quantitative understanding of either at present. Therefore, we develop a first order quantification of secular evolution of atmospheric O₂ with the assumptions that neither pH nor Zn content in seawater change significantly during the time intervals we investigated.

Sensitivity Tests of the Modelling

Here, we evaluate how temperature and pH changes influence the modelling results in seawater through Earth’s history.

First, we perform a sensitivity test that assuming three different temperatures:

By combining Equations S-7 and S-8, we have FeCO₃ + Zn²⁺ --> ZnCO₃ + Fe²⁺, where equilibrium constant K can be written as log K = log (a(ZnCO₃)/a(FeCO₃)) - log (a(Zn²⁺)/a(Fe²⁺)), where a are activities for different components. We then calculate K values at different temperatures using SUPCRT92 (Johnson et al., 1992

Johnson, J.W., Oelkers, E.H., Helgeson, H.C. (1992) SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bar and 0 to 1000°C. Computers & Geosciences 18, 899-947.

) and the results are shown in Figure S-6.

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Generally, when T increases, log K (also K) decreases, but still not so much.

When T = 25 ^oC, log K = -0.496, K = 0.32;

When T = 60 ^oC, log K = -0.575, K = 0.27;

When T = 100 ^oC, log K = -0.65, K = 0.22.

If we take the most extreme example: assuming modern T is 25 ^oC and Archean T is 100 ^oC. The fO₂ estimate in the Archean will increase by approximately 40 %. Therefore, even if we assume the most extreme ocean temperature (100 ^oC), the uncertainty caused by temperature change on fO₂ estimate is much smaller compared to uncertainties generated by Zn/Fe variations in carbonates (Fig. 3), which is at least one order of magnitude.

Second, we carry out a sensitivity test on pH change:

The modern day seawater has a pH of ~8 (M in the equation). If we assume a lower pH in the past (P in the equation), we can rearrange the equations as

Finally, we get the new pH dependent relationship as

Therefore, if the pH is lower in the past, this will cause the O₂ estimate to be lower. This also means that the O₂ level we estimate is the maximum values assuming the pH was lower in the past. Also, if we change the pH from 8.1 to 7.6, estimated O₂ is four orders of magnitude lower because the O₂ estimate is sensitive to pH change. Since we do not know how seawater pH changes through time, and there is no evidence that it changes dramatically on the order of hundreds of million years, we keep the pH constant in our model. However, if we understand better how pH change through time, we can plug in pH in above equations and get a more accurate estimate of O₂ evolution through time.


SI-5: Supplementary Information References

For Table S-1


For the rest of SI