The grand mirror of isotope anomalies

The existence of a pO₂ threshold that switched off one photochemical reaction, i.e., deep UV SO₂ photolysis in the troposphere, and switched on another photochemical reaction, i.e., the CO₂–O₂–O₃ reaction network in the stratosphere, has resulted in a grand mirror of isotope anomalies over the 4.6 Ga Earth history. The rock record reveals this grand pattern.

Financial support is provided by the National Natural Science Foundation of China (42494851, 42494852).

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The Grand Mirror of Isotope Anomalies-Supplementary Information

The Grand Mirror of Isotope Anomalies

Huiming Bao

1. International Center for Isotope Effects Research (ICIER), State Key Laboratory of Critical Earth Material Cycling and Mineral Deposits, Nanjing University, Nanjing 210023, China
2. Frontiers Science Center for Critical Earth Material Cycling, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

There are often simple and broad patterns in a seemingly chaotic history of a complex system that is filled with random events. Earth history is not an exception. For example, driven by mantle convection and plate tectonics, Earth's continents have repeatedly assembled into supercontinents and then broken apart—the supercontinent cycle (~400–600 million years) ^[1-3]. Episodic large igneous provinces may have triggered several mass extinction events of macroscopic life forms in the Phanerozoic Eon (since 541 Ma) ^{[4, 5]}. Identifying general patterns like these not only helps to reveal deep, underlying mechanisms but also offers excellent pedagogical frameworks in exploring Earth history. Here I highlight another grand pattern: The Grand Mirror of Isotope Anomalies (GMIA).

Earth surface has become more and more oxidized over time as evident from the geological record ^[6-8]. Hydrogen escape may have played an important role in the early stage ^{[9, 10]}. Subduction of organic matter and pyrite into the mantle may be the sustained driving force ^{[11, 12]}, with even deeper mantle-crustal processes playing a role ^[13]. A consequence of these processes is the rising of O₂ concentration (pO₂) in the atmosphere, followed by oxygenation of the shallow oceans and eventually the deep oceans. Associated changes include the scavenge of Fe(III) by O₂ and Fe(II) by sulfide as well as the increase of SO₄^2- concentration in the ocean overtime. Various positive and negative feedback loops are in play.

In the midst of this complex and irreversible evolution, there is a threshold of atmospheric O₂ concentration and therefore that of total atmospheric ozone column thickness. Below that threshold, deep ultraviolet can reach the ground or the troposphere and non-mass-dependent (NMD) sulfur isotope fractionation can be readily generated by the photolysis of volcanic SO₂ at Earth surface and the signatures (i.e., non-zero Δ³³S values) preserved in rock record. Above that threshold, NMD sulfur isotope signatures can no longer be generated readily in the troposphere and even more difficult to be preserved in rock record due to increasing intensity of microbial sulfur redox cycling. Instead, NMD oxygen isotope signatures appear in the atmosphere. This is because below the threshold, atmospheric CO₂-O₂-O₃ network reaction is insignificant; above the threshold, atmospheric O₂ bears non-mass-dependently ¹⁷O-depleted signature while O₃ bears ¹⁷O-positive signature (i.e., non-zero Δ¹⁷O values) ^[14]. This pO₂ threshold is likely at 10^-5 Present Atmospheric Level based on atmospheric chemistry models ^{[15, 16]}. The transition was coined the Great Oxidation Event (GOE), occurring at ~2.3 Ga ^[7], when atmospheric CO₂-O₂-O₃ network reaction was becoming significant. Thus, the Δ³³S and Δ¹⁷O of atmospheric compounds should constitute a mirror image of each other divided at ~2.3 Ga over the now ~4.6 billion-year Earth history, i.e., we will see a Grand Mirror of Isotope Anomalies over the axis of the entire geological time (Fig. 1). The GMIA hypothesis hinges upon the existence of a pO₂ threshold and its impacts on the underlying chemistries that produce and preserve the NMD ³³S and ¹⁷O anomalies.

The GMIA hypothesis has to imprint its atmospheric signatures into minerals and rocks in order to be preserved and tested. Making the atmosphere-rock connection is not straightforward. That’s where the geochemists step in. The proposed mechanism ^[6] is that when SO₂ is photolyzed by UV in the atmosphere, elemental sulfur and S(VI)O₃ are produced. The water-insoluble elemental sulfur, S₈ mostly, undergoes further disproportionation to generate S^- and subsequently to solid FeS/FeS₂ and S(IV) species while the S(VI)O₃ hydrolyzes to water-soluble SO₄^2- and subsequently locked in minerals like barite or anhydrite. The sulfide minerals bear Δ³³S-positive while the SO₄^2- bears Δ³³S-negative signatures. Importantly, due to the apparent dearth of sulfide oxidation and sulfate reduction in early Earth environment, these contrasting signatures were not homogenized or erased.

On the oxygen side, stratospheric CO₂-O₂-O₃ reaction results in O₃ and CO₂ bearing distinct positive Δ¹⁷O values while O₂ bearing negative Δ¹⁷O value ^[17]. Stratospheric ¹⁷O-depleted O₂ mixes with photosynthetically produced, ¹⁷O-normal O₂ to make ¹⁷O-depleted tropospheric O₂. Tropospheric O₃ also bears large positive Δ¹⁷O values. When sulfate forms via the oxidation of sulfide minerals or SO₂ gas, sulfate oxygen can carry tropospheric O₂ or O₃ signatures. Sulfate minerals are the most reliable recorder of the past atmospheric O₂ and O₃ signatures ^[18].

Although much of the geological time axis remains to be filled, the rock record has revealed a broad pattern of the expected GMIA. With a vertical ³³S- or ¹⁷O-anomaly axis centered at ~2.3 Ga on the time axis, we can divide the diagram into four quadrants. The 2^nd, the positive Δ³³S quadrant before 2.3 Ga has been occupied by sedimentary pyrite. We see ³³S anomalies in the sedimentary sulfides as old as the oldest rocks on Earth ^[19]. Only sporadic barite occurrences before 3.2 Ga and pyrite clusters around 2.5 Ga have occupied the 3^rd, the Δ³³S-negative quadrant before 2.3 Ga. Recent discovery of NMD ¹⁷O depletion in sulfate ^[20-23] has filled most of the time intervals in the 4^th, the Δ¹⁷O-negative quadrant after 2.3 Ga, while only younger-than-30-Ma volcanic-ash-bed ^[24] or hyperarid sulfate deposits ^{[25, 26]} dotted the 1^st, the Δ¹⁷O-positive quadrant after 2.3 Ga. The lack of positive Δ¹⁷O records in geological history is apparently a preservation issue: Δ¹⁷O-positive sulfate minerals can only accumulate in arid and hyperarid continental settings which can hardly survive erosion in a tectonically active planet.

Despite the confirmed pattern of the GMIA, a few exceptions to the GMIA do exist and their resolution may strengthen or potentially weaken the GMIA hypothesis. The most outstanding piece of the exceptions is the positive and negative Δ³³S values found in modern atmospheric sulfate of anthropogenic or volcanic sources ^[27-31]. Sulfur from the late Ordovician black shales in South China also display distinctly positive Δ³³S values ^[32]. Some of the Paleoproterozoic ^[33] and the latest Permian pyrite grains ^[34] were reported to be distinctly positive or negative in Δ³³S values. The modern atmospheric ³³S anomalies may be explained by the unprecedent high-spatial-temporal sampling resolution that has captured the reaction-transport effects which are otherwise homogenized in geological samples. The discoveries of Proterozoic and Phanerozoic sedimentary ³³S anomalies, while puzzling, have not yet constituted a critical challenge to the existing pO₂ threshold paradigm. The GMIA hypothesis will be continuously tested and enhanced with new data.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (42494851, 42494852).

References

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[5] S.E. Grasby and D.P.G. Bond, How large igneous provinces have killed most life on Earth—numerous times, Elements 19, 276 (2023). 
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[19] E. Thomassot, J. O'Neil, D. Francis, P. Cartigny, and B.A. Wing, Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec), Proc. Natl. Acad. Sci. U.S.A. 112, 707 (2015). 
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[25] H. Bao and D.R. Marchant, Quantifying sulfate components and their variations in soils of the McMurdo Dry Valleys, Antarctica, J. Geophys. Res. 111, D16301 (2006). 
[26] T. Sun, H. Bao, M. Reich, and S.R. Hemming, More than ten million years of hyper-aridity recorded in the Atacama Gravels, Geochim. Cosmochim. Acta 227, 123 (2018). 
[27] A.B. Romero and M.H. Thiemens, Mass-independent sulfur isotopic compositions in present-day sulfate aerosols, J. Geophys. Res. Atmos. 108, 4524 (2003). 
[28] X. Han, Q. Guo, H. Strauss, C. Liu, J. Hu, Z. Guo, R. Wei, M. Peters, L. Tian, and J. Kong, Multiple sulfur isotope constraints on sources and formation processes of sulfate in Beijing PM₂.₅ aerosol, Environ. Sci. Technol. 51, 7794 (2017). 
[29] I. Genot, D. Au Yang, E. Martin, P. Cartigny, E. Legendre, and M. De Rafelis, Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ³³S reservoir of sulfate aerosols?, Atmos. Chem. Phys. 20, 4255 (2020). 
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