Organic haze, glaciations and multiple sulfur isotopes in the Mid-Archean Era
Introduction
Detailed knowledge of the Earth's early atmosphere is important for constraining geochemical, geophysical, and biological evolution. Gaining a better understanding of the evolution of the atmosphere over the first 2 billion years of Earth's history will aid our understanding of the relationships between the atmosphere, climate, and biota, and will improve our ability to search for biospheres on other planets dissimilar from the one presently found on Earth. One tool that has proven useful in tracking photochemical processes on the early Earth has been the measurement of multiple sulfur (S) isotopes in sedimentary rocks. In particular, such studies have helped to better define the transition from an atmosphere with less than 10− 13 times the present atmospheric levels (PAL) of molecular oxygen (O2) to an atmosphere with at least 10− 5 PAL of O2 (Pavlov and Kasting, 2002) and probably considerably higher (Farquhar et al., 2000).
This major event in the evolution of the atmosphere occurred at ~ 2.45 Ga (billion years ago) (Cloud, 1972, Walker et al., 1983, Holland, 1994), and is marked by changes in sulfur isotope data, specifically by a decrease in the range of Δ33S (where Δ33S = δ33S − 1000 ∙ [(1 + δ34S / 1000)0.515 − 1], δ3iS = 1000∙(3iR/3iRV-CDT − 1), and 3iR = (3iS/32S) from greater than 10‰ to less than 0.4‰ (Farquhar et al., 2000, Farquhar et al., 2001, Bekker et al., 2004). Because this Δ33S signature measures a deviation from a relationship between δ33S and δ34S that is otherwise predictable from the masses of the isotopes involved, it is called a “mass-independent fractionation” (MIF, or in the specific case of S, S-MIF).
To date, the only tenable explanations for high Δ33S values require the absence of appreciable atmospheric O2 and ozone (O3). Although other explanations have been proposed for the origin of this signal (Ohmoto et al., 2006), they are inconsistent with other geological data and with 36S measurements — see Section 4.3. The experiments that best reproduce the multiple S isotope signals from the Archean are those that photolyze SO2 with photons that cannot penetrate the lower atmosphere when O2 and O3 are present, suggesting that those gases had to have been absent when the signal was created (Farquhar et al., 2000, Farquhar et al., 2001, Farquhar and Wing, 2003). The connection between experiments and the isotope record has recently been made at a theoretical level, as Lyons (2007) has shown that self-shielding of SO2 photolysis can explain the isotopic fractionations reported in experiments and the rock record. Accordingly, we use “S-MIF producing reaction” as shorthand for the following photolysis reaction:SO2 + hν ➔ SO + O,where SO2 is predissociated by a photon with wavelength λ = 1 / ν, such that 170 nm < λ < 220 nm, the range of the SO2 absorption spectrum that exhibits the band structure that leads to self-shielding (Lyons, 2007). Today, O2 and O3 are present in higher concentrations, causing these wavelengths of solar radiation to be absorbed in Earth's upper atmosphere. Because S-MIF needs to be present low in the atmosphere to have a significant chance of being preserved in the rock record, we can use the presence of S-MIF in the sedimentary record as a tracer for low atmospheric O2 and O3.
A second link between atmospheric oxygen levels and the Δ33S record is through the effect of atmospheric oxidation state on the ability to preserve S-MIF signals. The preservation of this atmospheric isotope signature requires that sulfur be deposited in at least two distinct chemical forms, each of which follows a separate pathway into sediments (Pavlov and Kasting, 2002). In their low-O2 Archean atmosphere model, Pavlov and Kasting (2002) predicted that significant amounts of sulfur would exit the atmosphere as SO2, H2S, and S8. Additionally, these calculations predicted that even a trace amount of atmospheric O2, 10− 5 PAL, would cause all sulfur to eventually be oxidized to sulfate, thereby eliminating the MIF signal. Hence, most authors (except the aforementioned study by Ohmoto et al., 2006) interpret the decrease in Δ33S at ~ 2.45 Ga as indicating an oxidation of the atmosphere via a rise in global atmospheric O2 concentrations (Pavlov and Kasting, 2002) or a decrease in CH4 concentrations (Zahnle et al., 2006).
Prior to the oxygenation event, the atmosphere probably contained appreciable concentrations of methane (CH4) and lesser amounts of molecular hydrogen (H2) (Walker, 1977, Kasting et al., 1983, Catling et al., 2001, Pavlov et al., 2001, Canfield, 2005, Kharecha et al., 2005). Kharecha et al. (2005) created a coupled atmosphere–biosphere model that predicted CH4 concentrations of ~ 1000 ppmv and H2 concentrations of 50–100 ppmv in the presence of a marine biota consisting of methanogens and/or anoxygenic photosynthesizers. Empirical support for the presence of methanogens in the Archean is provided by the recent discovery of low-13C methane in fluid inclusions in 3.4 Ga sediments (Ueno et al., 2006), although an abiotic origin for this CH4 is conceivable (Horita and Berndt, 1999). High CH4 concentrations, along with enhanced carbon dioxide (CO2) levels and significant amounts of ethane (C2H6), could have produced enough greenhouse warming to counter low solar luminosity and keep the Archean Earth warm (Pavlov et al., 2000; Haqq-Misra et al., submitted). Destruction of the methane greenhouse by rising O2 can thus explain why low-latitude Paleoproterozoic glaciations occurred at ~ 2.45 Ga (Pavlov et al., 2000, Bekker et al., 2005). Alternatively, the glaciations could have been caused by decreasing CO2, and the colder climate could have triggered the rise of O2 by permitting the proliferation of cyanobacteria (Lowe and Tice, 2007). But this scenario relies on extremely high Archean surface temperatures — of the order of 70 °C — inferred from O and Si isotopes in cherts (Knauth and Lowe, 2003, Robert and Chaussidon, 2006). In our view, an Archean climate with surface temperatures ~ 70 °C is implausible, and the isotopic data are best explained in other ways (Kasting and Howard, 2006, Kasting et al., 2006, Shields and Kasting, 2007).
Both climate and Δ33S production may have varied before the rise of O2. Diamictites have been identified in the ~ 2.9 Ga Pongola and Witwatersrand Supergroups of South Africa (Young et al., 1998, Crowell, 1999) and in the contemporaneous Belingwe greenstone belt in Zimbabwe (Nisbet et al., 1993). Diamictites by themselves cannot always be interpreted as glacial tillites, as they can also form by other processes (e.g., underwater landslides or terrestrial mudflows). However, the Pongola rocks also contain striated and faceted clasts interpreted as dropstones (Young et al., 1998), so it is likely that these sediments were indeed glacial.
Intriguingly, Δ33S values exhibit a distinct minimum at around this same time (Fig. 1). Ohmoto et al. (2006) suggested that this apparent decrease in Δ33S was caused by a transient increase in atmospheric O2 — the so-called “yo-yo atmosphere” model. But this hypothesis may be in conflict with conventional geological O2 indicators, e.g., detrital uraninite in the Witwatersrand gold deposits, which require that pO2 remained low, < 10− 2 times present (Holland, 1984, p. 315 ff.). Furthermore, it does not appear to be consistent with observed, non-zero Δ33S values during this interval or with the systematics of Δ36S versus 33ΔS variation (Farquhar et al., 2007). Here, we suggest that both the smaller range of mid-Archean Δ33S and the 2.9-Ga glaciations were caused by an entirely different mechanism, namely, the appearance of an optically thick organic haze. This organic haze, originally proposed by Sagan and Chyba (1997), could have shielded SO2 from S-MIF forming reactions while cooling the Earth's surface by creating an anti-greenhouse effect (Pavlov et al., 2001) similar to the one found on Titan (McKay et al., 1991). Thinning of this haze at ~ 2.7 Ga may have restored warm conditions and allowed for the large Δ33S values observed in the Late Archean.
Section snippets
Sample sources and experimental methodology
Our study is based partly on new multiple sulfur isotope data that are presented in Table 1. Here, we briefly describe how those data were obtained. PPRG (Precambrian Paleobiology Research Group) samples were graciously provided by W. J. Schopf; the remaining samples are from Claypool et al. (1980). All carbonate sulfur was extracted from crushed carbonate samples that were leached with sodium hypochlorite solution following the methods of Burdett et al. (1989). After leaching, samples were
Model description
In order to track the photochemical reactions that lead to S-MIF, we used the same one-dimensional, horizontally-averaged, photochemical model previously used by Pavlov and colleagues in predictions of the climatic effects of an organic haze (Pavlov et al., 2001). The model contains 72 chemical species that are divided into long-lived, short-lived, and well-mixed groups, and that are inter-connected by 337 chemical reactions. Transport was neglected for short-lived species, and the mixing
Proposed timeline for the evolution of Archean climate, atmospheric chemistry, and biology
The synthesis of climate calculations (Haqq-Misra et al., submitted), the Archean glacial record (Nisbet et al., 1993, Young et al., 1998, Crowell, 1999), the sulfur isotope record (Fig. 1), and photochemical calculations (Fig. 4) suggests a complex evolutionary sequence for atmospheric composition and climate during the first half of Earth history. Fig. 4 contains a proposed sequence of CH4 and CO2 concentrations for the Archean, marked by numbered circles that represent an evolutionary
Conclusions
We propose that temporal variations in the concentrations of atmospheric CH4 and CO2 during the Archean could have significantly affected the thickness of an organic haze, in turn triggering a rearrangement of both climate and photochemistry. Given this hypothesis and reasonable model constraints, we were able to reproduce many aspects of the Archean sedimentary record in our atmospheric models. In particular, we show how this organic haze can explain the Archean glacial and Δ33S records. While
Acknowledgements
We would like to thank the following groups for supporting this research: the NASA Astrobiology and Evolutionary Biology program, NSF grant EAR0348382, the CIW, PSARC, and VPL teams of the NASA Astrobiology Institute, the UMd Graduate School, and the Microbial Sciences Initiative at Harvard University for supporting this research. We also acknowledge J. W. Schopf and G. E. Claypool for samples. Finally, we would like to thank K. Zahnle and two anonymous reviewers for constructive critiques of
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