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relationship between atmospheric pO2 and MIF‐S was proposed (Farquhar et al., 2000). Subsequent experimental and modeling approaches revealed that the formation of MIF‐S and their preservation in sediments deposited at the Earth’s surface required that the atmospheric oxygen abundance was below 10–5 present atmospheric level (PAL; Pavlov and Kasting, 2002; but see also Zahnle et al., 2006).
Since the discovery paper by Farquhar et al. (2000), numerous research articles have added to a now sizeable record of Δ33S values for the Precambrian sedimentary record (Figure 2.7). This record displays clear temporal variations in the magnitude of mass‐independently fractionated sulfur isotopes. In line with the overall mechanism initially proposed for generating MIF‐S (i.e. the photochemistry of sulfur dioxide), respective variations in the magnitude of isotopic fractionation were related to photochemical reactions at different wavelengths (Johnston, 2011). However, the more important second feature that had already been noted in the discovery paper and is prominently apparent from the MIF‐S record available now, is the termination of mass‐independent sulfur isotopic fractionation in a narrow time interval between 2.4 and 2.2 billion years ago. Neither pyrite nor sedimentary sulfate minerals or carbonate‐associated sulfate in sedimentary rocks younger than 2.2 billion years display the two prominent features recognized as MIF‐S (i.e. a deviation of the Δ33S value from zero by more than 0.3‰ and a negative correlation between Δ33S and Δ36S following the Archean array). The demise of MIF‐S is clearly observed in stratigraphic successions from South Africa (Bekker et al., 2004; Guo et al., 2009), Canada (Papineau et al., 2007) and Russian Fennoscandia (Reuschel et al., 2013). This termination of MIF‐S in the sedimentary record is considered as the ‘smoking gun’ for the first rise in atmospheric oxygen abundance at that time in the early Paleoproterozoic.
Owing to the causal relationship between the presence of MIF‐S and atmospheric oxygen abundance, including a mechanistic understanding about formation of this signature as well as its transfer and preservation in the sedimentary rock record (Johnston., 2011), MIF‐S became a recorder of early atmospheric evolution. The latter always centered on the evolution of atmospheric oxygen abundance and with it questions related to the advent of oxygenic photosynthesis (Kurzweil et al., 2013). More recent work questions the synchronous disappearance of the MIF‐S‐signature as a clear sign for atmospheric oxygenation of the Earth, arguing for a re‐evaluation of placing the Great Oxidation Event at the last occurrence of the MIF‐S signature (Philippot et al., 2018). This argument follows an earlier modeling approach addressing the aspect of a possible long‐term sedimentary recycling of the MIF‐S signature (Reinhard et al, 2013).
Figure 2.7 Range in Δ33S for sedimentary sulfide and sulfate exhibiting mass‐independently fractionated sulfur isotopes in the Archean and early Paleoproterozoic and its disappearance around 2300 million years ago. (Modified from Reuschel et al., 2013.)
Recent work on other redox‐sensitive elements and their isotopic composition (e.g., Mo, Cr, U) has added substantially to this discussion, resulting in a refinement in our understanding and challenging in particular the timing of the first significant rise in atmospheric oxygen abundance.
2.7. SUMMARY AND DIRECTION OF FUTURE RESEARCH
It is without doubt that the sedimentary records of sulfide and sulfate have archived a vast range of information in respect to the evolution of our Earth System. Geological processes at the surface of our planet (e.g., deposition and weathering) as well as at depth leave their traces as much as microbially mediated processes of sulfur cycling. The sulfur isotopic composition has been and continues to be the most important source of information in respect to these processes, despite the fact that diagenesis tends to obscure to some extent any primary signal. Moreover, through the intimate relationship between the geochemical cycles of sulfur, carbon and oxygen, sulfur isotopes have become a prime proxy signal for the overall evolution of Earth’s redox state. Future efforts should continue to explore these possibilities in our quest to quantitatively understanding Earth System Evolution.
ACKNOWLEDGMENTS
I thank the editors for inviting me to contribute this review. Financial support for my research on sulfur isotope geochemistry by the Deutsche Forschungsgemeinschaft (DFG) throughout the years is gratefully acknowledged. Ted Present and two anonymous reviewers are thanked for their constructive comments that improved the clarity of this manuscript.
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