50 years of sulfur isotope studies of sedimentary iron sulfides, the major aim of respective research has always been to constrain pyrite formation with a strong focus on identifying the biogenic nature of the pyrite under study. This holds true in particular for Precambrian sedimentary rocks, but a characterization of ancient microbial sulfur cycling in the marine realm under variable environmental conditions was commonly the major driver behind respective studies of the geological past.
While most of the scientific focus was placed on the sulfur isotopic composition of sedimentary pyrite as a proxy for reconstructing sediment diagenesis or the temporal record of sulfur metabolism on Earth, organic sulfur and its sulfur isotopic composition were given comparatively little attention.
Seawater sulfate is also the principal source of organic sulfur in the marine realm. Incorporation of sulfur into organic matter via assimilatory reduction of sulfate (biosynthetic organic sulfur) represents one pathway, albeit of minor importance considering that sulfur represents <1% of marine biomass (Canfield, 2001a). Of much greater importance is the dissimilatory reduction of sulfate, resulting in highly reactive hydrogen sulfide. This can either react with iron and form a sedimentary iron sulfide or be oxidized to sulfur phases of intermediate valence (such as elemental sulfur, polysulfide, thiosulfate), or to sulfate, or be incorporated into organic matter. Thereby, an intermolecular reaction between sulfur and organic matter, resulting in organosulfur macromolecules, is distinguished from an intramolecular mechanism where sulfur becomes part of an organic molecule forming ring structures (Sinnighe‐Damsté et al., 1989). Prerequisites for the formation of organic sulfur include the abundance of reduced sulfur species, the presence of organic matter with functional sites for the reaction with sulfur, and a low concentration of metals, such as iron (Amrani, 2014; Shawar et al., 2018). The last of these is assumed to exert a strong influence because the reaction of hydrogen sulfide with iron is kinetically favored compared with the sulfurization of organic matter (Mossmann et al., 1991; Hartgers et al., 1997). However, several studies have reported the coformation of organic sulfur and pyrite (Brüchert and Pratt, 1996; Hennecke et al., 1997; Urban et al., 1999; Filley et al., 2002).
The sulfur isotopic composition of organic sulfur is dependent upon the isotope signature of the sulfur source, the isotopic fractionation associated with microbial sulfur metabolism resulting in a sulfur phase that is reactive towards organic matter (a possible isotope effect associated with the incorporation of sulfur into the organic matter), and a possible isotope effect related to the progressing maturation of the organic matter (Siedenberg et al., 2018). In 1963, Kaplan et al. had already noted a difference in δ34S between organic sulfur from marine animals and algae that was close to the isotopic composition of seawater sulfate and organic sulfur that displays a 34S‐depleted sulfur isotope signature. While the former suggests the assimilatory reduction/incorporation of sulfate into the biomass, the latter sulfur isotope signature suggests the incorporation of reduced sulfur likely resulting from microbial sulfate reduction. Reviews by Anderson and Pratt (1995) or Werne et al. (2004) indicated that organic sulfur appeared to always be 34S enriched (by 10–30‰) compared with sedimentary sulfide. The later timing of organosulfur compound formation, when progressive sulfate reduction had already resulted in sulfate limitation and an associated enrichment in 34S, was proposed as an explanation. Recent analytical advances, specifically compound‐specific sulfur isotope analysis (Werne et al., 2008) allowed for a significant advancement in our understanding of organic sulfur and its isotopic composition, as reviewed by Amrani (2014). Most notably, the formation of organic sulfur is much more complex than previously assumed. In addition, fractionations associated with the formation of organic sulfur, with the thermal maturation of organic matter as well as with preservation and mixing – all processes within the realm of progressing sediment diagenesis – exert a shift in the sulfur isotopic composition towards 34S‐enriched values of the bulk organic sulfur. Consequently, a substantial variability in isotopic composition for organic sulfur is to be expected, both in relation to coexisting iron sulfide as well as between different individual organic‐sulfur compounds. For the latter, compound‐specific sulfur isotope analyses revealed a variability in δ34S of as much as 50‰ (Amrani, 2014; Shawar et al., 2020). Two aspects became apparent: organic sulfur is an integral part of the diagenetic history of a sediment/sedimentary rock and organic sulfur as an important component of the total sedimentary sulfur pool is generally underestimated.
2.6. MASS‐INDEPENDENTLY FRACTIONATED SULFUR ISOTOPES – A RECORD OF EARTH’S OXYGENATION
Another milestone discovery in respect of the sulfur isotopic composition of sedimentary sulfur and its evolution through time was the observation of mass‐independently fractionated sulfur isotopes preserved in sedimentary sulfides and sulfates. This observation revolutionized the field of sulfur isotope research as much as it extended the importance of sulfur isotopes as a recorder of Earth System Evolution.
It started with the discovery of mass‐independently fractionated sulfur isotopes (MIF‐S) archived in pyrite and barite in sedimentary rocks older than 2.4 billion years (Farquhar et al., 2000). Respective sulfur phases were characterized by a Δ33S value more positive/negative than 0.3‰ and followed a regression line between Δ33S and Δ36S with a slope near –1, later called the Archean array. Inspired by mass‐independently fractionated oxygen isotopes recorded in ozone (Thiemens and Heidenreich, 1983), Farquhar et al. (2000) proposed UV‐driven photochemical disproportionation of volcanogenic SO2 as the principal reaction behind these sulfur isotope anomalies. Already early on, the exclusive occurrence of MIF‐S in rocks older than 2.4 billion years suggested at least a temporal relationship to the abundance of atmospheric oxygen on the early Earth. Following our understanding of an anoxic atmosphere during the early part of Earth history (Kasting, 1993) and a first significant rise in atmospheric oxygen abundance between 2.4 and 2.2 billion years ago, termed the Great Oxidation Event (Holland, 2006), not only a temporal but also a causal 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
