LITMIR.BIZ

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(Strauss, 1997). This enigmatic difference between nature and experiment persisted for nearly 30 years until a new type of microbial sulfur metabolism termed disproportionation of sulfur intermediates resulting from sulfide oxidation (such as thiosulfate or elemental sulfur) was identified as a major microbially mediated process of sulfur cycling in the marine realm (Jørgensen, 1990). Subsequent sulfur isotope studies of this type of microbial sulfur cycling (Cypionka et al., 1998; Habicht et al., 1998) revealed sizeable cumulative sulfur isotope effects that were greater than the canonical 46‰ defined by earlier studies for dissimilatory sulfate reduction. Consequently, large isotopic fractionations observed for marine sediments and sedimentary rocks above the canonical 46‰ (cf. Kaplan and Rittenberg, 1964) were attributed to disproportionation of intermediate sulfur compounds. Analyzing the temporal record of δ³⁴S_pyrite available at that time, Canfield and Teske (1996) concluded that the onset of such sizeable sulfur isotopic fractionations could be attributed to sulfide oxidation. Linking the temporal evolution of the global sulfur and oxygen cycles, these authors concluded that disproportionation of sulfur intermediates commenced around 850 million years ago as a reflection of the Neoproterozoic rise in atmospheric oxygen abundance in the dawn of animal life (Knoll et al., 1986). More recent work, however, places the onset of microbial sulfur disproportionation at around 635 million years, post‐dating the Marinoan glaciation (Kunzmann et al., 2017).

      Recently, Sim et al. (2011) reported a sulfur isotopic fractionation of nearly up to 70‰ for microbial sulfate reduction, both in a marine sediment on the New Jersey Shelf (USA) and in their cultivation experiments. The discovery of such a high magnitude of sulfur isotopic fractionation exerted solely by a sulfate‐reducing bacterium finally terminated the long‐standing discrepancy between sulfur isotope data from cultivation experiment and observational results from marine sediments and, thus, a long‐time paradigm of a canonical isotopic fractionation associated with microbial sulfate reduction of 46‰.

      Most recent work by Pellerin et al. (2019) challenged the assumption that only microbial sulfate reduction and disproportionation of sulfur intermediates are associated with sizeable isotopic fractionations. These authors reported a large sulfur isotope fractionation associated with bacterial sulfide oxidation. However, the implications of this new discovery for interpreting the deep time record of δ³⁴S_pyrite will have to be determined in future studies.

      Looking back on more than 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

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