values during Permian times. While the overall temporal trend was comparable to observations published earlier and based on evaporite minerals, this new time series of the sulfur isotopic composition of Phanerozoic seawater sulfate showed substantially more internal structure because of its significantly better temporal resolution. Numerous studies on δ34Ssulfate, published in the past 15 years, have provided a detailed view of the secular variations in the sulfur isotopic composition through time; these have most recently been compiled by Crockford et al (2019) extending the record back into the Proterozoic (Figure 2.6). A first‐order interpretation, based on isotope mass balance considerations, views these temporal variations as a reflection of changes in pyrite burial (fsulfide), with high rates in the early Paleozoic, minimum rates in the late Paleozoic and its transition into the Mesozoic, and an evolution towards modern values. However, additional information can be gathered from a detailed multiple sulfur isotope record through time.
Based on complimentary records of δ34Ssulfate and Δ33Ssulfate for Paleozoic and Mesozoic carbonate‐associated sulfate, and considering the temporal record of δ34Ssulfide through time, Wu et al. (2010, 2014) proposed significant changes in the operational mode of the global sulfur cycle through the Phanerozoic. Most notably, these authors highlighted a change in the magnitude of isotopic fractionation between sulfate and sulfide from lower values in the Paleozoic to higher values in the Cenozoic. Moreover, they argue for a change in the isotopic composition of the input function (δ34Sin) to the global sulfur cycle (i.e. the continental weathering signature), as previously discussed by Canfield (2013). Wu et al. (2014) attribute this to changes in the Earth surface sulfur pool, notably a rapid recycling of newly formed sulfate minerals. Discernible shorter‐term fluctuations (on the tens of million years scale) in the temporal records are interpreted as reflecting changes in the intensity of sulfide oxidation during cycling of sulfur and/or by rapid changes in sulfur influx to the oceans and its associated sulfur isotopes. Recently, Crockford et al. (2019) compiled and substantially extended the time series of δ34Ssulfate and Δ33Ssulfate for seawater sulfate (and δ18Osulfate and Δ17Osulfate) back in time into the late Archean, with new data mostly derived from gypsum or anhydrite. As in other previous studies, the observed large scale secular variations in the sulfur isotopic composition of seawater sulfate are attributed to temporal changes in burial/weathering of sedimentary sulfide. Moreover, these time series substantiate earlier suggestions (Melezhik et al., 2005) that the operational mode of the sulfur cycle as we know it today and, in particular, the continental weathering and riverine delivery of sulfate to the ocean only commenced in the early Proterozoic, postdating the first significant rise in atmospheric oxygen.
Two additional aspects related to the study of carbonate‐associated sulfate are noteworthy as they likely reflect on future research. An increasing number of studies have focused on critical time boundaries such as the Permian–Triassic transition (Schobben et al., 2015, 2017), and careful work has resulted in high‐resolution profiles across prominent rock successions allowing for a renewed view on the causes and consequences of short‐term perturbations of global sulfur cycling. The second aspect pertains to an analytical improvement, notably the measurement of sulfur isotopes at very low sulfate concentrations (nanomole level) using multi‐collector induced coupled plasma mass‐spectrometry (MC‐ICP‐MS) (Craddock et al., 2008; Paris et al., 2013). Applications of the latter will allow for an even higher temporal resolution for rock successions across critical time boundaries than presently available.
2.5. PYRITE AND ORGANIC‐BOUND SULFUR AS RECORDERS OF MICROBIAL SULFUR CYCLING IN THE PAST
Being a key element of life, sulfur is taken up by plants and microorganisms via assimilatory reduction of inorganic sulfate with little sulfur isotope fractionation associated. In contrast, dissimilatory sulfate reduction is an energy‐yielding process associated with the release of hydrogen sulfide. Kaplan and Rittenberg (1964) in their seminal paper on isotopic fractionation associated with dissimilatory sulfate reduction established the foundation for the application of sulfur isotopes as recorder of microbial activities in the geological past. Hartmann and Nielsen (1969) were the first to apply this new understanding that dissimilatory sulfate reduction is generally associated with a substantial isotopic discrimination against the heavy 34S isotope in their study of marine sediments. Subsequently, a negative δ34S value measured for a sedimentary pyrite or at least a sizeable apparent isotopic fractionation between the parental sulfate (preserved in the rock record as evaporite) and resulting sedimentary (iron) sulfide, as observed in natural marine settings as well as in the early laboratory experiments, were considered as evidence for the biogenicity of a sedimentary sulfide: it provided the basis for tracing microbial sulfur cycling through time.
Since the 1970s, numerous studies explored the antiquity of microbial sulfur cycling by studying sedimentary rocks as far back as 3.8 billion years ago (Monster et al., 1979). Time series of δ34S values for pyrite in sedimentary rocks were presented, among others, by Schidlowski et al. (1983), Hayes et al. (1992), Strauss (1999), and Canfield (2001a). Reviewing sulfur isotope research targeting sedimentary sulfides with the objective of identifying microbial sulfur cycling and trace it through time two milestone discoveries in the past 20 years by Sim et al. (2011) and Pellerin et al. (2019) are most notable.
In 1964, Kaplan and Rittenberg reported that a maximum sulfur isotopic fractionation of 46‰ associated with dissimilatory sulfate reduction, modern, and ancient marine sediments and sedimentary rocks frequently yielded a larger isotopic difference between a measured sedimentary pyrite and the coeval seawater sulfate (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 δ34Spyrite 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 δ34Spyrite will have to be determined in future studies.
