image courtesy of Zhiyong Lin).
Figure 2.4 (a) Variations in δ34S for chromium reducible sulfur from bulk rock extraction (CRS blue line) and differentiated for early to late stage diagenetic pyrite exhibiting different morphologies. The dashed line separates a zone to the left which is suggested to be dominated by organoclastic sulfate reduction (OSR) and a zone to the right suggested to be dominated by sulfate‐driven anaerobic oxidation of methane (SO4‐AOM). (b) Depth profiles for dissolved sulfate and methane concentrations from porewater. The gray field indicates the depth, which exhibits sulfate‐driven anaerobic oxidation of methane.
(Modified from Lin et al., 2016.)
By including the minor sulfur isotopes 33S and 36S (with results expressed as Δ33S and Δ36S values), Lin et al. (2017) were able to clearly distinguish the involvement of different microbially driven processes, i.e. sulfate reduction, disproportionation of sulfur intermediates and (partial) sulfide oxidation. Moreover, distinctly different data populations became apparent in a cross‐plot of Δ33Spy – Δ33SSO4 vs. δ34Spy – δ34SSO4 (Figure 2.5), allowing to distinguish sedimentary pyrite that formed via OSR vs. pyrite that formed as a result of SO4‐AOM. Most recently, Jørgensen et al. (2019) further advanced our understanding about spatiotemporal variations in the importance of organoclastic sulfate reduction vs. sulfate‐driven anaerobic oxidation of methane.
Studies such as those by Lin et al. (2016, 2017) clearly indicate the sometimes substantial complexity in sediment diagenesis that is recorded by sulfur isotopes. In terms of its application, the high variability in δ34S and additional information obtained from Δ33S offers a strong potential for unraveling this complexity. Yet, how detailed a respective (hi)story unveils strongly depends on the detail of the analytical approach.
2.4. OCEANIC SULFATE AND ITS EVOLUTION THROUGH TIME
Seawater sulfate is our prime recorder of secular changes in global sulfur cycling. Abundance and sulfur isotopic composition of oceanic sulfate reflect the delicate balance between inputs and outputs. Principal input parameters are the riverine delivery of sulfate from oxidative continental weathering of sulfide and the dissolution of sulfate as well as the interaction of seawater and ocean crustal rocks, most prominently exhibited at submarine hydrothermal vent sites along mid‐ocean ridges, island arcs and back‐arc settings. Dissolved sulfate leaves the ocean via precipitation of sulfate minerals or through microbially driven sulfate reduction and subsequent precipitation of the resulting hydrogen sulfide as iron sulfide or its incorporation into organic matter. In a simplified isotope mass balance
these two principal output functions (i.e. sulfate precipitation and biogenic sulfide formation) are balanced against an overall input function that supposedly reflects average crustal sulfur and that is generally considered to be invariable through time (Holser et al., 1988; for recent discussions, see Halevy et al., 2012 and Canfield, 2013).
As the precipitation of a sulfate mineral is not associated with a substantial sulfur isotope fractionation (Raab and Spiro, 1991), the sulfur isotopic composition of marine evaporitic sulfates faithfully records the isotopic composition of dissolved oceanic sulfate. In considering the simplified sulfur isotope mass balance, the temporal evolution in δ34Ssulfate has been interpreted as a reflection of secular changes in the balance between the principal output functions from the oceanic reservoir.
Figure 2.5 Differentiation of different forms of microbial sulfur cycling as demonstrated by multiple sulfur isotopes. (Redrawn after Lin et al., 2017). OSR: organoclastic sulfate reduction; SO4‐AOM: sulfate‐driven anaerobic oxidation of methane; DH‐CL 11 and HD 109 are drill cores.
Early accounts of the sulfur isotopic composition of oceanic sulfate through time were published by Nielsen (1965) and Holser and Kaplan (1966), followed by the seminal paper from Claypool et al. (1980). These studies used evaporitic sulfate minerals (generally gypsum or anhydrite) for reconstructing the temporal changes in δ34Ssulfate, thereby acknowledging the absence of a substantial isotope effect during mineral precipitation. Two observations were most apparent from these studies: the sulfur isotopic composition of oceanic sulfate was not homogeneous through time and the temporal record was not continuous, with gaps that reflect the nondeposition of evaporites or their loss during weathering (Strauss, 1997). Barite was considered as an alternative early on (Goodfellow and Johansson, 1984), and our present understanding about secular changes in the sulfur isotopic composition of oceanic sulfate during the past 130 million years is indeed based on a high‐resolution record of marine barite (Paytan et al., 2004).
The most promising and widely utilized alternative, however, is carbonate‐associated sulfate (CAS). During carbonate precipitation, sulfate is incorporated into the crystal lattice in concentrations of tens to thousands of parts per million (Busenberg and Plummer, 1985; Staudt and Schoonen, 1995), and this sulfate archives the sulfur isotopic composition of the dissolved sulfate at the time of carbonate precipitation. As with true sulfate minerals, it is important to independently constrain the marine nature of carbonates used, such as marine biogenic carbonates. Similar to the modern ocean, limited variability in δ34SCAS can be expected for multiple time‐equivalent carbonate samples, as carbonate associated sulfate would reflect the homogeneous nature of the seawater sulfate sulfur isotopic composition at any given time in Earth history. However, recent studies have reported significant differences in δ34SCAS for different sedimentary carbonate components within an individual stratigraphic unit (Present et al., 2015), indicating that diagenesis can exert control on the δ34SCAS value of carbonates (Present et al., 2019). Consequently, a prerequisite for successfully applying δ34SCAS is a thorough evaluation of the diagenetic history of the carbonate (Fichtner et al., 2017), with a rigid analytical procedure (Wotte et al., 2012) being mandatory.
Figure 2.6 Secular variations in δ34S for carbonate‐associated sulfate (CAS: blue circles) and evaporitic sulfate (orange circles). Red circles indicate 10 million year average values (0–500 Ma), 50 million‐year average values (500–1000 Ma) and 100 million‐year average values (1100–2600 Ma) calculated from δ34SCAS.
(Modified from Crockford et al., 2019.)
Kampschulte and Strauss (2004) published the first CAS‐based sulfur isotope time series of oceanic sulfate for the Phanerozoic. Clear differences were discernible such as high δ34Ssulfate
