Box 1 Isotopic notation (Bao et al., 2016; Cao and Liu, 2011; Hayles et al., 2017; Luo et al., 2010; Matsuhisa et al., 1978; Pack and Herwartz, 2014).
Invited review articleClaypool continued: Extending the isotopic record of sedimentary sulfate
Introduction
The Earth System has dramatically evolved across its 4.6 Ga history (Fig. 1) experiencing massive changes to the surface environment, including the oxygenation of the atmosphere (Farquhar et al., 2000; Holland, 2006; Bekker and Holland, 2012; Lyons et al., 2014), snowball glaciations (Hoffman et al., 1998; Kirschvink et al., 2000; Bekker, 2014a), and the evolution of the biosphere (El Albani et al., 2010; Brocks et al., 2017; Knoll and Nowak, 2017; Gibson et al., 2017). It is reasonable to assume that these events significantly impacted the cycles of oxygen, sulfur, iron and carbon, and hence the sedimentary sulfate isotope record (Rouxel et al., 2005; Farquhar and Wing, 2003; Johnston, 2011; Kunzmann et al., 2017). Sulfate (SO42−) is the second most abundant anion in modern marine environments, and although its abundance was much lower through most of Earth's history (Kah et al., 2004; Johnston et al., 2008; Canfield and Farquhar, 2009; Bekker and Holland, 2012; Crowe et al., 2014; Luo et al., 2015), it must have played a significant role in ancient biogeochemical cycles (Canfield and Raiswell, 1999). Sulfate is also the only oxy-anion known to be capable of preserving ancient atmospheric oxygen and sulfur isotope ratios, providing a window into atmospheric chemistry as well as the productivity of the ancient biosphere (Farquhar et al., 2000; Bao et al., 2008; Bao, 2015; Crockford et al., 2018).
Claypool et al. (1980) presented the first comprehensive survey of the major isotopes of sulfur (δ34S) and oxygen (δ18O) within sulfate over the last one billion years of Earth history. This record provided a foundation to evaluate secular variations in seawater sulfate concentrations and to constrain how the cycles of sulfur and oxygen operated over this interval. Since this pioneering work, much additional effort has gone into filling in the δ18O and δ34S records and extending them to earlier times (e.g. Strauss, 1993). The results have highlighted both links and disconnects between these two isotopic records (Utrilla et al., 1992; Strauss, 1999; Kampschulte and Strauss, 2004; Bottrell and Newton, 2006; Turchyn et al., 2009; Wu et al., 2014).
Advances in analytical capabilities have added new dimensions to interrogating these records through the ability to measure the minor isotopes of sulfur (33S, and 36S; Farquhar et al., 2000; Farquhar and Wing, 2003; Johnston, 2011) and oxygen (17O; Thiemens and Heidenreich, 1983; Luz et al., 1999; Thiemens, 2006; Bao, 2006). The new minor isotope datasets generated from sulfate-bearing sedimentary archives such as barite, gypsum and carbonate-associated sulfate (Bao et al., 2008, Bao et al., 2009; Crockford et al., 2018) as well as those bearing sulfide (Berner, 1984; Fischer et al., 2014; Scott et al., 2014; Kunzmann et al., 2017) and organic-bound sulfur (Bontognali et al., 2012; Raven et al., 2016, Raven et al., 2018) are rapidly improving our understanding of oxygen and sulfur cycling in the surface environment along with other processes that these isotopic records track. Nevertheless, because these techniques are more analytically demanding than traditional methods, the existing record is more fragmentary. This shortfall in data is particularly notable for the Proterozoic where large gaps exist particularly with respect to oxygen (δ18O, ∆17O) and multiple sulfur isotopic data (∆33S, ∆36S). One notable example is the potential for mass-independent oxygen isotope anomalies to provide quantitative constraints on ancient gross primary productivity (GPP; Crockford et al., 2018), a parameter that likely underlies many of the geochemical trends observed across the sedimentary record (Des Marais et al., 1992; Brasier and Lindsay, 1998; Partin et al., 2013; Lyons et al., 2014; Planavsky et al., 2018).
Here we briefly review the progress made to date in understanding Earth's ancient sulfur and oxygen cycles viewed through isotopic records of sedimentary sulfate as preserved in evaporite minerals and carbonate associated sulfate. A suite of 313 samples from 33 sedimentary formations spanning the Proterozoic were analyzed, and these data allow us to extend δ34S and δ18O age curves of sulfate through to the earliest Paleoproterozoic. These results are presented alongside new minor isotope (∆17O, and ∆33S) measurements. We then utilize these compilations to explore secular variations and links in all isotopic systems, highlighting potential causal mechanisms that may have driven revealed trends. Finally, we place a large degree of emphasis on the ∆17O record to generate a GPP curve spanning from the earliest Proterozoic to the modern and explore the implications of these results in the context of the emerging model for the evolution of the surface Earth (Bekker and Holland, 2012; Bekker, 2014a; Lyons et al., 2014; Payne et al., 2011; Planavsky et al., 2011; Sperling et al., 2015).
Section snippets
Fidelity of sulfate-bearing archives
While a great deal of effort has been applied to identifying primary isotopic signatures of marine, microbial and atmospheric reservoirs (e.g., Canfield, 2001; Johnston et al., 2005a; Sim et al., 2011; Johnston, 2011; Bradley et al., 2016), reliably screening such measurements within sulfate-bearing archives (gypsum, anhydrite, barite, carbonate associated sulfate) from compromised records that have been subjected to post-depositional alteration remains a challenge (Fike et al., 2015). Given
∆17O
Upon reaching sufficient levels of atmospheric oxygen to establish an ozone (O3) layer (pO2 = 10−3 pre-anthropogenic levels, hereafter PAL; Kasting and Donahue, 1980; Segura et al., 2003), O2 is imprinted with a mass-independent fractionation signature (O-MIF: ∆17O ≠ 0) imparted through the formation and destruction of O3 (Fig. 2). During photolysis, O3 dissociates into a single oxygen atom and one O2 molecule. Symmetry effects during recombination of O3 as well as reactions with other
Samples
For this study we analyzed 313 samples from 33 different formations on all continents but South America and Antarctica (Fig. 6, Fig. 7; Table 1). We rely on the most current literature estimates of ages for formations and summarize these along with sample locations and formation names in Fig. 6 and Table 1, respectively. This sample suite covers the oldest known sulfate evaporites from North America (2.32 Ga Gordon Lake Formation) and from South Africa (2.4 Ga Duitschland Formation) to
The isotopic record of sedimentary sulfate
In this section we discuss the isotopic record of sedimentary sulfate from the Archean through to the modern with an emphasis on the Proterozoic Eon, which we subdivide into five different intervals (Fig. 7, Fig. 9) that are discussed in separate sections. Given that large age uncertainties are endemic to evaporite deposits, together with significant gaps in the sulfate evaporite record, we set these boundaries with an eye toward highlighting changes in the sulfate isotope record. We also
A speculative synthesis
Here we have put forward a comprehensive isotopic record of Proterozoic sulfate from 313 samples from 33 different formations. This record, together with existing available data, confirms suggestions that atmospheric chemistry has significantly evolved through Earth's history and can be subdivided into three broad stages based on sulfate sulfur and oxygen multiple isotope records: S-MIF and no O-MIF (Archean), O-MIF and no S-MIF (Proterozoic), and no S-MIF or O-MIF (Phanerozoic). We construct
Acknowledgements
The authors thank Maya Gomes and Clint Scott for providing detailed and constructive reviews of this manuscript. Funding for this work was provided by the NSERC-CREATE CATP, NSERC PGS-D fellowship, McGill McGregor Fellowship, McGill Mobility and GREAT programs, Canadian Polar Continental Shelf Program, Northern Science Training Program, Mineralogical Association of Canada Foundation Scholarship and Travel Grant and the Agouron Geobiology Post-doctoral Fellowship Program. The Stable Isotope
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