Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate

Abstract

Microscopic sulfides with low 34S/32S ratios in marine sulfate deposits from the 3490-million-year old Dresser Formation, Australia, have been interpreted as evidence for the presence of early sulfate-reducing organisms on Earth. We show that these microscopic sulfides have a mass-independently fractionated sulfur isotopic anomaly (Delta33S) that differs from that of their host sulfate (barite). These microscopic sulfides could not have been produced by sulfate-reducing microbes, nor by abiologic processes that involve reduction of sulfate. Instead, we interpret the combined negative delta34S and positive Delta33S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

“…3b; cf. Philippot et al, 2007). Its derivation from a sedimentary source is also consistent with its marcasite-like texture and highly negative Fe isotope values.…”

Paleoarchean sulfur cycling: Multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

“…The possibility of a microbial origin of Paleoarchean (barite-hosted) microscopic pyrite was previously suggested, e.g., by Shen et al (2001) and Philippot et al (2007, but alternative views have been proposed (e.g., Bao et al, 2008;Philippot et al, 2012;see below).…”

“…Considering respective observations and underlying systematics of sulfur isotopic fractionation (reviewed, e.g., in Canfield and Raiswell, 1999;Johnston, 2011), the sedimentary records of Archean ı 34 S sulfate and ı 34 S sulfide have been regarded as generally reflecting (i) a low-sulfate ocean (Habicht et al, 2002;Crowe et al, 2014), whereas (ii) controversial views exist with respect to an early activity of biological sulfur cycling. Notably, the absence of consistently sizeable fractionations in 34 S has been regarded as evidence for a limited importance of biological sulfur cycling in Archean sedimentary surface environments (e.g., Strauss et al, 2003), despite individual reports of highly 34 S-depleted pyrite in Paleoarchean sedimentary rocks (e.g., Ohmoto et al, 1993;Shen et al, 2001Shen et al, , 2009Philippot et al, 2007;Wu and Farquhar, 2013) and the notion from molecular biology that bacterial sulfate reduction represents an ancient metabolic pathway (Shen and Buick, 2004;Blumenberg et al, 2006;Philippot et al, 2007;Ueno et al, 2008;Shen et al, 2009;Johnston, 2011). Several recent reports of highly 34 S-depleted pyrite occurrences of early Archean age (e.g., Philippot et al, 2007;Ueno et al, 2008;Shen et al, 2009;Wacey et al, 2011a,b;Roerdink et al, 2013) strengthen the case for early biological sulfur cycling, even utilizing diverse metabolic pathways such as bacterial sulfate reduction, elemental sulfur reduction, elemental sulfur disproportionation, and sulfide oxidation.…”

“…Three different pathways of microbial sulfur metabolism have been proposed previously for the Paleoarchean: elemental sulfur disproportionation (Philippot et al, 2007;Wacey et al, 2010Wacey et al, , 2011a, sulfate reduction (Shen et al, 2001(Shen et al, , 2009Ueno et al, 2008;Wacey et al, 2010Wacey et al, , 2011a and sulfide oxidation (Wacey et al, 2011b).…”

“…The guiding principle in respective discussions has been the observation that a negative 33 S value would be indicative of photolytic sulfate as the ultimate sulfur source (with a positive ı 34 S value as evident from the Archean barite deposits; e.g., Roerdink et al, 2012 In contrast, Philippot et al (2007Philippot et al ( , 2012 reported a substantial number of multiple sulfur isotope results for pyrite from Paleoarchean sedimentary successions in Western Australia (Dresser Formation) and South Africa (Mapepe Formation) exhibiting a negative correlation between ı 34 S and 33 S with a set of arrays going through the origin. These authors conclude that the entire range of multiple sulfur isotope values measured for Paleoarchean sulfides and sulfates, including strongly negative ı 34 S and positive 33 S values (the felsic volcanic array of Philippot et al, 2012) can be satisfactorily explained by a mixing of sulfide sulfur derived from a photolytic elemental sulfur source and sulfide sulfur derived from sulfate reduction of photolytic oceanic sulfate.…”