Pilbara Sulfur Isotopes

Sulfur isotopes are a promising additional tool in our armoury for investigating the early biosphere given that primitive bacteria which metabolize sulfur compounds are one of the most deeply rooted groups in the Tree of Life (e.g. Mojzsis, 2007, Figs. 7.5-12). Sulfur isotopes have also been utilized as a hotly debated tracer for the rise of atmospheric oxygen, an application that we will not discuss further here (see instead Kasting, 2006; Mojzsis, 2007). The analysis of sulfur isotopes preserved within ancient sulfides and sulfates can be used to recognize various processes in the sulfur cycle, in particular biological sulfate reduction and disproportionation of intermediate sulfur compounds (e.g. Shen and Buick, 2004). Evidence consistent with life at 3,490 Ma comes from the study of microscopic sulfides contained within barite crystals (BaSO4) pseudomorphing gypsum (CaSO4) in the Dresser Formation from North Pole, Western Australia (Shen et al., 2001). Fractionations of up to 21.1%o (mean 11.6%o) between the sulfides and co-existing sulfates, together with the association of the sulfides with organic carbon are used to argue that sulfate reducing bacteria had evolved by -3,490 Ma.

More recently, the 534S analyses of Shen et al. (2001) were repeated and corroborated using drill-core material from the North Pole by Philippot et al. (2007). More importantly, this study extended the analysis to include the minor sulfur isotope 33S which yielded a positive anomaly. This implies the involvement of micro-organisms which disproportionate elemental sulfur in the formation of these sulfide grains. This is an exciting result in itself but in addition, the strict environmental requirements of currently known S-disproportionating bacteria (an anoxic environment and temperatures below 40°C, at near neutral pH and low H2S concentrations) may help to settle discussions regarding the magnitude of hydrothermal inputs on the North Pole palaeo-environment. Caution is urged with respect to this inference, because this group of bacteria has received relatively little microbiological attention to date (Thamdrup, 2007).

4. Barberton Greenstone Belt

Ancient rocks of the Barberton Greenstone Belt (Fig. 3) include the Swaziland Supergroup, which comprises a lower mostly volcanic succession (Onverwacht Group) and an upper mainly clastic succession (Fig Tree Group and Moodies Group) (Anhaeusser, 1973; Lowe and Byerly, 1999). Here we focus on the oldest of these, the Onverwacht Group which spans the time interval —3,500-3,200 Ma (Armstrong et al., 1990). In detail (Fig. 3b), it is composed of komatiitic and tholeiitic basaltic rocks interbedded with thin sedimentary units of silicified ash and black chert, together with rare felsic volcaniclastic and intrusive rock. Here we review the literature from the last twenty years pertaining to the description of several putative biogenic structures from the Onverwacht Group. Some putative biological structures were described prior to this time (e.g. Schopf and Barghoorn, 1967; Pflug, 1967; Nagy and Nagy, 1969; Engel et al., 1968; Muir and Grant, 1976; Knoll and Barghoorn, 1977), but a comprehensive review of these has already been given by Schopf and Walter (1983) who concluded that none of these discoveries gave compelling evidence for ancient life.

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