Comment on "Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate"
Huiming Bao,*
Tao Sun,
Issaku Kohl,
Yongbo Peng
Philippot et al. (Reports, 14 September 2007, p. 1534) interpreted multiple–sulfur isotopic compositions of 
3.5-billion-year-old marine sulfide deposits as evidence that early Archaean microorganisms were not sulfate reducers but instead metabolized elemental sulfur. However, their data can be better explained by a scenario involving poor mixing of photochemical and surface sulfide sources.
Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803, USA.
* To whom correspondence should be addressed. E-mail: bao{at}lsu.edu
Philippot et al. (1) presented valuable ion-microprobe multiple–sulfur isotope compositions for microscopic pyrite grains located along overgrowth boundaries of early Archaean barite from Western Australia. Their data set shows that most of these pyrite grains display positive 
33S values, which is in sharp contrast with the coexisting barite sulfate. This contrast, together with a large spread of the negative 
34Ssulfides values, led them to the conclusion that early Archaean microorganisms preferred elemental sulfur, not sulfate. Their data, however, can be better explained by a mixing scenario, which renders their exclusive conclusion invalid.
One of the most distinct features of the Philippot et al. data set is the large scatter in positive 33S values [figure 3C in (1)] for microscopic sulfides [figure 2D in (1)]. The existence of such large 33S scatter in spatially limited growth zones can be explained if atmospheric S0 [or S8 aerosols (2)] of photochemical origin (which later transformed into sulfides in the ocean) was temporally or spatially heterogeneous in 33S. Even if the atmospheric source was initially homogeneous in 33S, poor mixing with sulfide pools of different 33S in the ocean could also result in a large 33S scatter. Thus, when we observe a large 34Ssulfides scatter in addition to the 33S scatter among the microscopic sulfides, the 34Ssulfides scatter cannot be attributed exclusively to secondary mass-dependent fractionation processes, be it bacterial S0 disproportionation or sulfate reduction. It could be entirely or partially due to poor mixing.
Philippot et al. (1) estimated the 34S for the photochemical S0 endmember [at between–3 and +3 per mil (
)] using mass-balance and assuming that the 33S-negative sulfate (i.e., barite) is a pure endmember representing 193-nm photochemical sulfate. The SO42– in early Archaean oceans, however, could itself be a mixture with a composition residing in Quadrant IV of the 33S-34S Cartesian plane (3, 4). Indeed, Philippot et al. state that "biologically derived gypsum or barite with a positive 33S anomaly inherited from parent elemental sulfur will be instantaneously mixed with a large reservoir of evaporative gypsum or hydrothermal barite with negative 33S anomalies" (1). Because a mixed sulfate pool as described above was used as an endmember, the calculated isotope parameters for sulfide should also be for a mixed pool. Thus, there exists a distinct possibility that a photochemical S0 endmember resides in Quadrant II (33S-positive and 34S-negative), far away from the origin, as supported by the 193-nm SO2 photolysis experiments (5).
In our mixing model (Fig. 1), we show that the 33S-34S data for microscopic pyrites in (1), including the three 33S-negative points, can be satisfactorily explained by different degrees of mixing between sulfides derived from S0 of photochemical sources and sulfides derived from SO42– in early Archaean oceans. The observed 33S-34S scatter in figure 3 in (1) can be achieved as long as the following two conditions are met for a 33S-negative sulfide endmember: (i) The magnitude 34Ssulfide-sulfate was large (up to about –25) during the reduction of the 33S-negative sulfate, be it metal-catalyzed thermal reduction or microbial reduction (the light-yellow area in Fig. 1) and (ii) The 33S-negative sulfide endmember dominated the total surface sulfide pool (close to the origin in Fig. 1). In our model, the 33S-positive sulfide component was derived from photochemical S0 with highly positive 33S value(s). The transformation from S0 to sulfide could have gone through inorganic or bacterial S0 disproportionation pathways, and the 34Ssulfide-sulfur could be large or small. These differences, however, will not have a considerable effect on the observed 33S-34S pattern as long as the mole fraction of the 33S-positive sulfide component in the total surface sulfide pool is small.
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Fig. 1. A two-endmember mixing model for sulfides in early Archaean oceans presented in a 33S-34S plane. A mixed sulfide pool would lie in the shaded area. Observed data for microscopic pyrites (1) are within the orange circle close to the origin.
[View Larger Version of this Image (25K GIF file)]
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In summary, Philippot et al. (1) used an innovative technique and revealed a remarkable heterogeneity in 33S and 34S values of the microscopic pyrite grains in early Archaean barite deposits. To attribute the scatter exclusively to a specific metabolic pathway or the lack of, the authors would have to rule out the mixing scenario we proposed here. Early Earth is still alien to us, both in its physical-chemical conditions and its biological activities. One thing we can confidently conclude is that the biological sulfur cycle, if present, was not as active in the early Archaean as it was in the Phanerozoic. The details of the biological sulfur cycle on early Earth, however, remain unknown.
References and Notes
- 1. P. Philippot et al., Science 317, 1534 (2007).[Abstract/Free Full Text]
- 2. A. A. Pavlov, J. F. Kasting, Astrobiology 2, 27 (2002). [CrossRef] [ISI] [Medline]
- 3. J. Farquhar, H. Bao, M. H. Thiemens, Science 289, 756 (2000).[Abstract/Free Full Text]
- 4. H. Bao, D. Rumble, D. R. Lowe, Geochim. Cosmochim. 71, 4868 10.1016/j.gca.2007.05.032 (2007). [CrossRef] [ISI]
- 5. J. Farquhar, J. Savarino, S. Airieau, M. H. Thiemens, J. Geophys. Res. [Planets.] 106, 32829 (2001). [CrossRef]
- 6. We acknowledge the Petroleum Research Fund (41374-G2) for financial support.
Received for publication 2 October 2007. Accepted for publication 8 February 2008.
