Technical Comments

Comment on "Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate"

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Science  07 Mar 2008:
Vol. 319, Issue 5868, pp. 1336
DOI: 10.1126/science.1151241

Abstract

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.

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 Δ33S values, which is in sharp contrast with the coexisting barite sulfate. This contrast, together with a large spread of the negative δ34Ssulfides 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 Δ33S values [figure 3C in (1)] for microscopic sulfides [figure 2D in (1)]. The existence of such large Δ33S 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 Δ33S. Even if the atmospheric source was initially homogeneous in Δ33S, poor mixing with sulfide pools of different Δ33S in the ocean could also result in a large Δ33S scatter. Thus, when we observe a large δ34Ssulfides scatter in addition to the Δ33S scatter among the microscopic sulfides, the δ34Ssulfides 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 δ34S for the photochemical S0 endmember [at between–3 and +3 per mil (‰)] using mass-balance and assuming that the Δ33S-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 Δ33S-δ34S Cartesian plane (3, 4). Indeed, Philippot et al. state that “biologically derived gypsum or barite with a positive Δ33S anomaly inherited from parent elemental sulfur will be instantaneously mixed with a large reservoir of evaporative gypsum or hydrothermal barite with negative Δ33S 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 (Δ33S-positive and δ34S-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 Δ33S-δ34S data for microscopic pyrites in (1), including the three Δ33S-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 Δ33S-δ34S scatter in figure 3 in (1) can be achieved as long as the following two conditions are met for a Δ33S-negative sulfide endmember: (i) The magnitude Δδ34Ssulfide-sulfate was large (up to about –25‰) during the reduction of the Δ33S-negative sulfate, be it metal-catalyzed thermal reduction or microbial reduction (the light-yellow area in Fig. 1) and (ii) The Δ33S-negative sulfide endmember dominated the total surface sulfide pool (close to the origin in Fig. 1). In our model, the Δ33S-positive sulfide component was derived from photochemical S0 with highly positive Δ33S value(s). The transformation from S0 to sulfide could have gone through inorganic or bacterial S0 disproportionation pathways, and the Δδ34Ssulfide-sulfur could be large or small. These differences, however, will not have a considerable effect on the observed Δ33S-δ34S pattern as long as the mole fraction of the Δ33S-positive sulfide component in the total surface sulfide pool is small.

Fig. 1.

A two-endmember mixing model for sulfides in early Archaean oceans presented in a Δ33S-δ34S 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.

In summary, Philippot et al. (1) used an innovative technique and revealed a remarkable heterogeneity in Δ33S and δ34S 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.

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