Response to Comment on "Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate"
Pascal Philippot1*,
Mark Van Zuilen1,
Kevin Lepot1,
Christophe Thomazo1,
James Farquhar2 and
Martin J. Van Kranendonk3
1 Equipe Géobiosphère Actuelle et Primitive, Institut de Physique du Globe de Paris, Centre National de la Recherche Scientifique and Université Denis Diderot, 4 place Jussieu, 75005 Paris cedex, France.
2 Earth System Science Interdisciplinary Center, Department of Geology, University of Maryland, College Park, Maryland 20742, USA.
3 Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia.
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Fig. 1. (A) Experimental results of photolyzed sulfur dioxide to elemental sulfur (+ 33S, large black diamonds) and sulfate (–33S, large black squares) (large empty circles correspond to the residual SO2) when it is exposed to ultraviolet radiation at a wavelength of 193 nm (5, 7). Superposed onto this diagram is the range of sulfide pools and array used in (1) ("Bao array"). These include: (i) photolyzed S0 (dark blue) formed from interaction with ultraviolet at 193 nm and associated sulfide (light blue) formed from atmospheric S0 reduction or disproportionation and (ii) "surface" oceanic sulfate pool (red) and associated sulfides formed from sulfate reduction of microbial or inorganic origin (yellow). The light gray array labeled "Philippot array" represents the range of the photolyzed sulfur pool considered in (4). This array passes through the mean of the photolyzed S0 and sulfate-residual SO2 pools defined by (5, 7). Also shown are S isotope data from the literature covering the period 3800 to 2700 million years ago [data from (2, 4, 5, 7–17)]. (B) S istope systematics of Archaean sulfides for the period 3800 to 2700 million years ago. Blue diamonds and red squares correspond to North Pole sulfides and barites, respectively. Strongly 34S-depleted microscopic sulfides are shown in light blue. Black and red crosses correspond to sulfides and barites from other localities. More than 90% of the S isotope data are comprised within the 193-nm array used in (4).
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Fig. 2. 33S versus  34S versus plots of sulfides (black diamonds) and barite (red squares) analyzed in (4). All analyses were performed on drill core samples shown as "rock sequence" on right [see (4) for details]. Diagrams on the left-hand side show the effect of mixing small amounts of extremely fractionated (33S  +70 ) atmospheric sulfur with a large volume of sulfides derived from the reduction of oceanic sulfates. The highest amount of particulate atmospheric S0 aerosols will be stored in the sedimentary layers located on the sea floor. This in turn should result in increasing mechanically the 33S scatter of the sedimentary sulfides (large dashed orange circle evolving toward the "S0 atmospheric pool") compared with the macroscopic and microscopic sulfides present in the underlying bedded barite. The range of measured 33S versus 34S values of North Pole sulfides are shown for comparison (black areas). Recognition that the sedimentary sulfides show systematic positive 33S anomalies indicates that the source of sedimentary sulfur was indeed almost exclusively derived from atmospheric S0, which agrees with the sulfide source defined in (1). However, the limited range of 33S and 34S values of sedimentary sulfides argue against their mixing scenario.
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