SULFUR-BASED METABOLISMS DURING THE EARLY ARCHEAN

Sulfur isotope ratios were measured by SIMS analysis of individual sulfides in the hydrothermally-altered sedimentary and volcanic sequence of the 3.5 Ga chert-barite deposit of the Dresser Formation, Western Australia. The rocks studied are drill cores that intercepted altered komatiites and associated network of silica- and barite-feeder veins, bedded barite and bedded carbonate. Sulfides in black chert veins yield positive δ³⁴ S values (~0 to 5.4‰), and positive (1.1 to 3.9‰) or negative (down to -1.1‰ ~ barite isotope composition) Δ³³S values. Sulfides from the host basalt show δ³⁴S and Δ³³ S values of -2.3 and 0.6‰, and -1.3 and 0.5‰, respectively, the least altered basalts plotting near 0‰. Microscopic sulfides along non-recrystallized barite overgrowth zones display ³⁴S-depleted values down to -22.6‰ and positive Δ ³³ S values up to 6.1‰. These were attributed to microbial disproportionation of elemental sulfur (S⁰) derived from the photolysis of volcanic SO₂ (Philippot et al., 2007).

These results contrast with bulk rock analysis (Shen et al.; 2001; 2009; Ueno et al.; 2008) for which black chert veins and sulfides in barite define two opposite linear trends intersecting at the mean isotope value of altered basalts. These trends are best attributed to mixing arrays between a slightly modified magmatic reservoir (altered basalts) and two atmosphere/hydrothermal-derived reservoirs, a positive δ ³⁴ S and Δ ³³ S reservoir (black chert veins) and a negative δ ³⁴ S and Δ ³³ S reservoir derived from the thermal and/or microbial reduction of sulfate.

Calculation of the stability limits of aqueous sulfur species (SO₄, HSO₄, H₂S, HS, S⁰) indicate that at low T (40-60°C), the stability field of S⁰ is restricted to a narrow range of low pH and logf(O₂). This implies that the use of S⁰ by microorganisms to form ³⁴S-depleted sulfides with positive Δ ³³ S values must have occurred as soon as atmospheric S⁰ reached the water column. In contrast, the recognition that the sulfate stability field shrinks to a narrow range of pH and redox conditions with increasing T (40 to 200°C) and that the ³⁴S-depleted sulfides with negative Δ ³³ S values are present in both the bedded and feeder vein barite is strong support for a sulfate reduction process of thermal rather than of microbial origin.