THE ABSENCE OF MASS INDEPENDENT SULFUR ISOTOPE FRACTIONATION IN ARCHEAN SEDIMENTARY ROCKS: EVIDENCE FOR AN OXIC ATMOSPHERE?

The current popular model for the Archean S-cycle (e.g., Farquhar, 2001; Kasting, 2001) predicts the signals of mass independent fractionation of sulfur isotopes (MIF-S: D³³S = d³³S -- 0.515d³⁴S >[0±0.3 permil]) are in all sulfide and sulfate minerals that precipitated from surface waters (oceans and lakes) prior to 2.32 Ga ago. To test this prediction, we have determined the d³⁴S, d³³S, and D³³S values of >70 bulk-rock samples from four Archean sedimentary formations recovered by the NASA Astrobiology Biosphere Drilling Project from the Pilbara district, Western Australia. These samples represent a ~150 m section of a lacustrine sandstone/shale unit (the 2.76 Ga Hardey Formation), a ~180 m section of a marine shale/sandstone unit (the 2.92 Ga Mosquito Creek Formation), and a ~100 m thick jasper/black-chert unit and an underlying ~30 m thick black-shale unit (the 3.46 Ga Towers Formation). Small MIF-S values (D³³S = ~0.5 permil) were observed on samples from the black shale unit of the Towers Formation. However, samples from the other three formations exhibit no MIF-S (D³³S = -0.3 to +0.3 permil), although their d³⁴S values vary from -3 to +8 permil.

The mineralogical and chemical characteristics of the studied samples (e.g., morphology, grain size, and distribution of pyrite crystals relative to clays and quartz; C/S ratio; trace element content) indicate that essentially all the sulfur resides in pyrite crystals, which are not detrital in origin. The pyrite crystals with some MIF-S (DD³³S = ~0.5 permil) appear to have formed by hydrothermal fluids during and/or after sediment diagenesis. However, those with no MIF-S formed within the host sediments during the early diagenetic stage (i.e., diagenetic pyrite) and/or in the overlying water bodies (i.e., syngenetic pyrite) by sulfate-reducing bacteria.

Possible interpretations of our data include, but are not restricted to, the following: (1) the Archean atmosphere fluctuated between oxic and anoxic, if atmospheric photochemical reactions are the only causes for MIF-S; or (2) large MIF-S values in some geologic samples were caused by mechanisms other than atmospheric photochemical reactions. Interpretation (2) implies that the presence or absence of MIF-S in geologic samples is not a measure of atmospheric pO₂ level.