Multiple sulfur isotopes from Paleoproterozoic Huronian interglacial sediments and the rise of atmospheric oxygen

Abstract

Mass-dependently fractionated (MDF) sulfur isotopes in sedimentary sulfides and sulfates can provide information on the past activity of microbial metabolisms and serve as a proxy for the concentration of seawater sulfate. In conjunction with these proxies, mass-independently fractionated (MIF) sulfur isotopes can be used to track the rise of atmospheric oxygen. The anoxic–oxic transition of the surface environment in the Paleoproterozoic has been constrained by MIF sulfur in 2.47 Ga sulfides from Western Australian banded iron-formations, and the absence of MIF in diagenetic sulfides from 2.32 Ga black shales in the Transvaal Supergroup (South Africa). We report new multicollector ion microprobe data for individual sulfides from water-lain sedimentary units in the 2.45–2.22 Ga Huronian Supergroup (Ontario, Canada). The authigenecity of sulfides in these units is interpreted on the basis of host lithology, mode of distribution, sulfide chemistry and MDF sulfur isotopes. Sulfides interpreted to have an authigenic-sedimentary origin from the McKim Fm. have a range of 0.7‰ in Δ 33S values and those in the Pecors Fm. preserve small magnitude MIF Δ 33S values up to + 0.88‰, which we propose represents the last remnants of an anoxic atmosphere. High δ 34S values (up to + 31.2‰) and near-zero Δ 33S values in authigenic sulfides from the stratigraphically higher Espanola and Gordon Lake Fms. suggest local variations of seawater sulfate concentrations and/or δ 34S sulfate, consistent with increased atmospheric O 2 levels. MIF sulfur is absent from the marine record after the second Huronian glaciation, which is analogous to the record of the Transvaal Supergroup where MIF is absent from the upper part of the interglacial sequence. Our results suggest that the possible simultaneous demise of MIF sulfur isotopes at these two localities may serve as a useful global geochemical marker to correlate Paleoproterozoic supergroups. We propose a new biogeochemical model where enhanced weathering rates during Paleoproterozoic post-glacial thawing served as a critical stimulus for interglacial blooms of oxygenic photosynthesis, the demise of methane, and ultimately to the irreversible rise in atmospheric oxygen.