Multiple sulfur isotopes of sulfides from sediments in the aftermath of Paleoproterozoic glaciations

Abstract

Geochemical evidence reported from Paleoproterozoic sediments has long been used to evaluate the transition from the anoxic Archean atmosphere to an oxygenated atmosphere. Sulfur isotopes ( 32S, 33S, 34S and 36S) in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition, such that the timing of the Paleoproterozoic “Great Oxidation Event” can be investigated using mass-independently fractionated (MIF) sulfur isotope systematics expressed as Δ 33S. Here we report data from 83 individual analyses of pyrite, pyrrhotite and chalcopyrite on a new suite of 30 different samples from Finland, South Africa, Wyoming and Ontario that span ∼600 My and follow one or several “Snowball Earth” events in the Paleoproterozoic. The samples were measured using a high-resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains (<30 μm) within individual sulfide grains while preserving petrographic context. We focused on sediments deposited in the aftermath of the Paleoproterozoic glaciations (between 1.9 and 2.2 Ga) to trace fluctuations in atmospheric O 2 concentrations that were likely affected by an interplay of O 2 sinks in the atmosphere and the upper ocean and continental crust, and by the emergence and diversification of aerobic organisms. Our results demonstrate that MIF sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the Paleoproterozoic and independently confirm observations that MIF ceased during this time. We interpret our results by integrating Δ 33S and δ 34S data in sulfides, δ 13C data in carbonates and the estimated timing of glaciation events in the Paleoproterozoic. Data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and/or in δ 34S sulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans. These changes modulated the population of primary producers, especially oxygenic photosynthesizers, and led to fluctuations in the abundance of atmospheric O 2, CO 2 and CH 4. Our results support the interpretation that the world-wide δ 13C carb excursion observed between ∼2.25 and 2.05 Ga ( Karhu and Holland, 1996) was a period of significant accumulation of O 2 in the atmosphere.