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Project Progress

In situ Sulfur Isotope studies Archean–Proterozoic sulfides and implications for atmospheric oxygenation

Project Progress

1. Sulfur isotope thermometry in Archean cherts

One potential application of the improved analytical precision of SIMS sulfur isotope analysis in sphalerite achieved by developments at WiscSIMS is sulfur isotope thermometry of pyrite-sphalerite pairs. We applied this technique to ~3.5 Ga cherts from the Old Exploration Camp and the Trendall Locality in the North Pole Dome of the Pilbara Craton, Western Australia. Our measurements revealed isotopic zoning within single grains, δ³⁴S variability in pyrite of up to 8‰ over less than 10 mm, and an average Δ_{pyrite-sphalerite} of 4.6‰ in one sample, leading us to conclude that pyrite and sphalerite in these samples are not in isotopic equilibrium and unsuitable for thermometry. This variability would be masked by conventional approaches using larger sample weights or multiple grains, potentially leading to erroneous conclusions.

2. Sulfur isotope distribution in pyrite from the Early Proterozoic Turee Creek Group, Western Australia

Along with similar deposits in North America, South Africa and Finland, the Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia records a glaciation that may have been global in extent and coincided with the rise of atmospheric oxygen during the so called “Great Oxygenation Event” (GOE) approximately 2.4 billion years ago. In the decade since the discovery of large, mass-independent fractionations of sulfur isotopes (S-MIF) preserved in Archean and Early Proterozoic sediments, this isotopic signal has been pursued as a tracer of one of the most profound biogeochemical changes in the history of the planet, namely the transition from an anoxic to an oxic atmosphere. The dominant model holds that S-MIF produced by high-energy reactions in the atmosphere disappeared with the appearance of appreciable ozone, while at the same time, enhanced oxidative weathering of sulfide minerals on the continents delivered increasing amounts of sulfate to the oceans. The latter effect led to an increase in the concentration of seawater sulfate above the threshold at which the isotopic fractionation associated with bacterial sulfate reduction is limited. As a result, normal, mass-dependent sulfur isotope fractionation increases during the same time interval in which S-MIF decreases.

Using new protocols developed at WiscSIMS, we made in situ measurements of three stable isotopes of sulfur (³²S, ³³S and ³⁴S) in pyrite from the Meteorite Bore Member with unprecedented small spot sizes and accuracy. We have found a moderate range of S-MIF (> 1‰) in authigenic pyrite before, during and after the Early Proterozoic glaciation as well as a 90‰ range of mass dependent sulfur isotope fractionation (δ³⁴S) larger than any observed in sediments older than 700 Ma. Furthermore, abundant detrital pyrite preserved in one glacial sandstone unit from the Meteorite Bore Member exhibits a range of mass-independent sulfur isotope fractionation slightly larger than the largest published range, from 2.5 Ga sediments of the Hamersley and Transvaal Basins (> 15‰), suggesting that these detrital grains may have originated in rocks of similar age. Taken together, these data imply that the Meteorite Bore Member was deposited during the transitional interval when atmospheric oxygen had risen sufficiently for enhanced continental weathering and ocean sulfate to occur, yet remained low enough to permit the preservation of detrital pyrite and moderate S-MIF.

We have achieved an analytical precision and spatial resolution that exceeds any so far reported for in situ sulfur three isotope measurements. Sulfur two isotope measurements with even smaller spots in individual pyrite grains have revealed isotopic gradients of 30‰ over less than 4 micrometers, sharper than any previously measured. Using published experiments on sulfur diffusion rates in pyrite, these data allow us to constrain the thermal maturation history of the host sediments.

The sulfur isotope data from Meteorite Bore pyrites provide important information about the history of atmospheric oxygenation and its connection to global glaciation. The analytical developments achieved in the process have significantly refined our ability to discover and detect biosignatures in ancient sediments on Earth as well as extraterrestrial materials.