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Project Progress

3. Prebiotic Chemical and Isotopic Evolution on Earth

1. Unravelling Earth’s Early Sulfur Cycle

The first focus of this project has been on the Archean sulfur cycle and understanding the significance of mass-independent sulfur isotope signals. Rumble, Farquhar, Postdoctoral Fellow Shuhei Ono, and Masters Student Margaret Baker studied Australian and South African Drill cores in collaboration with a number of international colleagues. Postdoctoral Fellows Boswell Wing and Andrey Bekker continued work on constraining the rise of atmospheric oxygen in the Huronian succession, Canada. Related progress was the development of a-state-of-the-art laser fluorination gas chromatograph mass spectrometer to measure multiple isotope ratios of nanomole levels of sulfur.

The second focus has been to deconvolve the mass-dependent sulfur isotope fractionations associated with biological metabolic and biogeochemical networks. This work has been the primary focus of Farquhar, Doctoral Student David Johnston, Wing, and Ono. Accomplishments in the past year include experiments with sulfate reducing, sulfur disproportionating, and thisulfate disproportionating bacteria and an extension of network models to biogeochemical systems. Analysis of four sulfur isotopes as a new tracer of post-Archean sulfur biogeochemical cycles promises to decouple mass-dependent and mass-independent fractionation.

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Related work by Andrey Bekker in collaboration with Olivier Rouxel (Woods Hole Oceanographic Institution) divided Precambrian time into three stages with different redox Fe cycles and suggested that the ocean Fe redox cycle quickly responded to the rise of atmospheric oxygen. In collaboration with Mark Barley and Bryan Krapez (University of Western Australia), Bekker attempted to integrate tectonic history with atmospheric and climatic changes during the period 2.45-2.2 Ga. Their preliminary data for the Jerome volcanic massive sulfide deposit open the possibility that the deep ocean was neither sulfidic nor fully oxygenated during 1.8-1.0 Ga.

2. The Critical Role of Sulfur in Prebiotic (Protometabolic) Organic Chemistry

Cody and Boctor continued their experimental studies of metal-sulfide-promoted prebiotic chemistry. Previous work outlined a potential entry point into a chemical reaction network that includes many compounds familiar to intermediary metabolism, e.g., citric acid and pyruvic acid. Their initial results show that transition-metal sulfides promote hydroformylation reactions that provide a suite of methylated dicarboxylic acids as intermediates, e.g., methyl fumaric (mesaconic) acid and methyl malic (citramalic) acid. They have recently discovered that by using pyrite (FeS₂) they are able to reductively demethylate these compounds to yield succinic acid. The specific mechanism is not yet known and is being explored. The fact that an abiotic pathway clearly links the methylated intermediates with the familiar four-carbon dicarboxylic acids appears to strengthen their contention that metal sulfide catalysis provided a critical connection between natural environmental chemistry and the emergence of life.

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Over the past year Cody and Boctor extended considerable effort in synthesizing a new suite of minerals to explore their catalytic qualities. In particular they synthesized tungstenite (WS₂), molybdenite (MoS₂), and awaruite (Ni₃Fe). These minerals are found in aqueously altered ultramafic rocks. They are initiating experiments to assay the catalytic performance of these phases and demonstrate hydrocarbon synthesis capability starting with CO₂ and H₂ directly. They aim ultimately to obtain stable isotopic data in order to refine their ability to distinguish between abiotic and biotic derived hydrocarbons.

Finally, over the past year Brandes and his group finished a study of metal-sulfide-catalyzed N₂ reduction. The group continued to examine different aspects of the prebiotic nitrogen cycle. Their efforts were concentrated on three areas. The first was method development to examine very small samples of organic nitrogen with the aim of examining organic inclusions in Archaean rocks and meteorites, as well as organic N generated during prebiotic synthesis experiments. This method allows high-precision natural abundance measurements of organic nitrogen isotopes on nanomole-size samples. The second was the development of techniques to quantify and characterize organic samples using soft X-ray microscopy, and they have applied the techniques to graphitic and amorphous carbon samples. Brandes and colleagues mapped C:N:O ratios in such samples and developed a detailed model used to interpret X-ray absorbtion spectra. Finally, they continued their efforts to examine mineral-catalyzed N cycling within hydrothermal systems. Their future goals in this area are to quantify and model the isotopic shifts associated with abiotic nitrogen transformations as a possible tool for investigating signatures in returned samples from Mars and other Solar System objects.