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Project Progress

1. Unravelling Earth’s Early Sulfur Cycle

In the past year Doctoral Student David Johnston and Co-I Farquhar determined for the first time the different types of isotopic signatures produced by different sulfur metabolisms. including sulfate-reducing bacteria, sulfite-disproportionating bacteria, and sulfur-disproportionating bacteria. These observations permitted calibration of the first global sulfur isotope models. These global models use multiple sulfur isotope data to constrain the relative proportions of sulfide burial and sulfide reoxidation in the sulfur cycle and to constrain the timing of the transition from a sulfur cycle that was dominated by sulfate reducers to a sulfur cycle that included a significant reoxidative cycle. In concert with isotopic analyses of proxies for oceanic sulfate (carbonate associated sulfate, evaporates, marine barite) these models place improved limits on the timing of this transition, changing its timing from ~800-1000 million years ago to between 1450 and 1300 million years ago. This transition is also thought to be connected with the history of oxygenation of the oceans and by implication the atmosphere. A similar approach was taken to study the transition from Fe-rich to sulfidic ocean chemistry for the ~1850 million year old sequences of the Animike Basin that host the Rove and Gunflint Formations. The results of this study appear to point to a fundamental change in the sulfur cycle and to a sulfur cycle with a significant amount of sulfide loss after the onset of sulfidic conditions associated with the deposition of the Rove Formation.

Postdoctoral Fellow Shuhei Ono received support from the Agouron Institute (Agouron Griqualand Paleoproterozoic Drilling Project) in addition to that from the NAI to constrain the Archean sulfur cycle by undertaking systematic sulfur (³²S /³³S /³⁴S /³⁶S) and carbon (¹³C/¹²C) isotope analyses of drill cores in rocks 2.4 to 2.9 billion years old from Western Australia and South Africa.

An important part of Ono’s effort has been implementing continuous-He flow technology for the Carnegie Institution’s sulfur isotope laser probe. Thanks to Ono’s work, required sample sizes were decreased from a few micromoles to approximately ten nanomoles. This new technology reveals micrometer-scale isotope heterogeneity in fresh drill core samples, which is critical to mapping the microstructure of Archean and Proterozoic microbial communities.

Postdoctoral Fellow Andrey Bekker worked over the last year on several projects related to atmospheric and oceanic redox states:

The first was the relation of atmosphere oxygenation to Paleoproterozoic glacial events. Global occurrences of Paleproterozoic glacial diamictites are found in Canada, Western Australia, and South Africa. Pyrite grains in conglomerates of the Missassagi Formation are sandwiched between the first and second glacial diamictites of the Huronian Supergroup, Ontario, Canada. The sulfur of the detrital pyrites is mass-independently fractionated, consistent with derivation from Archean sedimentary successions of the Superior Craton. Mass-independent fractionation of multiple S isotopes as found in pyrites from the ca. 2.47 Ga. Joffre and Whaleback Members of the Brockman Iron Formation that underlie the Paleoproterozoic glacial diamictite of the Turee Creek Group, Western Australia. The oxygen content of the atmosphere at that time was therefore likely below that required for oxidative pyrite weathering.

The second topic of study was the redox state of the Paleoproterozoic deep ocean. Volcanogenic massive sulfide deposits 1.76 Ga in age from Arizona grade into jaspers that have a positive Ce anomaly and a limited range of Fe isotope ratios consistent with a suboxic state of the deep ocean at that time. Bekker’s collaborators on this project include John Slack (US Geological Survey), Tor Grenne (Norsk Geologist Forening, or Geological Survey of Norway), and Olivier Rouxel (Woods Hole Oceanographic Institution).

The third project addressed the question of whether it is feasible to use analyses of multiple S isotopes in ore deposits and their wall rocks to trace the sources of sulfur in komatiite-hosted Archean nickel deposits. Preliminary results show that pentlandite deposits hosted by komatiite lavas have negative Δ³³S values, suggesting a surface origin for their sulfur. Collaborators on this project are Mark Barley and Marco Fiorentini of the University of Western Australia.

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

Co-Investigator George Cody and Collaborator Nabil 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. Over the past year Cody and Boctor were particularly interested in the facile synthesis of succinate starting with citrate under hydrothermal conditions. The obvious route involves the retroaldol reaction of citric acid to form oxalacetate and acetic acid; a complete reduction of oxalacetate first to malate and then to succinate could possibly occur. However, Cody and his team have ruled out this pathway. Rather they believe that succinate is formed through dehydration and subsequent decarboxylation of citrate to yield itaconate. Hydration of the exomethylene carbon yields methylsuccinyl alcohol. A retroaldol reaction of methylsuccinyl alcohol yields succinate and formaldehyde. Interestingly, Figure 1 shows that pyrite and ammonia both catalyze the formation of succinate. In the case of pyrite, it is likely that a weak organometallic complex forms between the exomethylene group of itaconate and surface iron, enhancing the synthesis of the methylsuccinyl alcohol relative to other hydrated isomers of the methylated C4 dicarboxylic acids. In the case of ammonia, Cody and his grouop are still not sure how the succinate reaction is catalyzed.

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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. Preliminary results reveal substantial catalytic activity for carbonyl insertion chemistry. The team is in the process of obtaining stable isotopic data in order to refine their ability to distinguish between abiotic and biotic synthesized hydrocarbons. Working in collaboration with Jennifer Stern, an NAI-Oak Ridge Postdoctoral Fellow based at NASA Ames Research Center, Cody and his group have initiated an exploration into the stable isotopic fractionation during carbonyl insertion reactions catalyzed by millerite (NiS). In order to highlight the isotopic fractionation, they are using butane thiol as a target molecule. Use of this highly volatile molecule has necessitated developing a new method for sample loading.

Finally, over the past year Co-Investigator Jay 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 Archean 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 absorption 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.