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Project Progress

3.1 Earth’s early sulfur cycle

Research done under the NAI initiative includes collaborative work between Co-Investigators Douglas Rumble (CIW) and James Farquhar (University of Maryland) measuring oxygen and sulfur isotopes on samples from a variety of geochemical environments and ages. A recent development has been to compare measurements of modern sea-floor environments with sulfur isotope analyses of Archean rocks of sea-floor magmatic origin. Published works by Olivier Rouxel and colleagues and Collaborator Shuhei Ono and colleagues are a foundation for interpreting the (unpublished) sulfur isotope data of Yuichiro Ueno (Tokyo Institute of Technology) on the Archean magmatic sea-floor system exposed in the North Pole Dome, Pilbara, Western Australia. Another new development by Collaborator Huiming Bao and coworkers is to perform triple oxygen isotope and quadruple sulfur isotope analyses on aliquots of the same Archean barite samples. The combined results of measuring both isotope systems provides a set of mutually supporting constraints leading to the conclusion that we don’t know quite as much as we think we know about the Archean ocean. An ongoing study on the ¹⁷O/¹⁶O composition of Earth’s oldest rocks reveals a consistency in the terrestrial fractionation line for oxygen isotopes that extends from the present to over 4 billion years ago. Collaborators Andrey Bekker and Axel Hofmann recently completed sulfur isotope analysis of Archean rocks collected in Zimbabwe.

3.2 The critical role of sulfur in prebiotic and protometabolic organic chemistry

More recently, Co-Investigator George Cody and colleagues have been exploring the link from sulfur-cycle pathways to the synthesis of pyrimidine and purines. The fact that living organisms only use the five bases found in RNA and DNA is intriguing, because other nucleobases may also work in the role of base pairing for molecular recognition (Figure 1). Cody’s studies point to plausible pathways to both classes of nucleobases that derive naturally from the transition-metal-sulfide-catalyzed reaction network. For example, in the case of pyrimidines, they find that in the presence of ammonia and, surprisingly, pyrite (FeS₂), significant quantities of succinic acid are generated from the aquathermolytic reaction of citrate and citramalate. Cody and his workers have demonstrated previously that certain minerals promote partial oxidation of saturated acids to olefinic acids; therefore, they surmised that fumaric acid might be a potential source of uracil formed through the stepwise condensation of urea, followed by dehydration and decarboxylation to orotic acid and uracil, respectively, under aqueous conditions. Preliminary experiments revealed a rapid synthesis of both orotic acid and uracil as well as providing evidence for an oxidative decarboxylation straight from dehydro-orotic acid to uracil. The ultimate yield was low, however, limited mostly by the hyrdrolysis of urea and partly by the side reaction to the hydantoin. Interestingly, Cody and his team observed a substantial increase in yield (> 100 X) at elevated pressures (e.g., 300 MPa), a result that they are currently exploring in greater detail.