Behavior of Marine Sulfate in the Early Paleozoic: Extending the Trace Sulfate Proxy

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Intellectual Merit: In the last decade, detailed investigation of the global carbon and sulfur cycles has increased our understanding of biospheric oxygenation. Progressive oxygenation of the Earth?s biosphere, driven by burial of organic matter, is interpreted to have resulted in the homogenization of photolytic mass-independent S-isotope ratios, followed by an increase in marine sulfate concentration and the enhanced redox cycling of marine sulfur. Many of these advances have relied on the development of new analytical capabilities, such as the simultaneous measurement of 34S/32S and 33S/32S, or the use of different geochemical proxies, such as the S-isotope analysis of carbonate-associated sulfate, or CAS. Availability of a CAS dataset, in particular, has resulted in a number of modeling efforts that seek to better understand sulfur cycle dynamics and its linkages with the marine C-isotope record. The most sophisticated of these models utilizes a time-dependent equation for isotopic change wherein the rate of S-(or C-) isotope change reflect not only the input and output fluxes, but also marine reservoir size, which acts to buffer the rate of isotopic change. Unfortunately, interpretation of this growing CAS record, and thus C-S linkages, is limited in the Precambrian by (1) our relatively poor understanding of potential variability of CAS isotopic compositions in restricted basins and epeiric sea environments that represent much of the preserved stratigraphic record, (2) the typically poor geochronologic control of otherwise well-studied successions, and (3) the extreme differences in the size of both C and S reservoirs, which affects the sensitivity of isotopic records and therefore limits their interpretation. Current data suggests, however, that by the end of the Precambrian, marine carbon and sulfate reservoirs reached sizes that should optimize the information preserved in their respective isotopic systems.In order to better understand the behavior of CAS in sedimentary systems and the linkages between C and S systems, we propose an integrated field, geochemical, and geochronological study of Late Cambrian and Mid-Late Ordovician strata of the Argentine Precordillera and Western Newfoundland. These localities (1) preserve open marine successions that should limit local overprinting of their isotopic records, (2) represent time intervals critical to understanding the evolution of C-S isotopic cycles in the transition from the Proterozoic to Phanerozoic worlds, (3) provide the opportunity to determine if S-isotope records from CAS and their timedependent modeling can distinguish small-scale changes in marine oxygenation potentially driven by oceanographic circulation and related cycles of nutrient limitation. Finally, numerous bentonites within the Argentine section provide the unique opportunity to interpret C-S records and constrain proposed modeling within the context of a high-resolution chronology.Broader Impacts: This project aims to advance discovery while promoting teaching, training, and learning by expanding the breadth of carbonate-based field and geochemical research at the University of Tennessee and involving students from differing educational levels in the process of scientific inquiry. The project supports one female Ph.D. student, and will continue the PIs strong history of involving undergraduates in research (9 in 5 years, leading to 5 peer-reviewed publications) by providing research opportunities for at least two students. Finally, the project will support international scientific collaboration through field research and U.S.-based geochemical training of Argentinian Ph.D. student Fernando Gomez. In addition to broader impacts supported by this proposal, the P.I. is a current participant in an NSF-funded GK-12 project aimed enhancing earth science education in rural TN middle schools. The PI has already used the initial field season as a case-study in exploring geologic time, and is currently using this project to train a local science teacher, Jay Bachman, on petrographic and geochemical techniques used in the geosciences. The PI expects to continue such activities through the duration of the project. Furthermore, results of geochronological and geochemical analysis will add to the EARTHIME initiative of creating a detailed, integrated time-scale for Earth history.