Reports: AC841866-AC8: Ocean Circulation, Nutrient Cycling, and the S-isotope composition of Early Paleozoic marine systems

Linda C. Kah , University of Tennessee

Summary: Evolution of the marine sulfur cycle and its linkage to oceanic carbon cycling is a critical link in understanding both the environments in which organic carbon is deposited and the time-temperature pathways experienced during hydrocarbon maturation. Many current models, however, are based on assumptions of marine sulfate concentrations similar to that of today, despite growing evidence that marine sulfate concentrations may have been much lower than present for much of Earth history (Kah et al. 2004; Hurtgen et al. 2006; Reis et al. 2009; Hurtgen et al. 2009). Changes in oceanic circulation may also have played a fundamental role oceanic productivity through limitation of bioessential nutrients such as phosphorous and nitrogen (Saltzman 2005). During the span of this project, we have conducted integrated field and geochemical analysis of Mid-Late Ordovician strata of the Argentine Precordillera to determine if coeval C- and S-isotope records from carbonate rocks, carbonate-associated sulfate (CAS), and pyrite. Our results indicate that we can successfully distinguish small-scale changes in marine oxygenation via the isotopic records of marine sulfate (preserved as carbonate-associated sulfate). Additionally, we have observed several important features that allow us to reconstruct a model for changing oceanic circulation that was potentially driven by oceanographic circulation and may have played a role in changing nutrient availability and the Great Ordovician biodiversification. Completed Tasks: As our final report on this project, we have summarized tasks only briefly, so that we may focus on our outstanding results: Completed all elements of fieldwork in Argentina and Newfoundland; completed petrographic analysis, cathodoluminescence analysis, microsampling, and analysis of C- and O-isotopes and major and trace elements for all samples; also completed S-isotope analysis of carbonate-associated sulfate and, when available, pyrite; finally completed a series of U-Pb analyses to place patterns of isotopic change into a rigid chronostratigraphic framework.Results: This project has been tremendously successful. We were not only able to complete our original goals, but the work has drawn the attention of the European International Innovation Report, published by Research Media. The 2011 issue of this report will cover innovation in the areas of Earth and Environmental Science and will feature a 2-page spread on this project. We are very excited by this international interest and thank ACS-PRF for funding this successful research. Our most exciting results are summarized as follows:(1) C-isotope compositions show relatively little isotopic variability, resulting from nutrient limitation of organic productivity in a redox stratified ocean. These results are similar to that which is observed elsewhere in the world (e.g. Saltzman et al. 2005), and is consistent with interpretations of global greenhouse conditions at this time. (2) S-isotopic analyses of carbonate-associated sulfate show rapid stratigraphic variation of nearly 6ä over intervals <20 meters. This pattern of variation is consistent between different measured stratigraphic sections, and has also been found in our associated examination of similarly aged strata of Newfoundland (Thompson & Kah 2007) and China (Kah & Zhan 2009). We interpret these data to represent short-term fluctuation of the marine oxycline and associated changes in the balance between bacterial sulfate reduction and sulfide oxidation. (3) An abrupt (<0.5 my), 15ä shift in CAS to lighter isotopic values is most reasonably interpreted as an abrupt, large-scale oxidation of a reactive hydrogen sulfide reservoir. Coincident with this abrupt change in CAS isotopic composition, the isotopic composition of marine pyrite becomes progressively heavier, ultimately resulting in an unusual case of “’isotopic inversion”“, wherein pyrite becomes isotopically heavier than coeval marine sulfate.(4) Relative rates of isotopic change of CAS and pyrite demand dual-reservoir geochemical modeling. Currently, Proterzoic and Paleozoic systems are generally modeled only with a single reservoir of marine sulfate, which cannot account for imbalances between rates of sulfate reduction and sulfate oxidation. Our two-box model results best fit the marine CAS and pyrite data when we have two reactive sulfur reservoirs: a relatively small reservoir of oxidized sulfur (sulfate), and a relatively larger reservoir of reduced sulfur (hydrogen sulfide). By investigating a dual-reservoir scenario, we can effectively model the combined observations of (1) a large and abrupt change in CAS composition (i.e. rapid oxidation of a portion of the sulfide reservoir and delivery of this isotopically light material to the relatively small sulfate reservoir); (2) a longer-term change in the isotopic composition of marine pyrite (i.e. slower response of the sulfide reservoir to changes in isotopic composition because of its relatively larger size); and (3) isotopic inversion of sulfide and sulfate isotopic values (i.e. increased oxidation of a reactive sulfide reservoir carries an isotopic fractionation that can lead to isotopically heavy sulfide, and the existence of the two reactive reservoirs effective decouples pyrite formation from the sulfate reservoir). Continuing Work: While conducting this work, we realized that the timing of isotopic inversion in the sulfur system (indicating substantial ventilation and oxidation of the deep-ocean hydrogen sulfide reservoir) is coincident with the largest change in marine Sr-isotopes in the Paleozoic. We are currently modeling this situation as an offshoot of this project. Publications: We have five publications underway associated with this work:Thompson, C.K., and Kah, L.C. Sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in Early to Middle Ordovician greenhouse oceans. Palaeogeography, Palaeoclimatology, Palaeoecology (accepted).Thompson, C.K., Kah, L.C., Astini, R., Bowring, S.A., Buchwaldt, R. Bentonite geochronology, marine geochemistry, and the Great Ordovician Biodiversification Event (GOBE). Palaeogeography, Palaeoclimatology, Palaeoecology (accepted).Thompson, C.K., Kah, L.C., and Kaufman, A.J. Did ventilation of euxinic oceans herald the end of Ordovician greenhouse climate? Nature Geosciences (in revision).Kah, L.C., Zhan, R., and Thompson, C.K. Anoxia, euxinia, and the timing of the Ordovician Biodiversification in China. Geology (submission planned, October 2011).Kah, L.C., and Thompson, C.K. Marine Sr-isotope change and ventilation of the Ordovician oceans. Nature Geoscience (submission planned, December 2011).
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