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Introduction: 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). For instance, lower sulfate concentrations in the Early Paleozoic would have been influenced by even small changes in marine oxygenation and redox cycling, potentially driven by changing oceanic circulation patterns. Redox cycling, in turn would result in substantially more variable isotopic compositions, and may even contribute, ultimately, to organic carbon availability. Changes in oceanic circulation associated with global icehouse-greenhouse climate cycles in the Early Paleozoic have been proposed to have played a fundamental role oceanic productivity through limitation of bioessential nutrients such as phosphorous and nitrogen (Saltzman 2005). In this scenario, nutrient limitation imposed by oceanic stratification and enhanced anoxia during greenhouse times is recorded by the general stability of marine carbon isotopic composition. By contrast, increased productivity associated with enhanced oceanic circulation and increased oxygenation during icehouse intervals is recorded by large positive C-isotope excursions.

The goals of this project, as originally proposed were to conduct an integrated field and geochemical study 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 can distinguish small-scale changes in marine oxygenation potentially driven by oceanographic circulation. Also proposed was the utilization of O-isotope signatures of CAS (Turchyn and Schrag, 2006; Bottrell and Newton 2006) and time-dependent modeling (Kah et al. 2004; Bartley and Kah 2004) to explore the behavior of the marine sulfur cycle and its linkage, via redox cycling, to the dynamic marine carbon cycle of the Mid-Late Ordovician.

Completed Tasks: In the first year of this two-year project, we have completed a large number of our original goals.

(1) We have ompleted all elements of fieldwork in Argentina. This has included detailed measurement and sampling of Cambrian and Ordovician sedimentary strata (over 1200 meters of total strata, sampled at ~6 meter resolution), two different localities focussing on Upper Cambrian strata and 4 Middle Ordovician localities that represent both onshore and offshore depositional environments.

(2) We have completed petrographic analysis, cathodoluminescence analysis, microsampling, and analysis of C- and O-isotopes and major and trace elements for all samples. This step of analysis is critical for determining the diagenetic history of the carbonate rocks that might compromise later analysis of carbonate-associated sulfate. Additionally, this data was used to establish a set of analytical priorities. We determined that the Late Cambrian C-isotope excursion (SPICE; Saltzman et al. 1998; Hurtgen et al. 2009) was not preserved, so we downgraded the importance of these sections to focus instead on Ordovician-aged strata.

(3) We have just completed preparation and S-isotope analysis of carbonate-associated sulfate (CAS) from all Lower and Middle Ordovician measured sections, and have completed much of the associated analyses on these samples. Associated analyses include the S-isotope analysis of pyrite (which gives us both oceanic reduced and oxidized elements), and analysis of the degree of pyritization (which allows us to examine the potential role of oceanic redox and organic carbon flux).

(4) Unrelated to this project, we are also in the midst of this same cohort of analyses for coeval Lower and Middle Ordovician strata from Newfoundland and China, which represent deeper-water depositional environments, for comparison to Argentine data.

Results: This project has produced high-resolution isotopic records of both oxidized and reduced forms of carbon (carbonate and organic matter) and sulfur (carbonate-associated sulfate and pyrite) from gas-producing Ordovician rocks of the Argentine Precordillera. C-isotope compositions are consistent with the global C-isotope curve for this time period (Saltzman 2005) and show relatively little isotopic variability. This isotopic stasis has earlier been attributed to nutrient limitation of organic productivity in a stagnant (likely redox stratified) greenhouse ocean circulation model. By contrast, isotopic analysis of carbonate-associated sulfate shows rapid stratigraphic variation of nearly 6‰ 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 and Zhan 2009).

Similar high-order variation in S-isotopes exhibited in Middle Ordovician successions from three globally and environmentally disparate successions indicates: 1) that recorded CAS isotopic signals are independent of depositional environment, 2) that high-order isotopic variation is a global phenomenon at this time period, and 3) that the mechanism responsible for this high-order isotopic variation acted on oceanographic, rather than geologic, time scales. Our initial S-isotope analysis of sedimentary pyrite shows typically heavy isotopic values (to <10‰) which suggests, further, that the marine water column remained, in the Ordovician, reltively sulfate-poor. We therefore suggest that it is likely that high-order d³⁴S variation reflects transient changes in the extent of oceanic bottom-water anoxia, resulting in variable redox cycling (BSR and sulfide oxidation) in deep-ocean environments.

Continuing Work: Year two of this project will consist of completing geochemical analyses (focussed on the isotopic analysis of reduced phases – organic carbon and pyrite – and the experimental analysis of sulfate O-isotopes for specific intervals); working on the radiometric dating of the succession to refine the time-scale of variation (high-resolution U-Pb dating of numerous bentonits in the unit is funded through different sources); integration of Argentine datasets with similar geochemical dadsets from Newfoundland and China; and geochemical modelling that will be focussed on determining the potential origins for geologically short-term isotopic variation.

Publications: We currently have one manuscript in preparation for submission (submission by January 2009), and expect at least 3 more peer-reviewed publications to arise from this project, to be submitted in August 2010, December 2010, and May 2011.