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ACS-PRF Narrative Report, Year 2 Ð

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 second year of this two-year project, we have completed most of our original goals, and we have requested an extension for 1 year to complete the remainder of these goals.

(1) We have completed all elements of fieldwork in Argentina: detailed measurement and sampling of Cambrian and Ordovician sedimentary strata, two different localities focusing 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.

(3) We have completed S-isotope analysis of carbonate-associated sulfate (CAS) and the associated analyses on these samples (S-isotope analysis of pyrite and analysis of the degree of pyritization, which allows us to examine the potential role of oceanic redox and organic carbon flux).

(4) We are in the process of attaining the last of the geochronological data for this project.

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 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 and Zhan 2009).

Similar high-order variation in ?34S 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 <10ä), 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 ?34S 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.

Significant results: The highlight of year two Year 2 of this project has been the recognition of a substantial change in the S-isotope composition of both Pyrite and CAS in the upper middle Ordovician (new data collected this year). Here we see sympathetic shifts in Pyrite S and CAS S that occur over a very short time frame that is also marked by a large shift in Sr-isotope composition. The Sr-isotopic shift is interpreted as reflecting increased sea-floor spreading and is coincident with a global sea-level rise. However, rather than seeing a rise in the oceanic chemocline that our data defines, we see a dramatic lowering of the chemocline. Our data is best interpreted to reflect a wholescale shift from warm-saline circulation to more traditional thermal circulation (and increased oxygenation of bottom waters), which restricted pyrite formation to substrate pore fluids. This change in circulation is coincident with published evidence for a cooling of sea surface temperatures, and occurs 10-15 Ma before the onset of late Ordovician glaciation.

Publications: We currently have two manuscripts in preparation for submission (one to Nature), and expect at least 2 more peer-reviewed publications to arise from this project; We expect submission in November 2010, December 2010, and two by May 2011.