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We investigate the expression of the Middle Darriwilian isotope carbon excursion (MDICE) across marine shelf environments in the Argentine Precordillera. Previous work identified the MDICE in the Las Chacritas Formation in Argentina, but did not recognize the expression of the MDICE in time-equivalent strata of the deeper-water Las Aguaditas Formation (Albanesi et al., 2013; Palaeogeography, Palaeoclimatology, Palaeoecology, v. 398, p. 48?66). Recent biostratigraphic investigations of these units have, for the first time, provided the opportunity for high-resolution correlation, which suggest that the MDICE, or at least the initiation of the MDICE, should be observed in both the Las Chacritas and Las Aguaditas formations. Here we present new paired carbon isotope data of carbonate and organic carbon from the Las Chacritas and Las Aguaditas formations. We identify a 2 ? positive shift in the isotopic composition of marine carbonate in the Las Chacritas Formation, whereas values abruptly fall to < −1 ? in equivalent strata of the Las Aguaditas Formation. This is the first record of divergence from the globally recognized MDICE event. There are also small, yet distinct differences in the isotopic composition of marine organic matter between these two sections. We suggest that the divergent C-isotope trends of carbonate in the Las Chacritas and Las Aguaditas formations represent deposition in fundamentally different parts of the water column above and below the marine chemocline, respectively during the MDICE interval. This interpretation is consistent with data from the Las Aguaditas Formation that shows elevated Mn and Fe concentrations in carbonate phases with little evidence for recrystallization, and with a growing consensus for regionally anoxic conditions for the Middle Ordovician. Our data indicate that redox gradients can play a critical role in the behavior of marine carbon isotope excursions. Fil: Henderson, Miles A.. University of Tennessee; Estados Unidos Fil: Serra, Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Feltes, Nicolás Alexis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina Fil: Albanesi, Guillermo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Museo de Paleontología; Argentina Fil: Kah, Linda C.. University of Tennessee; Estados Unidos

Patterns of change in the isotope composition of sulfur-bearing minerals play a key role in reconstructing the marine sulfur cycle. Determining marine sulfate concentration, in particular, is critical to understanding the linkages between oxygenation of the Earth's atmosphere and oceans, and the history of life. Much of our current understanding of marine oxygenation relies on a traditional single reservoir model, wherein the isotopic composition of marine sulfate is controlled primarily by burial of sedimentary pyrite. Utility of this model is limited, however, during times of persistent marine euxinia, which marks a fundamental decoupling between oxic and anoxic marine sulfur reservoirs. At these times, short-term fluxes that act between the two reservoirs (e.g., sulfate reduction and sulfide oxidation processes) often dominate over the long term fluxes (e.g., weathering and pyrite burial) that control the single reservoir ocean model. Ordovician strata from Argentina, western Newfoundland, and South China illustrate how the marine sulfur cycle is affected by the presence of a persistent euxinic reservoir. Regional euxinic reservoirs remain generally stable from the Floian through the Dapingian, despite evidence for periodic short-term oxygenation events. Dramatic reorganization of the marine sulfur cycle in the early Darriwilian, however, reflects dynamic disequilibrium between oxic and anoxic (euxinic) marine reservoirs, driven by ocean ventilation. Ventilation resulted in a rapid change in the isotopic composition of marine sulfate in surface oceans, and ultimately led to the near depletion of hydrogen sulfide within the anoxic reservoir, as marked by formation of superheavy pyrite. Ventilation occurred coincident with a decline in sea surface temperatures and may represent the onset of climatic change that ultimately led to late Ordovician glaciation.

Attribution of Ordovician climate forcing to explosive volcanism and the potential global importance of volcanism in Ordovician biodiversification suggest the necessity of evaluating the relationships between K-bentonite deposition and increasingly high-resolution records of marine biogeochemical change. Globally, Ordovician strata preserve an extensive record of explosive volcanism - including the widely recognized Lower to Middle Ordovician Famatina K-bentonite suite in Argentina and the Upper Ordovician Millbrig-Deicke-Kinnekulle suite of North America and Europe. Here, we present high-resolution ID-TIMS U-Pb zircon ages of K-bentonites from measured sections of the San Juan Formation (Talacasto and Cerro La Chilca section) of the Argentine Precordillera. K-bentonites from the Argentine Precordillera provide stratigraphically consistent (i.e., younging upward) ages that range from 473.45 ± 0.70. Ma to 469.53 ± 0.62. Ma, and constrain the age of a low-magnitude (2%), globally recorded, negative carbon-isotope excursion. Evaluation of the timing of K-bentonite deposition in the Argentina Precordillera relative to marine biostratigraphic and biogeochemical records provides insight into relationships between explosive volcanism and regional to global environmental change. From a regional standpoint, these ages provide critical direct evidence for a Dapingian to earliest Darriwilian age of the upper San Juan Formation at sampled localities. These ages are consistent with carbon-isotope data suggesting that the San Juan Formation in the region of its type section is coeval with only the base of the often-correlated Table Head Group of western Newfoundland. This data thus highlights the difficulties in using regional biostratigraphic data - particularly within erosionally truncated or otherwise diachronous units - to define the timeframe of carbon-isotope chemostratigraphy. New geochronological data also indicate that a discrete negative carbon-isotope excursion within the San Juan and Table Head formations is correlative to a globally recognized pre-MDICE negative excursion, and indicates that this aspect of the marine carbon isotope record can be used as a discrete chronologic marker. San Juan Formation bentonites, however, cannot be discretely correlated with observed, environmentally significant changes in the Middle Ordovician marine geochemical records of carbon, sulfur, strontium, or sea surface temperature. These results suggest that the extent of volcanism represented by the Famatina bentonite suite was insufficient to affect global surface environments and that the relationship between explosive volcanism and environmental change may not be straightforward as previously suggested. © 2012 Elsevier B.V. Fil: Thompson, Cara K.. University of Tennessee; Estados Unidos Fil: Kah, Linda C.. University of Tennessee; Estados Unidos Fil: Astini, Ricardo Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Bowring, Samuel A.. Massachusetts Institute of Technology; Estados Unidos Fil: Buchwaldt, Robert. Massachusetts Institute of Technology; Estados Unidos