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Distinguished Speaker Dr. Leanne Hancock Examines "Isotopes as Archives".

EES is proud to present our own Dr. Leanne Hancock, Research Associate in the MSU Hydrology Lab.

"Thallium and Molybdenum Isotopes as Archives of Miocene Seawater Paleoredox"

Several studies provide evidence for Miocene warming and ocean circulation changes, but there are few constraints on the related extent of global marine oxygen deficiency. Deposition of the Miocene Monterey Formation into numerous basins provides an ideal opportunity to study how preservation of geochemical signatures vary within and among basins as a function of local and global climatic and oceanic conditions. Commonly employed redox indicators, including iron speciation and redox-sensitive trace metal concentrations (Mo, U, V), are used to constrain paleoredox conditions from three Monterey locations: the Santa Barbara Basin, the Santa Maria Basin, and the San Joaquin Basin. These sections were deposited soon after a period of extreme warmth in the Miocene and under a range of local water-column redox conditions. In sediments deposited under euxinic (anoxic and sulfidic water column) conditions, the combination of sedimentary molybdenum (Mo) and thallium (Tl) isotopes can track trends in deoxygenation as linked to the global burial of redox-sensitive mineral phases. Despite a wide range of independently inferred paleo-depositional conditions—ranging from environments similar to both modern oxygen minimum zones to more reducing settings with appreciable water column sulfide accumulation—the Mo and Tl isotope records are relatively invariant within and among the three basins. Thallium isotope data closely match the expected seawater value from the independent Miocene ferromanganese crust record, suggesting that these data may be capturing a true seawater signal. Collectively, these results suggest that the Miocene oxic sink for Tl was similar to that of the modern ocean. In contrast, the Mo isotope records from the Monterey are offset from the modern seawater value of 2.3‰. Despite evidence for euxinia, a condition required for capturing seawater Mo isotope compositions, local factors including restriction and possibly low and variable sulfide concentrations likely fractionated the Mo isotope values relative to contemporaneous seawater. The paired Mo-Tl isotope approach, in combination with independent tracers of local conditions, provides a comprehensive picture of spatiotemporally varying Miocene environments along the California Margin. This redox landscape included regions and episodes of euxinia not seen today along the California Margin—all within an ocean that was otherwise oxygenated at or near modern levels. These data also illustrate that euxinic sediments may record significant Mo isotope variations that are unrelated to changes in the global marine Mo isotope composition.

Join us for this fasinating take on Isotopes as archeives on Friday, November 30,2018 at 12:30 PM in Room 204, Natural Science Building on the East Lansing Campus.

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