Collaborative Proposal: Molybdenum Isotopes as Paleoredox Proxies

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ABSTRACTThis proposal seeks funding for continued development and application of the molybdenum stable isotope system for the examination of changes in ocean redox through time. Research into ocean paleoredox is of great importance in paleoceanographic modeling, and in the coupled modeling of changes in the biogeochemical cycles through time. At present, the integration of multiple geochemical redox indicators in organic carbon-rich sediments identifies, with reasonable certainty, sediments deposited under anoxic-sulfidic (euxinic) bottom waters. However, beyond the areal distribution of such sediments, there is no means of estimating how prevalent euxinic bottom water conditions were at any particular time. Our initial survey of Mo isotopes in sediments from the Black Sea, ferromanganese nodules, seawater and continental materials indicated that the largest fractionation of Mo isotopes in the oceans occurs during preferential uptake of light Mo isotopes to Mn-oxyhydroxide sediments, but that Mo isotopes are relatively unfractionated during removal in euxinic environments (Barling et al., 2001). Therefore, Mo in seawater is isotopically lighter than Mo in oxic sediments and continent-derived Mo entering the oceans, but is similar to that of sediments accumulating under euxinic conditions. Because both oxic and euxinic sediments are important sinks in the ocean Mo budget, these findings led us to develop the following hypothesis: The Mo isotope composition of black shales should reflect that of seawater, and should vary with the extent of global ocean anoxia. Specifically 97Mo/95Mo (and other Mo isotope ratios) should shif toward isotopically lighter values during extended periods of expanded euxinic conditions in the oceans.We have explored key parts of this hypothesis as part of award EAR 0106712 which supported one year of further investigation of major reservoirs, and laboratory experiments designed to test the inference that Mo isotopes are fractionated during uptake by Mn-oxyhydroxides. This award will soon expire. Results to date demonstrate that Mo isotopes are fractionated during uptake by Mn-oxyhydroxides in the laboratory, in the expected direction and to the expected magnitude. In addition, we have found that the Mo isotopic compositions of euxinic sediments from the Cariaco Basin are similar to those of Black Sea sediments and seawater, consistent with the idea that such sediments provide a first order record of seawater Mo isotopes. Recent findings of other workers (e.g., Siebert et al., 2001b) also increase confidence in the paleoredox utility of Mo isotopesTo further test this hypothesis, we now propose to extend our research to examine the isotopic composition of Mo in the geologic record. Specifically, we intend to study Mo isotopes and other paleoredox indicators in three well-constrained black shale stratigraphic sequences from the Mesoproterozoic (Roper Group, McArthur Basin, N. Australia), the Paleozoic (Middle Devonian Oatka Creek Formation, northern Appalachian Basin) and the Mesozoic (Cenomanian-Turonian Boundary core, ODP Site 1138) Eras. These three sequences are selected because they may represent periods of substantial, but differing, perturbation of ocean redox conditions for periods of time comparable to, or longer than, the ocean residence time of Mo. These sequences are also attractive because they are well characterized by other geochemical techniques and have good stratigraphic and depositional context. Preliminary data obtained from the McArthur Basin, a Devonian black shale and the C-T Boundary are consistent with our hypothesis. We also propose to refine our laboratory experiments to better understand the mechanisms of Mo isotope fractionation in nature.The proposed research will strengthen ties between groups at U. Rochester and U. Missouri, which bring complementary approaches to paleoenvironmental questions of mutual interest. Paleoenvironmental research is of increasing societal importance as a result of public interest in probable anthropogenic climate change and its consequences. The complementary resources and approaches of these two groups will enrich the training of two graduate students, one at each institution. In addition, the proposed project facilitates the participation of a member of an underrepresented group (Co-PI Barling).