HR: 09:45h
AN: PP51G-08    [Abstracts]
TI: Formation and Preservation of Authigenic Metal Sedimentary Signatures: Carbon Cycle Proxies and Insights from Transition Element Stable Isotopes
AU: * McManus, J
EM: mcmanus@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University 104 Ocean Admin Bldg, Corvallis, OR 97331-5503 United States
AU: Siebert, C
EM: csiebert@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University 104 Ocean Admin Bldg, Corvallis, OR 97331-5503 United States
AU: Bice, A
EM: abice@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University 104 Ocean Admin Bldg, Corvallis, OR 97331-5503 United States
AB: The solubility of U, Cd, Re, and Mo decreases under the reducing conditions commonly encountered within the upper few centimeters of ocean margin sediments. Because of this decrease in solubility, the presence or absence of these elements in marine sedimentary deposits has been interpreted as a signature for reducing conditions in the past. However, our understanding of these elements has yet to mature to the point where their quantitative utility is possible. One difficulty in exploiting these elements derives from the fact that two processes may foster sedimentary reducing conditions: low bottom water oxygen concentrations and high organic carbon decomposition rates. These two factors create shallow oxygen penetration depths where diagenetic reaction zones (e.g., oxygen consumption, nitrate, Mn, and Fe reduction) can be spatially compressed to the point where they functionally overlap. It is suggested here that because of this diagenetic compression, the relationship between oxygen penetration into the sediments and U and Cd accumulation is non-linear. In addition, it appears that U, Cd, and Mo are each sensitive to the organic carbon flux arriving at the seafloor, perhaps indicating that under certain circumstances these elements may serve as quantitative proxies for carbon accumulation. Our emphasis for this study is on those processes that regulate the deposition of these elements in continental margin sediments. It is within these sediments where our target elements offer the brightest potential to serve as proxies. It is also these sediments where variations in biogenic material dilution and preservation wreak the most havoc on more traditional biogenic proxies thereby necessitating a reliance on alternative or complementary proxies to interpret the sedimentary record. Because it is often difficult to separate authigenic signatures from background signatures during the earliest stages of authigenesis (particularly for Mo) we present data on Mo isotope compositions that further allow us to refine our interpretations of the sedimentary record. It is clear from this work that Mo is initially taken up in sedimentary sequences during the earliest stages of diagenesis. This initial stage of authigenesis is associated with isotope fractionation resulting in solid-phase Mo being isotopically light relative to the initial material, and this authigenic phase continues to be enriched in the heavier isotope as diagenesis proceeds.
DE: 4267 Paleoceanography
DE: 1030 Geochemical cycles (0330)
DE: 1010 Chemical evolution
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting