HR: 09:30h
AN: PP51G-07 INVITED     [Abstracts]
TI: Combined Rock Magnetic/Geochemical Proxies of Redoxomorphic Iron Mineral Diagenesis Based on Fast Core Logging Techniques - Examples from the Equatorial Atlantic
AU: * von Dobeneck, T
EM: dobeneck@uni-bremen.de
AF: University of Bremen, Dept Geosciences, PO Box 330440, Bremen, 28334 Germany
AU: Funk, J
EM: funk@uni-bremen.de
AF: University of Bremen, Dept Geosciences, PO Box 330440, Bremen, 28334 Germany
AB: Many sediments show cyclic variations in organic carbon content representing climate controlled changes in paleoproductivity and bottom water oxygenation. In organically enriched layers Fe(3+) bearing magnetic minerals like magnetite and hematite are successively reduced to Fe(2+) and dissolved. The iron precipitates in situ as non-magnetic Fe(2+) sulfides/oxyhydroxides or diffuses upwards forming new magnetic and non-magnetic Fe(3+) minerals above the active Fe(2+)/Fe(3+) redox boundary. These processes leave characteristic imprints on all environmental magnetic parameters, in particular those related to absolute and relative magnetic mineral contents and magnetic grain size estimates. We propose new selective proxies to identify and quantify reductive iron oxide dissolution (Fe/kappa) and precipitation (kappa/Ti) based on logging data detected by X-ray fluorescence (Fe, Ti) and magnetic susceptibility (kappa) core scanning. Combining these rapid and non-destructive methods, diagenetically altered sediment sections can be detected at unprecedented speed and precision - provided that the primary composition of the terrigenous sediment fraction has been fairly constant through time. These integrated proxies help to distinguish climatic and diagenetic signal patterns, to quantify secondary magnetic mineral phases and to reconstruct the pristine magnetic susceptibility signal indicative of terrigenous content. XRF data are largely unbiased by mineralogy and do not follow susceptibility lows caused by conversion of ferri- into paramagnetic iron species. The (Fe/kappa) ratio therefore highlights intervals, where such alterations occur. An alternative hysteresis-based proxy is the ratio of high-field to low-field susceptibility. Sedimentary Ti is an inert and insoluble, hence conservative ion of terrigenous origin. Its accumulation is generally highly correlated with that of primary Fe. XRF-based Ti logs can therefore be used as normalizers for kappa and Fe logs. This yields two additional proxy ratios for ferrimagnetic iron precipitation (kappa/Ti) and total iron enrichment (Fe/Ti). All mentioned proxy parameters can be regionally calibrated by detailed rock magnetic and geochemical analyses on single samples. A collection of Late Quaternary sediment sequences from the Central Equatorial Atlantic Ocean serves as case study to demonstrate applications and interpretations.
DE: 9325 Atlantic Ocean
DE: 3022 Marine sediments--processes and transport
DE: 1512 Environmental magnetism
DE: 1615 Biogeochemical processes (4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting