HR: 0800h
AN: PP11A-0546    [Abstracts]
TI: Milankovitch Forcing at the Eocene/Oligocene Climate Transition from Benthic Foraminiferal Isotope Analysis
AU: * Riesselman, C R
EM: criessel@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University Braun Hall, Building 320 , Stanford, CA 94305-2115 United States
AU: Dunbar, R B
EM: dunbar@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University Braun Hall, Building 320 , Stanford, CA 94305-2115 United States
AB: A critical component missing from our knowledge of Antarctic cryosphere evolution is a comprehensive understanding of the role of orbital forcing in ice sheet development during the Eocene/Oligocene climate transition. Existing marine records lack the resolution and preservation to reconstruct variability in the high-frequency precessional band, thought to be potentially significant in early Oligocene ice sheet development and stability. A new E/O boundary section from Ocean Drilling Program Site 1263, on the northwest flank of Walvis Ridge in the eastern South Atlantic Ocean, yields a benthic foraminiferal isotopic record with an average time step of 3.5 kyrs, sufficient to resolve the 19- and 23-ky precessional Milankovitch bands. The E/O boundary at this site is associated with a gradual increase in sediment color lightness (L$\ast$),attributed to an increase in carbonate content and accumulation rate and a general improvement in microfossil preservation. The 1263 L$\ast$ increase is accompanied by an enrichment of $\sim$1.4$\permil$ in $\delta$$^{18}$O and $\sim$0.8$\permil$ in $\delta$$^{13}$C of bulk carbonate across the 5-meter interval that represents the period between 33.7 and 33.4 Ma. Multiple single-species isotope analyses of {\it Oridorsalis umbonatus}, {\it Nuttallides trumpeyi}, and {\it Cibicidoides spp.} produce consistent isotope values across a series of horizons, however deviation from these patterns observed in some larger specimens may be indicative of reworking. Using smaller specimens, we will present a high-resolution oxygen isotope record across the E/O transition interval in order to evaluate the significance of precessional forcing in early Oligocene glaciation.
DE: 9325 Atlantic Ocean
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 1040 Isotopic composition/chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting