HR: 0800h
AN: PP11A-0542    [Abstracts]
TI: Orbitally paced climate change across the middle Miocene climate transition
AU: * Shevenell, A E
EM: ashevenell@umail.ucsb.edu
AF: Department of Geological Sciences and the Marine Science Institute, University of California Santa Barbara, Santa Barbara, CA 93106-9630 United States
AU: Kennett, J P
EM: kennett@geol.ucsb.edu
AF: Department of Geological Sciences and the Marine Science Institute, University of California Santa Barbara, Santa Barbara, CA 93106-9630 United States
AB: Spectral analyses of an orbitally-tuned 3-my-long geochemical record of Miocene (16.5-13.5 Ma) climate and carbon cycling from the South Tasman Rise, Southern Ocean (paleolatitude: $\sim$$55\deg$S) provide insight into the processes and feedbacks involved in the middle Miocene climate transition (MMCT; 14.2-13.8 Ma), one of the three major steps in Earth's Cenozoic climate evolution. Spectral power in benthic foraminifer ({\it Cibicidoides mundulus}) $\delta$$^{18}$O and $\delta$$^{13}$C is concentrated in all primary Milankovitch frequency bands, with significant power in the 406- and $\sim$100-ky eccentricity bands. Cross-spectral analyses indicate that {\it C. mundulus} $\delta$$^{18}$O and $\delta$$^{13}$C co-vary and are coherent with orbital cyclicity. Variations in Earth's orbit preceded both Antarctic ice growth and global carbon cycling by $\sim$60-ky during the MMCT. At 14.5 Ma, a shift in power from the $\sim$100- to the 406-ky band occurred in both the $\delta$$^{18}$O and $\delta$$^{13}$C records. Band-pass filtering of these records reveals an increase in the amplitude of the 406-ky eccentricity cycle at 14.2 Ma and a decrease in the amplitude of the 100-ky eccentricity cycle at 14.5 Ma, even though the calculated eccentricity signal was relatively stable throughout the interval. Changes in eccentricity power and amplitude occur before the global $\sim$1$\permil$ $\delta$$^{18}$O increase at 13.9 Ma, suggesting that strong climate feedbacks were involved in the MMCT. Sensitivity to long-period eccentricity forcing increased at 14.2 Ma, immediately following peak warmth of the Miocene climatic optimum ($\sim$17-14 Ma). The shift in sensitivity may have resulted from changes in global carbon cycling associated with reorganization of the climate system, specifically a tectonically mediated reduction in meridional heat/vapor transport related to the constriction of the eastern Tethys Seaway. Inferred low atmospheric {\it p}CO$_{2}$ levels may have further enhanced the global response to this reorganization.
DE: 9604 Cenozoic
DE: 4806 Carbon cycling
DE: 4207 Arctic and Antarctic oceanography
DE: 4267 Paleoceanography
DE: 1050 Marine geochemistry (4835, 4850)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting