HR: 14:40h
AN: PP32C-05    [PDF]
TI: Estimates of Middle Miocene Ice Volume and Temperature Change from Southern Ocean Benthic Foraminiferal Mg/Ca and $\delta$$^{18}$O
AU: * Shevenell, A E
EM: ashevenell@umail.ucsb.edu
AF: Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA 93106 United States
AU: Kennett, J P
EM: kennett@geol.ucsb.edu
AF: Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA 93106 United States
AU: Lea, D W
EM: lea@geol.ucsb.edu
AF: Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA 93106 United States
AB: The middle Miocene global climate transition (16.8 to 12 Ma) represents a critical step in Cenozoic climate evolution, separating warmer early from cooler late Neogene climates. Between 14.0 and 13.8 Ma, Earth_s climate cooled abruptly, as manifested by the globally recognized $\sim$1$\permil$ $\delta$$^{18}$O increase. This increase is interpreted to reflect some combination of global cooling and increased Antarctic ice volume. Given limitations of $\delta$$^{18}$O as a temperature proxy, debate exists as to the relative proportions of ice growth and temperature change encompassed in the middle Miocene $\delta$$^{18}$O increase. Foraminiferal Mg/Ca ratios provide an independent geochemical temperature proxy that should assist with separating temperature and ice volume signals contained in middle Miocene $\delta$$^{18}$O records. Such separation is fundamental to improved understanding of mechanisms forcing this major Cenozoic climate reorganization. Previous studies of benthic foraminiferal Mg/Ca at relatively low resolution across the middle Miocene $\delta$$^{18}$O increase indicate that global deep waters cooled $\sim$2-3$\deg$C and Antarctic ice volume increased by $\sim$30$%$. Thus, benthic foraminiferal Mg/Ca studies conducted at moderate to high resolution between 16.8 and 12 Ma may provide insight into feedbacks involved in Antarctic cryosphere expansion and global climate evolution. Studies conducted in the high Southern latitudes may also provide critical information concerning changes in meridional heat flux to the Southern Ocean. We have generated a benthic foraminiferal (${C. mundulus}$) Mg/Ca record from the South Tasman Rise (STR) in the Pacific sector of the Southern Ocean (ODP Site 1171: paleolatitude- 55$\deg$S, paleowater depth- 1600m) to assess the potential for using Mg-paleothermometry to reconstruct middle Miocene deep water temperatures. For the Mg/Ca temperature proxy to be successful on pre-Quaternary timescales, several factors are crucial: very well preserved foraminiferal CaCO$_{3}$ and the presence of fossil species for which there are modern analogues with Mg/Ca temperature calibrations. At Site 1171, down core variations in ${C. mundulus}$ Mg/Ca are similar to those observed in the paired $\delta$$^{18}$O record. Both benthic foraminiferal Mg/Ca and $\delta$$^{18}$O records exhibit significant variability during the middle Miocene Climatic Optimum (MCO; 16.5-14.0 Ma) and less after the major $\delta$$^{18}$O increase (13.7-12.5 Ma). We employ the ${Cibicidoides}$ sp. Mg/Ca-temperature calibration of Martin et al (2002) to estimate STR bottom water temperatures ($\sim$1600m) from the Mg/Ca record. Paleotemperatures suggest that bottom waters fluctuated between 5 and 8$\deg$C during the MCO. Across the middle Miocene $\delta$$^{18}$O increase (1.2$\permil$; 14.0-13.7 Ma) STR bottom waters cooled $\sim$1.5$\deg$C, from 6.5 to 5$\deg$C. Calculations of seawater $\delta$$^{18}$O ($\delta$$^{18}$O$_{sw}$) from ${C. mundulus}$ $\delta$$^{18}$O and Mg/Ca-derived temperatures suggest an average $\delta$$^{18}$O$_{sw}$ increase of $\sim$0.5$\permil$ between 14 and 13.8 Ma. This preliminary estimate suggests a major expansion of the Antarctic cryosphere was encompassed in the middle Miocene $\delta$$^{18}$O increase. While the exact phasing of ice volume and temperature change has yet to be determined, our results suggest that the reorganization of heat and moisture supply to the high southern latitudes played a major roll in forcing the middle Miocene climate reorganization.
DE: 4267 Paleoceanography
DE: 4835 Inorganic marine chemistry
DE: 9310 Antarctica
DE: 9604 Cenozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting