HR: 13:55h
AN: PP53D-02 INVITED     [Abstracts]
TI: The evolution of atmospheric carbon dioxide since the middle Eocene: a biomarker perspective
AU: * Pagani, M
EM: mark.pagani@yale.edu
AF: Yale University, Department of Geology and Geophysics Yale University P.O. Box 208109 New Haven, CT 06520, New Haven, CT 06520 United States
AU: Zachos, J
EM: jzachos@es.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences Dept., Santa Cruz, CA 95064 United States
AU: Freeman, K H
EM: kate@essc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: The carbon isotopic fractionation that occurs during marine photosynthetic carbon fixation ($\epsilon$$_{p}$) is primarily a function of surface-water [CO$_{2aq}$], growth rate, and cell geometry. Therefore, if temporal variations in growth rate and cell geometry are minimal or constrained, $\epsilon$$_{p}$ values provide the potential to evaluate paleo-pCO$_{2}$ concentrations and trends. Alkenone-based $\epsilon$$_{p}$ ($\epsilon$$_{p}$$_{37:2}$) records have been used to estimate atmospheric carbon dioxide concentrations during the Miocene ($\sim$24-5 Ma). These records provide evidence that pCO$_{2}$ was substantially lower than previously anticipated and suggest that changes in CO$_{2}$ played a secondary role in forcing global climate change during the Neogene. Alkenone $\delta$$^{13}$C values ($\delta$$_{37:2}$) for the middle Eocene to the late Oligocene were measured from six ocean locations encompassing a range of growth environments (DSDP sites 511, 513, 516, 612, and ODP site 803). Data from sites 511, 513, and 803 are limited but overlap other records. Site 516 (Southwest Atlantic Ocean) represents the most continuous record ranging from the middle Miocene to the earliest Oligocene. The Eocene portion of the record is derived largely from site 612 (Northwest Atlantic Ocean). Our results reveal a clear secular trend in $\delta$$_{37:2}$ from the early Miocene to the latest Eocene ($\sim$45 Ma), with $\delta$$_{37:2}$ values leveling off to $\sim$-32 to -33.4$\permil$ by the latest Eocene. By $\sim$25-30 Ma and older, $\delta$$_{37:2}$ values are more negative than the lowest values recorded from similar environments in the modern ocean. $\epsilon$$_{p}$$_{37:2}$ values were calculated using the $\delta$$^{13}$C values of aqueous CO$_{2}$ estimated from the $\delta$$^{13}$C of coeval planktonic foraminifera or modeled from the isotopic composition of the $<$60 um fraction. Surface temperatures were estimated from the $\delta$$^{18}$O compositions of shallow-dwelling foraminifera and/or modeled from the isotopic composition of the $<$60 um fraction. All carbonates were assumed to be influenced by secondary carbonate and thus estimated temperatures were increased by $3\deg$ to $6\deg$C to account for diagenesis. In general, $\epsilon$$_{p}$$_{37:2}$ values track $\delta$$_{37:2}$ reaching maximum values of $\sim$23$\permil$ by the late Eocene. Atmospheric carbon dioxide concentrations can be estimated if we apply the modern calibration for $\epsilon$$_{p}$$_{37:2}$ as a function of surface-water [PO$_{4}$$^{3-}$] and [CO$_{2aq}$] and assume that the range of paleo-[PO$_{4}$$^{3-}$] for each site was similar to modern distributions. These criteria lead to minimum estimates of pCO$_{2}$ that are highly dependent on our assumed ocean temperatures. Our results suggest that middle Eocene atmospheric carbon dioxide concentrations were $\sim$1000 to 1500 ppmv. Atmospheric carbon dioxide concentrations appear to rapidly decline following the Eocene/Oligocene boundary reaching modern values near the end of the Oligocene.
DE: 4850 Organic marine chemistry
DE: 5407 Atmospheres--evolution
DE: 1040 Isotopic composition/chemistry
DE: 1635 Oceans (4203)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting