HR: 0800h
AN: V51A-0513    [Abstracts]
TI: Variations in the marine Ca cycle and implications for paleo-CO$_2$ levels over the past 24 Ma
AU: * Fantle, M S
EM: mfantle@eps.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Depaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science 307 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Depaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Earth Sciences Division, E.O. Lawrence Berkeley National Laboratory MS BLDG 70A-4418, Berkeley, CA 94720 United States
AB: A detailed record of the calcium isotopic composition of bulk nannofossil ooze from DSDP Site 590, based on 40 samples measured multiple times, shows variations of $\delta^{44}$Ca over the past 24 million years between --0.2 and --0.9$\permil$, relative to bulk Earth Ca (--1.15 to --1.85$\permil$, relative to seawater). These isotopic variations are inferred to reflect changes in $\delta^{44}$Ca of seawater between +1.1 and +0.4$\permil$. Fluctuations in $\delta^{44}$Ca, which occur in 1 to 4 Ma cycles with varying amplitude, are interpreted as resulting from imbalances between the input of Ca to the oceans by weathering processes and the biogenic removal of Ca. Using a model for oceanic inputs and outputs of Ca, we reconstruct past weathering fluxes and marine Ca concentrations. Combining this information with paleo-pH of the ocean, as estimated from B isotopes, we derive a record of atmospheric $pCO_2$ levels over the past 24 Ma. The concentration of Ca in the ocean was at a minimum at $\sim$20 Ma, during the Neogene climate optimum, increased until 6 Ma, and then decreased toward the present. Maxima in the record occur at 6.4 and 4 Ma, corresponding to a generally-recognized period of enhanced marine productivity and high mass accumulation rates at Site 590. Peaks in the inferred weathering flux occur at 18, 14, 8, 5, and 1 Ma. Our derived $pCO_2$ record indicates that the highest $CO_2$ levels during the 24 Ma period under consideration occurred at 20--24 Ma and were no more than about twice the present-day (pre-industrial) levels. The maximum in the $CO_2$ record occurs during the warming that accompanied the Neogene climate optimum. The $pCO_2$ minimum occurs at 6--4 Ma, during a period of enhanced marine productivity. The calculated marine paleo-Ca and paleo-$pCO_2$ values are sensitive to the assumed $\delta^{44}$Ca value for the weathering flux, but the shapes of the derived curves do not change greatly over the range of likely input values. The reconstructed $pCO_2$ curves depend significantly on pH, but the assumption of constant pH (which only barely violates the available constraints from B isotopes) yields substantially the same result. Although the paleo-$pCO_2$ curve is likely to be revised when more data are available, the present results do not show a simple linear relationship between $pCO_2$ and high-latitude temperature as reflected in the benthic oxygen isotope records.
DE: 4805 Biogeochemical cycles (1615)
DE: 6006 Atmospheres--evolution
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting