HR: 0830h
AN: PP21B-1175    [PDF]
TI: Isotopic and Climate Model Constraints on Paleo-CO$_{2}$ in the Late Paleozoic
AU: * Grossman, E L
EM: e-grossman@tamu.edu
AF: Texas A&M University, Texas A&M University, Dept. of Geology and Geophysics, College Station, TX 77843 United States
AU: Hyde, W T
EM: wthyde@duke.edu
AF: Duke University, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708 United States
AU: Pollard, D
EM: pollard@essc.psu.edu
AF: Pennsylvania State University, EMS Environment Institute, Pennsylvania State University, University Park, PA 16802 United States
AU: Scotese, C R
EM: chris@mail.scotese.com
AF: University of Texas at Arlington, Department of Geology, University of Texas at Arlington, Arlington, TX 76019 United States
AB: Atmospheric CO$_{2}$ is one of the most important drivers controlling ancient climate and one of the hardest to quantify. We have combined three methods for quantifying paleoclimate, a coupled energy balance-ice sheet model (EB/ISM), an atmospheric general circulation model (AGCM), and oxygen isotope analyses of fossils, to constrain late Paleozoic pCO$_{2}$ levels. Our estimated pCO$_{2}$ is that which yields the same ice volume determined using two independent approaches, a $\delta$$^{18}$O-AGCM method and an EB/ISM. We calculate ice volume from the $\delta$$^{18}$O of brachiopod shells and AGCM temperatures ($\delta$$^{18}$O-AGCM method). Brachiopod shell $\delta$$^{18}$O values depend on two variables, ambient temperature and seawater $\delta$$^{18}$O. Using the oxygen isotope paleotemperature equation and ambient temperatures derived from AGCM results, we calculate seawater $\delta$$^{18}$O. From this seawater $\delta$$^{18}$O we use $^{18}$O mass balance to calculate ice volume. We run the AGCM with various values of pCO$_{2}$, which produce different temperatures and different $\delta$$^{18}$O-derived ice volumes. Ice volumes deduced from brachiopod $\delta$$^{18}$O increase with pCO$_{2}$. Ice volumes as a function of pCO$_{2}$ are also determined from the ice sheet model in the EB/ISM, and those ice volumes decrease with increasing pCO$_{2}$. Our estimated pCO$_{2}$ is the intersection of the two ice volume-pCO$_{2}$ curves. Three different time slices and paleogeographies have been investigated in detail: 360, 320, and 280 Ma. GENESIS 2 AGCM simulations were performed at 1x and 4x modern preindustrial levels (280 ppm) for all time slices, and at 8x pCO$_{2}$ for 360 Ma. EB/ISM simulations were run with and without topography, with lapse rates of 5 and 7 $\deg$C/km, and with outgoing infrared radiation (OIR) ranging from 187.3 to 205.3 W/m$^{2}$, equivalent to pCO$_{2}$ levels of 1x to 16x. EB/ISM simulations yielded ice volumes ranging from 0 to greater than 129 x 10$^{6}$ km$^{3}$, depending on lapse rate, topography, and outgoing IR radiation. The highest ice volumes were obtained with topography, 7 $\deg$C/km lapse rate, and high OIR. The 320 Ma paleogeography generated the largest ice volume for a given input set. Interestingly, this is approximately the timing of initiation of major Carboniferous glaciation. Combining brachiopod $\delta$$^{18}$O values for North America and the Russian Platform with AGCM temperatures yielded ice volumes of 18 to 89 x 10$^{6}$ km$^{3}$ for 320 and 280 Ma, depending on pCO$_{2}$. Isotopic results for 360 Ma, a time generally considered to be ice-free, produced negative ice volumes. Using topography and a realistic lapse rate of 5 $\deg$C/km, the EB/ISM and oxygen isotope models for 320 and 280 Ma generated similar ice sheet volumes at 2x to 3x pCO$_{2}$. These values are similar to or slightly higher than results from geochemical models and pCO$_{2}$ proxies. Work is ongoing to explore variation in brachiopod $\delta$$^{18}$O values, and to investigate ice sheet volumes generated at different lapse rates and from uncoupled AGCM/ice sheet simulations.
DE: 1620 Climate dynamics (3309)
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 4870 Stable isotopes
DE: 9614 Paleozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting