HR: 17:15h
AN: OS34A-06    [Abstracts]
TI: Modeling Deep Sea Delta Calcite in the Eastern Equatorial Pacific Over the Last 25,000 Years: Normalizing Sedimentation Rate with Excess 230-Thorium Activity, or Not?
AU: * Mekik, F A
EM: mekikf@gvsu.edu
AF: Grand Valley State University, Department of Geology, Allendale, MI 49401 United States
AB: Accurately reconstructing deep sea delta calcite from the marine sedimentary record is important as a means for [1] assessing causes for millennial changes in atmospheric pCO2 and [2] tracing changes in the organic carbon to calcite rain ratio to the deep sea. Reconstructing paleo-delta calcite requires a reliable calcite dissolution proxy as well as correcting for lateral sediment focusing when calculating carbonate accumulation rate [CAR]. Here we demonstrate the impact of using 230Thorium normalized CAR estimates in reconstructing delta calcite versus using traditional CAR estimates. We reconstructed delta calcite in four cores from the eastern equatorial Pacific using the sedimentary diagenesis model, Muds. We used reverse modeling with the following input parameters: water depth, calcite flux, organic carbon flux, and percent calcite in bulk sediment. Organic carbon flux was estimated with Loubere's benthic foram productivity transfer function. Vertical calcite flux was estimated by dividing 230Th-normalized CAR by percent calcite preserved calculated with the G. menardii Fragmentation Index. Our modeled delta calcite estimates show a change of about 20umol/kg between the LGM and present in the four cores studied when 230Th-normalized CAR estimates are used. Further, delta calcite remains more or less constant in each core between the LGM and 8-10kyr and then shows a dramatic drop until present values are reached. When non-230Th corrected CAR values are used, the downcore delta calcite records diverge widely among cores and suggest changes in delta calcite between LGM and the present over 100umol/kg. These results indicate that it is essential to correct for sediment redistribution by normalizing to 230-Th.
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 1635 Oceans (1616, 3305, 4215, 4513)
DE: 4231 Equatorial oceanography
DE: 4806 Carbon cycling (0428)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005