HR: 0800h
AN: PP41B-0544    [Abstracts]
TI: Multi-proxy reconstruction of deep sea calcite dissolution from the subtropical South Atlantic Ocean: foraminifer fragmentation, shell weight and Mg/Ca
AU: Francois, R
EM: rfrancoi@eos.ubc.ca
AF: University of British University of British Columbia, Department of Earth and Ocean Sciences, Vancouver, BC V6T 1Z4, Canada
AU: * Mekik, F
EM: mekikf@gvsu.edu
AF: Grand Valley State University, Department of Geology, Allendale, MI 49401, United States
AU: Noll, N
EM: nolln@student.gvsu.edu
AF: Grand Valley State University, Department of Geology, Allendale, MI 49401, United States
AB: Tracing post-depositional deep sea calcite dissolution with planktonic foraminifers is challenging because their test structure and composition are often affected by environmental conditions in calcification waters. We attempt to resolve this problem by using a multi-proxy approach on core top sediment samples from a depth transect on the Rio Grande Rise (RGR) where surface ocean parameters are mostly uniform among our sampling locations. We find that we can expand the calibration range of the Globorotalia menardii fragmentation index (MFI) from the tropical Pacific to the subtropical Atlantic. We also demonstrate that the fragmentation trend of Globorotalia truncatulinoides, Neogloboquadrina dutertrei and Neogloboquadrina pachyderma follow both MFI and the drop in bottom water [CO3=]. The drop in Mg/Ca in G. truncatulinoides (a thermocline-dweller commonly found in Quaternary subtropical Atlantic sediments) follows the decrease in bottom water [CO3=] and, thus, supports MFI's dissolution trend. This observation bodes well for developing this tracer as an alternative means of quantifying downcore variations in calcite dissolution. Mg/Ca in G. menardii, another thermocline dweller, is largely insensitive to temperature and reflects mostly post-depositional calcite dissolution. If that also holds true for G. truncatulinoides, we would be able to use its Mg/Ca to assess carbonate dissolution in the glacial sections of Atlantic sediments, where the disappearance of G. menardii prevents the use of MFI. In contrast to all of these proxies, the loss of size normalized shell weight in G. truncatulinoides and Globorotalia inflata follows neither the trend of bottom water [CO3=] nor the other proxies on RGR.
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4825 Geochemistry
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting