HR: 0830h
AN: GP31D-0777    [PDF]
TI: ODP Site 1092: Revised Composite Depth Section has Implications for Upper Miocene 'Cryptochrons'
AU: * Evans, H F
EM: geohelen@ufl.edu
AF: Department of Geological Sciences, University of Florida, 241 Williamson Hall PO Box 112120, Gainesville, FL 32611 United States
AU: Westerhold, T
EM: tho@uni-bremen.de
AF: Fachbereich Geowissenschaften Universit„t Bremen, Postfach 33 04 40, Bremen, 28334 Germany
AU: Channell, J E
EM: jetc@nersp.nerdc.ufl.edu
AF: Department of Geological Sciences, University of Florida, 241 Williamson Hall PO Box 112120, Gainesville, FL 32611 United States
AB: ODP Site 1092 in the sub-Antarctic South Atlantic produced an Upper Miocene magnetic stratigraphy in the C3r-C5ABn interval (Evans and Channell, 2003). New data from X-Ray Fluorescence (XRF) scanning of half cores at the University of Bremen has led to revised composite depths for the site. This has some implications for the magnetostratigraphic interpretation. Using the shipboard-derived composite section, four polarity subchrons were identified within the normal polarity zone correlative to C5n.2n. The revision of the composite section indicates that there was a duplication of polarity subzones that is an artifact of the miscalculation of shipboard composite depths. Polarity subchrons ("cryptochrons") that were originally labeled C5n.2n-2 (recorded in core 1092A-13H) and C5n.2n-3 (recorded in core 1092C-14H) become a single subchron (relabeled as C5n.2n-2). The resulting modification of the composite section provides new composite depths for polarity zone boundaries at Site 1092 with new age estimates for subchrons not included in the standard geomagnetic polarity timescale (GPTS). Using the ages of the bounds of C5n.2n from Cande and Kent (1992, 1995) and assuming constant sedimentation rate within C5n.2n, the age estimates are C5n.2n-1 at 10.098 Ma, C5n.2n-2 at 10.258 Ma and C5n.2n-3 at 10.803 Ma.Normalized remanence (mean NRM/IRM), used as a proxy for geomagnetic paleointensity in Evans and Channell (2003), can also be well correlated between cores 1092A-13H and 1092C-14H after revision of the composite depths, thereby ratifying the adjustment of the shipboard composite section. The revised number of polarity subchrons within C5n.2n is now three, rather than four, consistent with the number of "cryptochrons" recognized in marine oceanic anomaly data.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting