Magnetic polarity stratigraphies for Sites 980-984 are based on shipboard measurements from the pass-through magnetometer after alternating field (AF) demagnetization at a peak field of 25 mT and shore-based stepwise AF demagnetization of discrete samples. The characteristic magnetization component was determined after AF demagnetization removed the steep downward drill-string-related magnetic overprint. Peak AF fields in the 20-30 mT range were required to resolve the component, carried by magnetite, that was used to produce unambiguous Pliocene-Pleistocene magnetic stratigraphies at all five sites. At Sites 980 and 983, magnetic stratigraphies were resolved to the base of the recovered advanced hydraulic piston corer (APC) section, which lies in the Matuyama Chron (1r.2r) and Olduvai Subchron (2n), respectively. At Sites 981 and 982, magnetization intensities decrease sharply in the normal polarity zone corresponding to the Gauss Chron (2An), and magnetic stratigraphies below this level could not be resolved. At Site 984, the resolution of magnetic stratigraphy was curtailed at ~250 meters below seafloor (Olduvai Subchron) by core deformation at the base of the APC section and in the underlying extended core barrel section.
As the magnetic stratigraphies at all four sites are unequivocal, polarity chron interpretations can be made without aid from the biostratigraphy. Mean sedimentation rates within polarity chrons have been calculated and Pliocene-Pleistocene biomagnetostratigraphic correlations tested.
1Raymo, M.E., Jansen, E., Blum, P., and Herbert, T.D.
(Eds.), 1999. Proc. ODP, Sci. Results, 162: College Station, TX (Ocean Drilling
Program).
2Department of Geology, University of
Florida, Gainesville, FL 32611-2120, U.S.A. jetc@nersp.nerdc.ufl.edu
3Laboratoire
des Sciences du Climat et de l'Environnement, CNRS-CEA, Avenue de la Terrasse, 91198
Gif-sur-Yvette, France.
Date of initial
receipt: 10 September 1997
Date of acceptance: 11 June 1998
Ms 162SR-002