HR: 17:20h
AN: PP42D-05    [PDF]
TI: Origin of Tropical Obliquity-Dominated Sequences and Secular Trends in Deep Ocean Chemistry
AU: * Norris, R D
EM: RNorris@ucsd.edu
AF: Scripps Institution of Oceanography, 308 Vaughan Hall, La Jolla, CA 92093 United States
AU: Roehl, U
EM: uroehl@allgeo.uni-bremen.de
AF: Bremen University, Geosciences Department, Bremen, D-28334 Germany
AU: Bice, K L
EM: kbice@whoi.edu
AF: Woods Hole Oceanographic Institution, Mail Stop #23, Woods Hole, MA 02543 United States
AB: A long-standing puzzle is the presence of a strong obliquity cycle in Paleogene tropical sediment cores, since orbital theory predicts that precession should be the major signal in tropical deposits while obliquity should predominate at mid and high latitudes. Drilling in tropical and warm subtropical sequences of Paleogene and Cretaceous age shows that precession-dominated and obliquity-dominated intervals may be present in the same cores. Data are based on XRF core scanning of Paleogene sections in cores form several Atlantic sites, including recently drilled, Leg 207 sites. The seeming alternation of high and low latitude signals in tropical deposits at least partly owes its origin to variations in the extent to which the record reflects surface and bottom water processes. Because bottom waters are formed mostly at high latitudes, their flow strength, corrosiveness, and other physical properties should carry a dominant obliquity signal which can be superimposed on precession-dominated tropical sedimentary records. When the sedimentary record is not highly modified by benthic processes, the precession signal dominates tropical sedimentary records, but where there are changes in deep water flow strength (which may winnow a deposit) or corrosiveness of bottom water (which dissolves surface-derived carbonate), then the mid/high latitude orbital period predominates. Obliquity signals also dominate in Neogene and late Paleogene records in part because of the onset of polar ice growth. We might expect tropical records to show some depth dependence in the kinds of orbital signals that are recorded in sediments, with shallower, less dissolved, sequences displaying a stronger precessional cycle than deeper, more dissolved sequences. The general tendency for tropical Cretaceous and early Paleogene deep sea sediments to display precessional dominance and later Paleogene and Neogene sediments to display obliquity dominance suggests an increase in deep water corrosiveness over the Cenozoic. This increase started in the middle Eocene in agreement with global estimates of changes in the depth of the carbonate compensation depth.
DE: 3319 General circulation
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 9604 Cenozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting