HR: 1340h
AN: PP13B-1486    [Abstracts]
TI: North Atlantic Deep Water Formation over the past 20 kyr as Recorded by Nd Isotopes in the Authigenic Fraction of Pelagic Sediments Along the Blake Ridge
AU: * Gutjahr, M
EM: gutjahr@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Zurich, 8092 Switzerland
AU: Frank, M
EM: mfrank@ifm-geomar.de
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Zurich, 8092 Switzerland
AU: Frank, M
EM: mfrank@ifm-geomar.de
AF: now at: IfM-GEOMAR, Leibniz Institute for Marine Sciences at the University of Kiel, Kiel, 24148 Germany
AU: Stirling, C H
EM: stirling@erdw.ethz.ch
AF: ETH Zurich, Institute for Isotope Geology and Mineral Resources, Zurich, 8092 Switzerland
AU: Keigwin, L D
EM: lkeigwin@maill.whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Halliday, A N
EM: alexh@earth.ox.ac.uk
AF: now at: Department of Earth Sciences, University of Oxford, Oxford, OX1 3PR United Kingdom
AB: Various lines of evidence, including the Nd isotope composition of Southern Atlantic seawater as reconstructed from authigenic ferromanganese coatings in pelagic sediments from the Cape Basin, suggest that the strength of the thermohaline overturning circulation in the North Atlantic was significantly reduced during the Last Glacial Maximum (LGM) as well as during Heinrich event 1 and the Younger Dryas. The Nd isotope composition and variability of these ferromanganese coatings was employed as a direct tracer for mixing between North Atlantic Deep Water (NADW) and Pacific-derived water masses in the Southern Ocean, despite the fact that no such record exists for the North Atlantic mixing end-member NADW. This study seeks to answer two major questions: First, can the temporal variability seen in the South Atlantic be identified in the North Atlantic? Second, was the Nd isotope composition of the interglacial NADW identical to the shallower Glacial North Atlantic Intermediate Water (GNAIW)? High-resolution LGM to present-day Nd isotope records have been extracted from the authigenic fraction in pelagic sediments from the Blake Ridge in the western North Atlantic (ca. 30°4N) from one shallow (1790 m) and two deep core sites (3410 m and 4250 m). Additionally, Holocene and LGM Nd isotope depth transects were produced for a direct comparison of the present-day water masses and the LGM situation. To avoid misinterpretations induced by sediment redistribution down-slope the Blake Ridge, ²³⁰Th-excess data were determined on samples from the shallow site. Our data suggest that southern source waters dominated the deeper North Atlantic at the Blake Ridge throughout most of the deglacial and that interglacial NADW-typical εNd signatures were not reached until after the Younger Dryas. During the LGM the two prevailing water masses along the Blake Ridge (i.e., GNAIW and southern source water) were virtually identical in Nd isotope composition, ranging from ca. -9.5 in εNd at 1700 m to ca. -10.5 in 4250 m. If these values are considered representative, then the Nd isotope composition of GNAIW was indeed different from the interglacial NADW by about 3 εNd units and could thus explain the observed South Atlantic Nd isotope variability over the past 20 kyr without a major weakening of deep water export from the North Atlantic.
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4924 Geochemical tracers
DE: 4936 Interglacial
DE: 4962 Thermohaline
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005