HR: 0830h
AN: PP31B-0259    [PDF]
TI: Deep Ocean Circulation Changes During the Transition to the Last Ice Age
AU: * Zylberberg, D R
EM: dzop@dartmouth.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: * Zylberberg, D R
EM: dzop@dartmouth.edu
AF: Dartmouth College, Dept. of Earth Sciences, Hanover, NH 03755 United States
AU: Piotrowski, A M
EM: arcturus@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AB: The transition between marine isotope stages (MIS) 5a and 4 appears in the stacked benthic foraminferal $\delta^{18}$O SPECMAP record as a gradual increase in ice volume. In contrast, the transition occurs in the Greenland ice core $\delta^{18}$O records with two well-developed interstadial events (I19 and I20), which are the first Dansgaard-Oescheger events of the last ice age. The MIS 5b/5a transition appears as a much more rapid warming in both the Greenland ice and benthic $\delta^{18}$O records. Recent work (Lehmann et al. 2002, Chapman et al. 1999) indicates that climate variability in MIS 5 as indicated in the Greenland ice record was closely interconnected with iceberg discharges, surface temperature changes, and deep ocean circulation in the North Atlantic. In order to determine the response of deep ocean circulation to climate changes from late in MIS 5 to full glacial MIS 4, we have measured Nd isotope ratios from the Fe-Mn portion of core TNO57-21 from the Cape Basin in the South Atlantic. Nd isotopes, unlike nutrient water mass proxies, are not affected by biological fractionation, and reflect the strength of the North Atlantic Deep Water (NADW) signal in the seawater above the core site. Results from cores TNO57-21 and RC11-83 (also from the Cape Basin) indicate that the NADW export to the Southern Ocean has varied on time scales reflecting glacial-interglacial cycles through MIS 4 (Rutberg et al. 2000) and during interstadial events through MIS 3 (Piotrowski et al. Fall AGU), and was stronger and weaker during warmer and colder Northern Hemisphere climate intervals, respectively. The extension of the Nd isotope record to MIS 5a and 5b indicates an increased NADW signal during MIS 5, therefore the long-term pattern of strong and weak NADW export during warm and cold periods persists beyond the last ice age. The Nd isotope pattern during MIS 4 through 5b generally corresponds to the benthic foraminferal $\delta^{13}$C record from Cape Basin cores (Ninnemann et al. 1999), indicating that the pattern of the carbon isotope record also generally reflects ocean circulation changes. Over the transition to the last ice age (MIS 5a to 4) the NADW signal rapidly decreases toward LGM levels, and displays a smaller decrease between the MIS 5b and 5a peaks. However, during MIS 4 the NADW signal is stronger than during the LGM, and during the MIS 5a and 5b peaks it is weaker than during the Holocene. There is a larger degree of millennial-scale variability in THC intensity during MIS 5a than MIS 4, a pattern that is also observed in MIS 3 and 2, respectively (Piotrowski et al. Fall AGU). The sharp decrease in NADW intensity over the MIS 5a/4 transition appears to correspond to the end of interstadial 19 at $\sim$70 ka, in contrast with the gradual increase in ice volume, which approached its maximum at that time. This may indicate that the system reached a threshold that forced a rapid change to a different ocean circulation mode.
DE: 1040 Isotopic composition/chemistry
DE: 1724 Ocean sciences
DE: 4207 Arctic and Antarctic oceanography
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting