HR: 14:55h
AN: OS33D-06    [Abstracts]
TI: Atlantic Ocean Circulation During the Last Ice Age: A 231Pa/230Th Record of Marine Isotope Stage 3
AU: * Major, C O
EM: cmajor@whoi.edu
AF: Woods Hole Oceanographic Institution, Dep't of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: McManus, J F
EM: jmcmanus@whoi.edu
AF: Woods Hole Oceanographic Institution, Dep't of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Keigwin, L D
EM: lkeigwin@whoi.edu
AF: Woods Hole Oceanographic Institution, Dep't of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Francois, R
EM: rfrancois@eos.ubc.ca
AF: University of British Columbia, Dep't of Earth and Ocean Sciences, Vancouver, BC V6T 1Z4 Canada
AU: Brown-Leger, S
EM: sbrownleger@whoi.edu
AF: Woods Hole Oceanographic Institution, Dep't of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Millennial-scale oscillations in climate-sensitive geochemical proxies are seen throughout Marine Isotope Stage 3 (MIS3, ~30 to 60 ka BP) in marine and ice-core records, with variability nearly as large as that seen on the last deglaciation. Nutrient proxies, such as δ¹³C, have been widely used to reconstruct water mass reorganizations associated with this so-called stadial-interstadial variability. While such passive tracers are invaluable in determining the configuration of water masses they cannot provide direct information about the rate of thermohaline circulation. The burial ratio of unsupported ²³¹Pa:²³⁰Th in bulk North Atlantic sediments is a function of the residence time of water in this basin, and thus serves as a dynamical proxy sensitive to the vigor of thermohaline circulation and an important complement to passive proxies. We present a suite of data, including ²³¹Pa/²³⁰Th, ²³⁰Th-normalized fluxes, and stable isotopes, from the Blake Outer Ridge (BOR) (core KNR140-8JPC, ~3400 m), a drift deposit underlying the deep western boundary current in the subtropical north Atlantic. ²³¹Pa/²³⁰Th data indicate moderate-amplitude millennial-scale variations in circulation rate during MIS3. ²³¹Pa/²³⁰Th ranges between maximum values less than the production rate ratio (0.093) and minimum values close to the core-top ratio (~0.058), with high and low ratios associated with stadials and interstadials, respectively. These data indicate continuous but on average less vigorous ventilation of the western basin compared to present, with substantially reduced circulation during stadials. Heinrich events during MIS3 do not appear to be associated with the particularly dramatic drop in circulation rate seen in the deglacial H1 interval on the Bermuda Rise (McManus et al., 2004). ²³⁰Th-normalized fluxes account for only 10 to 50 percent of the total mass accumulation, indicating significant sediment focusing at this site particularly during the stadials. Benthic δ¹⁸O from this western Atlantic site is similar to but of slightly greater amplitude than that documented at comparable water depth on the Iberian margin, where the low frequency variability appears more similar to the Antarctic ice core records (Shackleton et al., 2000) than to the planktic δ¹⁸O, which generally follows the northern hemisphere temperature record. The benthic δ¹³C in our core is consistent with a stronger southern source water influence at the core site during stadials, although productivity effects may overprint this signal. In all, the data from the BOR indicate increased residence time of water in the north Atlantic during stadials, and an apparent lag of deep water chemistry relative to the surface conditions.
DE: 4860 Radioactivity and radioisotopes
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4924 Geochemical tracers
DE: 4940 Isotopic stage
DE: 4962 Thermohaline
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005