Rapid Climate Oscillations of the Last Glacial Cycle in the Western North Atlantic
L D Keigwin (WHOI, Woods Hole, MA 02543; Internet: LKeigwin@whoi.edu); E A Boyle (MIT, Cambridge, MA 02139; Internet: email@example.com)
Sediment drifts in the western North Atlantic preserve a high resolution
geological record of the climatically important exchanges of heat, salt and water with other ocean basins. As yet, however, the exact linkages between changes in surface ocean hydrography, deep ocean hydrography and high latitude air temperature have not been defined for millennial and century timescales in this region. This talk will focus on the history of marine isotope stage 3 on the Bermuda Rise (BR) in the northern Sargasso Sea. In particular, we will address three observations: (1) In a 4 m interval of BR core GPC-5, ten oscillations in the d18-O of the planktonic foraminifer G. ruber (amplitude ~1o/oo), indicate ~4 degree temperature change, and correlate with interstadial (IS)/stadial cycles 7-16 in Greenland ice (~35 to 60 ka). Stage 3 maxima are close to the d18-O of the last glacial maximum (LGM) on the BR. Making reasonable assumptions about sea level change during stage 3, it appears that sea surface temperature during stadial events must have been colder than during the LGM. (2) Sedimentation during stage 3 on the BR was variable and repetitive. Although 14C dating of % CaCO3 maxima suggests a constant sedimentation rate of ~20 cm/ka, each stadial event is marked by low bulk density. This suggests high clay flux and very high sedimentation rates, as occurred during the LGM and the Little Ice Age (the most recent millennial-scale climate event). Heinrich events 4 and 5 are clearly identified at the end of IS 9 and 13 as peaks of ice rafting of ~1/gram. H3 and H4 lasted at most a few centuries in the northern Sargasso Sea but the exact duration of these and other events must await new Th-230 data. (3) Although benthic foraminiferal abundance is variable, d13C results are complete enough to show low values during each stadial event with an amplitude of nearly 1o/oo. Some 13C depletion may result from reduced production of lower North Atlantic Deep Water, and some may be an artifact of a higher carbon rain rate during stadials. New trace metal data will be used to factor out these and other variables.