HR: 10:20h
AN: PP22A-01 INVITED     [Abstracts]
TI: The Dispersal of the 8.2 ka Lake Agassiz Drainage Waters Into the NW Atlantic: Working Hypothesis
AU: * HILLAIRE-MARCEL, C
EM: chm@uqam.ca
AF: GEOTOP - UQAM & McGill, CP 8888, succursale Centre-Ville, Montreal, Qc H3C 3P8 Canada
AU: de VERNAL, A
EM: devernal.anne@uqam.ca
AF: GEOTOP - UQAM & McGill, CP 8888, succursale Centre-Ville, Montreal, Qc H3C 3P8 Canada
AU: WEAVER, A
EM: weaver@uvic.ca
AF: Earth and Ocean Science University of Victoria, PO Box 3055, Victoria, BC V8W 3P6 Canada
AU: FISHER, D A
EM: Fisher@nrcan.gc.ca
AF: Geological Survey of Canada, 601 Booth Street, Ottawa, Ont K1A 0E8 Canada
AU: SOLIGNAC, S
EM: solignac.sandrine@courrier.uqam.ca
AF: GEOTOP - UQAM & McGill, CP 8888, succursale Centre-Ville, Montreal, Qc H3C 3P8 Canada
AB: The so-called "8.2 ka event", often associated with the drainage of glacial Lake Agassiz through Hudson Strait into the Labrador Sea that occurred some 8.4 ka ago, has been "seen" in various types of records. Absent in the Antarctica ice records, it appears in some Arctic ice records but not others. It is clearly recorded in the GRIP and North GRIP records, although with distinct features (notably a reverse deuterium-excess trend, suggesting that a proximal source of moisture could be involved; V. Masson pers. com.). Several paleoclimate sequences from Western Europe depict highly variable climate oscillations that are often attributed to this event. Such oscillations generally differ in their duration (from less than 40 years up to several hundreds of years) and, in many cases, do not differ significantly from the subsequent climate oscillations of the Holocene. Elsewhere, continental evidence for a major climate event is often unclear or questionable. Most scientists evoke a collapse or a strong reduction in the rate of the Atlantic Meridional Overturning (AMO) as an explanation for their observations, thus linking the pulses of freshwater release, the thermohaline circulation (THC), and the atmospheric response that would have resulted in largely spread "climatic anomalies". Some model experiments seem to support this scenario, but they have not necessarily been run with realistic freshwater release rates or routes. Most however would not allow a short "THC-excursion" thus raising concern about a less than 40 yr-event as reported in some records. Therefore, some strong marine "evidence" for at least a reduction of the AMO is needed. Unfortunately, most marine records have a time resolution unsuitable to ascertain a short duration reduction of the AMO, and/or are smoothed due to benthic mixing, thus dampening high frequency signals into the stochastic noise. When reported in marine records from the North Atlantic, the presence of this 8.2 ka event seems generally inconclusive. However, off Hudson Strait, near the outlet of the final drainage of Lake Agassiz, a few marine records show a very high time resolution (up to 15 cm/year), and minimum benthic mixing. In these records, some evidence for a very short "sedimentological event" has been found, but none show any indication for a significant concomitant change in sea-surface salinity or temperature, based on micropaleontological or isotopic data. The question thus arises of the fate of the near 160 * 103 km3 of drainage waters released into the Labrador Sea, possibly in a couple of years, based on glaciological models. We hypothesize here that this water merged into the Labrador Sea, after a long subglacial tunnel trajectory, as a dense turbiditic flow, with possibly two spreading routes: one surface branch, nearshore, on the Labrador Shelf, and a deep underflow branch flowing practically along the NAMOC route. In both cases, the influence on open ocean sea surface conditions would have been negligible. In this scenario, the opening of Hudson Bay and the final collapse of the Laurentide ice sheet into two small residual ice caps would have resulted in a major atmospheric reorganization that could explain both long distance correlative climate responses and the influence of more proximal moisture sources indicated by the GRIP records.
DE: 9325 Atlantic Ocean
DE: 3344 Paleoclimatology
DE: 1040 Isotopic composition/chemistry
DE: 1620 Climate dynamics (3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting