The deep-sea record of rapid late pleistocene paleoclimate change and ice-sheet dynamics in Labrador sea sediments /

• Main Author: Rashid, Harunur
• Other Authors: Hesse, Reinhard (advisor), Piper, David (advisor)
• Format: Thesis
• Language: English
• Published: McGill University 2002
• Subjects: Geology; Paleoecology; Hudson; Hudson Strait; Ice Sheet; Labrador Sea
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=38540

Thirty eight piston cores and numerous seismic profiles collected by two R/V Marion Dufresne and several CSS Hudson cruises were examined and cores from other cruises reinterpreted in the course of this study of Late Pleistocene ice-rafting events (Heinrich events) in the Labrador Sea. In the northwestern Labrador Sea, close to the margin of the former Laurentide Ice Sheet, Heinrich layers are unusually thick. Thicknesses of up to 4.8 m result from deposition from nepheloid-flows and low-density turbidity flows and from iceberg melting. Heinrich layers formed in three stages, explained by a proposed model that also attempts to explain ice-sheet dynamics, ice-stream behavior, and sediment transfer mechanisms from the ice-sheet to the deep sea. Nepheloid-flow deposits seem to result largely from fresh water buoyantly rising out of giant turbidity currents generated at the Hudson Strait outlet. High-resolution stratigraphic records of oxygen isotopes, concentration of total carbonate, coarse-fraction content, and magnetic susceptibility, and 70 new 14C-AMS dates establish the presence of eight Heinrich events, H0 to H6 in the northwest Labrador Sea, including a new event, termed H5a (between H5 and H6). This refined chronology shows that the recurrence intervals between Heinrich events are fairly evenly spaced, with an average of ∼7 ka, as postulated by the binge and purge model for their origin. High-resolution oxygen isotope, concentration of total carbonate, and coarse-fraction content data suggest that both meltwater and iceberg supply increased during Heinrich events. This is the first time that increased meltwater supply by direct discharge from a major ice outlet during Heinrich events has been documented, because ice-proximal areas tend to be barren of foraminifera. The presence of H3 between 46° and ~63°N in the Labrador Sea, previously debated, has been proved. Thickness variations for H3 were compiled on an isopach map showing a maximum thickness of 4.8 m near Hudson Strait and thinning to 30 cm in the central Labrador Sea. H0 is present in upper Labrador slope cores, compared to other Heinrich layers which were found widely on the slope, rise, and in the basin. These findings suggest that during or prior to H0, the Hudson Strait ice-stream had retreated landward, so that glaciomarine sediments were delivered to the shelf and iceberg trajectories were following the course of the already established Labrador Current.