GLACIAL-MARINE SEDIMENT RECORD OF NE LAURENTIDE ICE MARGIN ACTIVITY FROM 55-10 KA IN THE NW LABRADOR SEA

We analyzed lithostratigraphic parameters and the isotopic composition of planktonic foraminifera in a 12-m core from the upper continental slope off southeastern Baffin Island. Six C-14 dates constrain the chronology down to 35 ka at 717 cm depth. Detrital carbonate-rich (DC) layers corresponding to Heinrich layers 1, 2, 4, 5 and 5a are present. Heinrich layer 3 is present but has lower DC concentrations. In addition to Hudson Strait-sourced DC layers, the core contains numerous dark gray, DC-poor diamict layers enriched in total organic carbon (TOC). These diamicts are traced to a former ice stream source in Cumberland Sound based on provenance data from bulk Sr-Nd-Pb isotopic compositions and U-Pb ages of detrital zircons. Stratigraphic relationships between the TOC-rich diamicts and DC-layers suggest that the increases in ice discharge and/or sediment flux from Cumberland Sound occurred at different times than those from Hudson Strait. The presence of relatively thick and coarse-grained TOC-rich diamict layers interspersed among H4, H5 and H5a corroborates the hypothesis that the SE Baffin Island margin of the NE Laurentide ice sheet had advanced onto the continental shelf by the middle of Marine Isotope Stage 3. Planktonic foraminifera are sparse in the DC- and TOC-rich layers, but where present, oxygen isotopic compositions are depleted, consistent with increased iceberg and/or meltwater discharges during deposition of these units.

In a sample subset (n=53), we compared two common ice-rafted detritus (IRD) proxies: 1) the number of grains larger than 2 mm visible in 2 x 2 cm windows in core x-radiographs, and 2) the weight percent greater than 150 microns in sieved grain-size samples. In this environment the two proxies produce similar broad trends, but also show notable differences. This may result from the different sample sizes and/or slope processes that concentrate medium-coarse sand in the absence of IRD deposition. Peaks in detrital carbonate associated with Heinrich events do not always coincide with, and sometimes precede, peaks in the IRD proxies. The relatively IRD-poor nature of some H-layers in the NW Labrador Sea illustrates that these glaciological events involve more than a simple increase in the flux of icebergs and ice-rafted material.