GLACIAL-MARINE SEDIMENT RECORDS OF NE LAURENTIDE ICE MARGIN ACTIVITY FROM 60(?)-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. The upper part of the core is well radiocarbon dated and contains detrital carbonate-rich (DC) layers correlative to Heinrich layers 1, 2 and 4. Heinrich layer 3 is present but has lower DC concentrations. In addition to Hudson Strait-sourced DC layers, the core contains dark gray, total organic carbon-rich (TOC), DC-poor diamict layers. These diamicts have been traced to a former ice stream source in Cumberland Sound based on comparisons of Sr-Nd-Pb isotopic compositions. Stratigraphic relationships between the TOC-rich diamicts and DC-layers suggest that ice discharges from Cumberland Sound occurred more frequently and at different times than those from Hudson Strait. Sediments underlying Heinrich layer 4 include at least two additional DC-rich layers, implying that H5 and H5a or H6 are present. Three TOC-rich diamict layers are present beneath H4. Planktonic oxygen isotope values are depleted in both types of layers, consistent with major iceberg and/or meltwater discharges during deposition. Both the DC layers and the TOC-rich diamicts are thicker in the lower, pre-H4 part of the core than those above H4. This observation is consistent with the hypothesis that the Laurentide ice sheet margin advanced farther seaward during the earlier part of the last glaciation. In any case it appears that the southeast Baffin Island margin of the Laurentide ice sheet delivered more sediment to the slope during the early part of Marine Isotope Stage 3 than during Stage 2. Using samples from the lower half of the core, we compared two commonly used proxies of ice-rafted detritus (IRD): 1) the number of grains larger than 2 mm visible in 2 x 2 cm windows in the core x-rays, and 2) the weight percent greater than 150 microns in sieved samples. In this environment the proxies deviate from one another, possibly due to slope sedimentation processes. Peaks in detrital carbonate associated with Heinrich events do not coincide with peaks in either IRD proxy. The IRD-poor nature of H-layers in the NW Labrador Sea illustrates that Heinrich events comprise more than simple iceberg discharges.