LITHOLOGIC CHANGES IN LABRADOR SEA HEINRICH LAYERS AS A FUNCTION OF DISTANCE FROM THEIR HUDSON STRAIT ICE-STREAM SOURCE

During Late Pleistocene Heinrich events distinct, decimetre to centimetre thick layers of ice-rafted debris (IRD) were deposited in North Atlantic that are characterized by high detrital carbonate concentration, low foraminifera content, a high percentage of Neogloboquadrina pachyderma (sinistral) among the planktonic foraminifera, high magnetic susceptibility and high grey colour values. In the Labrador Sea, Heinrich layers (H-layers) reach meter thickness in some proximal core sites near the iceberg source off the Hudson Strait ice stream (HSIS) and show low magnetic susceptibility and low grey levels on the colour scale. The great thickness of ice-proximal Labrador Sea H-layers is due to the supply of vast amounts of terrigenous sediments eroded from the country rocks underlying the north-eastern sector of the Laurentide Ice Sheet (LIS) and transported to the deep-sea by processes much more efficient than ice rafting involving sediment lofting and low-density turbidity currents. Four distinct depositional facies termed Heinrich layers type I to IV have been differentiated: Type I H-layers occur within 300 km from the presumed HSIS terminus and consist of stacked thin layers of graded muds containing IRD. The graded muds spiked with IRD resulted from the deposition of fine-grained lofted sediment that collected dropstones and –grains under the iceberg drift route. Sediment lofting occurs when fresh-water generated turbidity currents rise buoyantly to the surface after having lost sediment by deposition. Type II and III H-layers on the levees of the Northwest Atlantic Mid-Ocean Channel (NAMOC) and its tributary canyons occur at greater distance from the Hudson Strait outlet on the slope and rise south of the strait . They consist of alternations of thin mud turbidites with intercalated laminae or layers of IRD, respectively. Type IV is made up of bioturbated hemipelagic muds with coarser IRD and occurs in regions between canyons not reached by spill-over turbidity currents and in the distal open ocean or on seamounts. Isopach maps for H-layers 1-3 give hints on the drift routes of the lofted suspended sediment during its ascent to the surface and on iceberg drift directions in the Labrador Sea.