HESSE, Reinhard, Earth & Planetary Sci., McGill Univ., 3450 University St., Montreal, Quebec, Canada, H3A 2A7, rein_h@geosci.lan.mcgill.ca; and NAMOC study group.

A bird's eye view of the giant submarine drainage-system of the Northwest Atlantic Mid- Ocean Channel (NAMOC) of the Labrador Sea, provided by state-of-the-art HAWAII-MR1 sidescan sonar imagery, reveals the far-reaching marine effects of ice-cap drainage of the Pleistocene LIS in the deep-sea. The converging tributary system of NAMOC is the submarine continuation of the subglacial LIS drainage on land, the combined system being one of the largest interconnected land/sea drainage systems in the world. Large-scale mud- dominated and sand-dominated depositional systems (the NAMOC and a giant submarine braidplain, respectively) occur juxtapposed side-by-side to one another. This dichotomy is the result of grain-size separation on an enormous scale, induced by ice-margin sifting off the Hudson Strait Outlet through the south-flowing Labrador Current. Deposition of fine suspended sediment, dispersed by buyoantly rising or floating meltwater plumes, blanketed the slope south of the Hudson Strait, whose high relief was caused by retrograde canyon erosion. The muddy NAMOC system was sourced from this remobilized, predominantly fine-grained material. Coarse, sand-rich sediment supplied by bedload-rich meltwater discharges produced hyperpycnal flows on the low-relief slope off the Strait, which further basinwards formed the braidplain onlapping the left NAMOC levee, the most remarkable feature of the sidescan imagery. Periodic surging of the Hudson Strait Ice-stream, which may also have triggered extraordinary large, bedload-rich meltwater discharges through subglacial-lake outburst flooding, released unusual quantities of icebergs, which supplied the ice-rafted debris in Heinrich layers and caused short-term climate oscillations marked by times of warming followed by abrupt cooling. The new role of the LIS in deep-sea sedimentation and Pleistocene paleoclimate change can be seen as one of an active pacemaker, rather than passive recorder. Laurentide Ice-sheet, deep-sea channel system, subglacial outburst flooding, Heinrich events, Labrador Sea.