PALEOCEANOGRAPHY AND DISTAL PLUME SEDIMENTATION ASSOCIATED WITH HEINRICH EVENTS: EASTERN CANADIAN MARGIN

Heinrich Events (HE) on the eastern Canadian margin represent rapid episodic iceberg discharges and melt-water plumes during the late Quaternary from Hudson Strait. These HE deposited poorly sorted sediment, rich in detrital carbonate with few foraminifera. Such beds occur throughout the Labrador Sea and NW Atlantic. This broad range has previously been ascribed to an increase in the velocity of the Labrador Current combined with suppressed flocculation due to high carbonate content.

The mean sortable silt (10-63 mm) current proxy; however, suggests that the Labrador Current was weak or absent during HE. A 25-ka current proxy record established from cores on the northern Grand Banks margin indicates that the modern Labrador Current was initiated ~7 ka and has steadily increased in intensity to the present. The record captures three HE over which no increases in mean sortable silt grain size is observed.

In large river plumes, which are a modern analogue for the melt-water plumes associated with HE, flocculation removes most of the fine sediment from suspension within 10 km of river mouth. After this initial rapid loss, low sediment concentrations limit flocculation; thus, slowing the removal of fine sediment. The remaining sediment in the diluted plume may travel hundreds of km, as observed through satellite imagery. Grain size analyses of HE 1 (14.5 ka) within cores along the Labrador, Newfoundland and Scotian margins indicate that these units fine distally, indicating that iceberg input is reduced and that melt-water plumes become the primary source of sediment for these units along the distal margins. Scaled analysis calculations indicate that it is possible for a small but significant fraction of plume sediment to travel several orders of magnitude farther from source than the flocculated sediment. The time required to produce observed thicknesses of HE beds, given the assumed sediment and plume properties, is consistent with time scales calculated for HE durations by other methods.

New studies of detrital petrography of sediment in piston cores from the Scotian Margin show distinct H1, H2, and possibly H3 as far SW as 67º W. These do not require the presence of a strong paleo- Labrador Current: the dynamics of a freshwater plume, fed along the entire paleo-ice margin, is sufficient to produce observed distribution.