Ice-proximal Labrador Sea Heinrich layers: a sedimentological approach

Abstract

During Late Pleistocene Heinrich events (H-events), distinct, decimetre- to centimetre-thick layers of ice-rafted debris (IRD) were deposited in the North Atlantic as Heinrich layers (H-layers). These layers are characterized by high detrital carbonate content, low foraminifera content, a high percentage of Neogloboquadrina pachyderma (sinistral) among the planktonic foraminifera, high magnetic susceptibility, and high grey colour values. In contrast, H-layers in the Labrador Sea reach metre thickness at core sites proximal to the iceberg source off the Hudson Strait ice stream (HSIS), and show low magnetic susceptibility and relatively low grey levels on the colour scale. To provide the reader with some background information, four hypotheses concerning the origin of H-events are discussed at the outset: (1) the binge–purge (internal forcing) model, (2) the subglacial outburst flood model, (3) the external forcing model, and (4) the catastrophic ice shelf breakup model. The higher thickness of ice-proximal H-layers is due to the supply of large amounts of terrigenous sediments that were eroded from country rocks underlying the northeastern sector of the Laurentide Ice Sheet (LIS). These sediments were transported to the deep Labrador Sea by the efficient processes of bottom-following mass and surface plume movement, where they mixed with ice-rafted sediment. Four distinct depositional facies of H-layers (Types I to IV) have been identified: 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 that are spiked with IRD are the result of deposition of fine-grained sediment from lofting sediment columns that collected dropstones and grains under the iceberg route. Type II H-layers occur on the slope and rise at a greater distance south of the Hudson Strait outlet, on the levees of tributary canyons to the Northwest Atlantic Mid-Ocean Channel (NAMOC). These layers consist of alternating thin mud turbidites with intercalated laminae of IRD. Type III H-layers exist on the levees of the main channel of the NAMOC, and consist of layers of IRD alternating with fewer fine-grained spillover turbidites, reflecting the lower spillover frequency from the deep channel compared to the less deep slope canyons. Type IV H-layers are made up of bioturbated hemipelagic muds with IRD, and occur in regions between canyons not reached by spillover turbidity currents, and in distal regions of the open ocean or on seamounts. The anomalously high thickness of individual H-layers on the slope and rise off Hudson Strait is explained by the transport of significant portions of H-layer sediment by suspended sediment columns lofted from sand-carrying freshwater turbidity currents (Type I), and by low density turbidity currents (Types II and III). Isopach maps for H-layers 1–3 give hints of the drift routes of the lofted suspended sediment during its ascent to the surface, and of iceberg drift directions in the Labrador Sea.