Significance of fine-grained sediment lofting from melt-water generated turbidity currents for the timing of glaciomarine sediment transport into the deep sea

Abstract

Sediment lofting from turbidity currents is a process that occurs in density currents generated from fresh-water discharges into the sea and other gravity currents that can produce reversed buoyancy. Settling of suspended sediment from the top or deposition from the bottom of the flows may lower the density below that of ambient seawater causing the currents to lift up from their substrate either in part or as a whole through buoyancy reversal, as is well known from experiments. Fine-grained sediment lofting on a large scale during times of Heinrich events has been inferred for ice-proximal regions of the northwest Labrador Sea adjacent to the margin of the Late Pleistocene Laurentide Ice Sheet (LIS). The evidence comes from a distinct depositional facies of stacked graded mud layers containing ice-rafted debris (IRD). The significance of the lofted fine-grained depositional facies arises from its genetic link to large sand and gravel transporting turbidity currents that were generated by melt-water discharges from the Hudson Strait outlet of the LIS and are the likely source of the lofted sediment. The observed lofted depositional facies is exclusively found in Heinrich layers. Through this stratigraphic relationship the lofted facies ties the main pulses of sediment supply in the Labrador Basin to Heinrich events. Heinrich events thus appear to be the times of the main depositional activity in the Labrador Sea, a hitherto unknown aspect of these phenomenal events besides their known role as intense iceberg discharges.