Distribution, thickness and origin of Heinrich layer 3 in the Labrador Sea

The presence of Heinrich layer 3 (HL-3) in the northwest Labrador Sea has been debated in the literature. Calypso giant piston core MD99-2233, five new standard piston cores, and re-interpretation of 34 cores from previous cruises confirm the presence of HL-3 in the Labrador Sea. It is identified by high total carbonate concentration (up to 45%), an increase in coarse fraction content, and lighter δ18O values in polar species planktonic foraminifer Neogloboquadrina pachyderma (left-coiling) as low as 3.1‰. The age of HL-3 of ∼27 ka was bracketed in the various cores by about 50 14C-accelerator mass spectrometer dates. Where it is present in ice-proximal regions, it consists of nepheloid-flow deposits at the base and mud turbidites at the top. The thickness of HL-3 varies between 4.8 m (proximal to Hudson Strait) and 0.9 m (distal), decreasing rapidly seaward. On the upper continental slope, HL-3 was too deeply buried to be sampled. Elsewhere, HL-3 is absent in some cores, probably due to slumping or erosion associated with sandy turbidity currents or debris flows.