The magnetic signature of rapidly deposited detrital layers from the deep Labrador Sea: Relationship to North Atlantic Heinrich layers

Rock magnetic parameters from deep Labrador Sea piston core (P-094) are useful for recognizing rapidly deposited detrital layers, some of which correlate with North Atlantic Heinrich layers. Variations in magnetic properties and lithology distinguish two classes of rapidly deposited detrital layers during the last ice age: (1) seven detrital carbonate (DC) layers have high carbonate content and mean magnetite grain diameters about 4 times greater than the background sediments; (2) six low detrital carbonate (LDC) layers have very similar magnetic properties to DC layers but low carbonate content (similar to the background sediments). The magnetite associated with DC and LDC layers is well sorted and relatively uniform within and between layers. Ice-rafted detritus (IRD), recognized by increased percentage of the >125 micron grain-size fraction and by high magnetic susceptibility, is associated with most DC and LDC layers but is not the dominant detrital constituent. The correspondence of DC and LDC layers with increased grain size of well-sorted magnetite, but not with coarse fraction content, precludes ice rafting as the primary depositional mechanism. DC and LDC layers may have been deposited from suspended sediment derived from turbidite activity in the nearby Northwest Atlantic Mid-Ocean Channel (NAMOC). This interpretation is supported by a piston core (P-013) from a Greenland Rise site largely, but not completely, outside the influence of the NAMOC. The association of DC layers with IRD and the apparent age correlation of DC layers to North Atlantic Heinrich layers suggest that the ice advances which produced the IRD in P-094 and in correlative North Atlantic Heinrich layers also triggered turbiditic flows down the NAMOC. Several DC and LDC layers, however, do not correlate with recognized Heinrich layers but appear to be coeval with recently discovered high-frequency IRD-rich layers in the North Atlantic.