Ice‐Rafted Detritus of the Southeast Grand Banks Slope, Newfoundland, Throughout Heinrich Layers 1 to 5a: 2. Magnetic Properties

Ocean-wide “Heinrich event” layers marked by weakly magnetic dolomitic ice-rafted detritus (IRD) from the Hudson Bay region are a prominent signature of Laurentide Ice Sheet surges. Contrary to intuition, the Heinrich layers of the Ruddiman Belt IRD exhibit maxima of magnetic susceptibility. Building on IRD petrology and abundance in sediments from the southeast Grand Banks Slope of Newfoundland described in Part I of this study, this Part II presents bulk rock magnetic, major element and grain-size data as well as rock magnetic analyses of 22 classified IRD lithologies. By stratigraphically, computationally and statistically comparing magnetic with other bulk sediment properties and lithology-specific IRD counts, we analyze, how the various magnetic IRD species and the glaciomarine background sedimentation generate the observed magnetic susceptibility record. How to infer from microscopically identifiable, but subordinate coarse IRD to the apparently greater impact of the unclassifiable fine IRD fraction is critical for this quest. We show that multi-domain magnetite, originating from plutonic Canadian Shield rocks, especially muscovite-biotite granite, dominates and shapes bulk susceptibility records more than all other IRD lithologies. Inverse correlations of Ca/Sr and K/Fe ratios, proxies for dolomitic and granitic IRD, delineate that the magnetic enhancement by the granitic detritus is partly compensated by co-deposited weakly magnetic dolomitic detritus in the mid-phase of Heinrich events. Modeling bulk susceptibility from various non-magnetic records suggests that large parts of fine IRD did not settle directly from melting overpassing icebergs, but seem to have been redistributed and sorted by ocean currents and gravity flows.