European contribution of ice-rafted sand to Heinrich layers H3 and H4

This paper reports the Sr-Nd isotopic composition of the carbonate-free sand (>63 mu m-1 mm) fraction of Heinrich layers 3 and 4 from about 17 cores from the northern Atlantic Ocean (38 degrees N to 68 degrees N). The results indicate that the transporting icebergs were calved from many ice sheets from around the North Atlantic (at least the Fennoscandian, Icelandic and Laurentide). Our isotopic data show a more distinctly European origin for ice-rafted debris (IRD) deposited during the 'atypical' Heinrich event H3 than for background IRD. In contrast, Heinrich layer H4, a representative of 'typical' Heinrich events H1, H2, H4, and H5, presents a clear dominant Laurentide signal. This result confirms that these 'typical' Heinrich events are associated with a massive discharge of icebergs from the Laurentide ice sheet. However, the theory that the Laurentide ice sheet set the events into motion [Alley, R.B., MacAyeal, D.R., 1993. Ice-rafted debris associated with binge/purge oscillations of the Laurentide Ice Sheet. Paleoceanography 9 (4), 503-511] cannot explain the fact that a European signal of an early-H4 sample has been observed prior to the Laurentide signature of a late-H4 sample in the same core, along the European margin. This observation indicates that the European ice sheets may have surged before the North American ones, and suggests that the Laurentide ice sheet was not the sole motor behind the Heinrich events. Moreover, in the Norwegian Sea, IRD has a Scandinavian origin in both H3 and H4. (C) 1999 Elsevier Science B.V. All rights reserved.