Did the European Ice Sheet Surges Trigger the North Atlantic Heinrich Events?

F E Grousset (DGO, UMR 5805, University Bordeaux I, Avenue des FacultTs, 33405, Talence cedex, France; ph. 33 556 84 8731; fax 33 556 84 0848: Internet: grousset@geocean.u-bordeaux.fr); H Snoeckx (Environmental Studies Program, University of West Florida, Pensacola, FL 32514-5751, USA; ph. 850 474 2746; fax 850 857 6063; Internet: snoeckx@uwf.edu); M Revel (LGCA, Institut Dolomieu, 15 rue M. Gignoux, 38031 Grenoble, France; ph. 33 476 63 5908; fax 33 476 87 8243; Internet: marie.revel@ujf-grenoble.fr).

Abrupt, massive discharge of icebergs during Heinrich events transported and deposited large quantities of ice rafted debris (IRD) in the North Atlantic. Theories explaining the origins of the particles and their distribution generally point to the Laurentide ice sheet as the culprit behind the events. Doubt was cast by Revel et al. (1996), who,by measuring the Sr-Nd isotopic compositions on the bulk lithic fraction of the Heinrich layers, found that the sediments originated not only from the Laurentide ice sheet as heretofore envisioned but also from other European sources.

In order to be sure that we fingerprint the source of IRD only, we isolated the carbonate-free coarse (>630) lithic fraction of Heinrich layers and non-Heinrich "ambient" sediments in about 20 cores from the northern Atlantic (38N to 68N) and analyzed its Sr-Nd isotopic composition. The results confirm that the transported icebergs were calved from many ice sheets from around the North Atlantic (at least Fennoscandian, Icelandic and Laurentide). Isotopic data show a more distinctly European origin for IRD deposited during the "atypical" Heinrich event H3 than for ambient IRD. In contrast, Heinrich layer H4, a representative of "typical" Heinrich events H1, H2, H4, and H5, presents a dominant Laurentide signal. These results confirm that these "typical" Heinrich events are associated with a massive discharge of icebergs from the Laurentide ice sheet.

Nevertheless, the theory that the Laurentide ice sheet sets the events

into motion (Alley and MacAyeal, 1993) is challenged by the European signal of the early-H4 deposits followed by the Laurentide signature of the late-H4 deposits in the same cores. This observation indicates that the European ice sheets surged before the North American, which disqualifies the Laurentide ice sheet as the unique motor behind the Heinrich events. Hypotheses on the Heinrich event origins have to be reconsidered.