EVIDENCE FROM AR-40 AR-39 AGES FOR A CHURCHILL PROVINCE SOURCE OF ICE-RAFTED AMPHIBOLES IN HEINRICH LAYER-2/

Ar-40/Ar-39 ages of most single ice-rafted amphiboles from Heinrich la ver 2 (H2) from a core in the Labrador Sea, a core in the eastern Nort h Atlantic and a core In the western North Atlantic range from 1600 to 2000 Ma. This range is identical to that for K/Ar ages from the Churc hill province of the Canadian Shield that outcrops at Hudson Strait an d forms the basement of the northern part of Hudson Bay. The ambient g lacial sediment includes some younger and older grains derived from Pa leozoic, Mesoproterozoic and Archean sources, but still the majority o f the amphiboles have ages in the 1600-2000 Ma interval. The Ca/K rati os of these 1600-2000 Ma old amphiboles, however, have a bimodal distr ibution in contrast with the uniformity of the Ca/K ratios of H2 amphi boles. This indicates that 1600-2000 Ma old amphiboles of the ambient sediment were derived from an additional Early Proterozoic source besi des Churchill province. In H2, Churchill-derived grains constitute 20- 40% of the ice-rafted debris (IRD). The fraction in the ambient glacia l sediment is 65-80%. Results presented here are consistent with the h ypothesis that Heinrich events were produced by a sudden intensificati on of the iceberg discharge through Hudson Strait that mixed, in the N orth Atlantic, with icebergs that continued to calve from other ice sh eets. The shift from mixed sources in the background sediment to a lar ge dominance of Churchill province grains in H2 indicates that, even i f calving of other ice sheets intensified during the Heinrich episode, the increase in the iceberg discharge via Hudson Strait from the Huds on Bay drainage basin of the Laurentide ice sheet was by far the large st.