40Ar/39Ar Age Constraints on the Sources of Ice-rafted Hornblende Grains in the Heinrich Layers

S. R. Hemming Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA


W. S. Broecker Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA

W. D. Sharp Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, USA

R. H. Gwiazda Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA

G. C. Bond Lamont-Doherty Earth Observatory, Rt. 9W, Palisades, NY 10964, USA

Heinrich events 1, 2, 4 and 5 have an intriguing stratigraphic relationship with records of climate proxies in the same cores (Bond et al., 1992; Broecker et al., 1992; Bond and Lotti, 1995). These rapidly accumulated, coarse detrital layers occur at the terminations of cooling cycles and are immediately followed by abrupt warming to almost interglacial temperatures. Surging of ice through Hudson Strait has been proposed as a way to produce the enormous volume of detritus required (MacAyeal, 1993). This explanation provides a very specific prediction about the provenance of ice-rafted detritus in the Heinrich layers. Continental crust in this region was formed during the Late Archean, and much of it was regionally metamorphosed during the Early Proterozoic (Hoffman, 1989). Previous researchers have interpreted isotopic compositions of bulk, decarbonated sediments from the Heinrich layers as reflecting mixing between Icelandic sources and continental crust from Baffin Island (Grousset et al., 1993; Revel et al., 1996). Analyses of individual grains provide important constraints on the nature of the continental end member. We report here 40Ar/39Ar age constraints on individual hornblende grains from Heinrich layers 1, 2, 3, 4, and 5 in V28-82 (49o27'N, 22o16'W). Although the sedimentation rates are relatively low, the stratigraphy of this core is well preserved as demonstrated by our results for an 0.5 cm layer within H3 that has only Archean hornblende ages (n = 8) and one 60 Ma age. Additionally, we report data from the Gulf of St. Lawrence, as North Atlantic circulation patterns would allow it to be a contributor.

Single step laser fusion Ar analyses of individual hornblende grains from detrital carbonate-bearing Heinrich layers (H1, 2, 4, 5) in V28-82 yield Early Proterozoic ages of 1.6 to 1.9 Ga (n=56 of 64 grains analyzed). This is the same range as found in hornblende samples from the Labrador Sea (Gwiazda, 1996a) and Baffin Island moraines. Of the grains from the Heinrich layers that are not Early Proterozoic, 6 are between 2.4 and 2.7 Ga and 2 are between 0.9 and 1 Ga. Our results are consistent with the observations of increased proportions in Heinrich layers of detrital carbonates in the coarse lithogenic fraction (Bond et al., 1992). They further corroborate Pb isotopic data from single feldspar grains (Gwiazda et al., 1996b) and 40Ar/39Ar data grains (Gwiazda et al., 1996a) from Heinrich layer 2, interpreted to record derivation from the Hudson Strait region (Churchill Province) of the Canadian shield grains (Gwiazda et al., 1996a, 1996b). The results contrast strongly with our 40Ar/39Ar ages from the Gulf of St. Lawrence which are all between 0.9 and 1 Ga (n = 13, including one step-heated sample), and with individual feldspar grains that have Grenville and Appalachian Pb isotopic compositions. Additionally, Pb isotopic compositions of composite samples of 75 to 300 feldspar grains from Heinrich layers 1, 2, 4, and 5 from V28-82 are consistent with their all having dominantly the same feldspar and hornblende sources (Gwiazda et al., subm.). Together these data provide strong support for the origin of Heinrich layers 1, 2, 4, and 5 from ice surges through the Hudson Strait.


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