Provenance of Greenland Ice Sheet icebergs constrained by Ar-Ar radioisotope data from late glacial-early Holocene records

Radiogenic Ar-Ar dating of hornblende has been used to constrain the sources of ice-rafted detritus on the southwest Greenland margin and central Davis Strait. Single hornblende grains (total of 248) were picked from sand-size fractions of two marine cores, DA04-31P and TTR462G, covering the early Holocene to late glacial interval including Heinrich events (HE) 1-3. Paleoceanographic analyses of DA04-31P have indicated that the last deglaciation of the southern Greenland Ice Sheet occurred in multiple steps during the early deglacial/HE1, Allerød and early Holocene intervals (Knutz et al., in review). Each of these collapse stages were preceded by upper ocean warming related to enhanced Irminger Current activity. Pre-HE1, Allerød and Holocene hornblende samples show a bimodal signature of late Archean (2.5-3.0 Ga) and early Proterozoic (1.6-2.2 Ga) ages consistent with southern Greenland bedrock sources. In contrast hornblende grains from detrital carbonate HE layers and within the Younger Dryas interval show a dominant early Proterozoic spectrum that is similar to results obtained from the central North Atlantic IRD belt. The early Proterozoic age of Younger Dryas samples from the Davis Strait site and the high amounts of basalt in this core, presumably derived from the Disko-Nussuaq province, suggests that icebergs carrying Archean material from southern Greenland did not enter the Baffin Bay or at least not for this specific time interval. The conclusion of our study is that the Greenland Ice Sheet appears to have been a much larger contributor to non-Heinrich IRD in the North Atlantic than thought previously. P. C. Knutz, H. Ebbesen, S. Christiansen, M.-A. Sicre and A. Kuijpers. Multiple deglaciation steps of the southern Greenland Ice Sheet driven by vigorous Irminger Current: Significance for the Younger Dryas cooling. Submitted to Quaternary Science Reviews.