The deposition of voluminous ice-rafted detritus (IRD) comprising Late Cenozoic Heinrich events in the North Atlantic has generally been attributed to instabilities in the Laurentide and Fennoscandian ice sheets. Little consideration has been given to the role of the Greenland Ice Sheet (GIS) in delivering IRD to the North Atlantic, even though the occurrence of past instabilities in the GIS would have obvious implications for predictions of the behavior of the ice sheet during global warming. To assess the importance of the GIS as a source of North Atlantic IRD, we obtained Nd and Sr isotopic data from glacimarine detritus and IRD deposited along the southeast margin of Greenland. Our approach was to use tills and ice proximal glacimarine sediments to define the isotopic compositions of detritus actually delivered to the North Atlantic by ice streams active in SE Greenland, and then to compare these isotopic compositions to those determined for IRD deposited offshore of the continent (Farmer et al., 2003). We concentrated on the <63 micron size fraction, given that this sediment size range represents the dominant volume of sediment produced by glaciation (Andrews and Principato, 2002). Tills and glacimarine sediments were sampled in the vicinity of Scoresby Sund and Kangerlussuaq, the likely sites of large ice streams in SE Greenland during glacial advances (Fig. 1). From the southern portion of the Scoresby Sund trough mouth fan (cores JR 51GC-31, -32; Fig. 1) we sampled dark grey massive diamicts that form the lower most units in these cores and that have been interpreted as glacigenic debris flows deposited during glacial advance(s) through Scoresby Sund (Dowdeswell et al., 1997). Measured eNd values and 87Sr/86Sr ratio for these sediments range from -11.8 to -15.8, and 0.719 to 0.725 for both <63m and >63m size fractions and overlap the isotopic compositions of Paleoproterozoic basement rocks exposed in the Caledonian orogenic belt of northeastern Greenland (Fig. 2a, b). Archean, and not Paleoproterozoic basement rocks comprise much of the inner, western portions of Scoresby Sund, itself, so our data indicate that Paleoproterozoic basement rocks must exist further to east along continental shelf, and that the Scoresby Sund TMF represents the products of the erosion of this shelf by grounded ice during a past glacial advance. Tills sampled from cores positioned at various portions of Kangerlussuaq fiord and offshore troughs (Fig. 1) show a range of isotopic compositions that correlate with changes in the age and composition of underlying basement rocks. Tills from the inner portion of the fiord have lower eNd values, consistent with their derivation from rocks similar to the local Precambrian basement (Fig. 2a,b). Further to the west within the inner Kangerlussuaq trough, eNd values are considerably higher (-2 to+2.5) and 87Sr/86Sr lower (0.705 to 0.707) reflecting the erosion of underlying Cenozoic basalts of the East Greenland volcanic province. At the outer trough, however, eNd are lower (-5.6 to -8.4) and 87Sr/86Sr higher (0.710 to 0.712) and suggest Precambrian rocks must comprise the continental shelf in the southern reaches of the Kangerlussuaq trough and these rocks were eroded by grounded ice during glacial advance(s) across this region. Overall, the data from both Scoresby Sund and Kangerlussuaq reveal that the majority of the glacial detritus delivered to the shelf margin these portions of SE Greenland was the product of erosion of the continental margin, itself, a conclusion also reached for other ice sheets worldwide.

Fine-grained (<63 m) sediments from deeper marine sites adjacent to SE Greenland were sampled from cores in the Blosseville Basin (core PS62-017-4), north of Denmark Strait, and from further south within the Irminger Basin (SU 9024, HU 91-045) (Fig. 1). In the Blosseville Basin, sediments corresponding to H-1 through H-3 have eNd values (-7 to -12) and 87Sr/86Sr (0.711 to 0.717; Fig. 2a, b). Sediments from H-1 and H-4 in core SU 9024 have a range of eNd and 87Sr/86Sr (-3.8 to -11.3, 0.709-0.714). These data reveal that Heinrich events sediments both north and south of Denmark Strait are similar isotopically to glacial sediments at Scoresby Sund and Kangerlussuaq. Based on the limited amount of isotopic data available we cannot determine from where along the SE margin of Greenland the various H-event sediments were eroded, but our observations suggest that all of the IRD present in these cores was likely provided by the southern GIS. Further south in the Irminger Basin, at core HU91-045, both ambient and H-1, -3 and -4 sediment have significantly lower eNd values (-12 to -28) than determined for IRD further north. These Nd isotopic compositions overlap those of North Atlantic IRD considered to be derived from the northern Laurentide ice sheet. However, the 87Sr/86Sr of the southern Irminger Basin sediments (0.7097 -0.7194) are significantly lower than those of glacial detritus delivered through the Hudson Strait by the Laurentide ice-sheet (Fig. 2b) and suggest that IRD in these cores was not derived from North America. Instead, high metamorphic grade, low 87Sr/86Sr. gneisses exposed along portions of continental margin of southeast Greenland represent a more likely IRD source. We conclude, then, that much of the IRD deposited in the Late Cenozoic in the northern North Atlantic was delivered by ice emanating from the southern portions of the GIS, indicating that significant instabilities in at least this portion of the ice sheet have occurred over the past 40 Ka.