THE ND, SR AND PB ISOTOPIC COMPOSITIONS OF ICE PROXIMAL SEDIMENTS: IMPLICATIONS FOR ICE SHEET DYNAMICS AND THE SOURCES OF QUATERNARY IRD IN THE NORTH ATLANTIC
FARMER, G. LANG . University of Colorado.
Barber, Donald C.. Bryn Mawr College.
Andrews, John T.. University of Colorado.
The Nd, Sr and Pb isotopic compositions of Quaternary, ice-proximal sediments from around the northern North Atlantic have been determined to fingerprint the isotopic characteristics of potential sources of ice-rafted detritus (IRD) in this region, as well as to study the dynamics of the various Quaternary, circum-northern Atlantic ice sheets. Targeted for study were sediments deposited adjacent to the Laurentide (Baffin Bay, western Hudson Strait, Cartwright Saddle and the Gulf of St. Lawrence), Greenland and Iceland (Denmark Strait and vicinity), and Fennoscandian (Bear Island fan, mid-Norwegian continental shelf) ice sheets. In each area we obtained isotopic data primarily from the <63 um size fraction, but in some cases the >63 um size fraction was analyzed as well.
From the eastern margin of the Laurentide ice sheet (LIS), <63um sediments from Baffin Bay have low eNd (-23.1 to –27.2; Fig. 1), high 87Sr/86Sr (0.7259 to 0.7356), and variable Pb isotopic compositions (206Pb/204Pb=16.82 to 17.73). Those samples from the central and southern Baffin Bay have lower eNd, older Nd model ages (~2.4 b.y.) and higher 87Sr/86Sr. The latter also plot along a ~2.0 b.y. secondary Pb errorchron, whereas samples from northern Baffin Bay plot above this line. Fine-grained glacial sediments from western Hudson Strait have uniformly low eNd (-28.1 to –28.9), 87Sr/86Sr=0.720 to 0.726, and variable Pb isotopic composition that plot along the same ~2.0 b.y. errorchron defined by the Baffin Bay samples. At Cartwright Saddle, offshore of northern Labrador, glacial sediments also have low eNd (-21 to –22.5; TDM=2.0 to 2.2 b.y.) but lower 87Sr/86Sr (0.7159-0.7169) than glacial detritus further north. In contrast, both coarse- and fine- grained sediment from the Gulf of St. Lawrence (GSL) have significantly higher eNd values (-11.9 to –15.1; TDM=1.5 to 1.8 b.y.), and generally lower 87Sr/86Sr ratios (0.7171 to 0.7227) than glacial sediment deposited at higher latitudes. The coarse GSL sediments also define a 450 m.y. Sr errorchron. These data reveal that the isotopic compositions of ice-proximal marine sediment offshore eastern Canada reflect the isotopic compositions of adjacent Precambrian crustal terranes and so illustrate that sediment derived from different portions of the eastern LIS, including the Hudson Strait and GSL ice streams, are isotopically distinguishable.
Sediments from the Bear Island fan in the Norwegian Sea have Nd, Sr, and Pb isotopic compositions that are essentially indistinguishable from those found in the GSL (Fig. 1). Sediments from the mid-Norway continental shelf have isotopic compositions similar to those in the GSL and Bear Island fan, but have slightly lower eNd (~-15, TDM~1.8 b.y.) and more radiogenic Pb and Sr. Glacial sediments deposited off western Iceland, in contrast, have high (eNd =5.7 to 6.2), low 87Sr/86Sr (0.7038 to 0.7050) and Pb isotopic compositions that plot to the right of the geochron close to the Northern Hemisphere Reference Line. In contrast, on the east Greenland continental slope, glacial sediment has variable isotopic compositions, with eNd values ranging from +4 to –2.5 and 87Sr/86Sr from 0.7040 to 0.7085.
Overall, our data confirm that there are significant temporal and spatial variations in the isotopic compositions of ice-proximal sediments throughout the circum Northern Atlantic. Radiogenic isotopic data from such sediments should therefore provide an important proxy record of Quaternary ice sheet dynamics and aid in better identifying the sources North Atlantic IRD. For example, previous studies have suggested that the relatively high eNd values of “atypical” H3 ice-rafted detritus (IRD) in the North Atlantic was derived from the Fennoscandian ice sheet (Snoeckx al., 1999). While a possibility, our data reveal that, on isotopic grounds, ice emanating from the Gulf of St. Lawrence is an equally plausible source for this detritus. A source for high eNd detritus in the southern Laurentide ice sheet removes the need for a European component to the detritus, and opens the possibility that most of the detritus in the IRD belt of the North Atlantic was ultimately derived from the Laurentide ice sheet. As a result, the H-3 IRD in the North Atlantic may contain information regarding the timing of major surges in the north vs. southern portions of the LIS, but provides little information regarding the relative phasing of ice surges in the North American and northern European ice sheets.
Snoeckx, H., Grousset, F., Revel, M., and Boelaert, A., 1999: European contribution of ice-rafted sand to Heinrich layers H3 and H4. Marine Geology, 158: 197-208.
Figure 1. eNd vs. 147Sm/144Nd for ice-proximal sediments throughout the northern North Atlantic region.
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