DETRITAL SILICATES IN NORTHEAST ATLANTIC DEEP-SEA SEDIMENTS DURING THE LATE QUATERNARY - MAJOR-ELEMENT, REE AND RB-SR ISOTOPIC DATA

The major element content and the Rb-Sr isotopic composition of non-ca rbonate sediment residues have been determined on different size fract ions of 14 samples and on 4 ice rafted dropstones from several deep-se a cores of the NE Atlantic Ocean (47-degrees-30'N, 19-degrees-30'W), r ecovered within the framework of the NOAMP project (Northeast Atlantic Monitoring Program). Rare-earth elements compositions have also been measured on 15 bulk sediment samples. These results provide complement ary data of a previous mineralogical and K-Ar isotopic study, carried out on the same samples (Jantschik & Huon 1992). The geochemical as we ll as the Rb-Sr isotopic composition of the detrital silicate fraction s support the hypothesis of mixtures of several different mineral sour ces. according to the prevailing transport mechanisms. During the warm climatic periods (foraminiferal ooze) throughout the last 140 ky, the Sr-87/Sr-86 ratios of the clay-size fractions are lower (0.7241 0.729 1) than during the cold climatic periods (diamicton, 0.7321 0.7407). T his increase is best explained by a strong dilution of the basalt-deri ved material supply from the province of Iceland by an increase of the terrigenous input from the surrounding land masses (North America, Gr eenland). This change is not recorded by the K-Ar ages, due to the low K-40 and Ar-40 contributions of Tertiary-Quaternary basaltic material to the mineralogical mixtures. On the contrary, during several phases of enhanced ice-rafting in the Late Pleistocene (cemented marl) and o f previous glacial periods (at about 250 and 400 ky), the K-Ar apparen t ages of the silicate material increase to 1138 +/- 18 Ma for the cla y-size fractions and to 1338 +/- 22 Ma for the coarsest size fractions (> 63 mum), whereas the Sr-87/Sr-86 ratios remain comparable to those of the diamicton sediments. These ice-rafted layers display a differe nt mineralogical composition, characterized by low Rb/Sr ratios and by high Sr-87 radiogenic content due to occurrence of Precambrian detrit al minerals. The K-Ar and Rb-Sr isotopic signatures of detrital silica te fractions of deep-sea sediments provide complementary and substanti al information for Late Pleistocene paleoceanographic and paleoclimati c reconstructions in the NE Atlantic ocean.