ALBERT

Description: Stable and radiogenic isotopic and sedimentological data from sub-Antarctic deep sea sediment cores reveal a temporal link between changes in seawater 87Sr/86Sr ratios and major episodes of late Eocene-early Oligocene climate change. The 87Sr/86Sr records show two major inflections, one at 38-39 Ma near the middle/late Eocene boundary, followed by another at 33.4 Ma. Similarly, the oxygen isotope, ice-rafted debris, and clay assemblage records indicate two important climatic events: the appearance of alpine glaciers and/or small ice-sheets on Antarctica in the late Eocene at 38-39 Ma, followed by a rapid transition to larger and more permanent temperate ice-sheets in the early Oligocene at 33.4 Ma. Moreover, during the early Oligocene 30-33 Ma. three to four inferred peaks in glacial activity appear to coincide with subtle steps in the 87Sr/86Sr record. The coupled variations in climate and seawater Sr isotope ratios during the Eocene/Oligocene imply a strong causal link between the two. Either changes in climate directly influenced patterns of continental weathering and hence seawater chemistry, and/or a tectonic event e.g., uplift.as reflected in weathering and seawater chemistry triggered relatively abrupt changes in global climate.

Description: The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt and Pd were determined in 11 abyssal peridotites from ODP Sites 895 and 920, as well in six ultramafic rocks from the Horoman peridotite body, Japan, which is generally thought to represent former asthenospheric mantle. Individual oceanic peridotites from ODP drill cores are characterized by variable absolute and relative PGE abundances, but the average PGE concentrations of both ODP suites are very similar. This indicates that the distribution of the noble metals in the mantle is characterized by small-scale heterogeneity and large-scale homogeneity. The mean Ru/Ir and Pt/Ir ratios of all ODP peridotites are within 15% and 3%, respectively, of CI-chondritic values. These results are consistent with models that advocate that a late veneer of chondritic material provided the present PGE budget of the silicate Earth. The data are not reconcilable with the addition of a significant amount of differentiated outer core material to the upper mantle. Furthermore, the results of petrogenetic model calculations indicate that the addition of sulfides derived from percolating magmas may be responsible for the variable and generally suprachondritic Pd/Ir ratios observed in abyssal peridotites. Ultramafic rocks from the Horoman peridotite have PGE signatures distinct from abyssal peridotites: Pt/Ir and Pd/Ir are correlated with lithophile element concentrations such that the most fertile lherzolites are characterized by non-primitive PGE ratios. This indicates that processes more complex than simple in-situ melt extraction are required to produce the geochemical systematics, if the Horoman peridotite formed from asthenospheric mantle with chondritic relative PGE abundances. In this case, the PGE results can be explained by melt depletion accompanied or followed by mixing of depleted residues with sulfides, with or without the addition of basaltic melt.