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Accurate determination of the absolute ages of the oldest Solar System objectschondrules and Ca-Al-rich inclusions (CAIs), requires knowledge of their 238 U/ 235 U ratios. This ratio was assumed to be invariant in all U-Pb dating of meteorites so far, but the recent discovery of U isotope variations in CAIs (Brennecka et al. 2010a) shows that this assumption is invalid. We present the first combined high precision U and Pb isotopic data for a CAI, and U isotopic data for chondrules and whole rock fractions of the Allende meteorite. The Pb-Pb isochron age of the CAI SJ101 is 4567.18±0.50 Ma, calculated using the measured 238 U/ 235 U=137.876±0.043 (2), reported relative to 238 U/ 235 U=137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: 238 U/ 235 U=137.821±0.014.The error in the age includes uncertainties in the Pb-Pb isochron intercept and in the 238 U/ 235 U ratio. Allende bulk rock and chondrules have 238 U/ 235 U=137.747±0.017 (2), distinctly lower than the CAI. The difference in the 238 U/ 235 U ratio of 0.129±0.046 (2) between the CAI and chondrules and bulk meteorite increases the 207 Pb-206 Pb age difference by ~1.4 million years, and eliminates apparent disagreement between the CAI-chondrule formation time interval determinations with the U-Pb and extinct nuclide (26 Al-26 Mg and 182 Hf-182 W) data. We discuss standardization of 238 U/ 235 U measurements for U-Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAIs in the context of 238 U/ 235 U variability, and possible causes of U isotopic variations in CAIs and meteorites.

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A high-resolution authigenic Nd isotope record has been extracted from the Fe-Mn oxyhydroxide fraction of drift sediments along the Blake Ridge in the North Atlantic.These sediments facilitate reconstruction of the timing and extent of major hydrographic changes in the western North Atlantic since the Last Glacial Maximum (LGM). This is one of the few locations where sediments were deposited in the major flow path of the Western Boundary Undercurrent (WBUC), which transports North Atlantic Deep Water (NADW) southward at the present day. The hydrodynamic setting, however, also causes problems. Authigenic Nd isotope compositions similar to the typical present-day NADW ε Nd value of -13.5 ± 0.5 were only extracted from sediments located within the main water body of the WBUC coinciding with the highest along slope current velocity below 3200 m water depth. Above this depth the authigenic Nd isotopic composition is more radiogenic than measured in a nearby seawater profile and appears to be influenced by downslope and lateral sediment redistribution. Our data suggest that these radiogenic signals were formed at shallow depths in Florida current waters, compromising the 2The LGM and deglacial sediments below 3400 m water depth bear no evidence of an ambient deep water ε Nd as unradiogenic as -13.5. Although the deep core sites also experienced enhanced degrees of sediment focusing before the Younger Dryas, the ε Nd values of between -11 and -10 are more readily explained in terms of increased presence of Southern Source Waters. If this is the case, the change to Nd isotopic compositions that reflect a modern circulation pattern, including the presence of Lower NADW, only occurred after the Younger Dryas.