U-SERIES DATING TARGETS IN ARID AND SEMI-ARID ENVIRONMENTS

U-series dating requires few, often testable, assumptions and, if applied to dense, pure authigenic carbonate or silica, can be precise to 2-3 per cent or better over the last ca. 350 ka. In particular, travertine, pedogenic carbonate, and silicified rhizoliths may be dated via U-series, with applications to geomorphology, glacial chronology, and Quaternary stratigraphy. With J. Pederson, K. Karlstrom, L. Crossey and G. O'Brien, we have dated travertine that cements ancient alluvium of the Colorado River and its tributaries, thereby determining timing and rates of incision and aggradation in Grand Canyon. Reliable travertine U-series ages that preserve stratigraphic order on outcrop and map scale result if initial Th-230 is shown to be negligible in modern samples, and if travertine preserves dense primary textures and yields concordant ages for sub-samples. With R. Amundson, O. Chadwick, and J. Owen, we are dating pedogenic carbonate clast-rinds in soils of glacial and glacio-fluvial landforms in Wind River basin, Wyoming to refine the chronology at the type locality of the Bull Lake glaciation. To reduce age averaging and examine the behavior of the U-Th system at high spatial resolution in rinds with 5 ppm or more U, we have determined ages on 100-micron spots via laser ablation multi-collector ICP-MS. Visually pristine rind laminae yield clustered, and therefore reliable, ages. Continuous radial age traverses reveal that selected, texturally simple clast-rinds become monotonically younger away from the clast, confirming that ages of the innermost intact laminae of such rinds most nearly approach the age of the host deposit. Rinds from various terraces and depths display similar episodes of accelerated growth at ca. 80-60 and 35-15 ka, corresponding broadly with local glacial maxima and suggesting a brief lag between gravel deposition and initiation of rind formation in Wind River terraces. With R. Potts, K. Behrensmeyer, and A. Deino, we are dating silicified rhizoliths in Olorgesailie basin, Kenya to provide minimum ages for fluvial strata in a hominid-tool bearing sequence. After appropriate pretreatment, multiple fractions of individual rhizoliths yield clustered ages, consistent with closed U-Th systems and rapid formation after deposition of the host strata.