SPEED DATING WITH A YOUNG, DIVERSE CROWD: U-TH GEOCHRONOLOGY USING HYDROGENIC AND BIOGENIC MATERIALS

The U-Th dating method is applicable to a diverse range of hydrogenic and biogenic materials formed during the last 500 ka and has been widely used in geomorphic, tectonic, paleoclimate, paleohydrologic, and archaeologic studies. Calcite, aragonite, and dolomite from speleothems, corals, soil carbonate, tufa, travertine, and fault-related veins are most commonly used. However, opal, sulfates, phosphates, and hydroxides (including ice) can also be dated, as can fossil bones, teeth, and ratite eggshells when enriched with secondary U. Dating of hydrogenic material relies on the large fractionation between U and Th in most near-surface water: U is relatively soluble while Th is not. Consequently, materials precipitated under these conditions typically incorporate U (notably ²³⁸U and ²³⁴U) but little or no ²³⁰Th (the alpha-decay progeny of ²³⁴U), thus starting the U-Th “clock”. Amenable material can yield precise Quaternary dates (better than ±1%), and because ages depend only on intrinsic sample properties, both newly collected and archival materials can be dated. Simple assumptions inherent to radioactive decay allow levels of accuracy to approach precision, and the system includes inherent checks on the reliability of resulting dates. A key assumption is that samples remain closed to U and Th after formation, or in the case of bones, teeth and eggshells, since U uptake. This assumption is likely valid if U-Th ages preserve micro- or macro-stratigraphic order, and if samples of coeval material produce concordant dates. Visual criteria applied during subsampling increase the likelihood of success. A second assumption is that initial ²³⁰Th is absent or can be quantitatively subtracted using common Th corrections or isochron techniques. Contributions of ²³⁰Th from detrital sources can be reduced by careful selection of small (0.1-30 mg), “clean” subsamples. The veracity of U-Th dates can be further assessed by evaluating age versus initial ²³⁴U/²³⁸U results in a suite of related samples. In situ methods using laser ablation or ion probe analyses are capable of high spatial resolution (10-100 µm) and can provide increased accuracy for ages of materials with slow growth rates, albeit at reduced precision. We will present examples of dated materials and approaches for obtaining U-Th ages on a variety of materials.