2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 6
Presentation Time: 9:15 AM


BLACKWELL, Bonnie A.B.1, BLICKSTEIN, Joel I.B.2 and SKINNER, Anne R.1, (1)Dept. of Chemistry, Williams College, Williamstown, MA 01267, (2)RFK Sci Rsch Institute, 7540 Parsons Bvd, Flushing, NY 11366, bonnie.a.b.blackwell@williams.edu

Electron spin resonance (ESR) dating has been developed for many materials, including hydroxyapatites in enamel, bone and some fish scales, aragonite and calcite in speleothems, travertine, molluscs, and calcrete, and quartz from ash, which have many potential applications in karst settings. Although the complexity of the signals in some materials has hampered routine application, research is solving these problems to make the method more widely applicable. When tested against other dating techniques, age agreement has usually been excellent. Generally, the most reliable applications seem to be tooth enamel, some mollusc species, calcite deposits, and quartz minerals.

ESR dating uses signals resulting from trapped charges created by radiation in crystalline solids. Ages are calculated by comparing the accumulated dose in the dating sample with the internal and external radiation dose rates produced by natural radiation in and around the sample. For fossils and authigenic minerals, no zeroing is necessary to obtain accurate ages. In sediment which contains reworked mineral clasts, such as quartz, ESR can be used to date the age of the mineral grain itself if not zeroed. For dating the age for the deposition, however, ESR signals must have been zeroed in order to give the correct age for sedimentation. High pressure, heating, and from some minerals, light exposure and grinding can zero an ESR signal. For materials that absorb uranium (U) during their burial history, such as teeth, bones, or mollusc shells, the age calculation considers their U uptake by cross-calibrating with U series or U/Pb dating or by assuming different uptake models. Some difficulties in calculating the external dose rate can be overcome by applying the ESR isochron method, in which the sample acts as its own dosimeter. In open-air environments, changes in the external dose rate due to changing sediment cover depths, and hence, changing cosmic dose rates need to be modelled. For all karst environments, sedimentary water concentrations and mineralogy also need to be considered. Tooth enamel, the most commonly used chronometer in caves, can date fossils ranging from 5-10 ka to 4-5 Ma, and for molluscs and speleothem from 1-5 ka to 300-500 ka. Fault gouge can be dated ranging from 1 ka to 2 Ma in age.