PROBING THE MAXIMUM AGE LIMITS FOR ESR DATING: COMPARATIVE ANALYSES FOR MIOCENE FOSSILS FROM SHANXI CHINA AND THE SNAKE CREEK FAUNA, NEBRASKA

The maximum age limit for ESR dating has been estimated to lie between 5 and 10 Ma for most teeth. Since the only sediment available with these museum samples was that still attached to the teeth, 5-8 subsamples were independently dated by ESR in order to calculate an isochron for each tooth (Blackwell & Schwarcz, 1993, Applied Radiation & Isotopes 44: 243-252). Since the tooth acts as its own dosimeter, an isochron date can provide both the external dose rate and the age for the tooth. Fossils from the Miocene Snake Creek fauna from Nebraska, and the classic faunal locality at Shanxi China, also thought to be Miocene, were tested to see if ESR ages could be obtained. If the stratigraphic correlations are correct, both these sites should predate 5.2 Ma.

For all the teeth tested, the accumulated dose (A_S) curves showed that the ESR signal was close to saturation. Consequently, the A_S determinations required added doses up to 3-4 Mrad, and are associated with larger uncertainties than would normally occur for tooth enamel dating. Because the teeth contained large U concentrations in the dentine and enamel, the internal dose rates dominate over the external dose rates. The calculated ages depend strongly on the assumed U uptake model. Assuming early uptake, these teeth gave ages ranging from 200 to 550 ka, while a linear uptake assumption yielded ages from 0.4 to 1.1 Ma. Given the stratigraphy of these two sites, these ages are clearly too young. Assuming recent U uptake (RU) with an uptake parameter p=10 gave ages ranging from 1.6 to 4.5 Ma, but using p=20 resulted in ages from 2.8 to 7.0 Ma. Assuming p=200 yielded ages from 6.4 to 13.1 Ma. The isochrons do suggest secondary U uptake has occurred, which can be best modeled by an RU model.

Two scenarios may apply here: Either the traps which form the ESR signals have already reached saturation in these teeth, meaning that the teeth are beyond the maximum age limit for ESR dating, or they acquired 99% of their U within the last few thousand years, meaning that an ultra-RU model applies. Understanding U uptake in old teeth will be critical in deciding between these possibilities.