Q-band ESR spectra as indicators of fossilization in tooth enamel

Abstract

Electron spin resonance (ESR) dating of the hydroxyapatite (HAP) in tooth enamel uses an ESR signal that remains stable for approximately 10 19 years at 25 °C, and is particularly useful in the time range between the maximum 14C and the minimum 39Ar/ 40Ar age limits. Because the ESR dating signal results from radiation damage to samples during burial, the ESR age depends on the radioisotope concentrations, within both the sample and its surrounding sediment. Since teeth are not closed systems with respect to U uptake, however, ESR dates must measure an U uptake model by coupled 230Th/ 234U-ESR dating, or assume one based on paleoclimatic and geological factors. Experiments to find criteria that might be used to determine the model in teeth too old for 230Th/ 234U dating began using samples whose abnormal HAP ESR spectra in the X-band hinted that fossil diagenesis might be the culprit. Unlike X-band, Q-band ESR spectroscopy resolves spectral peaks better, by separating signals with very similar g values for individual study. When examining abnormal enamel spectra from well fossilized Early Pleistocene and Pliocene teeth in the Q-band, the problematic signal can be resolved as two signals, both with g∼2.002, but one considerably broader than the other. How much broader, however, appears to be a function of age. ESR theory suggests that the broader signal results from crystal distortion, as might be expected in fossilized samples. To test this, modern tooth enamel was artificially fossilized by prolonged heating in a buffered aqueous solution. To test if the teeth had developed characteristics typical in naturally fossilized teeth, the samples were analyzed by HPLC and GC to determine their amino acid racemization (AAR) ratios. Although the Q-band spectra from artificially fossilized teeth did not entirely mimic those from natural fossilization, similar ESR signal broadening occurred and correlated with the aIle/Ile ratios from the AAR analyses. Therefore, fossil diagenesis appears to cause distorted HAP crystals, which would make teeth more likely to absorb U through microcracks in the enamel. This suggests that samples exhibiting a broad peak might have absorbed much of their U relatively recently, and that accurate ESR dating requires a recent uptake model.