FOSSILIZATION EFFECTS ON URANIUM UPTAKE BY TOOTH ENAMEL: INSIGHTS INTO AN ELECTRON SPIN RESONANCE (ESR) DATING PROBLEM

Electron spin resonance (ESR) dating of tooth enamel uses an ESR signal that remains stable for approximately 10¹⁹ y, 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 the enamel during burial, the ESR age depends on the radioisotope content both inside and outside the sample. Since teeth absorb U as they fossilize, ESR ages either must measure the U uptake via ²³⁰Th/²³⁴U dating or assume an U uptake model based on paleoclimatic and geological factors. To find absolute criteria that might be used to determine the appropriate model, when teeth were too old for Th/U analysis, began while studying samples whose ESR spectra differed from the standard enamel spectrum.

Q-band ESR spectroscopy (rather than the more usual X-band) increases the spectral resolution by separating overlapping "convoluted" signals, allowing them to be studied separately. When the non-standard enamel spectra from well fossilized, early Pleistocene and Pliocene teeth were examined in the Q-band, the abnormal signal was discovered to combine two peaks from the standard signal, one considerably broader than the other. In a Q-band spectrum for young enamel, the one peak was much narrower. ESR theory suggests that this broad signal results from crystal distortion, as might be expected in more fossilized samples.

To test this, modern tooth enamel was artificially fossilized by prolonged heating in buffered solution. To test if the teeth had developed characteristics typical in naturally fossilized samples, the samples were analyzed by HPLC and GC to determine their amino acid racemization (AAR) ratios. The Q-band spectra from the artificially fossilized teeth did not entirely mimic those from natural fossilization, but had some of the same signal broadening, and correlated with the aIle/Ile ratios from AAR. Therefore, fossilization appears to cause distortion in the hydroxyapatite crystals, which would make the teeth more likely to absorb U through microcracks in the enamel and dentine. This suggests that samples exhibiting a broad peak might have absorbed much of their U relatively recently, and hence, require a recent uptake model.