Quaternary dating studies using ESR signals, with emphasis on shell, coral, tooth enamel and quartz

Abstract

In this study, the electron spin resonance (ESR) method is examined as a suitable dating tool to estimate the age of Quaternary materials. Although ESR dating has contributed substantially to earth science and archaeological studies, a standard procedure has not yet been developed for mollusc shell. Also, there has not been any practical application of ESR to the dating of sediments using solar-reset quartz. In this study, recently published and newly developed ESR methods are tested and compared with independent age controls (e.g. obtained by 14C, U-series, thermoluminescence and optically stimulated luminescence) for mollusc shell, coral and quartz. Annealing experiments on Q-band spectra suggest that the g=2.0006 signal in aragonite shell contains thermally unstable components but these appear to be removed by annealing at 150°C for 14-15 hours. For both aragonite and calcite-containing shells, DE­ values based on the ESR signals at g=2.0006 (X-band, 200 mW, aragonite shells), g=2.0006 (Q-band, aragonite shells), g=2.0006 (X-band, calcite-containing shells) and g=2.0014 (X-band, aragonite shells) appear to be comparable after annealing at 150°C for 14-15 hours. In contrast, the g=2.0007 signals observed in the X-band (microwave power of 5 mW and 200 mW) and Q-band in coral samples appear to yield comparable DE­ values both before and after annealing; these signals thus may not include thermally unstable components. For the coral samples examined, the ESR ages based on the annealed g=2.0058 signal and the g=2.0007 signal (before and after annealing) are in agreement with the independent age controls; both signals appear to be suitable for dating. However, the ESR ages based on the annealed g=2.0058 signal in aragonite and calcite-containing shells are often much greater than those based on the g=2.0006 and g=2.0014 signals. At this stage, it has not been determined which, if any, signal yields the most reliable estimate of age. Three new light-sensitive ESR signals in quartz have been discovered in this study. These signals are observed at g=l.9870, 1.9842 and 1.9162 at liquid nitrogen temperature (77K) with an X-band spectrometer. Sunlight bleaching experiments using an orange filter (> 510 nm wavelength transmission) suggest that the bleaching behaviour of these signals is comparable to the bleaching response of the 375°C TL signal, but is unlike that of the 325°C TL and OSL signal. The ESR ages based on these signals are in close accord with independent age estimates obtained by 14C, TL and OSL. The Do values (the characteristic saturation doses) based on the light-sensitive ESR signals are much greater than those indicated by OSL. This implies that these signals have potential application to the dating of quartz sediments deposited as long ago as 1-2 Ma (given an environmental dose rate of 0.5-1 Gy ka-1 typical of many Australian sandy deposits), a time period beyond the range of 14C, U-series, TL and OSL dating.