Testing the reliability of ESR dating of optically exposed buried quartz sediments
Introduction
Electron spin resonance (ESR) dating is based on the measurement of radiation-sensitive ESR signals which can begin to form after crystallization or can regrow after being reset to zero or reduced to a residual level by exposure during depositional processes. In ESR optical dating, characteristic ESR signals in detrital quartz grains are reduced or zeroed by daylight exposure, and then regrow during burial of the quartz in sediments due to exposure to natural radiation. The method resembles the better-known techniques of thermoluminescence (TL) and optically stimulated luminescence dating (Prescott and Robertson, 1997, Wintle, 1997), but has the potential to date much older deposits, possibly up to more than 5 Ma, due mainly to a slower growth to saturation of the signals compared with OSL or TL.
Walther and Zilles (1994) showed that a signal associated with aluminum (Al) defects can be reduced to between 40% and 60% of its initial value by extensive bleaching. Yoshida (1996) showed that the titanium (Ti) signal is fully zeroed after 10–20 days of sunlight, while the Al signal can only be reduced to a nearly constant residual value after more than about 100 days of sunlight. Further experiments by Toyoda et al. (2000) confirmed that the Ti–Li signal (used in the present study) can be fully zeroed, and concluded that when it and one of the two other types of Ti signals (Ti–H and Ti–Na) give concordant results, then the age obtained is reliable. However, in many cases these other signals are small or non-existent in our experience.
Laurent et al., 1994, Laurent et al., 1998 showed that aluminum ESR signals in detrital quartz grains could be used to date daylight-exposed detrital sediments with ages up to , based on ESR ages in good agreement with biostratigraphic age estimates of alluvial sands in the Somme Basin of France. However, it still remained to be seen if ESR optical dates agreed with other independent methods of dating.
Previous studies have concluded that the dates obtained by this method are generally consistent with other estimates of age and therefore appear to be reliable. It remains to compare ESR ages with those obtained by other accepted dating methods over the multi-million year time range. Here we have carried out ESR analyses on three types of samples: (1) a dune sand from Cape Kidnappers, New Zealand whose age can be firmly established by and fission track ages on an associated volcanic stratum, (2) nearshore lacustrine sands from ancient Lake Bungunnia in the Murray Basin of Australia whose ages are constrained by paleomagnetic analyses, and (3) along-shore lacustrine sands in conglomerate from the archaeological site of ‘Ubediya in the Jordan Valley of northern Israel whose age is constrained by a combination of paleomagnetic, stratigraphic and biostratigraphic data.
In this paper, rather than attempting to determine the age of sites, we have assumed that the chronological age is well established by the other age criteria available at each site. We can then test the agreement between expected and observed values for the equivalent dose using various treatments of the data. The purpose of this exercise is to identify the optimal treatment of ESR data that gives the best agreement between expected and observed . This would then provide us with the appropriate techniques to use for ESR optical dating where the age is otherwise unknown.
Section snippets
Sample material and independent age controls
To yield accurate dates, ESR optical dating of quartz requires that the sediment sample being dated has been fully exposed to direct sunlight for periods of tens to hundreds of days. We believe that along-shore environments, where sands are generally reworked for decades of residence time in the near shore environment, provide one of the best testing grounds for this method. Both aeolian sands and waterlain sands from these environments are very likely to have received many days of accumulated
Cape Kidnappers
Sample CKDS was collected by one of us (PS) and Dave Huntley (Simon Fraser University, Physics Department) who kindly provided the sample. The external dose rate was determined by Huntley using a portable -ray spectrometer. The cosmic dose rate was zero due to the thick overburden in the cliff exposure which has been eroded landward in the very recent geological past. The particle dose was determined by instrumental neutron activation analysis (INAA) of the bulk sediment for U, Th
Results
We tested the effects of bleaching and thermal annealing on the CKDS sample. We first compared the decay of the Al and Ti signals during light exposure in the natural sample and one that had received an additional dose of 6000 Gy from the source (Fig. 2). In both samples, the Ti signal was reduced to zero after about 30 h (equivalent to 20 days of 12 h of sunlight per day). After about 80 h of SOL2 exposure, the Al signal in the natural sample was reduced to a constant residual value equal
Discussion
The analysis of the dose response curves shows that added doses well above Do in almost every case yield reliable values. Thus we have only weak evidence that this may be used to gain accurate age results, namely the better agreement obtained when omitting doses above Do for sample Au 9. This example suggests that in some cases Do may be useful to define the upper limit for added doses. The increased error in in this case may also be a consequence of having insufficient added dose points
Conclusions
Overall we find excellent promise for ESR optical dating of sediments in quartz-bearing, sand sized sediments deposited by wind and water in coastal settings. In most of the cases we have studied the Al (bleachable portion only) and Ti signals yield values in good agreement with values expected from the known age of samples. Further studies are needed to better understand the questions of whether added dose range can influence correct determination for the Ti signal. Despite this question
Acknowledgments
We thank the Natural Sciences and Engineering Research Council of Canada (NSERC), and a Premier's Research Excellence Awards of Ontario for financial support to WJR. We thank the Department of Earth Sciences, Melbourne University, Mike Sandiford (University of Melbourne) and Derek Feibel for logistical support in collecting the Australian samples. We thank K. Chancellor-Maddison for assistance with the ESR measurements of the Australian samples.
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Current address: Stiftung Preußische Schlösser und Gärten, Berlin-Brandenburg, Abteilung Restaurierung, Postfach 601462, 14414 Potsdam, Germany.