TESTING THE VIABILITY OF ESR DATING ON MIDDLE QUATERNARY STALAGMITES

For dating stalagmites, the preferred method is ²³⁰Th/²³⁴U, but it can rarely be applied to any material older than about 500 ka. Thus, early Middle Quaternary and older stalagmites often remain undated, despite the important climatic data that they may house. ESR can date carbonate fossils ranging from 10 ka to > 2 Ma in age with 5-10% uncertainty using carbonate ESR signals centered near g = 2.0007. Because the ESR signals are not destroyed while performing the dating, unlike TL and OSL signals, ESR signals can be reread as desired to check for reproducibility of the spectral analysis. Because speleothem deposited in deep caves contains clean aragonite or calcite with little associated non-carbonate detritus and relatively low amounts of organic contamination, it should theoretically also be datable by ESR using one of the standard carbonate radical signals. If accurate, reproducible, and precise, ESR may provide a reliable way to date speleothem that far exceeds the ²³⁰Th/²³⁴U dating limit at ~ 500 ka. To test the viability of ESR dates on speleothem, six stalagmites collected from deep within caves ranging in age from 9.1 to > 500 ka as determined by TIMS ²³⁰Th/²³⁴U dating. Where samples permitted, 3-5 subsamples were analyzed per stalagmite. Samples were prepared by subsampling the top in each stalagmitic growth horizon using a hand-held Dremel drill. Sections with minimal visible colouration and large crystal sizes were selected to bracket or parallel the slices dated by ²³⁰Th/²³⁴U. After powdering to 150-74 µm, the powder was sonicated in 1.0 vol% CH₃COOH) for 1.0 minutes, rinsed 15 times in distilled deionized water, once with methanol, once with acetone, and air-dried overnight. Using ⁶⁰Co γ, 16 aliquots weighing 70.0 ± 0.1 mg were artificially irradiated to 0-128 krads at 14.2 rad/s, and annealed for 1.0 hr at 90˚C to remove any unstable interference signals. At 5 mW, the derivative spectra were scanned over 10 mT centered at 336 mT without deconvolution. To assess dose rates, 1.0-2.0 g of powdered stalagmite analyzed by NAA. Reproducibility for the ESR analyses was excellent, while uncertainties were low, averaging < 8%, and accuracy was high for the late Quaternary ages. This suggests the ages for the Early Middle Quaternary stalagmites may also be accurate.