4.4 Absolute chronology

Absolute chronology is just becoming available for the Sima de las Palomas breccia column where uranium-thorium determinations of two aragonite crystals from its base correspond to the last interglacial period that lasted from 128,000 to 116,000 years ago, and a third, much higher up (from just below CG-1) of 50-60,000. The three dates lie within a period that had already been suggested in 1994 by three electron spin resonance determinations on unstratified material: namely, (a) 83,000 or 42,000 BP, (b) 146,000 or 73,000 BP, and (c) 532,000 or 266,000 BP - in each case depending on whether background irradiation was 1 or 2 Gray per millennium; Dr Peter Pomery, responsible for those determinations at the Department of Chemistry, Queensland University, considered 1 Gy/kyr a likely level for limestone, though maybe Cabezo Gordo magnetite lodes raised it higher.

Under the control of a portable gamma-ray spectrometer Dr. John Mitchell and Professor Derek Roe (Oxford University, Donald Baden-Powell Quaternary Research Centre) took a sedimentary sample one-third of the way down the exposed Sima de las Palomas column, where the background dose-rate was 1.25±0.04 mGy/year. It gave an age of 157,000±22,500 BP when OSL (optically stimulated luminescence) dated by Professor Michael Tite and Dr. Sara Hall (pers. comm.) at the Research Laboratory for Archaeology and the History of Art (Oxford University). Dr. Ed Rhodes of the laboratory has subsequently said (pers. comm.) that this should really be regarded as a 'maximum age', because

"looking at the luminescence characteristics for the ... sample (1526b)... there are signs suggesting that the sample grains may not have been exposed to sufficient light to zero fully the signal at the time of deposition."


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Last updated: Wed Dec 23 1998