The Effects of Contamination of Calcareous Sediments on their Radiocarbon Ages

Abstract

Two principal reasons for the inherent uncertainty in 14C dating of calcareous sediments such as tufa or those of lacustrine origin are the unknown initial 14C activity (Ao) of the sediment, mainly affecting younger (Holocene) samples, and contamination of older sediments with recent carbonate, causing 14C ages to be excessively young. To assess the contamination effect, samples of old tufa from the Riss/Würm interglacial were examined. These sediments contain essentially no 14C except that contributed by surface contamination. Tufa samples were crushed and grains ranging in size from <1 mm, 1 to 2mm, up to 4 to 5mm were separated for analysis; 2M HCl was then used to dissolve the samples in successive steps. 14C measurements indicated that each subsequent soluble fraction obtained from porous tufa gave a successively older age, indicating that the surface of the sample was contaminated by younger carbonates. No consistent effect of grain size on the 14C age was observed. Compact tufa proved to be less subject to contamination. 14C ages obtained on this material were also too young, yet older than the age obtained from porous tufa samples.14C ages of interglacial tufa were cross-checked with the 230Th/234U dating method, using samples of very clean calcite which overlies the tufa blocks. Inferred 230Th/234U ages of the interglacial tufa (which had yielded 14C dates ranging from 25,000 to 37,000 yr) coincided with the last interglacial (Riss/Würm, Stage 5). Samples of Holocene tufa, in which contributions of recent 14C from surface contamination would pose less of a problem, yielded 14C and 230Th/234U dates which were in excellent agreement.