engleski

The effects of contamination of calcareaous sediments on their radiocarbon age

Two principal reasons for the inherent uncertainty in 14^C dating of calcareous sediments such as tufa or those of lacustrine origin are the unknown initial 14^C activity (A0) of the sediment, mainly affecting younger (Holocene) samples, and contamination of older sediments with recent carbonate, causing 14^C 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 14^C except that contributed by surface contamination. Tufa samples were crushed and grains ranging in size from <1 mm, 1 to 2 mm, up to 4 to 5 mm were separated for analysis ; 2M HCl was then used to dissolve the samples in successive steps. 14^C 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 14^C age was observed. Compact tufa proved to be less subject to contamination. 14^C ages obtained on this material were also too young, yet older than the age obtained from porous tufa samples. C ages of interglacial tufa were cross-checked with the 230^Th/234^U dating method, using samples of very clean calcite which overlies the tufa blocks. Inferred 230^Th/234^U ages of the interglacial tufa (which had yielded 14^C 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 14^C from surface contamination would pose less of a problem, yielded 14^C and 230^Th/234^U dates which were in excellent agreement.

radiocarbon; tufa; Plitvice

nije evidentirano