Robust chronological reconstruction for your speleothems using radiocarbon

Hua, Q; McDonald, J; Redwood, D; Drysdale, RN; Lee, S; Fallon, SJ; Hellstrom, JC

Robust chronological reconstruction for your speleothems using radiocarbon

Date

2011-07-21

Authors

Publisher

18th INQUA Congress

Abstract

U/Th dating method is usually employed to build precise and reliable chronologies for speleothems. However, for some speleothems U-series dates may not be useful. We have studied two young speleothems, SC4 from Smiths Cave (Christmas Island, eastern Indian Ocean and WM7 from Wollondilly Cave (Wombeyan caves, SE Australia), with the aim to get a better understanding of past climate and rainfall variability beyond the instrumental records. Attempts to date SC4 by the U/Th method have proved unsuccessful with some age reversals due to multiple sources of non-authigenic Th. In addition, a sample close to the base of speleothem WM7 dated by U-series resulted in an imprecise age of ~4000 ± 500 cal BP (2?), which is due to the fact WM7 has low uranium concentrations (<10 ppb) and consequently contains very low levels of authigenic 230Th for a good U/Th age determination. In this paper, we investigate the possibility of using radiocarbon for the reconstruction of reliable chronologies for these young speleothems. Approximately 100 carbonate samples from the two stalagmites were analysed for 14C using the AMS facilities at ANSTO and ANU. The AMS results indicated that bomb 14C was evident in the youngest parts of both stalagmites. Dead carbon fraction (DCF) values for these stalagmites for the pre-bomb period were estimated based on the timing of 14C dates for that period determined by high-resolution stable isotopes recorded in the speleothems (?18O for SC4 and ?13C for WM7), and the timing of the onset of bomb 14C. Chronologies of these speleothems were built based on a dense sequence of AMS dates using 3 different age-depth models, OxCal and Bacon (Bayesian statistical approach) and Clam (non-Bayesian statistical model). The result of our radiocarbon-based chronologies and their reliability will be discussed for different DCF values including the mean DCF for the pre-bomb period and for different age-depth models. Copyright (c) 2011 INQUA 18

Keywords

Uranium concentrates, Carbon 14, Thallium, Age estimation, Biological models, Sampling

Citation

Hua, Q., McDonald, J., Redwood, D., Drysdale, R., Lee, S., Fallon, S., Hellstrom, J., (2011). Robust chronological reconstruction for young speleothems using radiocarbon. Presented to the 18th International Union for Quaternary Research Congress, 21th-27th July 2011, Berne Switzerland.