Browsing by Author "Redwood, D"
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- ItemRobust chronological reconstruction for young speleothems using radiocarbon(Elsevier Science Ltd, 2012-12-01) Hua, Q; McDonald, J; Redwood, D; Drysdale, RN; Lee, S; Fallon, SJ; Hellstrom, JCWe have studied two young speleothems, SC4 from Smiths Cave (Christmas Island, eastern Indian Ocean) and WM7 from Wollondilly Cave (Wombeyan caves, SE Australia). Attempts to date these speleothems by the Th/U method have proved unsuccessful with some age reversals for SC4 due to multiple sources of non-authigenic Th. This method has also resulted in imprecise ages for WM7 because of low U concentrations (<10 ppb) and consequently very low levels of authigenic Th-230 relative even to the very low levels of detrital Th-230 present. Here, we present an alternative method for reliable dating of these young speleothems using radiocarbon. Approximately 100 carbonate samples from SC4 and WM7 were analysed for C-14 by accelerator mass spectrometry (AMS). The AMS results indicate that bomb C-14 was evident in the youngest parts of both stalagmites. Two different approaches were used to estimate dead carbon fraction (DCF) values for these stalagmites for the pre-bomb period. For SC4, the DCF values were estimated based on the timing of C-14 dates for that period determined by high-resolution delta O-18 recorded in the speleothem, and the timing of the onset of bomb C-14. For WM7, a "maximum" range of pre-bomb DCF was determined. Chronologies of these speleothems were built based on a dense sequence of DCF-corrected ages using three different age-depth models: Clam (Classical method), and Bacon and OxCal (Bayesian statistical approach). Good agreement between these age-depth models were observed indicating that the top 170 mm of SC4 and the top 50 mm of WM7 grew during the past 550-750 years and 1360-1740 years, respectively. © 2012, Elsevier Ltd.
- ItemRobust chronological reconstruction for your speleothems using radiocarbon(18th INQUA Congress, 2011-07-21) Hua, Q; McDonald, J; Redwood, D; Drysdale, RN; Lee, S; Fallon, SJ; Hellstrom, JCU/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
- ItemUsing the 14C bomb pulse to date young speleothems(University of Arizona, 2011-04-01) Hodge, E; McDonald, J; Fischer, MJ; Redwood, D; Hua, Q; Levchenko, VA; Drysdale, RN; Waring, CL; Fink, DMany factors may influence the radiocarbon age results of lacustrine sediments, among which the hardwater effect is particularly important. Daihai Lake is a closed lake located in the semi-arid region of Inner Mongolia, China. High concentrations of (HCO3)- and (CO3)2- and high pH values in the lake water imply that there is a hardwater effect when using bulk lacustrine sediment samples for 14C dating. To correct the apparent 14C age, we present a pilot study based on a series of 14C ages of lake surface sediment, lake water, submerged aquatic plant (Myriophyllum), fish bone (Cyprinus carpio), and surface soil samples from and around Daihai Lake. Assuming that the relationship between the 14C/12C ratio of DIC and of atmospheric CO2 was constant (at 0.816), the hardwater effect ages calculated for the past 8000 yr would have varied from 949 to 1788 yr. Together with the reservoir effect and soil organic matter input, the hardwater effect is a major factor causing changes in apparent age when using bulk organic matter for 14C dating. © 2011, University of Arizona.