HR: 1330h
AN: PP32B-0296    [PDF]
TI: Initial $^{230}$Th and isotopic compositions of U and Th in a Holocene stalagmite from Ninh Binh, Vietnam
AU: * Shen, C
EM: river@ntu.edu.tw
AF: National Taiwan University, Department of Geosciences, Taipei, 106 Taiwan
AU: Sung, Y
EM: r89224205@ntu.edu.tw
AF: National Taiwan University, Department of Geosciences, Taipei, 106 Taiwan
AU: Chen, Y
EM: ygchen@ccms.ntu.edu.tw
AF: National Taiwan University, Department of Geosciences, Taipei, 106 Taiwan
AU: Ho, W
EM: howk@ms35.url.com.tw
AF: National Taiwan University, Department of Geosciences, Taipei, 106 Taiwan
AU: Wang, X
EM: wang0452@tc.umn.edu
AF: University of Minnesota, Department of Geology & Geophysics, Minneapolis, MN 55455 United States
AU: Galang, C
EM: star_galaxy_me@msn.com
AF: University of Arizona, Department of Physics, Tucson, AZ 85721 United States
AU: Cheng, H
EM: cheng021@umn.edu
AF: University of Minnesota, Department of Geology & Geophysics, Minneapolis, MN 55455 United States
AU: Edwards, R
EM: edwar001@tc.umn.edu
AF: University of Minnesota, Department of Geology & Geophysics, Minneapolis, MN 55455 United States
AU: Burr, G
EM: burr@u.arizona.edu
AF: University of Arizona, Department of Physics, Tucson, AZ 85721 United States
AU: Lam, D
EM: geoins@ncst.ac.vn
AF: National Centre for Natural Sciences and Technology, Instituteof Geology, Hanoi, 84 Viet Nam
AB: The application of the $^{238}$U-$^{234}$U-$^{230}$Th disequilibrium dating technique to cave carbonates has provided access to a valuable source of Quaternary environmental information. For some speleothems, however, the initial Th associated with the incorporation of detrital materials into the growing carbonate matrix can bias the true ages, which play a pivotal role in the paleo-sciences. Isochron techniques and development diagrams (Cheng et al., 2000, {\ G.C.A.}, {\bf 64}, 2401-2416) were used on a stalagmite with unusually high thorium concentrations of 5-130 ppm and low uranium concentration of 100-300 ppb, collected from Ninh Binh, Vietnam, in order to evaluate the initial $^{230}$Th values, $^{230}$Th/$^{232}$Th ratios and ages. Uranium and thorium concentrations and isotopic compositions for forty subsamples collected from eight horizons of this 4600-yr old speleothem were analyzed by ICP-MS techniques (Shen et al., 2002, {\ Chem. Geology}, {\bf 185}, 165-178). Results from the application of isochron techniques were: (1) 50-90% of the $^{230}$Th was deposited initially and must be subtracted from measured $^{230}$Th in order to obtain accurate ages; (2) in some cases, discordant ages were obtained among subsamples on the same horizon if the same initial of $^{230}$Th/$^{232}$Th was used for correction; (3) there was some variability in the initial $^{230}$Th/$^{232}$Th values for different layers. These observations indicate that there are many Th sources with different concentrations and $^{230}$Th/$^{232}$Th ratios introduced into growing layers during crystallization. Accordingly, the initial values of $^{230}$Th/$^{232}$Th ratios vary between and within horizons. Such heterogeneous coprecipitation of detrital materials places some limits on the utility of simple two-dimensional isochron approaches to speleothems with large and complicated Th contamination, which enhances the uncertainty of estimated ages. If the development diagram technique is also employed, the initial values of $^{230}$Th/$^{232}$Th ratios are more tightly constrained, varying from 17 to 26 ppm. However, even this sample, with very high thorium levels, yields ages that are generally consistent with AMS-$^{14}$C ages of this stalagmite, which has only a trivial dead carbon problem. Therefore, speleothems with moderate Th-levels can likely be dated precisely using the approaches that we outlined here.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 9320 Asia
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting