Larry Edwards: U-Th Dating of Cave Deposits скачать в хорошем качестве

Larry Edwards: U-Th Dating of Cave Deposits

12th Institute for Rock Magnetism Conference Session 3: Speleothem Magnetism Conveners: Josh Feinberg (IRM, University of Minnesota) and Ricardo Trindade (University of São Paulo) Keynote Presentation - U-Th Dating of Cave Deposits Author: R. Lawrence ‘Larry’ Edwards Institution: Department of Earth and Environmental Sciences, University of Minnesota, Twin Cities, USA Abstract: Uranium-Thorium (U-Th) or 230Th dating is appropriate for dating many types of carbonates precipitated from surface or subsurface waters. This method involves the measurement of 238U (t1/2 = 4.468 x 109 y), 234U (t1/2 = 245,620 y), and 230Th (t1/2 = 75,590 y), all nuclides in the 238U decay chain. All are typically present in trace quantities, but the latter two generally have extremely low natural concentrations, with 230Th concentrations as low as parts in 1015. With current technical capabilities, the method covers an age range from 1 year to 650,000 years. It is well-suited to the dating of cave calcite and aragonite (speleothems) with the right set of characteristics. Carbonate that is a few to 100 years old can be dated to a precision of one year. Carbonate deposited 10 ka ago can reach 2σ uncertainties as low as ±10 years, ±300 years at 130 ka, ±800 years at 200 ka, ±3 ka at 300 ka, ±8 ka at 400 ka, ±15 ka at 500 ka, and ±40 ka at 600 ka. Because of the low natural concentrations of 230Th and 234U, 230Th dating has always been limited technically, with respect to precision and sample-size requirements. The first 230Th dates [1] were determined by alpha-counting. Edwards [2] developed mass spectrometric techniques for 230Th dating, increasing the sensitivity of the method by 4 orders of magnitude. Subsequently, the use of inductively-coupled plasma ionization techniques has increased sensitivity another order of magnitude [e.g., 3] Although the alpha-counting approach led to important early contributions, applications related to 230Th dating of speleothems dating jumped forward with the development of mass spectrometric techniques, evolving into today’s vibrant field. Particularly significant contributions have been and continue to be made to fields of climate and environmental change and to archeology and cultural change. In addition, this approach has led to the calibration of the full 14C timescale, a goal of the scientific community for 7 decades. This calibration is, in part related to the geomagnetic field’s modulation of 14C-producing cosmic rays. In addition, this approach has been used to constrain the timing of the Laschamps Excursion and to establish the chronology for environmental magnetism studies. [1] Barnes, J.W., Lang, E.J., and Potratz, H.A. 1956. The ratio of ionium to uranium in coral limestone. Science 124, 175-176. [2] Edwards, R.L., Chen, J.H. and Wasserburg, G.J. 1987. U-238, U-234, Th-230, Th-232 systematics and the precise measurement of time over the past 500,000 years. Earth Planet. Sci. Lett., 81, 175-192. [3] Cheng, H., Edwards, R.L., Shen, C.C., Polyak, V.J., Asmerom, Y., Woodhead, J., Hellstrom, J., Wang, Y.J., Kong, X.G., Spotl, C., Wang, X.F., and Alexander, E.C. 2013. Improvements in 230Th dating, 230Th and 234U half-life values, and U-Th isotopic measurements by multi-collector inductively coupled plasma mass spectrometry. Earth Planet. Sci. Lett. 371, 82-91. doi.org/10.1016/j.epsl.2013.04.006