SESSION NO. 50 50-4 9:10 AM Farley, K.A. [53636] SESSION NO. 50, 8:00 AM APPLICATIONS AND LIMITATIONS OF (U-TH)/HE DATING FOR QUATERNARY GEOCHRONOMETRY Monday, July 28, 2003 FARLEY, K.A., Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, CA 91125, [email protected] The recent resurgence of interest in dating by ingrowth of 4He from U and Th decay has been S30. New Developments in Quaternary Numeric driven by thermochronometry applications, but a few studies have demonstrated its strengths and Dating Methods weaknesses for “absolute” dating, especially of very young materials. The main attraction of 4He for this application is that ingrowth in many minerals is rapid relative to blanks and detection limits; e.g., in just 1 Myr the 4He produced in 1 mg of material with 1 ppm U at secular equilibrium is Reno Hilton Resort and Conference Center, Carson 3&4 >100x typical blank. Furthermore, it is commonly possible to trade-off lower U, Th contents against larger sample size, so even materials with ppb levels of parent nuclides may be datable. However the method suffers from two major limitations: 1) Some materials diffuse He at Earth 50-1 8:10 AM Steig, Eric J. [55603] surface temperatures, including volcanic glass and whole rock, quartz, feldspars, and Mn oxides, THE MYSTIQUE OF ICE CORE CHRONOLOGIES while apatite, zircon, titanite, garnet, fluorite, amphibole, pyroxene, and some Fe oxides are STEIG, Eric J., Quaternary Research Center, University of Washington, 19 Johnson Hall, thought to be sufficiently retentive to be potential dating targets (though “excess He” may be a Box 351360, Seattle, WA 98195, [email protected] problem in some of these minerals). 2) Secular disequilibrium in the U series may cause He pro- Chronologies from ice cores are one of the most important benchmarks for Quaternary dating up duction to be time-varying as the intermediate nuclides decay into equilibrium. For volcanic sam- to 100 ka. In addition to being used to define the timing of specific events such as the Younger ples the nuclide of concern is 230Th (t1/2=75 kyr), while in fluid precipitated samples 234U Dyras termination, the GISP2 ice core in particular has been critical for radiocarbon calibration (t1/2=247 kyr) must also be considered. Secular equilibrium is achieved within ~5t1/2 of the prior to 10 ka, since its provides the basis of the chronology of the Cariaco basin varve sequence. longest lived daughter in initial disequilibrium, but He concentrations integrate disequilibrium Furthermore, paleoclimate records from ice cores, due to their high resolution and the general ingrowth and so “remember” it for much longer periods. Methods for determining ages under sec- agreement that they are representative of climate over broad areas, are frequently used as ular disequilibrium are straightforward if the initial conditions are known or assumed; if they are benchmarks for the interpretation of other Quaternary records as far back as 400 ka. For all of not known it is possible to obtain precise but potentially inaccurate ages, with the inaccuracy these reasons, it is important that uncertainties in ice core chronologies be well understood and, decreasing with time since mineral formation. Examples of He ages of Quaternary apatite, zircon, where possible, improved upon. garnet, fluorite and goethite obtained at Caltech and by other groups will be discussed. Over the In this talk, I will briefly review the layer-counting method used to develop the most widely-used range from ~1 kyr to ~ 2 Myr, He ages typically are precise to ±5-10%. Accuracy ranges from benchmark ice core chronologies – GISP2 and GRIP in central Greenland – the trace-gas-match- equally good to unacceptably poor depending on age and knowledge of secular equilibrium state. ing technique used to transfer these chronologies to cores in Antarctica, and published estimates of the uncertainties. I will then discuss possible improvements to the latter chronologies, based on the requirement of self-consistency among time series of ice accumulation rate, temperature and 50-5 9:30 AM verosub, Kenneth L. [55113] the difference between gas age and ice age (\delta age). Finally, I will highlight our recent work on RECENT ADVANCES IN THE USE OF PALEOMAGNETISM FOR DATING SEDIMENTS an ice core from Mt. Logan, Yukon, which demonstrates the reliability of the chronology of this VEROSUB, Kenneth L., Geology, Univ of California - Davis, One Shields Ave, Davis, CA particular core at the sub-annual timescale. 95616, [email protected] The Earth’s magnetic field varies over many different time scales. Over tens of years to hundreds of years, the direction of the field can change by as much as 30 or 40 degrees at mid-latitudes. 50-2 8:30 AM Ku, Teh-Lung [55039] These changes, known as paleosecular variation, are coherent over distances of at least a few U-SERIES DATING OF CONTINENTAL SEDIMENTS: RECENT PROGRESSES AND thousand kilometers, and records of paleosecular variation from different sites in the same region APPLICATIONS can be compared and compiled into a master curve that can then be used to provide a chronolo- KU, Teh-Lung, Earth Sciences, Univ Southern California, 3651 University Ave, Los gy for a new site. In recent years, researchers have used paleomagnetic records from Holocene Angeles, CA 90089-0740, [email protected] lacustrine sequences to develop high-quality master curves for several regions of the world. They Among the many numerical dating tools of the Quaternary, those based on uranium-series dise- have also shown that when master curves from lacustrine sequences are compared to paleomag- quilibria enjoy the widest range in terms of applicable material even if we limit ourselves to con- netic records from nearby marine environments with high deposition rates, the agreement is usu- sidering only the 230Th-234U-238U systematics, as we shall do so in this presentation. Here, we ally quite remarkable. Thus, the use of master curves of paleosecular variation for the dating of will further confine our discussion to the dating of sedimentary deposits from the continental envi- both lacustrine and marine sediments deposited during the past 10,000 years is well-established. ronment. As the 230Th-234U method dates the time of U incorporation into a given sample, the However, the Earth’s magnetic field is a vector which has a magnitude as well as a direction. nearly ubiquitous presence of traces of uranium in natural deposits explains the method’s wide Determining the magnitude (or intensity) of the field is more difficult than determining its direction applicability. In this regard, recent advancement in the analytical capability (precision and sensitiv- because the strength of magnetization of a sediment reflects both the intensity of the magnetic ity) of the long-lived U and Th isotopes of 238U, 234U, 232Th and 230Th using mass spectrometry field at the time of deposition and the concentration of magnetic material in the sediment. In the has played an important role. In addition to having improved the age precision, this atom-counting past fifteen years, methods have been developed to separate the influence of these two factors, technique has significantly expanded the dating range of 10-350 ka commonly quoted for the and it is now possible to obtain reliable records of the paleointensity variations of the Earth’s mag- method when the radiation-counting technique of alpha-spectrometry is used. Examples will be netic field. These records show that the field varies on time scales on the order of a few thousand given to show efforts in the dating of lake evaporites and carbonate, groundwater-precipitated and to a few tens of thousands of years and that these variations are globally coherent. Moreover, pedogenic carbonate and silica, and permafrost peat deposits. These efforts entail attempts at composite records of paleointensity variations now span the last million years. Although these coping with the problem of correcting the non-in-situ produced 230Th often present in terrestrial records were developed from and have been applied mainly to marine sediments, the global deposits, using isochron schemes or micro-sampling of high U/Th aliquots to minimize the correc- coherence of the paleointensity signal means that paleointensity dating can also be used for tion effect on ages. The general validity of results through external and internal cross-checks will lacustrine sediments deposited during the past 1 million years. also be examined. 50-6 10:10 AM Roberts, Richard G. [53695] 50-3 8:50 AM Fairbanks, Richard G. [55791] THE REVOLUTIONARY SMALL-SAMPLE PHOTON-STIMULATED LUMINESCENCE DATING RADIOCARBON CALIBRATION USING HIGH PRECISION 230TH/234U/238U AND 14C METHODS MEASUREMENTS ON PRISTINE CORALS ROBERTS, Richard G., School of Geosciences, Univ of Wollongong, Wollongong 2522 FAIRBANKS, Richard G., Earth & Environmental Sciences, Lamont-Doherty Earth Australia, [email protected]. Observatory of Columbia Univ, 210 Isotope Laboratory, Rt. 9W, Palisades, NY 10964, Huntley et al. (Nature 313, 105-107, 1985) proposed that the time since grains of quartz and [email protected], MORTLOCK, Richard A., Lamont-Doherty Earth feldspar were last exposed to sunlight could be estimated from their photon-stimulated lumines- Observatory, Columbia Univ, Palisades, NY 10964-8000, GUILDERSON, Thomas P., cence (PSL) signals and knowledge of the environmental dose rate. They noted the potential to Center for Accelerator Mass Spectrometry, Lawrence Livermore National Lab, Livermore, obtain ages from individual sub-samples (aliquots), but a working single-aliquot protocol was not CA 94551, and CHIU, Tzu-chien, Earth & Environmental Sciences, Lamont Doherty Earth devised for K-feldspars until 1991, and this was later extended to individual sand-sized grains of K- Observatory, Columbia University, Palisades, NY 10964-8000 feldspar.