Now showing items 20699-20709 of 20709

    • ZAGRADA—The New Zagreb Radiocarbon Database

      Porter, Antun; Obelić, Bogomil; Krajcar Bronić, Ines (Department of Geosciences, The University of Arizona, 2010-01-01)
      After introducing the liquid scintillation counting (LSC) method and graphite target preparation for accelerator mass spectrometry (AMS) measurements in the Zagreb Radiocarbon Laboratory, the existing database software designed only for gas proportional counting (GPC) (using a DOS operating system) could not satisfy the requirements for parallel conduction of several techniques. This has been enabled recently by the development of the new relational database ZAGRADA, which--using SQL scripts and constraints defined by primary and foreign keys--enforces high data integrity and provides better performances in data filtering and sorting. The structural scheme of this database conceptually comprises 4 basic modules with data on the samples, chemical pretreatment and preparation, measurements and data on the final results. A user-friendly graphical user interface has been designed to perform various actions and data manipulation to the database. The implementation of a new database for 14C samples has significant contribution to scientific research performed in the Radiocarbon and Tritium Laboratory and to quality assurance and quality control, and will enable better and easier communication with customers.
    • Zea diploperennis: A Primitive Relative Offers New Traits to Improve Corn

      Nault, L. R.; Findley, W. R.; Department of Entomology, Ohio Agricultural Research and Development Center; Agricultural Research Service, U.S. Department of Agriculture (University of Arizona (Tucson, AZ), 1982-06)
    • Zen and the Art of Plant Communities

      Siegwarth, Mark; Boyce Thompson Arboretum (University of Arizona (Tucson, AZ), 2012-06)
    • Zero BP Plus 34: 25 Years of Radiocarbon

      Deevey, Edward S. (American Journal of Science, 1984-01-01)
    • Zinc Reduction as an Alternative Method for AMS Radiocarbon Dating: Process Optimization at CIRCE

      Marzaioli, F.; Borriello, G.; Passariello, I.; Lubritto, C.; De Cesare, N.; D'Onofrio, A.; Terrasi, F. (Department of Geosciences, The University of Arizona, 2008-01-01)
      The pretreatment of samples for radiocarbon measurements, transforming a variety of materials into graphite solid targets, represents a critical point in the accelerator mass spectrometry (AMS) procedure. We describe the new, state-of-the-art CIRCE AMS preparation laboratory, particularly the setup and optimization of an alternative method, the zinc reduction method, for graphite target production, compared to the more common hydrogen reduction method. Measured 14C values on standard and blank samples reduced via zinc reaction revealed mean background levels, accuracy, and sensitivity comparable to those obtained by our conventional hydrogen reaction lines. Zinc line reduction at the CIRCE laboratory represents an effective and powerful alternative to the conventional hydrogen reduction, ensuring higher sample throughput with lower costs at a comparable performance level.
    • Zonation of Understory Vegetation Around a Juniper Tree

      Arnold, J. F. (Society for Range Management, 1964-01-01)
    • Zone Tillage Saves on Time and Energy

      LePori, W. A.; Stapleton, H. N.; Department of Agricultural Engineering; Department of Agricultural Engineering (College of Agriculture, University of Arizona (Tucson, AZ), 1966)
    • Zoned chondrules in Semarkona: Evidence for high- and low-temperature processing

      Grossman, Jeffrey N.; Alexander, Conel M. O'D.; Wang, Jianhua; Brearley, Adrian J. (The Meteoritical Society, 2002-01-01)
      At least 15% of the low-FeO chondrules in Semarkona (LL3.0) have mesostases that are concentrically zoned in Na, with enrichments near the outer margins. We have studied zoned chondrules using electron microprobe methods (x-ray mapping plus quantitative analysis), ion microprobe analysis for trace elements and hydrogen isotopes, cathodoluminescence imaging, and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved. Mesostases in these chondrules are strongly zoned in all moderately volatile elements and H (interpreted as water). Calcium is depleted in areas of volatile enrichment. Titanium and Cr generally decrease toward the chondrule surfaces, whereas Al and Si may either increase or decrease, generally in opposite directions to one another; Mn follows Na in some chondrules but not in others; Fe and Mg are unzoned. D/H ratios increase in the water-rich areas of zoned chondrules. Mesostasis shows cathodoluminescence zoning in most zoned chondrules, with the brightest yellow color near the outside. Mesostasis in zoned chondrules appears to be glassy, with no evidence for devitrification. Systematic variations in zoning patterns among pyroxene- and olivine-rich chondrules may indicate that fractionation of low- and high-Ca pyroxene played some role in Ti, Cr, Mn, Si, Al, and some Ca zoning. But direct condensation of elements into hot chondrules, secondary melting of late condensates into the outer portions of chondrules, and subsolidus diffusion of elements into warm chondrules cannot account for the sub-parallel zoning profiles of many elements, the presence of H2O, or elemental abundance patterns. Zoning of moderately volatile elements and Ca may have been produced by hydration of chondrule glass without devitrification during aqueous alteration on the parent asteroid. This could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material. Calcium was mainly lost during this process, and other nonvolatile elements may have been mobile as well. Some unzoned, low-FeO chondrules appear to have fully altered mesostasis.
    • Zoning patterns of Fe and V in spinel from a type B Ca-Al-rich inclusion: Constraints on subsolidus thermal history

      Paque, J. M.; Burnett, D. S.; Beckett, J. R. (The Meteoritical Society, 2007-01-01)
      We obtained two-dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS-34 with a 4-5 micrometer resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS-34 contained V-rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ~8 micrometer and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600-700 degrees C, this leads to a time scale for calciumaluminum- rich inclusion (CAI) alteration in the range of decades to centuries.
    • Łódź Radiocarbon Dates III

      Kanwiszer, Andrzej; Trzeciak, Pawet (Department of Geosciences, The University of Arizona, 1991-01-01)
    • Δ14C and δ13C of Seawater DIC as Tracers of Coastal Upwelling: A 5-Year Time Series from Southern California

      Santos, Guaciara M.; Ferguson, Julie; Acaylar, Kayla; Johnson, Kathleen R.; Griffin, Sheila; Druffel, Ellen (Department of Geosciences, The University of Arizona, 2011-12-16)
      Marine radiocarbon (14C) is a widely used tracer of past ocean circulation, but very few high-resolution records have been obtained. Here, we report a time series of carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from the Newport Beach pier in Orange County, within the Southern California Bight, from 2005 to 2010. Surface seawater was collected bimonthly and analyzed for 14C, 13C, and salinity. Results from May 2005 to November 2010 show no long-term changes in 13C DIC values and no consistent variability that can be attributed to upwelling. 14C DIC values have lowered from ~34 to about ~16, an 18 decrease from the beginning of this project in 2005, and is consistent with the overall 14C depletion from the atmospheric thermonuclear bomb pulse at the end of the 1950s. 14C DIC values, paired with salinity, do appear to be suitable indicators of upwelling strength with periods of upwelling characterized by more saline and lower DIC 14C values. However, a similar signal was not observed during the strong upwelling event of 2010. These results were obtained in the Southern California Bight where upwelling is fairly weak and there is a complex oceanographic circulation in comparison with the remaining western USA coastline. It is therefore likely that the link between DIC 14C, salinity, and upwelling would be even stronger at other sites. These data represent the longest time series of 14C data from a coastal Southern California site performed to date.