Now showing items 3409-3428 of 20709

    • ¹⁴C Bomb Effect in Tree Rings of Tropical and Subtropical Species of Brazil

      Lisi, Claudio S.; Pessenda, Luiz C. R.; Tomazello, Mario; Rozanski, Kazimiers; ¹⁴C Laboratory, Center for Nuclear Energy in Agriculture, University of São Paulo, Picacicaba, Brazil; Tree-Ring Laboratory, Department of Forest Sciences, University of São Paulo, Piracicaba, Brazil; University of Mining and Metallurgy, Faculty of Physics and Nuclear Techniques, Krakow, Poland (Tree-Ring Society, 2001)
      Atmospheric nuclear tests in the early 1960s introduced large amounts of radiocarbon into the atmosphere, which resulted in an increase of tropospheric ¹⁴CO₂ concentration by nearly 100% during the years 1964-1965. The bomb-produced ¹⁴C was then gradually incorporated within the global carbon cycle. The history of ¹⁴C concentration in the troposphere is preserved within annual growth layers of trees and can be reconstructed for those areas where direct measurements of 14C in the atmosphere were not performed. The paper presents results of ¹⁴C activity measurements in tree rings of tropical and subtropical species from Brazil, for the period 1945-1997. We investigated two species ( Araucaria angustifolia and Parkia sp.) growing at three sites covering the latitudinal band between 7 °S and 24 °S. The results indicate that the maximum ¹⁴C activity in the Southern Hemisphere occurred in 1965, with the Δ¹⁴C values reaching around 700%. Significant differences in Δ¹⁴C were recorded among the studied sites for the period of maximal ¹⁴C levels in the atmosphere, with the highest level observed at the tropical site and lowest in the subtropical zone. This reflects the dynamics of interhemispheric transport of ¹⁴C during the years of high spatial and temporal gradients of this isotope in the atmosphere.
    • C. E. Buck, W. G. Cavanagh and C. D. Litton. Bayesian Approach to Interpreting Archaeological Data

      Scott, Marian (Department of Geosciences, The University of Arizona, 1997-01-01)
      Reviewed by Marian Scott, Department of Statistics, University of Glasgow
    • C3/C4 Production Shift on Seasonal Burns: Northern Mixed Prairie

      Steuter, A. A. (Society for Range Management, 1987-01-01)
      This study investigates the potential of fire to manipulate the balance of C3 (cool-season) and C4 (warm-season) herbage in 2 northern Mixed Prairie communities. The xeric high prairie community and mesic low prairie community were chosen to represent regional moisture extremes. Treatments included dormant spring burn, mid-summer burn, dormant fall burn, and untreated. The high prairie community appears to be a C3-dominant type. All 3 burn treatments increased the C3 herbage fraction relative to untreated sites. Total production, however, was unaffected by treatment. The C3/ C4 ratio of high prairie communities appears to be the result of long-term adaptation rather than short-term adjustments to fire or weather effects. Spring burning shifted low prairie communities towards C4 herbage relative to other treatments. This was due to an increase in C4 herbage (and total) rather than to a decrease in C3 herbage. The C3/C4 ratio of low prairie communities did appear to respond to short-term adjustments in moisture, temperature, and light caused by the spring burn. The response of low prairie C3/C4 ratios to mid-summer and dormant fall burns appeared to be related to phenological and indirect weather effects rather than to changes in site microclimate caused by the fires.
    • Ca,Al-rich inclusions in Rumuruti (R) chondrites

      Rout, S. S.; Bischoff, A. (The Meteoritical Society, 2008-01-01)
      Rumuruti chondrites (R chondrites) constitute a well-characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al-rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al-rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al-rich objects (101 CAIs, 19 Al-rich chondrules, and 6 spinel-rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel-rich inclusions (42), (2) fassaite-rich spherules, (3) complex spinel-rich CAIs (53), (4) complex diopside-rich inclusions, (5) Al-rich chondrules, and (6) Al-rich (spinel-rich) fragments. The simple concentric and complex spinel-rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al-(Cl)-rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R-chondrite group. Some Ca,Al-rich inclusions appear to be primitive (e.g., low FeO-contents in spinel, low abundances of Na,Al-(Cl)-rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al- (Cl)-rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R-chondrite lithologies have abundant secondary Ab-rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.
    • Caatinga vegetation dynamics under various grazing intensities by steers in the semi-arid Northeast, Brazil

      de Albuquerque, S. G. (Society for Range Management, 1999-05-01)
      The effects of cattle grazing were evaluated on range dynamics of the Caatinga which is a deciduous dry woodland, covering most of the semi-arid Brazilian Northeast. Three stocking rates (SR) were studied (heavy, 1 steer 6.7 ha(-1); moderate 1 steer 10 ha(-1); light, 1 steer 13.3 ha(-1)), in addition to an ungrazed exclosure (zero stocking). In the first phase (1978-81) each stocking rate was tested under continuous and deferred grazing. In the second phase (1981-84), deferred grazing was eliminated, so that pastures became replications of continuous grazing. Six steers per pasture were used, and pasture size was used to vary stocking rate. There was no effect of stocking rate or grazing system period on the frequency of the herbaceous species. They were, however, influenced by rainfall in the period, and could be divided into 3 groups. Sixteen species increased with increasing rainfall during the last months of the rainy season, and reached the highest frequency in 1984. Eleven species also increased with increasing rainfall but reached the highest frequency in 1983. Rainfall had no effect on the frequency of 2 important species. Herissantia crispa (L.) Briz. and Selaginella convoluta Spring. Death rate of 5 shrubs (Lippia microphylla Cham., Croton rhamnifolius (Kunth em.) Mull. Arg. Calliandra depauperata Benth. Cordia leucocephala Moric., and Bauhinia cheilantha (Bong.) Steud.) decreased with decreasing stocking rate, 11.7, 9.3, 7.7. and 4.5%, respectively on heavy, moderate, light, and zero stocking. Death rates were higher in easily broken shrub species. L. microphylla and C. leucocephala. Stocking rate also influenced the height growth rate of the tagged shrubs, being respectively -2.7 and 9.8% for heavy and zero stocking. Mean density of shrubs and trees, determined by the Point-Centered Quarter Method, was respectively 21,109, and 447 plants ha(-1) in 1982, and 13,230 and 401 plants in 1984; the main cause of the high shrub death (37.3%) was probably the 1982 drought. Density was not affected by stocking rate. Considering the 7 experimental areas separately, there was no regression between 1982 and 1984 shrub densities. There was, however, regression between 1982 density and the difference between 1982 and 1984 densities.
    • Cacti in the Living Plant Collection of the Boyce Thompson Southwestern Arboretum

      Newland, Kent C.; Crosswhite, Frank S.; Boyce Thompson Southwestern Arboretum (University of Arizona (Tucson, AZ), 1985)
    • Cactus Invasion; Can We Stop It?

      Brown, A. L. (College of Agriculture, University of Arizona (Tucson, AZ), 1951)
    • Cairo Natural Radiocarbon Measurements I

      Nakhla, Shawki M.; Mohammed, Ferial M. (American Journal of Science, 1974-01-01)
    • Calcisols In Arizona

      Buol, S. W.; Department of Agricultural Chemistry and Soils (College of Agriculture, University of Arizona (Tucson, AZ), 1964)
    • Calcium Polysulphides For Root-Rot Control

      Streets, R. B.; Plant Pathology (College of Agriculture, University of Arizona (Tucson, AZ), 1953)
    • Calcium-aluminum-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites

      Aléon, J.; Krot, A. N.; McKeegan, K. D. (The Meteoritical Society, 2002-01-01)
      Calcium-aluminum-rich refractory inclusions (CAIs) in CR chondrites are rare (<1 vol%), fairly small (<500 micrometers) and irregularly-shaped, and most of them are fragmented. Based on the mineralogy and petrography, they can be divided into grossite +/- hibonite-rich, melilite-rich, and pyroxene-anorthite-rich CAIs. Other types of refractory objects include fine-grained spinel-melilite-pyroxene aggregates and amoeboid olivine aggregates (AOAs). Some of the pyroxene-anorthite-rich CAIs have igneous textures, and most melilite-rich CAIs share similarities to both the fluffy and compact type A CAIs found in CV chondrites. One major difference between these CAIs and those in CV, CM, and CO chondrites is that secondary mineral phases are rare. In situ ion microprobe analyses of oxygen-isotopic compositions of 27 CAIs and AOAs from seven CR chondrites demonstrate that most of the CAIs are 16O-rich (Delta-17O of hibonite, melilite, spinel, pyroxene, and anorthite <-22 ppm) and isotopically homogeneous within 3-4 ppm. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, 16O-rich compositions (-24 ppm < Delta-17O < -20 ppm). In contrast, objects which show petrographic evidence for extensive melting are not as 16O-rich (Delta-17O less than -18 ppm). Secondary alteration minerals replacing 16O-rich melilite in melilite-rich CAIs plot along the terrestrial fractionation line. Most CR CAIs and AOAs are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes, which is reflected in their relatively homogeneous 16O-rich compositions. It is likely that these objects (or their precursors) condensed in an 16O-rich gaseous reservoir in the solar nebula. In contrast, several igneous CAIs are not very enriched in 16O, probably as a result of their having melted in the presence of a relatively 16O-poor nebular gas. If the precursors of these CAIs were as 16O-rich as other CR CAIs, this implies either temporal excursions in the isotopic composition of the gas in the CAI-forming regions and/or radial transport of some CAI precursors into an 16O-poor gas. The absence of oxygen isotope heterogeneity in the primary minerals of melilite-rich CAIs containing alteration products suggests that mineralogical alteration in CR chondrites did not affect oxygen-isotopic compositions of their CAIs.
    • Calculating Damages in Investment Arbitration: Should Tribunals Take Country Risk into Account [Article] [Comment]

      Dominguez, Marcos D. Garcia (The University of Arizona James E. Rogers College of Law (Tucson, AZ), 2017)
    • Calculating Foraging Area Using Global Navigation Satellite System (GNSS) Technology

      Anderson, Dean M. (Society for Range Management, 2014-12-01)
      On the Ground • Optimum forage utilization on animal-dominated landscapes can only occur when stocking rate (SR) and stocking density (SD) are considered and managed simultaneously. • Landscapes with foraging animals contain vegetation ranging from unused to over-used even under a proper SR. • The global navigation satellite system (GNSS) technology has catapulted our understanding of spatial–temporal management of free-ranging livestock into a 24/7 opportunity. • Location-specific data will improve management of stocked landscapes, both ecologically and economically. • GNSS data from instrumented animals provides an opportunity to understand when and where a landscape is used to improve animal distribution. • A proper SR and management of animal distribution (i.e., SD) will facilitate adaptive management of animal dominated landscapes.
    • Calculating Grazing Intensity for Maximum Profit on Ponderosa Pine Range in Northern Arizona

      Pearson, H. A. (Society for Range Management, 1973-07-01)
      The profit formula is based on forage production, digestibility and utilization, animal weight and daily gain, costs per animal day, and beef prices. Rangeland producing 500-1,000 lb forage per acre would produce maximum profit with moderate utilization.
    • Calculating Sediment Compaction for Radiocarbon Dating of Intertidal Sediments

      Bird, M. I.; Fifield, L. K.; Chua, S.; Goh, B. (Department of Geosciences, The University of Arizona, 2004-01-01)
      This study estimates the maximum and minimum degrees of autocompaction for radiocarbon-dated Holocene mangrove sediments in Singapore, in order to correct apparent sediment accretion rates for the effects of sediment compression due to autocompaction. Relationships developed for a suite of modern (surface) sediment samples between bulk density, particle-size distribution, and organic matter content were used to estimate the initial (uncompacted) bulk density of buried and variably compressed Holocene sediments, based on the grain-size distribution and organic matter content of the sediment. The difference between measured (compacted) and initial (uncompacted) bulk density of each buried sediment interval can be interpreted as the amount of length shortening experienced by each interval since burial. This allows the elevation of samples selected for 14C dating to be corrected for the effects of autocompaction of the underlying sediment sequence, so that accurate estimates of vertical sediment accretion rates can be calculated. The 3 Holocene mangrove sequences analyzed and dated for this study ranged in age from 2000 to 8500 cal BP. The effects of autocompaction are significant, even in comparatively thin sequences, with subsidence of up to 56 cm calculated for carbon-dated samples presently 2 m above incompressible basement. The vertical sediment accretion rates for these mangrove sequences ranged from 0.99 to 6.84 mm/yr and carbon sequestration rates ranged from 0.9 to 1.7 t/ha/yr, all within the range observed for comparable Holocene and modern mangrove sediments elsewhere.
    • Calculating Yearlong Carrying Capacity: An Algebraic Approach

      Workman, J. P.; MacPherson, D. W. (Society for Range Management, 1973-07-01)
      Estimates of yearlong carrying capacity obtained by three different techniques are compared in terms of accuracy as measured by actual carrying capacity of a northern Utah cattle ranch. A new "algebraic" approach appears superior to two established techniques currently in use.
    • Calculation of Past Dead Carbon Proportion and Variability by the Comparison of AMS 14C and TIMS U/Th Ages on Two Holocene Stalagmites

      Genty, Dominique; Massault, Marc; Gilmour, Mabs; Baker, Andy; Verheyden, Sophie; Keppens, Eddy (Department of Geosciences, The University of Arizona, 1999-01-01)
      Twenty-two radiocarbon activity measurements were made by accelerator mass spectrometry (AMS) on 2 Holocene stalagmites from Belgium (Han-stm lb) and from southwest France (Vil-stm lb). Sixteen thermal ionization mass spectrometric (TIMS) U/Th measurements were performed parallel to AMS analyses. The past dead carbon proportion (dcp) due to limestone dissolution and old soil organic matter (SOM) degradation is calculated with U/Th ages, measured calcite 14C activity and atmospheric 14C activity from the dendrochronological calibration curves. Results show that the dcp is different for the 2 stalagmites: between 10,800 and 4780 yr from present dcp = 17.5% (sigma = 2.4; n = 10) for Han-stm lb and dcp = 9.4% (sigma = 1.6; n = 6) between 3070 and 520 yr for Vil-stmlb. Despite a broad stability of the dcp during the time ranges covered by each sample, a slight dcp increase of about 5.0% is observed in the Han-stmlb sample between 8500 and 5200 yr. This change is synchronous with a calcite delta-13C increase, which could be due to variation in limestone dissolution processes possibly linked with a vegetation change. The dcp and delta-13C of the 2 studied samples are compared with 5 other modern stalagmites from Europe. Results show that several factors intervene, among them: the vegetation type, and the soil saturation leading to variable dissolution process systems (open/closed). The good correlation (R2 = 0.98) between the U/Th ages and the calibrated 14C ages corrected with a constant dcp validates the 14C method. However, the dcp error leads to large 14C age errors (i.e. 250-500 yr for the period studied), which is an obstacle for both a high-resolution chronology and the improvement of the 14C calibration curves, at least for the Holocene.
    • Calendar Age of Lisakovsky Timbers Attributed to Andronovo Community of Bronze Age in Eurasia

      Panyushkina, Irina P.; Mills, Barbara J.; Usmanova, Emma R.; Cheng, Li (Department of Geosciences, The University of Arizona, 2008-01-01)
      We measured radiocarbon ages of 22 decadal replications and 1 bulk group from 5 tree-ring specimens using acid-base-acid pretreatment and accelerator mass spectrometry (AMS). The study has the goal of refining the precision and resolution of a segment of the conventional Bronze Age chronology in the Eurasian steppe attributed to the multicultural community known as Andronovo. The archaeological timbers were gathered from 3 cemeteries at the Lisakovsky cluster of sites in Kazakhstan, where there is a prominent Andronovo occurrence that appears to show evidence of overlapping Alakul and Fedorovo cultures in the southern margin of the Eurasian steppe. The new set of Andronovo calendar dates derived from 14C wiggles and a composite floating tree-ring chronology places the cultural overlap from 1780 to 1660 cal BC. Results indicate older ages of artifacts from the Lisakovsky site than were previously determined by the typological chronology, shifting them from the Late Bronze Age to also include the transition between the Middle and Late Bronze Age. The chronological order of the Lisakovsky cemeteries provides strong evidence of contemporaneity of the Alakul and Fedorovo cultures in the Tobol River Valley for a portion of the 120-yr period of occupation. We discuss an application of the dated Alakul-Fedorovo overlap to the relationship and origin of different groups of the Andronovo community in the Ural region. Our results demonstrate the substantial power that tree rings from Bronze Age timbers provide for developing a precise and highly resolved calendar chronology of prehistoric human occupation in the Eurasian steppe during the 2nd millennium BC.
    • Calf and Lamb Feeding Experiments at U. of A. Farm

      Stanley, E. B. (College of Agriculture, University of Arizona (Tucson, AZ), 1926-12)
    • Calf Feeding Test Explained to Stockmen

      Stanley, E. B.; University of Arizona (College of Agriculture, University of Arizona (Tucson, AZ), 1926-05)