Now showing items 21-40 of 53

    • WOCE Pacific Ocean Radiocarbon Program

      Key, Robert M. (Department of Geosciences, The University of Arizona, 1996-01-01)
      Fieldwork for the World Ocean Circulation Experiment (WOCE) radiocarbon program was recently completed. Ca. 9000 samples were collected for analysis using both conventional beta-counting techniques and the newer AMS technique. The mean uncertainty for the beta analyses is 3 per mil; for AMS analyses, ca. 4.5 per mil degrees.
    • WOCE AMS Radiocarbon I: Pacific Ocean Results (P6, P16 and P17)

      Key, Robert M.; Quay, Paul D.; Jones, Glenn A.; McNichol, A. P.; Von Reden, K. F.; Schneider, Robert J. (Department of Geosciences, The University of Arizona, 1996-01-01)
      AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14C since the time of the GEOSECS survey (8/73-6/74). Nearsurface 14C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25-50 per mil, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25 per mil. Changes in the 250-750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75 per mil since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50 per mil.
    • Using Bulk Soil Radiocarbon Measurements to Estimate Soil Organic Matter Turnover Times: Implications for Atmospheric CO2 Levels

      Harrison, Kevin G. (Department of Geosciences, The University of Arizona, 1996-01-01)
      Although soil contains about three times the amount of carbon present in the preindustrial atmosphere, determining how perturbations (e.g., changing land use, CO2 fertilization, changing climate and anthropogenic nitrogen deposition) alter soil carbon storage and influence atmospheric CO2 levels has proved elusive. Not knowing the soil carbon turnover times causes part of this uncertainty. I outline a strategy for using radiocarbon measurements to estimate soil organic matter turnover times and inventories in native soil. The resulting estimates of carbon exchange produce reasonable agreement with measurements of CO2 fluxes from soil. Furthermore, derivatives of the model are used to explore soil carbon dynamics of cultivated and recovering soil. Because the models can reproduce observed soil 14C measurements in native, cultivated, and recovering ecosystems (i.e., the underlying assumptions appear reasonable), the native model was modified to estimate the potential rate of additional carbon storage because of CO2 fertilization. This process may account for 45-65% of the "missing CO2 sink."
    • Transect Along 24 Degrees N Latitude of 14C in Dissolved Inorganic Carbon in the Subtropical North Atlantic Ocean

      Severinghaus, Jeffrey P.; Broecker, Wallace S.; Pen, Tsung-Hung; Bonani, Georges (Department of Geosciences, The University of Arizona, 1996-01-01)
      The distribution of bomb-produced 14C in the ocean provides a powerful constraint for circulation models of upper ocean mixing. We report 14C measurements from an east-west section of the main thermocline at 24 degrees N latitude in the subtropical North Atlantic Ocean in summer 1992, and one profile from the Gulf of Mexico in 1993. Observed gradients reflect the transient invasion of bomb 14C into the thermocline via mixing along isopycnals from the poleward outcrop, with progressively more sluggish mixing at greater depths. A slight deepening of the profile is observed over the 20-yr period since the GEOSECS survey at one location where the comparison is possible.
    • Toward an Absolute Chronology at Elk Lake, Minnesota

      Aardsma, Gerald E. (Department of Geosciences, The University of Arizona, 1996-01-01)
      Radiocarbon measurements on organic carbon from the varved cores of Elk Lake, Minnesota suggest that the varve count may underestimate calendar years by 18% for the most recent 3800 varve years.
    • The Use of Carbon Isotopes (13C, 14C) in Soil to Evaluate Vegetation Changes During the Holocene in Central Brazil

      Pessenda, L. C. R.; Aravena, Ramon; Melfi, A. J.; Telles, E. C. C.; Boulet, René; Valencia, E. P. E.; Tomazello, Mario (Department of Geosciences, The University of Arizona, 1996-01-01)
      This paper presents carbon isotope data measured in three soil profiles from the Salitre area, Central Brazil. The study forms part of a research project on tropical and subtropical soils in Brazil, in which the main objective is to use carbon isotopes to provide information about vegetation changes that have occurred in relation to climate changes during the Holocene. 14C data from charcoal samples and soil organic matter (SOM) indicate that the organic matter in the soils studied is of Holocene age at least. Furthermore, the presence of a significant amount of charcoal in the soils suggests that forest fire was a significant ocurrence during the Holocene and probably had an important role in determining the dynamics of forest vegetation in the study area. Correspondingly, 13C data indicate that C3 plants provided the dominant vegetation of the study area, even during the dry periods when savanna vegetation is supposed to have replaced the forest communities. This study contributes to our better understanding of the relation between climatic changes and vegetation in the subtropical region of Brazil.
    • The Effect of Tillage on Soil Organic Matter Using 14C: A Case Study

      Rutberg, Randye L.; Schimel, David S.; Hajdas, Irena; Broecker, Wallace S. (Department of Geosciences, The University of Arizona, 1996-01-01)
      We compared four adjacent soil plots in an effort to determine the effect of land use on soil carbon storage. The plots were located at the High Plains Agricultural Research Laboratory near Sidney, Nebraska. We measured 14C, total carbon, total nitrogen and 137Ce to determine the size and turnover times of rapid and stable soil organic matter (SOM) pools, and their relation to land-use practices. Results were consistent with the model produced by Harrison, Broecker and Bonani (1993a) in that the 14C surface soil data fell on the time trend plots of world 14C surface soil data, indicating that the natural sod and non-tilled plots had a rapidly turning over SOM pool, comprising ca. 75% of surface soil carbon, and the tilled plots had a rapidly turning over SOM pool, comprising only 50% of surface soil carbon.
    • Requirements for an International Radiocarbon Soils Database

      Becker-Heidmann, Peter (Department of Geosciences, The University of Arizona, 1996-01-01)
    • Reservoir Ages in Eastern Pacific Coastal and Estuarine Waters

      Ingram, B. Lynn; Southon, John R. (Department of Geosciences, The University of Arizona, 1996-01-01)
      We have refined marine reservoir age estimates for eastern Pacific Coastal waters with radiocarbon measurements of mollusk shells collected prior to 1950. We have also investigated interspecific variability in 14C ages for historic and ancient shells from San Francisco Bay.
    • Report: Summary of the Workshop "Aspects of High-Precision Radiocarbon Calibration"

      Kromer, Bernd; Ambers, Janet; Baillie, M. G. L.; Damon, Paul E.; Hesshaimer, Vago; Hofmann, Jutta; Jöris, Olaf; Levin, Ingeborg; Manning, S. W.; McCormac, F. G.; et al. (Department of Geosciences, The University of Arizona, 1996-01-01)
    • Regional Radiocarbon Effect Due to Thawing of Frozen Earth

      Damon, P. E.; Burr, George; Peristykh, A. N.; Jacoby, G. C.; D'Arrigo, R. D. (Department of Geosciences, The University of Arizona, 1996-01-01)
      Accelerator mass spectrometry (AMS) measurement of 25 single-year tree rings from AD 1861-1885 at ca. +/3.5 precision shows no evidence of an anomalous 11-yr cycle of 14C near the Arctic Circle in the Mackenzie River area. However, the Delta-14C measurements are lower on average by 2.7 +/0.9 (sigma) relative to 14C measurements on tree rings from the Pacific Northwest (Stuiver and Braziunas 1993). We attribute this depression of 14C to thawing of the ice and snow cover followed by melting of frozen earth that releases trapped 14C-depleted CO2 to the atmosphere during the short growing season from May through August. Correlation of Delta-14C with May-August estimated temperatures yields a correlation index of r = 0.60. The reduction in Delta-14C is dominated by seven years of anomalous depletion. These years are 1861, 1867-1869, 1879-1880 and 1883. The years 1867-1869 are coincident with a very strong ENSO event.
    • Radiocarbon Updates

      Department of Geosciences, The University of Arizona, 1996-01-01
    • Radiocarbon Dating of Soils: Database Contribution by Bonn and Hamburg

      Scharpenseel, H. W.; Pietig, Franz; Schiffman, Heinrich; Becker-Heidmann, Peter (Department of Geosciences, The University of Arizona, 1996-01-01)
      We present a compilation of 14C soil dates measured at the University of Hamburg through 1984 (HAM-1597).
    • Parameters of a Radiocarbon Installation

      Khait, Vladimir Z. (Department of Geosciences, The University of Arizona, 1996-01-01)
      I aim to define instrumental parameters of a radiocarbon laboratory installation whereby one can estimate its precision and a maximum age up to which its measuring results are reliable. The commonly accepted factor of merit (FM) relates the precision of measurement to Poisson statistics. Unlike the FM, the proposed parameters show the extent to which a 14C laboratory is affected by destabilizing factors that could cause additional measurement errors. Assuming that all destabilizing factors produce either a change in counting efficiency or additional fluctuations of the background counting rate, I have derived two parameters for consideration.
    • Post-Bomb Radiocarbon Records of Surface Corals from the Tropical Atlantic Ocean

      Druffel, Ellen R. M. (Department of Geosciences, The University of Arizona, 1996-01-01)
      Delta-14C records are reported for post-bomb corals from three sites in the tropical Atlantic Ocean, In corals from 18 degrees S in the Brazil Current, Delta-14C values increased from ca. -58 per mil in the early 1950s to +138 per mil by 1974, then decreased to 110 per mil by 1982. Shorter records from 8 degrees S off Brazil and from the Cape Verde Islands (17 degrees N) showed initially higher Delta-14C values before 1965 than those at 18 degrees S, but showed lower rates of increase of Delta-14C during the early 1960s. There is general agreement between the coral results and Delta-14C of dissolved inorganic carbon (DIC) measured in seawater previously for locations in the tropical Atlantic Ocean. Delta-14C values at our tropical ocean sites increased at a slower rate than those observed previously in the temperate North Atlantic (Florida and Bermuda), owing to the latter's proximity to the bomb 14C input source in the northern hemisphere. Model results show that from 1960-1980 the Cape Verde coral and selected DIC Delta-14C values from the North Equatorial Current agree with that calculated for the North Atlantic based on an isopycnal mixing model with a constant water mass renewal rate between surface and subsurface waters. This is in contrast to Delta-14C values in Bermuda corals that showed higher post-bomb values than those predicted using a constant water mass renewal rate, hence indicating that ventilation in the western north Atlantic Ocean had decreased by a factor of 3 during the 1960s and 1870s (Druffel 1989).
    • Natural Radiocarbon Measurements in Brazilian Soils Developed on Basic Rocks

      Pessenda, L. C. R.; Valencia, E. P. E.; Camargo, P. B.; Telles, E. C. C.; Martinelli, L. A.; Cerri, C. C.; Aravena, Ramon; Rozanski, Kazimierz (Department of Geosciences, The University of Arizona, 1996-01-01)
      This paper presents 14C, 13C and chemical data of soil organic matter (SOM) in three soil profiles under native forests from Brazil: Londrina (southern), Piracicaba (southeastern) and Altamira (northern). The main objective is to use carbon isotopes in tropical and subtropical soils of Brazil to provide information about vegetation changes that occurred in relation to climate changes during the Holocene. 14C data from SOM indicate that the organic matter in the soils studied is of at least Holocene age. 13C data indicate that C4 plants probably provided the dominant vegetation in Londrina and Piracicaba during the early and mid-Holocene and that C3 plants provided the dominant vegetation in the Altamira region during the Holocene.
    • Large-Radiocarbon, Volume WOCE Radiocarbon Sampling in the Pacific Ocean

      Stuiver, Mine; Östlund, H. G.; Key, Robert M.; Reimer, Paula J. (Department of Geosciences, The University of Arizona, 1996-01-01)
      At the University of Miami Tritium Laboratory and the University of Washington Quaternary Isotope Laboraptory, more than 1000 large-volume Pacific Ocean radiocarbon samples were measured for the WOCE program. Here we present a comprehensive data set, and a brief discussion of our findings.
    • Introduction: Challenges in the Soil

      Harkness, Doug (Department of Geosciences, The University of Arizona, 1996-01-01)
    • Inorganic Radiocarbon in Time-Series Sediment Trap Samples: Implication of Seasonal Variation of 14C in the Upper Ocean

      Honda, Makio C. (Department of Geosciences, The University of Arizona, 1996-01-01)
      In order to verify sediment trap samples as indicators of upper ocean 14C concentrations, particulate inorganic radiocarbon (PIC-Delta-14C) collected by time-series sediment traps in the Sea of Okhotsk and the Bering Sea was measured by accelerator mass spectrometry (AMS). All of the PIC-Delta-14C measurements were <0%, in contrast to GEOSECS 14C data in the upper ocean from the northwestern North Pacific. This difference is attributed to the upwelling of deepwater that contains low Delta-14C of dissolved inorganic carbon (DICAI4C) and to the decrease over time of surface DIC-Delta-14C owing to the decrease of atmospheric Delta-14C values. In addition, PIC-Delta-14C values showed significant seasonal variability: PIC-Delta-14C collected in the fall was the greatest (-22 per mil on average), whereas PIC-Delta-14C collected in winter showed an average minimum of -48 per mil. It is likely that this difference was caused by changes in mixed layer thickness. Although some uncertainties remain, further study on PIC-Delta-14C will enable us to estimate seasonal variability in DIC-Delta-14C and air-sea CO2 exchange rate.
    • Historic Measurements of Radiocarbon in New Zealand Soils

      Lassey, K. R.; Tate, K. R.; Sparks, R. J.; Claydon, J. J. (Department of Geosciences, The University of Arizona, 1996-01-01)
      Extensive measurements of radiocarbon have been used in New Zealand since the mid-1960s to follow carbon (C) movement and turnover in soils. We present here unpublished radiocarbon (14C) measurements on a range of eight New Zealand soils with details of the sites, ecosystems, climates, soil descriptions and associated analytical data. An overview is also given of published 14C measurements on soils, and the use of these measurements to model soil C turnover.