• Coral Reef Evolution at the Leeward Side of Ishigaki Island, Southwest Japan

      Yamano, Hiroya; Abe, Osamu; Kitagawa, Hiroyuki; Niu, Etsuko; Nakamura, Toshio (Department of Geosciences, The University of Arizona, 2001-01-01)
      In comparison with windward coral reefs, the facies and evolution of leeward coral reefs has been discussed to a lesser extent. By accelerator mass spectrometry (AMS) carbon-14 dating of coral specimens collected from the trench excavated across a modern coral reef during a fishery port repair, we revealed the internal facies and Holocene evolution of a leeward reef in Ishigaki Island, Ryukyu Island, southwest Japan. The reef facies can be split into three facies: the tabular Acropora reworked facies first formed a ridge by 3500 BP. Then, the tabular Acropora framework facies grew both upward and seaward. The accumulation rates of the tabular Acropora framework facies ranged from 2.2 to 8.3 m/ka. Thus, the reef framework facies and accumulation rates of this leeward reef is similar to those of windward reefs, although the age of the reef top is younger than that of windward reefs.
    • Development of Accurate and Reliable 14C Chronologies for Loess Deposits: Application to the Loess Sequence of Nussloch (Rhine Valley, Germany)

      Hatté, Christine; Pessenda, Luiz Carlos; Lang, Andreas; Paterne, Martine (Department of Geosciences, The University of Arizona, 2001-01-01)
      Due to very high accumulation rates, loess sequences are best suited archives for the continental paleoclimate of glacial periods. Accurate chronologies cannot be easily established by radiocarbon-dating, because of the lack of organic macrorests, the only material for reliable 14C dating so far. A chemical protocol is reported to extract the organic matter of loess (organic carbon content lower than 0.1% by weight) for accelerator mass spectrometry (AMS) 14C dating. Sediments were taken from the loess sequence of Nussloch, for which a large dataset of luminescence ages (TL, IRSL/OSL) is available. The 14C chronology of the organic matter extracted from loess is in good agreement with the corresponding luminescence ages. It allows high resolution correlations with climatic proxy signals (magnetic susceptibility, malacological assemblages, delta-13C on organic matter, etc.) derived from the loess sequence and global environmental proxy records.
    • High-Resolution AMS 14C Dating of Post-Bomb Peat Archives of Atmospheric Pollutants

      Goodsite, Michael E.; Rom, Werner; Heinemeier, Jan; Lange, Todd; Ooi, Suat; Appleby, Peter G.; Shotyk, William; van der Knapp, W. O.; Lohse, Christian; Hansen, Torben S. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Peat deposits in Greenland and Denmark were investigated to show that high-resolution dating of these archives of atmospheric deposition can be provided for the last 50 years by radiocarbon dating using the atmospheric bomb pulse. 14C was determined in macrofossils from sequential one cm slices using accelerator mass spectrometry (AMS). Values were calibrated with a general-purpose curve derived from annually averaged atmospheric 14CO2 values in the northernmost northern hemisphere (NNH, 30 degrees to 90 degrees N). We present a through review of 14C bomb-pulse data from the NNH including our own measurements made in tree rings and seeds from Arizona as well as other previously published data. We show that our general-purpose calibration curve is valid for the whole NNH producing accurate dates within 1-2 years. In consequence, 14C AMS can precisely date individual points in recent peat deposits within the range of the bomb-pulse (from the mid-1950s on). Comparing the 14C AMS results with the customary dating method for recent peat profiles by 210Pb, we show that the use of 137Cs to validate and correct 210Pb dates proves to be more problematic than previously supposed. As a unique example of our technique, we show how this chronometer can be applied to identify temporal changes in Hg concentrations from Danish and Greenland peat cores.
    • In-Situ Radiocarbon Production by Neutrons and Muons in an Antarctic Blue Ice Field at Scharffenbergbotnen: A Status Report

      van der Borg, K.; van der Kemp, W. J. M.; Alderliesten, C.; de Jong, A. F. M.; Lamers, R. A. N.; Oerlemans, J.; Thomassen, M.; van de Wal, R. S. W. (Department of Geosciences, The University of Arizona, 2001-01-01)
      In the radiocarbon accelerator mass spectrometry (14C AMS) analysis of gases obtained in a dry extraction from a 52-m Antarctic ice core, we observed 14CO2 and 14CO concentrations decreasing with depth. The concentrations are explained in terms of in-situ production by neutrons and captured muons in ablating ice. The ratio of the 14CO2 concentration to that of 14CO has been found to be constant at 1.9 +/0.3. The ablation rates obtained of 42 +/18 cm.yr-1 and 40 +/13 cm.yr-1 for the neutron and muon components, respectively, are about three times higher than observed from stake readings. The discrepancy may point to an incomplete extraction of the dry extraction method. Using the constant ratio in 14CO2 and 14CO concentrations we correct for the in-situ component in the trapped 14CO2 and deduce an age of 10,300 +/900 BP for the ice core.
    • Radiocarbon Age Profiles and Size Dependency of Mixing in Northeast Atlantic Sediments

      Brown, Louise; Cook, Gordon T.; MacKenzie, Angus B.; Thomson, John (Department of Geosciences, The University of Arizona, 2001-01-01)
      In recent years, the most common technique for radiocarbon dating of deep-ocean sediments has been accelerator mass spectrometry (AMS) analysis of hand-picked planktonic forminifera (forams). Some studies have exposed age offsets between different sediment size fractions from the same depth within a core and this has important implications when establishing a chronological framework for palaeoceanographic records associated with a particular sediment component. The mechanisms generating the age offsets are not fully understood, a problem compounded by the fact that the fraction defined as "large"varies between different studies. To explore this problem, we dated samples of hand-picked forams from two Biogeochemical Ocean Flux Study (BOFS) cores, for which the presence of an offset between the bulk carbonate and >150 micrometers foraminiferal calcite had already been demonstrated. The presence of a constant age offset between bulk carbonate and coarse fraction material at the two BOFS sites has been confirmed, but the magnitude of the offset is dependent on whether a simple size-separation technique or hand-picking of well-preserved forams is applied. This may be explained if the selection of well preserved forams biases the sample towards those specimens that have spent least time in the surface mixed layer (SML) or have undergone less size selective mixing. Modeling of the 14C profiles demonstrates that SML depth and sediment accumulation rates are the same for both the bulk and coarse sediment fractions, which is consistent with the hypothesis that size-selective mixing is responsible for the age offset.
    • Radiocarbon Dating of delta-18O-delta-D Plots in Late Pleistocene Ice-Wedges of the Duvanny Yar (Lower Kolyma River, Northern Yakutia)

      Vasil'chuk, Y. K.; Vasil'chuk, A. C.; Rank, Dieter; Kutschera, Walter; Kim, Jong-Chan (Department of Geosciences, The University of Arizona, 2001-01-01)
      The Duvanny Yar cross-section located in the Lower Kolyma River valley of Northern Yakutia (69 degrees N, 158 degrees E, height above the Kolyma River level 55 m), has been studied and dated in detail by radiocarbon. The sequence mainly consists of sandy loam sediments with large syngenetic ice wedges. Their width at the top is 1-3.5 m. Allochthonous organic material occurs in high content, concentrating as 0.5-0.7 m lenses. Shrub fragments, twigs, and mammoth bones are accumulated in peaty layers. Through interpolation based on a series of 14C dates, dating of the host sediments provides an approximate age for the ice wedges. The 14C dates of various types of organic material are sometimes very close, but not all in agreement. Therefore, the dates do not accurately show the age of the delta-18O and delta-D plots. A new approach is developed to a 14C dating strategy of syncryogenic sediments with high admixture of allochthonous organic material. The main purpose of this study is to consider detection of inversions or disturbances in the syngenetic permafrost sediment at the Duvanny Yar cross-section by 14C date series. Direct accelerator mass spectrometry (AMS) dating of the ice confirmed the relatively young age of ice wedges.
    • Radiocarbon in Seawater at Radioactive Waste Dumping Sites in the Northeast Atlantic and Northwest Pacific

      Povinec, P. P.; Jull, A. J. T.; Burr, G. S. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Radiocarbon has been an important constituent of radioactive wastes dumped in the open ocean and marginal seas as well as wastes released from nuclear reprocessing plants. Therefore, in some regions these sources could have a greater impact on 14C concentration in seawater than global fallout. The high analytical sensitivity of 14C measurement by accelerator mass spectrometry (AMS) allows even tracer amounts of 14C to be found in seawater at radioactive waste dumping sites and their comparison with the global distribution of this radionuclide. Data on measurements of 14C in samples taken at former dumping sites in the northeast Atlantic and northwest Pacific Oceans and in the open ocean are discussed and compared with other anthropogenic radionuclides, namely 3H. Small increases in 14C concentrations observed in some bottom and surface seawater samples collected at the Northwest Pacific Ocean dumping sites require further 14C analyses before final conclusions can be made on possible leakages from dumped radioactive wastes.
    • Towards a Radiocarbon Chronology of the Late-Glacial: Sample Selection Strategies

      Walker, M. J. C.; Bryant, C.; Coope, G. R.; Harkness, D. D.; Lowe, J. J.; Scott, E. M. (Department of Geosciences, The University of Arizona, 2001-01-01)
      This paper outlines a dating program designed to test the reproducibility of radiocarbon dates on different materials of Late-Glacial age (plant macrofossils, fossil beetle remains, and the "humic" and "humin" chemical fractions of limnic sediments) using a combination of radiometric (beta counting) and accelerator mass spectrometry (AMS) techniques. The results have implications for the design of sampling strategies and for the development of improved dating protocols, both of which are important if a high-precision 14C chronology for the Late-Glacial is to be achieved.