• Assessing Open-System Behavior of 14C in Terrestrial Gastropod Shells

      Rech, Jason A.; Pigati, Jeffrey S.; Lehmann, Sophie B.; McGimpsey, Chelsea N.; Grimley, David A.; Nekola, Jeffrey C. (Department of Geosciences, The University of Arizona, 2011-01-01)
      In order to assess open-system behavior of radiocarbon in fossil gastropod shells, we measured the 14C activity on 10 aliquots of shell material recovered from Illinoian (~190-130 ka) and pre-Illinoian (~800 ka) loess and lacustrine deposits in the Midwestern USA. Eight of the 10 aliquots yielded measurable 14C activities that ranged from 0.25 to 0.53 percent modern carbon (pMC), corresponding to apparent 14C ages between 48.2 and 42.1 ka. This small level of open-system behavior is common in many materials that are used for 14C dating (e.g. charcoal), and typically sets the upper practical limit of the technique. Two aliquots of gastropod shells from the Illinoian-aged Petersburg Silt (Petersburg Section) in central Illinois, USA, however, yielded elevated 14C activities of 1.26 and 1.71 pMC, which correspond to apparent 14C ages of 35.1 and 32.7 ka. Together, these results suggest that while many fossil gastropods shells may not suffer from major (1%) open-system problems, this is not always the case. We then examined the mineralogy, trace element chemistry, and physical characteristics of a suite of fossil and modern gastropod shells to identify the source of contamination in the Petersburg shells and assess the effectiveness of these screening techniques at identifying samples suitable for 14C dating. Mineralogical (XRD) and trace element analyses were inconclusive, which suggests that these techniques are not suitable for assessing open-system behavior in terrestrial gastropod shells. Analysis with scanning electron microscopy (SEM), however, identified secondary mineralization (calcium carbonate) primarily within the inner whorls of the Petersburg shells. This indicates that SEM examination, or possibly standard microscope examination, of the interior of gastropod shells should be used when selecting fossil gastropod shells for 14C dating.
    • Balanced-Energy Counting Window for Stable Liquid Scintillation Radiocarbon Dating

      Theodórsson, Pall (Department of Geosciences, The University of Arizona, 2011-01-01)
      This paper describes an optimal radiocarbon counting window for liquid scintillation (LS) 14C dating that secures for unquenched as well as for heavily quenched dating samples maximal stability of 14C counting efficiency and theoretically minimal quench correction. In high-precision dating, a balanced counting window with fixed channel limits is frequently used, where about 3% of the highest part of the 14C spectrum is sacrificed for high 14C counting stability. The stability is, however, diminished for quenched samples. Therefore, this window is here replaced by a balanced fixed-energy 14C counting window where the channel limits depend on the quench level. The LS system used must have a linear amplifier and a multichannel analyzer. All samples are measured at a fixed high voltage. For energy calibration and determination of the quench level, the channel number of the middle of the 59.5-keV peak from an external 241Am gamma source is determined before and after measuring each sample. This counting mode is valuable in high-precision dating. It could be widely applied if adapted to systems with a logarithmic amplifier, generally used in LS dating.
    • Bayesian Analysis of High-Precision AMS 14C Dates from a Prehistoric Mexican Shellmound

      Kennett, Douglas J.; Culleton, Brendan J.; Voorhies, Barbara; Southon, John R. (Department of Geosciences, The University of Arizona, 2011-01-01)
      We establish a precision accelerator mass spectrometry (AMS) radiocarbon chronology for the Archaic period Tlacuachero shellmound (Chiapas, Mexico) within a Bayesian statistical framework. Carbonized twig samples were sequentially selected from well-defined stratigraphic contexts based on iterative improvements to a probabilistic chronological model. Analytical error for these measurements is 15 to 20 14C yr. This greater precision and the absence of stratigraphic reversals eclipses previous 14C work at the site. Based on this, we establish a chronological framework for a sequence of 3 clay floors dating to between 4930 and 4270 cal BP and determine that the bedded shell deposits that formed the mound accumulated rapidly during 2 episodes: a lower 2-m section below the floors that accumulated over a 0-150 cal yr period at 5050-4875 cal BP and, an upper 3.5-m section above the floors that accumulated over a 0-80 cal yr period at 4380-4230 cal BP.
    • Chronology of the Danish Bronze Age Based on 14C Dating of Cremated Bone Remains

      Olsen, Jesper; Hornstrup, Karen Margrethe; Heinemeier, Jan; Bennike, Pia; Thrane, Henrik (Department of Geosciences, The University of Arizona, 2011-01-01)
      The relative Bronze Age chronology for Scandinavia was established as early as 1885. It is traditionally divided into 6 periods (I-VI). Earlier attempts to make an absolute Bronze Age chronology for southern Scandinavia were derived from burials and settlements and were mainly based on radiocarbon-dated charcoal or carbonized cereals, often with undefined archaeological periods. Here, we present high-precision 14C dating on burials with well-defined associated archaeological periods in order to improve the absolute chronology of the Danish Bronze Age. Our results are in broad agreement with the traditional absolute chronology of the Danish Bronze Age. However, our results do indicate that the onset of period III likely occurred earlier than previously thought.
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      Department of Geosciences, The University of Arizona, 2011-01-01
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      Department of Geosciences, The University of Arizona, 2011-01-01
    • Evidence for a Solar Flare Cause of the Pleistocene Mass Extinction

      LaViolette, Paul A. (Department of Geosciences, The University of Arizona, 2011-01-01)
      The hypothesis is presented that an abrupt rise in atmospheric radiocarbon concentration evident in the Cariaco Basin varve record at 12,837 +/- 10 cal yr BP, contemporaneous with the Rancholabrean termination, may have been produced by a super-sized solar proton event (SPE) having a fluence of ~1.3 x 1011 protons/cm2. A SPE of this magnitude would have been large enough to deliver a lethal radiation dose of at least 3-6 Sv to the Earth's surface, and hence could have been a principal cause of the final termination of the Pleistocene megafauna and several genera of smaller mammals and birds. The event time-correlates with a large-magnitude acidity spike found at 1708.65 m in the GISP2 Greenland ice record, which is associated with high NO-3 ion concentrations and a rapid rise in 10Be deposition rate, all of which are indicators of a sudden cosmic-ray influx. The depletion of nitrate ions within this acidic ice layer suggests that the snowpack surface at that time was exposed to intense UV for a prolonged period, which is consistent with a temporary destruction of the polar ozone layer by solar cosmic rays. The acidity event also coincides with a large-magnitude, abrupt climatic excursion and is associated with elevated ammonium ion concentrations, an indicator of global fires.
    • Examining the Inherent Variability in ΔR: New Methods of Presenting ΔR Values and Implications for MRE Studies

      Russell, N.; Cook, G. T.; Ascough, P. L.; Scott, E. M.; Dugmore, A. J. (Department of Geosciences, The University of Arizona, 2011-01-01)
      Currently, there is significant ongoing research into the temporal and spatial variability of marine radiocarbon reservoir effects (MREs) through quantification of R values. In turn, MRE studies often use large changes in R values as proxies for changes in ocean circulation. R values are published in a variety of formats with variations in how the errors on these values are calculated, making it difficult to identify trends or to compare values, unless the method of calculating the R is explicitly described or all of the data are made available in the publication. This paper demonstrates the large range in R values (+34 to -122) that can be obtained from a single, secure archaeological context when using the multiple paired sample approach, despite the fact that the terrestrial entities were of statistically indistinguishable 14C ages, as were the marine samples. This demonstrates the inherent variability in the R calculations themselves and we propose that, together with calculation of mean R, the distribution of R values should be displayed, e.g. as histograms in order to illustrate the full data range. This spread is only apparent when employing a multiple paired sample approach as the uncertainty derived on a single pair of samples, taking account only of the errors on the individual 14C ages, will never truly represent the overall variability in R that results from the intrinsic variability in the population of 14C ages in samples that might have been used. Consequently, R values and the associated uncertainty calculated from single pairs should be treated with some caution. We propose that, where possible, when using paired archaeological samples, that a multiple paired approach should be employed as it will test the context security of the material used in the R calculations. When summarizing the values by the weighted average, we also propose that the standard error for predicted values should be employed as this will fully encompass the uncertainty of a future R calculation, using different samples for a similar time and location. Finally, we encourage future publishing of R values using the histogram format, making all of the data available. This will help ensure that R values are comparable across the literature and should provide a framework for standardization of publication methods.
    • Marine Radiocarbon Reservoir Effect of Coastal Waters off Cape Verde Archipelago

      Monge Soares, Antonio M.; Matos Martins, Jose M.; Cardoso, Joao Luis (Department of Geosciences, The University of Arizona, 2011-01-01)
      Quantification of the marine radiocarbon reservoir effect (Delta-R) is essential in order to calibrate conventional 14C dates from marine shell samples with reliability. Delta-R also provides information concerning the intensity of coastal upwelling in marine regions influenced by this phenomenon. 14C ages of closely associated marine samples (mollusk shells) and terrestrial samples (goat bones) from So Vicente Island, Cape Verde Archipelago, permitted the first calculation of the marine 14C reservoir effect in this region. A Delta-R weighted mean value of 70 +/- 70 14C yr was obtained. This value is in accordance with the previously published oceanographic conditions of the region indicating the existence of a seasonal active upwelling regime.
    • Paired AMS 14C Dates on Planktic Foraminifera from a Gulf of Mexico Sediment Core: An Assessment of Stratigraphic Continuity

      Flower, B. P.; Hastings, D. W.; Randle, N. J. (Department of Geosciences, The University of Arizona, 2011-01-01)
      A series of recent papers has called for multiple radiocarbon dates on planktic foraminifera to assess stratigraphic continuity in deep-sea sediment cores. This recommendation comes from observations of anomalous 14C dates in planktic foraminifera from the same stratigraphic level. Potential reasons include bioturbation, downslope transport, secondary calcification, carbonate dissolution, and differential preservation. In this study, paired 14C dates on dissolution-susceptible Globigerinoides ruber and dissolution-resistant Neogloboquadrina dutertrei are used to evaluate a Gulf of Mexico sediment core. Fourteen of 15 pairs (between 8815 and 12,995 uncorrected 14C yr BP) yield concordant uncorrected 14C ages (mean difference -2 +/- 75 yr), attesting to continuous deposition at high accumulation rates (>35 cm/kyr). For 1 pair, N. dutertrei is nearly 1000 yr younger, which is difficult to explain by any combination of dissolution and bioturbation or downslope transport, given the excellent carbonate preservation and persistent laminations. The concordant ages underscore the utility of paired 14C dates in planktic foraminifera as a means of assessing stratigraphic continuity in deep-sea sediment sequences.
    • Possible Factors Causing Older Radiocarbon Age for Bulk Organic Matter in Sediment from Daihai Lake, North China

      Reuther, Joshua D.; Wu, Yanhong; Gerlach, S. Craig; Wang, Sumin; Zhou, Liping (Department of Geosciences, The University of Arizona, 2011-01-01)
      Many factors may influence the radiocarbon age results of lacustrine sediments, among which the hardwater effect is particularly important. Daihai Lake is a closed lake located in the semi-arid region of Inner Mongolia, China. High concentrations of (HCO3)- and (CO3)2- and high pH values in the lake water imply that there is a hardwater effect when using bulk lacustrine sediment samples for 14C dating. To correct the apparent 14C age, we present a pilot study based on a series of 14C ages of lake surface sediment, lake water, submerged aquatic plant (Myriophyllum), fish bone (Cyprinus carpio), and surface soil samples from and around Daihai Lake. Assuming that the relationship between the 14C/12C ratio of DIC and of atmospheric CO2 was constant (at 0.816), the hardwater effect ages calculated for the past 8000 yr would have varied from 949 to 1788 yr. Together with the reservoir effect and soil organic matter input, the hardwater effect is a major factor causing changes in apparent age when using bulk organic matter for 14C dating.
    • Royal Institute for Cultural Heritage Web-Based Radiocarbon Database

      Van Strydonck, Mark; De Roock, Edwin (Department of Geosciences, The University of Arizona, 2011-01-01)
      The radiocarbon dating laboratory at KIK-IRPA in Brussels was founded in the 1960s. From the beginning, dates were reported at more or less regular intervals in the journal Radiocarbon (Schreurs 1968) as did most of the other 14C laboratories.
    • Rudjer Boškovic Institute Radiocarbon Measurements XVI

      Obelić, Bogomil; Krajcar Bronić, Ines; Horvatinčić, Nada; Barešić, Jadranka; Rajtarić, Anita (Department of Geosciences, The University of Arizona, 2011-01-01)
      In this paper, we present dating of archaeological samples performed since our last report (Obelic et al. 2002). Included are results of samples measured by the gas proportional counting (GPC) method until the abandonment of this technique in 2007, as well as results of several series measured by both GPC and liquid scintillation counting (LSC) methods.
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      Department of Geosciences, The University of Arizona, 2011-01-01
    • Tell Sabi Abyad, Syria: Radiocarbon Chronology, Cultural Change, and the 8.2 ka Event

      Van der Plicht, J.; Akkermans, P. G.; Nieuwenhuyse, O.; Kaneda, A.; Russell, A. (Department of Geosciences, The University of Arizona, 2011-01-01)
      At Tell Sabi Abyad, Syria, we obtained a robust chronology for the 7th to early 6th millennium BC, the Late Neolithic. The chronology was obtained using a large set of radiocarbon dates, analyzed by Bayesian statistics. Cultural changes observed at ~6200 BC are coeval with the 8.2 ka climate event. The inhabitation remained continuous.
    • The Canadian Archaeological Radiocarbon Database (CARD): Archaeological 14C Dates in North America and Their Paleoenvironmental Context

      Gajewski, K.; Muñoz, S.; Peros, M.; Viau, A.; Morlan, R.; Betts, M. (Department of Geosciences, The University of Arizona, 2011-01-01)
      Databases of accumulated paleoecological and archaeological records provide a means for large-scale syntheses of environmental and cultural histories. We describe the current status of the Canadian Archaeological Radiocarbon Database (CARD), a searchable collection of more than 36,000 14C dates from archaeological and paleontological sites from across North America. CARD, built by the late Dr Richard Morlan of the Canadian Museum of Civilization, consists of uncalibrated 14C data as well as information about the material dated, the cultural association of the date (e.g. Paleoindian, Archaic, Woodland), and its geographic location. The database can be used to study questions relating to prehistoric demography, migrations, human vulnerability to environmental change, and human impact on the landscape, but biases relating to sampling intensity and taphonomy must first be accounted for. Currently, Canada and the northern United States are well represented in the database, while the southern United States is underrepresented. The frequency of 14C dates associated with archaeological sites increases through time from 15,000 cal yr BP until European contact, which likely reflects, among other factors, both the destruction of older cultural carbon due to erosion and dissolution and increasing population numbers through time. An exploratory analysis of the dates reveals their distribution in both time and space, and suggests that the database is sufficiently complete to enable quantitative analysis of general demographic trends.
    • Using the 14C Bomb Pulse to Date Young Speleothems

      Hodge, Ed; McDonald, Janece; Fischer, Matthew; Redwood, Dale; Hua, Quan; Levchenko, Vladimir; Drysdale, Russell; Waring, Chris; Fink, David (Department of Geosciences, The University of Arizona, 2011-01-01)
      Three modern speleothems were sampled at high resolution for radiocarbon analysis to identify their bomb-pulse signatures and to construct chronologies. Each speleothem exhibited a different 14C response, presumed to be related to site characteristics such as vegetation, temperature, rainfall, depth below the surface, and water pathway through the aquifer. Peak 14C activity for WM4 is 134.1 pMC, the highest cited thus far in the literature and suggestive of a lower inertia at this site. Dead carbon fractions for each stalagmite were calculated and found to be relatively similar for the 3 speleothems and lower than those derived from Northern Hemisphere speleothems. An inverse modeling technique based on the work of Genty and Massault (1999) was used to estimate soil carbon residence times. For each speleothem, mean soil 14C reservoir ages differed greatly between the 3 sites, ranging from 2-6.5 to 32-46 yr.
    • Who's That Lying in My Coffin? An Imposter Exposed by 14C Dating

      Sowada, Karin; Jacobsen, Geraldine E.; Bertuch, Fiona; Palmer, Tim; Jenkinson, Andrew (Department of Geosciences, The University of Arizona, 2011-01-01)
      In the 19th and early 20th centuries, many museums acquired Egyptian coffins containing mummies from private donors who bought them from dealers in Egypt. Owing to the unknown context of such acquisitions, it cannot be assumed that the mummified individual inside the coffin is the same person named on it. Radiocarbon dating is a key diagnostic test, within the framework of a multidisciplinary study, to help resolve this question. The dating of an adult mummy in the Nicholson Museum at the University of Sydney was therefore checked using 14C dating. For over 150 yr, mummy NM R28.2 was identified as Padiashaikhet as per his coffin, dated to the 25th Dynasty, about 725-700 BC. 14C results from samples of linen wrappings revealed that the mummy was an unknown individual from the Roman period, cal AD 68-129. The mummification technique can now be understood within its correct historical context.