Browsing Radiocarbon, Volume 36, Number 1 (1994) by Journal
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15th International Radiocarbon ConferenceDepartment of Geosciences, The University of Arizona, 19940101

Advances in Liquid Scintillation SpectometryDepartment of Geosciences, The University of Arizona, 19940101

Apparent Ages of Marine Shells: Implications for Archaeological Dating in Hawai'iThe conventional 14C ages of 8 marine shells of known age and 11 marine shells stratigraphically associated with dated wood charcoal show considerable variation from expected ages. One source of this variation is seashore geology; comparison of 6 AMS dates on 3 species of shallowwater, herbivorous gastropod shells from Pleistocene limestone and Holocene volcanic coasts shows that shells from Pleistocene limestone coasts can have apparent, or reservoir, 14C ages up to 620 yr greater than shells of the same species from volcanic coasts. The relatively great variation in apparent ages of Hawaiian marine shells poses problems for their use in dating archaeological sites. For best results, an archaeological marine shell should be sourced to a particular local environment, and the apparent age of shells in that environment determined by dating wellprovenienced shells of known age.

Associate EditorsDepartment of Geosciences, The University of Arizona, 19940101

Calibration 1993Department of Geosciences, The University of Arizona, 19940101

Calibration of 14C Histograms: A Comparison of MethodsThe interpretation of 14C histograms is complicated by the nonlinearity of the 14C time scale in terms of calendar years, which may result in clustering of 14C ages in certain time intervals unrelated to the (geologic or archaeologic) phenomenon of interest. One can calibrate 14C histograms for such distortions using two basic approaches. The KORHIS method constructs a 14C histogram before calibration is performed by means of a correction factor. We present the CALHIS method based on the Groningen calibration program for individual 14C ages. CALHIS first calibrates single 14C ages and then sums the resulting calibration distributions, thus yielding a calibrated 14C histogram. The individual calibration distributions are normalized to a standard Gaussian distribution before superposition, thus allowing direct comparison among various 14C histograms. Several experiments with test data sets demonstrate that CALHIS produces significantly better results than KORHIS. Although some problems remain (part of the distortions due to 14C variations cannot be eliminated), we show that CALHIS offers good prospects for using 14C histograms, particularly with highly precise and accurate 14C ages.

delta13C Measurements from the Irish Oak ChronologyMeasurements of the stable isotope ratio 13C/12C, relative to PDB, were made for fractionation correction purposes on all oak samples used in the Irish oak 14C calibration curve. Stable isotope data have not been published previously. We have collated the stable isotope data from the calibration work, carried out some further measurements to investigate anomalies in the original results, and generated tables of data that include site and tree information pertaining to both stable isotopes and source material for 14C calibration measurements. The data suggest that landgrown trees tend to be isotopically lighter than boggrown wood, and that the Irish trees used in the calibration exercise tend to. be isotopically heavier than those from Scotland and England. Preliminary analysis of the data is given.

How Smooth Should Curves Be for Calibrating Radiocarbon Ages?We show that smoothed versions of the highresolution calibration curve should be used when 14C ages are calibrated with large (> ~30 14C yr) measurement errors (represented by standard deviation sigmam) or are mixtures of elements of variable age (natural sample error with standard deviation sigman). The degree of smoothing should agree with the standard deviation of total sample error, sigma1 the square root of the quadratic sum of sigmam and sigman. However, in most cases, sigma1 is not well known, especially due to difficulties in quantifying sigman. We present an inverse method that gives a measure of mean sigma1 for different materials that are widely used in (conventional) 14C dating. Calculations with large (>100) data sets of wood, charcoal, ombrotrophic peat and minerotrophic peat/gyttja samples indicate that sigma1 of such materials is generally much larger than previously assumed, mainly because of large values of sigman. This means that particularly in organic deposits, strongly smoothed calibration curves should be used where mediumterm 14C variations (wiggles) are completely straightened. This has especially major consequences for calibrating 14C histograms for natural 14C variations. We conclude that 14C histograms consisting of samples of organic deposits do not require correction for mediumterm 14C variations and that uncalibrated 14C histograms need not be as suspect as is usually believed.

Late Quaternary Chronology and Paleoclimates of the Eastern MediterraneanDepartment of Geosciences, The University of Arizona, 19940101

LSC 92Department of Geosciences, The University of Arizona, 19940101