• Hydropyrolysis: Implications for Radiocarbon Pretreatment and Characterization of Black Carbon

      Ascough, P. L.; Bird, M. I.; Meredith, W.; Wood, R. E.; Snape, C. E.; Brock, F.; Higham, T. F. G.; Large, D. J.; Apperley, D. C. (Department of Geosciences, The University of Arizona, 2010-01-01)
      Charcoal is the result of natural and anthropogenic burning events, when biomass is exposed to elevated temperatures under conditions of restricted oxygen. This process produces a range of materials, collectively known as pyrogenic carbon, the most inert fraction of which is known as black carbon (BC). BC degrades extremely slowly and is resistant to diagenetic alteration involving the addition of exogenous carbon, making it a useful target substance for radiocarbon dating particularly of more ancient samples, where contamination issues are critical. We present results of tests using a new method for the quantification and isolation of BC, known as hydropyrolysis (hypy). Results show controlled reductive removal of non-BC organic components in charcoal samples, including lignocellulosic and humic material. The process is reproducible and rapid, making hypy a promising new approach not only for isolation of purified BC for 14C measurement but also in quantification of different labile and resistant sample C fractions.
    • Spatial Variation in the Marine Radiocarbon Reservoir Effect throughout the Scottish Post-Roman to Late Medieval Period: North Sea Values (500-1350 BP)

      Russell, N.; Cook, G. T.; Ascough, P. L.; Dugmore, A. J. (Department of Geosciences, The University of Arizona, 2010-01-01)
      The marine radiocarbon reservoir effect (MRE) occurs as a spatially and temporally dependent variable owing to localized changes in oceanic water composition. This study investigates Delta-R values (deviations from the global average MRE whose Delta-R = 0) during the period 500-1350 BP for the east coast of Scotland, where a complex estuarine system exists that drains into the semi-enclosed North Sea basin. Due to the availability of suitable archaeological samples, the data set has a distinct Medieval focus that spans the area from Aberdeen in the north to East Lothian in the south. Many of the Delta-R values are not significantly different from 0 (the global average), but there are occasional excursions to negative values (max -172 +/- 20) indicating the presence of younger water. These values show greater variability compared to other published data for this general region, suggesting that considerable care must be taken when dating marine derived samples from archaeological sites on the east coast of Scotland.
    • Temporal and Spatial Variations in Freshwater 14C Reservoir Effects: Lake Mývatn, Northern Iceland

      Ascough, P. L.; Cook, G. T.; Church, M. J.; Dunbar, E.; Einarsson, Á.; McGovern, T. H.; Dugmore, A. J.; Perdikaris, S.; Hastie, H.; Friðoriksson, A.; et al. (Department of Geosciences, The University of Arizona, 2010-01-01)
      Lake Mvatn is an interior highland lake in northern Iceland that forms a unique ecosystem of international scientific importance and is surrounded by a landscape rich in archaeological and paleoenvironmental sites. A significant freshwater reservoir effect (FRE) has been identified in carbon from the lake at some Viking (about AD 870-1000) archaeological sites in the wider region (Mvatnssveit). Previous accelerator mass spectrometry (AMS) measurements indicated this FRE was about 1500-1900 14C yr. Here, we present the results of a study using stable isotope and 14C measurements to quantify the Mvatn FRE for both the Viking and modern periods. This work has identified a temporally variable FRE that is greatly in excess of previous assessments. New, paired samples of contemporaneous bone from terrestrial herbivores and omnivores (including humans) from Viking sites demonstrate at least some omnivore diets incorporated sufficient freshwater resources to result in a herbivore-omnivore age offset of up to 400 14C yr. Modern samples of benthic detritus, aquatic plants, zooplankton, invertebrates, and freshwater fish indicate an FRE in excess of 5000 14C yr in some species. Likely geothermal mechanisms for this large FRE are discussed, along with implications for both chronological reconstruction and integrated investigation of stable and radioactive isotopes.