• Radiocarbon and Stable Carbon Analysis of Dissolved Methane and Carbon Dioxide from the Profile of a Raised Peat Bog

      Garnett, M. H.; Hardie, S. L.; Murray, C. (Department of Geosciences, The University of Arizona, 2011-01-01)
      We developed and tested a new method to separate CO2 and CH4 from bulk gas samples for radiocarbon and stable-carbon analysis that utilizes a zeolite molecular sieve. To validate the technique, tests were performed using a suite of standard gases, composed of CO2 and CH4 of distinctly different isotopic composition. We employed the method to investigate the carbon isotopic composition of samples of dissolved CO2 and CH4 collected in situ from the near surface to deep layers of an ombrotrophic raised peat bog. Results showed that the age of both the CO2 and CH4 components of the dissolved gases increased with depth from ~0-300 BP at 0.25 m to ~4000 BP at 4 m. CH4 was mainly similar or slightly older in age compared to CO2, with the greatest difference in ages occurring at 1 m depth where CH4 was older by 430-615 yr. The 13C values of CO2 increased with depth from -12.4 and -8.0 at 0.25 m to +6.9 and +8.3 at 4 m, whereas the 13C of CH4 stayed in the range -58.4 to -70.6. The 14C results from the deepest layers are consistent with a similar source for both gases. 14C ages for the CO2 component were younger compared to CH4, within the shallower depths of the peat bog (1 m) and demonstrate the incorporation of acrotelm-derived respired CO2 into the catotelm.
    • Radiocarbon Anomalies from Old CO2 in the Soil and Canopy Air

      Soter, Steven (Department of Geosciences, The University of Arizona, 2011-01-01)
      The canopies of forests and cultivated fields can retard the ventilation of CO2 respired from the soil. The plants in dense canopies can then acquire a small fraction of their carbon by recycling some of the respired CO2. Furthermore, some plants can assimilate a small fraction of their carbon by uptake of CO2 in the soil via their roots. In tectonically active areas, the diffuse flux of CO2 from geological sources may be comparable to that from normal soil respiration. In such areas, both the canopy and root uptake effects may allow plants to acquire a measurable fraction of their carbon from geological sources. Because this "old" carbon lacks radiocarbon, its assimilation would increase the apparent 14C ages of the plants. These effects may account for some of the discrepancies between archaeological and 14C dates.
    • Radiocarbon Concentrations of Wood Ash Calcite: Potential for Dating

      Regev, Lior; Eckmeier, Eileen; Mintz, Eugenia; Weiner, Steve; Boaretto, Elisabetta (Department of Geosciences, The University of Arizona, 2011-01-01)
      Ash is formed when plant calcium oxalate crystals (CaC2O4) decompose to form calcite (CaCO3). We found that ash does retain the original calcium oxalate radiocarbon concentration, but in addition, there is another minor 14C source. This is shown by the presence of a consistent small shift in the pMC and 13C levels when comparing cellulose and ash from modern and archaeological woods. Possible mechanisms for 14C exchange during combustion or due to diagenesis are considered in order to define parameters for identifying better-preserved wood ash samples.
    • Radiocarbon Evidence from the Middle Bronze Age Settlement at Portella (Aeolian Islands, Italy): Chronological and Archaeological Implications

      Alberti, Gianmarco (Department of Geosciences, The University of Arizona, 2011-01-01)
      This paper deals with radiocarbon determinations from the Middle Bronze Age site of Portella on the island of Salina (Aeolian Archipelago, Italy). The available 14C evidence is taken into account, in a simple Bayesian model, in order to explore the issue of the absolute chronology of both the settlement and the stage of the local cultural sequence to which Portella belongs. A high date is proposed for the start of the Aeolian (and Sicilian) Middle Bronze Age: 1556-1422 cal BC (95.4% confidence), with a a most likely (modal) date of about 1450 cal BC. Further, the analysis suggests that the Portella phase is likely to have been a very short one, with a span of 0-65 yr (68.2%) or 0-131 yr (95.4%). The archaeological implications are explored. The relation of these results to the evidence of ceramic phasing is also considered. Since Aegean datable ceramic imports are documented in Aeolian/Sicilian Middle Bronze Age contexts, the connection between Portella's chronology and the absolute dating of one of the Aegean phases (namely, Late Helladic IIIA1) is also investigated.
    • Radiocarbon-Dated Paleoenvironmental Changes on a Lake and Peat Sediment Sequence from the Central Great Hungarian Plain (Central Europe) During the Last 25,000 Years

      Sümegi, Pál; Molnár, Mihály; Jakab, Gusztáv; Persaits, Gergo; Majkut, Péter; Páll, Dávid G.; Gulyás, Sándor; Jull, A. J. Timothy; Törcsik, Tünde (Department of Geosciences, The University of Arizona, 2011-01-01)
      One of Hungary's geological and environmental treasures is nestled in the heart of the Great Hungarian Plain. The catchment basin of Lake Kolon was subjected to detailed environmental historical studies starting in 2005. Undisturbed cores taken along transects of the basin were subjected to detailed sedimentological, paleoecological, and geochemical studies. To establish a reliable timeframe of the lacustrine and marshland sedimentary sequence identified, 22 samples were analyzed by accelerator mass spectrometry (AMS) in the radiocarbon laboratories of Poznań and Tucson. With the new results in hand, we had the opportunity to elucidate the geological evolution of the area for the past 25,000 yr. This sequence is highly beneficial, as it is probably the most well-dated profile of the Quaternary from the area studied. The new absolute dates enabled the comparison of local geological evolution of the studied area with those of global climatic changes. As seen from our findings, the geological evolution of the catchment basin was congruent with major climatic events during the Pleistocene and the entire Holocene. However, a very peculiar trajectory was identified for the terminal part of the Pleistocene and the opening of the Holocene regarding the evolution of the landscape, the vegetation, and the fauna of this part of the Great Hungarian Plain.
    • Sampling Iron for Radiocarbon Dating: Influence of Modern Steel Tools on 14C Dating of Ancient Iron Artifacts

      Hüls, Matthias; Grootes, Pieter M.; Nadeau, Marie-Josée (Department of Geosciences, The University of Arizona, 2011-01-01)
      Before the 17th century, charcoal was regularly used in the production of iron (smelting and forging) and some of this charcoal carbon was incorporated into the iron. Depending on the age of the wood used to produce the charcoal, the age of the carbon incorporated in the iron lattice can reflect the age of manufacture of the iron artifacts. A reliable preparation method allowing for the routine dating of iron artifacts would permit the dating of numerous objects for which now the age can only be estimated. In an earlier work (Hls et al. 2004), we tested the extraction of carbon from iron samples by closed-tube combustion. The samples were cut in small pieces to ease the release of the carbon from the lattice. During the tests, it became clear that the steel tools used to cut the samples can add contamination at the surface. As modern steel is made using coal, this leads to erroneously old ages. We have tested ways to reduce or eliminate this surface contamination from the sampling tools using iron artifacts of known ages. In order to quantify the contamination, we produced standard test materials from pure iron (99.998% Fe) melted with carbon of known 14C content and prepared samples using different cutting tools. The results of these tests indicate that the proper choice of cutting technique and tool, combined with an additional cleaning of the freshly cut surface, reduces sample contaminations to low levels; measured sample 14C concentrations are close to the 14C content of the charcoal used to produce these standard iron samples.
    • Table of Contents

      Department of Geosciences, The University of Arizona, 2011-01-01
    • The Influence of Soil Organic Matter Age Spectrum on the Reconstruction of Atmospheric 14C Levels via Stalagmites

      Fohlmeister, J.; Kromer, B.; Mangini, A. (Department of Geosciences, The University of Arizona, 2011-01-01)
      The imprint of the radiocarbon bomb peak was detected in the top of stalagmite ER-77 from Grotta di Ernesto (NE Italy). This recently grown stalagmite reveals a reservoir age, also known as dead carbon fraction (dcf), of ~1050 14C yr, or 12%. By applying a 14C soil-karst model, the age spectrum of soil organic matter (SOM) as well as the CO2 contribution of the single SOM reservoirs to the total soil CO2 can be derived. Under the assumption of constant vegetation, meaning both vegetation density and the age spectrum of SOM, it is possible to derive the soil-air 14C activity of the past using the 14C calibration curve (IntCal04). Hence, it is also possible to calculate an artificial stalagmite 14C data set covering the last 25,000 yr with parameters determined for stalagmite ER-77. With this artificially constructed data set, we derived the hypothetical atmospheric 14C activity by using the common method of applying a constant dcf on the modeled 14C data set of the stalagmite. This theoretical approach allows to analyze the impact of a constant and variable SOM age spectrum on atmospheric 14C reconstructions performed with real stalagmite 14C measurements. We observe deviations between IntCal04 and the atmospheric 14C activity as derived with our modeled 14C data set, which are larger for older SOM than for younger SOM and vary in time up to 2 pMC, depending on the strength of the variations in the atmospheric 14C level. This value is comparable with the 1-delta uncertainty given by IntCal04 for the last glacial. For a varying SOM age spectrum, the deviations between the calibration curve and 14C level of the atmosphere reconstructed with a stalagmite exceed 3 pMC, which is larger than the 1-delta uncertainty of IntCal04. In general, the SOM has smoothing, shifting, and 14C-depleting effects on the stalagmite 14C record and, therefore, on the stalagmite-derived atmospheric 14C activity. In this study, changes in soil-air pCO2 and carbonate dissolution conditions, which have also an important impact on the 14C record of a stalagmite, are not accounted for.