• Carbonate 14C Background: Does It Have Multiple Personalities?

      Nadeau, Marie-Josée; Grootes, Pieter M.; Voelker, Antje; Bruhn, Frank; Oriwall, Alexander (Department of Geosciences, The University of Arizona, 2001-01-01)
      Measurements of the radiocarbon concentration of several carbonate background materials, either mineral (IAEA C1 Carrara marble and Icelandic double spar) or biogenic (foraminifera and molluscs), show that the apparent ages of diverse materials can be quite different. Using 0.07 pMC obtained from mineral samples as a processing blank, the results from foraminifera and mollusc background samples, varying from 0.12 to 0.58 pMC (54.0–41.4 ka), show a species-specific contamination that reproduces over several individual shells and foraminifera from several sediment cores. Different cleaning attempts have proven ineffective, and even stronger measures such as progressive hydrolization or leaching of the samples prior to routine preparation, did not give any indication of the source of the contamination. In light of these results, the use of mineral background material in the evaluation of the age of older unknown samples of biogenic carbonate (>30 ka) proves inadequate. The use of background samples of the same species and provenance as the unknown samples is essential, and if such material is unavailable, generic biogenic samples such as mixed foraminifera samples should be used. The description of our new modular carbonate sample preparation system is also introduced.
    • Chemical Removal of Conservation Substances By 'Soxhlet'-Type Extraction

      Bruhn, Frank; Duhr, Alexander; Grootes, Pieter M.; Mintrop, Annette; Nadeau, Marie-Josée (Department of Geosciences, The University of Arizona, 2001-01-01)
      At the Leibniz radiocarbon lab, art and archaeological objects, often chemically conserved and thus potentially contaminated with respect to their 14C content, are treated using a computer-controlled “Soxhlet”-type series extractor. This device uses a continuous procedure of boiling and condensation of different solvents for extraction and vacuum filtration under constant process conditions. An elutrope sequence of five solvents that dissolve most customary conservation chemicals was selected. A study of these different contaminants applied to reference wood samples with subsequent accelerator mass spectrometry (AMS) measurements demonstrates that their effective removal is dependent on the use of adequate solvents. For many contaminants (e.g. Wood glue, methyl cellulose, Klucel(R), sugar, and polyethylene glycol), routine acid-alkali-acid (AAA) treatment already yields satisfactory results, whereas for Caparol(R) and beeswax a relatively “mild” treatment with acetone, methanol, water, and subsequent standard AAA extraction is sufficient. Complete removal of rubber glue, epoxyresin, and paraffin can only be accomplished with our full set of solvents. The latter procedure is also appropriate when no or only incomplete information about the type of conservation material is available. For epoxy resin the contamination appears to be enriched in the alkali residue, and the easily soluble “humic acid” fraction, even after standard AAA treatment, gives satisfactory results. Two case studies on the application of the extraction procedures are presented.
    • Correlation of Marine 14C Ages from the Nordic Seas with the GISP2 Isotope Record: Implications for 14C Calibration Beyond 25 ka BP

      Voelker, Antje L.; Sarnthein, Michael; Grootes, Pieter M.; Erlenkeuser, Helmut; Laj, Carlo; Mazaud, Alain; Nadeau, Marie-Josée; Schleicher, Markus (Department of Geosciences, The University of Arizona, 1998-01-01)
      We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous 14C ages up to 54 14C ka BP. Based on various lines of evidence, the local 14C reservoir effect was restricted to 400-1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the delta-18O spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1-6. This correlation indicates large fluctuations in the calibration of 14C ages between 20 and 54 14C ka BP. Generally the results confirm the 14C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our 14C shifts after 45 14C ka BP clearly exceed the geomagnetic prediction near 40-43 and 32-34 calendar (cal) ka BP. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic 14C and 10Be production, giving rise to excess 14C in the atmosphere of up to 1200 per mil.
    • How Clean is Ultrafiltration Cleaning of Bone Collagen?

      Hüls, Matthias C.; Grootes, Pieter; Nadeau, Marie-Josée (Department of Geosciences, The University of Arizona, 2007-01-01)
      As part of our bone dating development, we have tested the ultrafiltration of bone gelatin using 2 different filtersVivaspin 20 (VS20), a polyethersulfone, and Vivaspin 15R (VS15R), a cellulose, both with a 30,000 molecular weight cutoffand bone collagen from dated samples ranging in age from 1.5 to 50 kyr BP. A direct accelerator mass spectrometry (AMS) measurement yielded radiocarbon concentrations of ~0.5 pMC (~42 kyr) for the polyethersulfone, ~14.417.5 pMC (~15.614 kyr) for the cellulose, and ~107.4 pMC for the glycerin. The filters were cleaned before use similar to the Oxford protocol (Bronk Ramsey et al. 2004), and a series of freeze-dried archaeological bone gelatin samples and a modern pig-skin gelatin were passed through VS20 and VS15R filters (Vivascience). We recovered both the eluent (30-kD fraction) and the liquid that stayed above the filter (30 kD) in order to obtain a carbon mass and isotope balance. While the 30-kD collagen fraction that is usually selected for AMS analysis does not appear to be significantly contaminated, measurements show significant age differences between the eluent 30 kD and the unfiltered bone collagen, indicating that, despite cleaning, both glycerin and filter still give off contaminants in the eluent. Ultrafiltration with young collagen from pig skin generally confirms these results for the 30-kD fraction but also shows the possibility of small contaminations in the 30-kD fraction. Until a contamination with filter carbon of the 30-kD collagen fraction can be excluded, we would recommend caution in the use of ultrafiltration for cleaning bone collagen with VS20 or VS15R ultrafilters.
    • Mussels with Meat: Bivalve Tissue-Shell Radiocarbon Age Differences and Archaeological Implications

      Fernandes, Ricardo; Bergemann, Stefanie; Hartz, Sönke; Grootes, Pieter M.; Nadeau, Marie-Josée; Melzner, Frank; Rakowski, Andrzej; Hüls, Matthias (Department of Geosciences, The University of Arizona, 2012-10-15)
      Local reservoir ages are often estimated from the difference between the radiocarbon ages of aquatic material and associated terrestrial samples for which no reservoir effect is expected. Frequently, the selected aquatic material consists of bivalve shells that are typically well preserved in the archaeological record. For instance, large shell middens attest to the importance of mussel consumption at both coastal and inland sites. However, different physiological mechanisms associated with tissue and shell growth may result in differences in reservoir effects between the surviving component (shell) and the component relevant to dietary reservoir effects in consumers (tissue). The current study examines bivalve tissue-shell age differences both from freshwater and marine contexts close to archaeological sites where human consumption of mollusks has been attested. Results exhibited significant 14C age differences between bivalve tissue and shell in a freshwater context. In a marine context, no significant bivalve tissue-shell age differences were observed. The results also showed that riverine and lacustrine shells show large and variable freshwater reservoir effects. The results have important implications for establishing local reservoir effects especially in a freshwater environment. For good a priori knowledge of expected 14C differences in organic and inorganic water, carbon is thus necessary. Furthermore, the high variability in freshwater shell 14C ages implies the need for representative sampling from the archaeological record.
    • Radiocarbon Intercomparison Program for Chauvet Cave

      Cuzange, Marie-Thérèse; Delqué-Količ, Emmanuelle; Slar, Tomasz; Grootes, Pieter Meiert; Higham, Tom; Kaltnecker, Evelyne; Nadeau, Marie-Josée; Erlin, Christine; Paterne, Martine; van der Plicht, Johannes; et al. (Department of Geosciences, The University of Arizona, 2007-01-01)
      We present the first results of an accelerator mass spectrometry (AMS) radiocarbon intercomparison program on 3 different charcoal samples collected in one of the hearths of the Megaceros gallery of Chauvet Cave (Ardche, France). This cave, rich in parietal decoration, is important for the study of the appearance and evolution of prehistoric art because certain drawings have been 14C dated to the Aurignacian period at the beginning of the Upper Paleolithic. The new dates indicate an age of about 32,000 BP, which is consistent with this attribution and in agreement with the results from the same sector of the cave measured previously at the Laboratoire des Sciences du Climat et de lEnvironnement (LSCE). Six laboratories were involved in the intercomparison. Samples were measured in 4 AMS facilities: Center for Isotope Research, Groningen University, the Netherlands; the Oxford Radiocarbon Accelerator Unit, UK; the Centre de datation par le carbone 14, Univ. Claude Bernard Lyon 1, France (measured by AMS facilities of Poznan University, Poland); and the LSCE, UMR CEACNRS-UVSQ, France (measured by the Leibniz-Labor of Christian-Albrechts-Universitt Kiel, Germany).
    • Radiocarbon Levels in the Iceland Sea from 25-53 kyr and Their Link to the Earth's Magnetic Field Intensity

      Voelker, Antje L.; Grootes, Pieter M.; Nadeau, Marie-Josée; Sarnthein, Michael (Department of Geosciences, The University of Arizona, 2000-01-01)
      By correlating the climate records and radiocarbon ages of the planktonic foraminifera N. Pachyderma(s) of deep-sea core PS2644 from the Iceland Sea with the annual-layer chronology of the GISP2 ice core, we obtained 80 marine 14C calibration points for the interval 11.4-53.3 ka cal BP. Between 27 and 54 ka cal BP the continuous record of 14C/cal age differences reveals three intervals of highly increased 14C concentrations coincident with low values of paleomagnetic field intensity, two of which are attributed to the geomagnetic Mono Lake and Laschamp excursions (33.5-34.5 ka cal BP with maximum 550 per mil marine Delta-14C, and 40.3-41.7 ka cal BP with maximum 1215 per mil marine Delta-14C, respectively). A third maximum (marine Delta-14C: 755 per mil) is observed around 38 ka cal BP and attributed to the geomagnetic intensity minimum following the Laschamp excursion. During all three events the Delta-14C values increase rapidly with maximum values occurring at the end of the respective geomagnetic intensity minimum. During the Mono Lake Event, however, our Delta-14C values seem to underestimate the atmospheric level, if compared to the 36Cl flux measured in the GRIP ice core (Wagner et al. 2000) and other records. As this excursion coincides with a meltwater event in core PS2644, the underestimation is probably caused by an increased planktonic reservoir age. The same effect also occurs from 38.5 to 40 ka cal BP when the meltwater lid of Heinrich Event 4 affected the planktonic record.
    • 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.
    • The Carbonate 14C Background and Its Components at the Leibniz AMS Facility

      Schleicher, Markus; Grootes, Pieter M.; Nadeau, Marie-Josée; Schoon, Axel (Department of Geosciences, The University of Arizona, 1998-01-01)
      After routine accelerator mass spectrometry (AMS) radiocarbon dating had been established at the Leibniz-Labor with the completion of systems for CO2 production, graphitization, and target making, a systematic investigation was conducted to find the sources of 14C concentrations observed in background materials. We quantified the contributions of the AMS-system, the reduction, CO2 production from carbonate, carbonate contamination, and combustion. Carbonate contamination appears to be the dominating factor. Improvements in the pretreatment of foraminifer carbonate have led to the elimination of most of this contamination.