• Sample Dilution for AMS 14C Analysis of Small Samples (30-150 μg C)

      de Rooij, M.; van der Plicht, J.; Meijer, H. J. (Department of Geosciences, The University of Arizona, 2008-01-01)
      We investigated sample dilution as a technique for accelerator mass spectrometry (AMS) radiocarbon analysis of very small samples (down to 30 mu-g). By diluting such samples up to a total weight of 200 mu-g, we can still perform reliable AMS measurements and improve the success rate significantly for targets that are difficult to measure. A disadvantage of this dilution technique is a loss of measurement precision. In addition, calculations of the 14C/12C isotope ratios and the uncertainties therein are not straightforward because of peculiarities in isotope fractionation processes in the AMS system. Therefore, to make sample dilution a routine method in our laboratory, we did extensive theoretical and experimental research to find the optimum conditions for all relevant parameters. Here, we report on the first detailed study dealing with all aspects of sample dilution. Our results can be applied in general. As an illustrative test case, we analyze 14C data for CO2 extracted from an ice core, from which samples of 35 mu-g C or less are available.
    • Very Long-Lived Mollusks Confirm 17th Century AD Tephra-Based Radiocarbon Reservoir Ages for North Icelandic Shelf Waters

      Wanamaker, Alan D., Jr.; Heinemeier, Jan; Scourse, James D.; Richardson, Christopher A.; Butler, Paul G.; Eiríksson, Jon; Knudsen, Karen Luise (Department of Geosciences, The University of Arizona, 2008-01-01)
      Marine sediment records from the north Icelandic shelf, which rely on tephrochronological age models, reveal an average Delta-R (regional deviation from the modeled global surface ocean reservoir age) of approximately 150 yr for the last millennium. These tephra-based age models have not hitherto been independently verified. Here, we provide data that corroborate Delta-R values derived from these sediment archives. We sampled the youngest portion (ontogenetic age) of a bivalve shell, Arctica islandica (L.), for radiocarbon analysis, which was collected alive in 2006 from the north Icelandic shelf in ~80 m water depth. Annual band counting from the sectioned shell revealed that this clam lived for more than 405 yr, making it the longest-lived mollusk and possibly the oldest non-colonial animal yet documented. The 14C age derived from the umbo region of the shell is 951 +/- 27 yr BP. Assuming that the bivalve settled onto the seabed at AD 1600, the corresponding local value of Delta-R is found to be 237 +/- 35 yr by comparison of the 14C age with the Marine04 calibration curve (Hughen et al. 2004) at this time. Furthermore, we cross-matched a 287-yr-old, dead-collected, A. islandica shell from AD 1601 to 1656 from the same site with the live-caught individual. 14C analysis from the ventral margin of this shell revealed a Delta-R of 186 +/- 50 yr at AD 1650. These values compare favorably with each other and with the tephra-based Delta-R values during this period, illustrating that 14C from A. islandica can effectively record 14C reservoir changes in the shelf seas.