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Assessing Open-System Behavior of 14C in Terrestrial Gastropod Shells

Published online by Cambridge University Press:  18 July 2016

Jason A Rech*
Affiliation:
Department of Geology, Miami University, Oxford, Ohio 45056, USA.
Jeffrey S Pigati
Affiliation:
US Geological Survey, Denver Federal Center, Box 25046, MS-980, Denver, Colorado 80225, USA.
Sophie B Lehmann
Affiliation:
Department of Geology, Miami University, Oxford, Ohio 45056, USA.
Chelsea N McGimpsey
Affiliation:
Department of Geology, Miami University, Oxford, Ohio 45056, USA.
David A Grimley
Affiliation:
Illinois State Geological Survey, University of Illinois, 615 East Peabody Drive, Champaign, Illinois 61820, USA.
Jeffrey C Nekola
Affiliation:
Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA.
*
Corresponding author. Email: [email protected].
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Abstract

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In order to assess open-system behavior of radiocarbon in fossil gastropod shells, we measured the 14C activity on 10 aliquots of shell material recovered from Illinoian (~190–130 ka) and pre-Illinoian (~800 ka) loess and lacustrine deposits in the Midwestern USA. Eight of the 10 aliquots yielded measurable 14C activities that ranged from 0.25 to 0.53 percent modern carbon (pMC), corresponding to apparent 14C ages between 48.2 and 42.1 ka. This small level of open-system behavior is common in many materials that are used for 14C dating (e.g. charcoal), and typically sets the upper practical limit of the technique. Two aliquots of gastropod shells from the Illinoian-aged Petersburg Silt (Petersburg Section) in central Illinois, USA, however, yielded elevated 14C activities of 1.26 and 1.71 pMC, which correspond to apparent 14C ages of 35.1 and 32.7 ka. Together, these results suggest that while many fossil gastropods shells may not suffer from major (>1%) open-system problems, this is not always the case. We then examined the mineralogy, trace element chemistry, and physical characteristics of a suite of fossil and modern gastropod shells to identify the source of contamination in the Petersburg shells and assess the effectiveness of these screening techniques at identifying samples suitable for 14C dating. Mineralogical (XRD) and trace element analyses were inconclusive, which suggests that these techniques are not suitable for assessing open-system behavior in terrestrial gastropod shells. Analysis with scanning electron microscopy (SEM), however, identified secondary mineralization (calcium carbonate) primarily within the inner whorls of the Petersburg shells. This indicates that SEM examination, or possibly standard microscope examination, of the interior of gastropod shells should be used when selecting fossil gastropod shells for 14C dating.

Type
Soils and Sediments
Copyright
Copyright © The American Journal of Science 

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