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USGS National Research Program: Tucson AZ

Past Research Highlights

January 2004

Radiocarbon Dating of Gastropod Shells

Jeff Pigati, University of Arizona, Department of Geosciences
Jay Quade, University of Arizona, Department of Geosciences
Tim Shanahan, University of Arizona, Department of Geosciences
C. Vance Haynes, Jr., University of Arizona, Department of Anthropology

Gastropods are found today in a variety of environmental conditions, from marshes and wet meadows, to grasslands and high-elevation forests. Their shells are abundantly preserved in diverse geological and archeological deposits, and could potentially be used in radiocarbon dating. Gastropod shell carbonate contains ~12% carbon by weight, but many taxa consume limestone and incorporate old carbon during shell formation. Shell dates can be too old by up to ~3,000 14C yrs and are considered less trustworthy than dates on terrestrial plant materials.

Researchers at the Desert Lab have found that a previously overlooked group of gastropods, termed "minute" because of their small body size (<10 mm in length), yield reliable 14C ages even in the presence of limestone. The 14C activity was measured in living gastropods collected in two geologic settings that maximize the potential for ingestion of "old" carbon: (1) alluvium dominated by Paleozoic carbonate rocks, and (2) the margins of active springs with highly 14C-deficient water present at the surface.

Drawings of Pupilla blandi, Euconulus fulvus and Succineidae shells.
Pupilla blandi, Euconulus fulvus and Succineidae shells, from Burch, J.B., 1962, How to Know the Eastern Land Snails: Wm. C. Brown Company Publishers, Dubuque IA, 213 pp.

Several minute gastropods do incorporate old carbon (~2 to >30%) during shell formation, and would therefore yield 14C ages that are too old. The 14C activities of the common land snails Pupilla blandi and Euconulus fulvus, however, are indistinguishable from the 14C activity of live plants. The 14C activity of the semi-aquatic gastropod Succineidae deviated from modern values by only ~10% of the local carbon-reservoir effect (i.e. the amount of old carbon dissolved in the available surface water). The reliability of Pupilla, Euconulus, and Succineidae for 14C dating, even in settings where highly 14C-deficient carbon is ubiquitous, demonstrates their potential for dating a variety of Quaternary sediments worldwide.

Pigati, J. S., Quade, J., Shanahan, T. M., and Haynes, C. V. 2003. Radiocarbon dating of minute gastropods and new constraints on the timing of late Quaternary spring discharge deposits in southern Arizona. Palaeogeography, Palaeoclimatology, and Palaeoecology v. 3244, p. 1-13.

For more information contact Jeff Pigati at jpigati@geo.arizona.edu.

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