2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 9
Presentation Time: 4:00 PM


PIGATI, Jeffrey S., SHANAHAN, Timothy M. and QUADE, Jay, Department of Geosciences, Univ of Arizona, Tucson, AZ 85721, jpigati@geo.arizona.edu

Minute gastropods live in a variety of environmental conditions, from marshes and wet meadows, to grasslands and high-elevation forests. Their distribution in the fossil record is equally diverse, yet they are often avoided for 14C dating because many larger gastropods are known to incorporate 14C-deficient carbon during shell formation. Some minute varieties appear to yield 14C ages comparable to late Pleistocene organic matter, but their potential for dating is not based on live specimens. We conducted a more rigorous evaluation, which included measuring the 14C activity of minute gastropods collected live in two geologic settings that maximize the potential for ingestion of “old” carbon: (1) alluvium dominated by Paleozoic carbonate rocks, and (2) near extant springs with highly 14C-deficient water present at the surface. The 14C activities of Pupilla blandi and Euconulus fulvus collected from setting 1 are indistinguishable from the 14C activity of live plants, and therefore these taxa will yield reliable 14C ages in the fossil record. A semi-aquatic gastropod, Catinella sp. (family: Succineidae), collected from setting 2 deviates from modern by an amount equivalent to ~10% of the local hard-water effect. Several other minute taxa, including Cochlicopa, Discus, Gastrocopta, Vallonia, and Vitrina, incorporate variable amounts (2 to >30%) of old carbon during shell formation, and therefore provide maximum 14C ages. As an example of the potential utility of minute gastropods for dating in the fossil record, we 14C-dated shells of E. fulvus and Succinea sp. (family: Succineidae) to constrain the age of the Coro Marl, a late Pleistocene spring-fed marsh deposit exposed at Murray Springs in the San Pedro Valley of southern Arizona, USA. 14C ages obtained from the minute gastropods indicate deposition of the marl occurred between ~25 and 13 14C ka. These ages can be used to constrain the timing of elevated water tables throughout the valley.