Radiocarbon dating of minute gastropods and new constraints on the timing of late Quaternary spring-discharge deposits in southern Arizona, USA

Gastropod shells are commonly preserved in Quaternary sediments, but are often avoided for radiocarbon dating because some taxa incorporate C-14-deficient carbon during shell formation. Recently, Brennan and Quade [(1997) Quat. Res. 47, 329-336] found that some minute taxa (Vallonia, Pupilla, and Succineidae) appear to yield reliable C-14 ages for late Pleistocene samples. A more rigorous evaluation of the C-14 inventory of minute gastropods is presented here, which involved measuring the C-14 activity of specimens collected live in two geologic settings that maximize the potential for ingestion of old' carbon: (1) alluvium dominated by Paleozoic carbonate rocks, and (2) adjacent to extant springs with highly C-14-deficient water present at the surface. We found that several minute taxa, including Vallonia, incorporate significant and variable amounts of old carbon (similar to 2 to > 30%) during shell formation. The C-14 activities of the land snails Pupilla blandi and Euconulus fulvus, however, are indistinguishable from the C-14 activity of live plants. The C-14 activity of the semi-aquatic gastropod Catinella sp. (Family: Succineidae) deviates from modern values in the presence of C-14-deficient water by an amount equivalent to similar to 10% of the local carbon-reservoir effect. These results imply that at least some minute gastropods can provide reliable, 4 C ages even when C-14-deficient carbon is readily available. To demonstrate an application of our findings, we C-14-dated shells from P. muscorum, E fulms, and Succinea sp. (Family: Succineidae) recovered from the Coro Marl, a late Pleistocene spring-fed marsh deposit exposed at the Murray Springs Paleoindian site in the San Pedro Valley of southern Arizona, USA. Radiocarbon ages obtained from the minute gastropods show that the unit was deposited between similar to25000 and 13000 C-14 years ago. The marl is situated > 15 m above the modern water table at Murray Springs, and is similarly positioned in discontinuous outcrops along a similar to 150-km stretch of the San Pedro Valley. Thus, the C-14 ages of minute gastropods presented here may be used to infer the timinsg of high water-table levels throughout the valley. (C) 2003 Elsevier B.V. All rights reserved.