Radiocarbon dating of minute gastropods and new constraints on the timing of late Quaternary spring-discharge deposits in southern Arizona, USA
Abstract Gastropod shells are commonly preserved in Quaternary sediments, but are often avoided for radiocarbon dating because some taxa incorporate 14 C-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 14 C ages for late Pleistocene samples. A more rigorous evaluation of the 14 C inventory of minute gastropods is presented here, which involved measuring the 14 C 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 14 C-deficient water present at the surface. We found that several minute taxa, including Vallonia , incorporate significant and variable amounts of old carbon (∼2 to >30%) during shell formation. The 14 C activities of the land snails Pupilla blandi and Euconulus fulvus , however, are indistinguishable from the 14 C activity of live plants. The 14 C activity of the semi-aquatic gastropod Catinella sp. (Family: Succineidae ) deviates from modern values in the presence of 14 C-deficient water by an amount equivalent to ∼10% of the local carbon-reservoir effect. These results imply that at least some minute gastropods can provide reliable 14 C ages even when 14 C-deficient carbon is readily available. To demonstrate an application of our findings, we 14 C-dated shells from P. muscorum , E. fulvus , 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 ∼25 000 and 13 000 14 C 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 ∼150-km stretch of the San Pedro Valley. Thus, the 14 C ages of minute gastropods presented here may be used to infer the timing of high water-table levels throughout the valley.