Reliable Late-Pleistocene Stratigraphic Ages and Shorter Groundwater Travel Times from ¹⁴C in Fossil Snails from the Southern Great Basin

Both aquatic and land snails are common in the geologic record, but their utility in dating is greatly restricted by their well-documented tendency to yield ¹⁴C dates inconsistent with true ¹⁴C ages. In this study, we examine the use of ¹⁴C ages from (1) small, previously unstudied, terrestrial snails to date hosting spring deposits and from (2) cooccuring aquatic snails to constrain ground-water travel times during the last glacial period. Our study area in the southern Great Basin encompasses Yucca Mountain, site of the proposed high-level nuclear waste repository, where information on the age and extent of past high water tables and on ground-water flow times is crucial to several licensing issues. Our results show that shells of small terrestrial snails belonging to Vallonia sp. yield ¹⁴C dates consistent with ¹⁴C ages of associated carbonized wood. These results imply that these taxa can provide reliable ¹⁴C age control on the broadly distributed deposits in which they have been described. In contrast, cooccurring aquatic snails from fossil spring deposits yield ¹⁴C ages generally greater than the control age. This is because the aquatic shells often formed in spring waters that had an initial ¹⁴C deficiency. However, the magnitude of the deficiency is much less than that observed in nearby modern springs, arguing for much higher average ¹⁴C contents in late Pleistocene groundwaters in these basins. If representative, this implies shorter groundwater travel times through aquifers in southern Nevada during late-glacial time.