Reliable late-Pleistocene stratigraphic ages and shorter groundwater travel times from C-14 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 C-14 dates inconsistent with true C-14 ages. In this study, we examine the use of C-14 ages from (1) small, previously unstudied, terrestrial snails to date hosting spring deposits and from (2) cooccuring aquatic snails to constrain groundwater 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 groundwater flow times is crucial to several licensing issues. Our results show that shells of small terrestrial snails belonging to Vallonia sp. yield C-14 dates consistent with C-14 ages of associated carbonized wood. These results imply that these taxa can provide reliable C-14 age control on the broadly distributed deposits in which they have been described. In contrast, cooccurring aquatic snails from fossil spring deposits yield C-14 ages generally greater than the control age. This is because the aquatic shells often formed in spring waters that had an initial C-14 deficiency. However, the magnitude of the deficiency is much less than that observed in nearby modern springs, arguing for much higher average C-14 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. (C) 1997 University of Washington.