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 th eir utility in dating is greatly restricted by their well-documented t endency 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 unst udied, terrestrial snails to date hosting spring deposits and from (2) cooccuring aquatic snails to constrain groundwater travel times durin g the last glacial period. Our study area in the southern Great Basin encompasses Yucca Mountain, site of the proposed high-level nuclear wa ste repository, where information on the age and extent of past high w ater tables and on groundwater flow times is crucial to several licens ing issues. Our results show that shells of small terrestrial snails b elonging to Vallonia sp. yield C-14 dates consistent with C-14 ages of associated carbonized wood. These results imply that these taxa can p rovide reliable C-14 age control on the broadly distributed deposits i n which they have been described. In contrast, cooccurring aquatic sna ils from fossil spring deposits yield C-14 ages generally greater than the control age. This is because the aquatic shells often formed in s pring waters that had an initial C-14 deficiency. However, the magnitu de of the deficiency is much less than that observed in nearby modern springs, arguing for much higher average C-14 contents in late Pleisto cene groundwaters in these basins. If representative, this implies sho rter groundwater travel times through aquifers in southern Nevada duri ng late-glacial time. (C) 1997 University of Washington.