Gastropod shells are commonly preserved in Quaternary eolian, lacustrine, and fluvial sediments, and are often (super 14) C dated to provide chronologic control. A persistent challenge in geochronology is determining the accuracy and precision of (super 14) C ages on terrestrial gastropods, particularly for genera that live on limestone-rich terrain, like in central Texas. A previous study by G. Goodfriend reported (super 14) C age anomalies of 580 to 1060 years for the pre-bomb gastropod Rabdotus dealbatus from catchments across central Texas. In contrast, recent studies of modern gastropods living on limestone terrain in the Midwest and the western U.S. indicate an inconsistency in the presence and magnitude of this age anomaly. Thus, there is a need to date gastropod shells collected prior to atomic bomb contamination to resolve the age anomaly at the catchment level. This study focused on (super 14) C dating of gastropods collected alive between 1907 and 1945 from the Brazos River catchment, curated at the Mayborn Museum, Baylor University. The inner and outer-most whorl sections were (super 14) C dated by AMS dating method for gastropod species Rabdotus dealbatus, Anguispira alternata, Vitrea indentata, and Rumina decollate. Also, the outer and inner whorl was (super 14) C dated for a modern gastropod R. decollate collected in Feb. 2015 to assess continuity of age offsets into the 21st century. Larger robust species such as R. decollate, A. alternata and R. dealbatus gave age anomalies between 170 and 1070 yr. Surprisingly, the inner and outer whorls of these species show a consistent offset of 316+ or -53 years (n=4). The smallest species V. indentata returned the largest age anomaly of 2250 yr. Currently, we are considering the following processes to explain these age anomalies (1) gastropods living on limestone rich environments incorporate (super 14) C depleted limestone carbon source into their aragonite, or (2) during shell formation gastropods discriminate against (super 14) C in the early stages of shell secretion and to a lesser extent with age. We believe that this research will establish what species and shell areas (aperture to inner whorl) yield (super 14) C ages with the lowest and most consistent age anomaly and errors, and will also provide a statistical basis for a numeric correction on the reservoir effect in central Texas to enhance (super 14) C dating accuracy of terrestrial gastropods.
|Title of host publication
|1-4 November GSA 2015
|Geological Society of America (GSA), Boulder, CO, United States
|Published - 2015