Assessing the Radiocarbon Landscape Reservoir Effect of Gastropods from Central Texas to Increase the Accuracy and the Precision of Radiocarbon Dating of Past River Systems

摘要

A persistent challenge in geochronology is determining reliable ~(14)C ages of terrestrial gastropods, particularly for genera that live on limestone-rich terrain, like in central Texas. Modern gastropods may yield apparent ~(14)C ages up to 3400 years old, reflecting the old carbon added into the shell aragonite, by ingestion of old CO_2 in surface waters and limestone particles. This age offset for gastropods is called the Landscape Reservoir Effect (LRE), and if uncompensated, it reduces the accuracy and precision of ~(14)C dating. The LRE for gastropods from the Brazos and other rivers in the region range from 580 to 1060 years. However, other studies of modern gastropods living on limestone terrain in the upper Midwest and western U.S. indicate an inconsistency in the presence and magnitude of the LRE. Thus, there is a need to date gastropod shells collected prior to atomic bomb contamination (July 1945) to resolve the LRE for a specific catchment. This study will utilize gastropod species, collected between 1907 and 1945, from the catchment of the Brazos River in the collections of the Mayborn Museum, at Baylor University. The research will determine the LRE for gastropod species Rabdotus dealbatus, Anguispira alternate, Vitrea indentata, and Rumina decollate through ~(14)C and ~(13)C analyses of shells by accelerator mass spectrometry. Also, carbon isotope composition for modern analogous shells will indicate the variability in carbon cycling with a more human-altered river system, than pre-bomb times. Ultimately, this research will establish what species and shell areas (aperture to inner whorl) yield ~(14)C ages with the lowest and most consistent LRE and associated errors. The data generated would provide a statistical basis for a numeric correction to enhance ~(14)C dating of terrestrial gastropods, and improve the understanding of the timing of past climate changes.