Environmental significance of 13C/12C and 18O/16O ratios of modern land-snail shells from the southern great plains of North America

C-13/C-12 and O-18/O-16 ratios of aragonite shells of modem land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow cast-west Corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell 6180 values (PDB scale) ranged from -4.1parts per thousand to 1.2parts per thousand, while delta(13)C values ranged from -13.2parts per thousand to 0.0parts per thousand delta(18)O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1parts per thousand from locality averages of measured delta(18)O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the delta(18)O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated delta(13)C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average delta(13)C Values of the modem aragonite shells are related to variations in the type of photosynthesis (i.e., C-3, C-4) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the "neighborhood" scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems. (C) 2004 University of Washington. All rights reserved.