13C/12C and 18O/16O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east–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 δ18O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ13C values ranged from −13.2‰ to 0.0‰. δ18O 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 1‰ from locality averages of measured δ18O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ18O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ13C 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 δ13C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C3, C4) 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.

Significance In 2007, scientists proposed that the start of the Younger Dryas (YD) chronozone (10,900 radiocarbon years ago) and late Pleistocene extinctions resulted from the explosion of a comet in the earth’s atmosphere. The ET event, as it is known, is purportedly marked by high levels of various materials, including nanodiamonds. Nanodiamonds had previously been reported from the Bull Creek, Oklahoma, area. We investigate this claim here by quantifying the distribution of nanodiamonds in sediments of different periods within the Bull Creek valley. We found high levels of nanodiamonds in YD boundary deposits, supporting the previous claim. A second spike in nanodiamonds during the late Holocene suggests that the distribution of nanodiamonds is not unique to the YD.

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