Pronghorn (Antilocapra americana) enamel phosphate δ(18)O values reflect climate seasonality: Implications for paleoclimate reconstruction

Stable oxygen isotope (δ(18)O) compositions from vertebrate tooth enamel are widely used as biogeochemical proxies for paleoclimate. However, the utility of enamel oxygen isotope values for environmental reconstruction varies among species. Herein, we evaluate the use of stable oxygen isotope compositions from pronghorn (Antilocapra americana Gray, 1866) enamel for reconstructing paleoclimate seasonality, an elusive but important parameter for understanding past ecosystems. We serially sampled the lower third molars of recent adult pronghorn from Wyoming for δ(18)O in phosphate (δ(18)O(PO4)) and compared patterns to interpolated and measured yearly variation in environmental waters as well as from sagebrush leaves, lakes, and rivers (δ(18)O(w)). As expected, the oxygen isotope compositions of phosphate from pronghorn enamel are enriched in (18)O relative to environmental waters. For a more direct comparison, we converted δ(18)O(w) values into expected δ(18)O(PO4*) values (δ(18)O(W)‐(PO4*)). Pronghorn δ(18)O(PO4) values from tooth enamel record nearly the full amplitude of seasonal variation from Wyoming δ(18)O(W‐PO4*) values. Furthermore, pronghorn enamel δ(18)O(PO4) values are more similar to modeled δ(18)O(W‐PO4*) values from plant leaf waters than meteoric waters, suggesting that they obtain much of their water from evaporated plant waters. Collectively, our findings establish that seasonality in source water is reliably reflected in pronghorn enamel, providing the basis for exploring changes in the amplitude of seasonality of ancient climates. As a preliminary test, we sampled historical pronghorn specimens (1720 ± 100 AD), which show a mean decrease (a shift to lower values) of 1–2‰ in δ(18)O(PO4) compared to the modern specimens. They also exhibit an increase in the δ(18)O amplitude, representing an increase in seasonality. We suggest that the cooler mean annual and summer temperatures typical of the 18th century, as well as enhanced periods of drought, drove differences among the modern and historical pronghorn, further establishing pronghorn enamel as excellent sources of paleoclimate proxy data.