Stable Isotope Investigation of Vertebrates From Hagerman Fossil Beds National Monument

Stable oxygen and carbon isotopes of fossil teeth are commonly used to investigate paleoclimate of past environments and paleoecology of past faunas. Tooth enamel from Hagerman Fossil Beds National Monument was analyzed for δ¹⁸O and δ¹³C to investigate paleoclimatic/paleoecological conditions 3.2Ma (Pliocene) in southern Idaho. This was one of Earth’s warmest recent climates. Because climate trends suggest an increase in temperature over the next several hundred years, it may serve as an analogue for future climate dynamics. Taxa analyzed include relatives of proboscidieans, horses, camels, and beavers. Average δ¹⁸O_enamel ranged between 21.81‰-VSMOW for a camel to 17.42‰ for a horse. Overall, beavers have the lowest average δ¹⁸O_enamel values (17.55‰). δ¹³C_enamel range between -7.75‰-VPDB for a beaver to -10.56‰ for a camel. High δ¹⁸O_enamel for camels suggest they consumed more plants and drank less water than other taxa, while low δ¹⁸O values for beavers suggest they were water dependent. Low δ13C_enamel camel values suggest they selected food with higher water content, whereas high δ¹³C beaver values are consistent with a dry diet (bark).  Modern δ¹⁸O_{precipitation} in Hagerman is -16‰ whereas δ¹⁸O_{precipitation} calculated from horses was -15.0‰ and -14.1‰ for proboscidieans. This change in isotope composition is interpreted to result from lower elevations of the Pliocene Cascade Range, because higher mountain ranges produce lower δ¹⁸O_{precipitation} (as seen today). The modern precipitation rate in Hagerman is 250 mm rain/yr. Using δ¹³C_enamel, Pliocene precipitation rates are calculated at 100 mm rain/yr.  This suggests future warm climates in southern Idaho may be drier than today.