Stable Isotope Investigation of Vertebrates From Hagerman Fossil Beds National Monument
Matthew J. Kohn, Dr. Celina Suarez (University of Colorado-Boulder), and Samantha Evans
Stable oxygen and carbon isotopes of fossil teeth are commonly used to investigate paleoclimate (rainfall, temperature) of past environments and paleoecology (diet, habitat) of past faunas. Tooth enamel from Hagerman Fossil Beds National Monument was analyzed for oxygen and carbon stable isotopes (δ18O and δ13C) to investigate paleoclimatic and paleoecological conditions 3.2 million years ago in southern Idaho. This time period, the Pliocene climatic optimum, 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 elephants (mastodon and gomphothere), horses, camels, and beavers, and data were collected in the Stable Isotope Laboratory, Department of Geosciences, Boise State University. Average δ18O values for taxa ranged between 21.81‰ (VSMOW) for a camel to 17.42‰ for an outlier horse. Overall, beavers have the lowest average δ18O(enamel) values (17.55‰). δ13C values range between -7.75‰ (VPDB) for a beaver to -10.56‰ for a camel. The high δ18O values for camels suggest they consumed more plants and drank less water than the other taxa, while low δ18O values for beavers suggest they drank more water or had a higher water flux. The low δ13C values for camels suggest they selected food that had higher water content, whereas high δ13C values for beavers are consistent with a relatively dry diet of bark. Modern precipitation δ18O in Hagerman is -16‰ whereas the precipitation composition calculated from horses was -15.0‰, and -14.1‰ for combined gomphothere and mastodon. This change in isotope composition is interpreted to result from lower elevations of the Cascade Range in the Pliocene, because higher mountain ranges produce lower δ18O values (as seen today). The modern precipitation rate in Hagerman is 250 mm rain/yr. Using δ13C(enamel), Pliocene precipitation rates are calculated to be 100 mm rain/yr. This suggests that past warm climates in southern Idaho may have been drier. A potential consequence of modern warming trends may be drier conditions in southern Idaho.