Stable Isotopes in Large Herbivore Tooth Enamel Capture a Mid-Miocene Precipitation Spike in the Interior Pacific Northwest

Stable carbon and oxygen isotopes of fossil ungulate tooth enamel were used to investigate the paleoclimate of the Pacific Northwest from the mid-Oligocene through Pleistocene, particularly targeting the mid-Miocene Climatic Optimum (MMCO). We specifically test whether precipitation, as inferred from tooth enamel δ ¹³ C values relative to the δ ¹³ C values of atmospheric CO ₂ (Δ ¹³ C), increased with increasing partial pressure of atmospheric CO ₂ ( p CO ₂ ). Analysis emphasized water-dependent equids, rhinos, and proboscideans, but several artiodactyls and Castor were also analyzed as available. Values of Δ ¹³ C > + 19.5‰ in the lower John Day (~ 27 and ~ 18 Ma) and Mascall (15–16 Ma) localities imply relatively high mean annual precipitation (MAP = ca. 550–800 mm/yr). Values of Δ ¹³ C < + 18.5‰ at ~ 18 Ma and at four levels between 15 and 3 Ma imply low MAP (≤ 250 mm/yr), similar to modern climate. High vs. low MAP generally correlates with high vs. low p CO ₂ , implicating p CO ₂ as a probable driver of MAP in the Pacific Northwest. A climate oscillation model best explains data, such that warm-wet conditions during high p CO ₂ events alternated with cool-dry conditions during low p CO ₂ events on timescales of 100 kyr. Because of poor time constraints associated with many of the specimens, changes in p CO ₂ are mainly inferred from marine isotopic data, rather than direct comparison between terrestrial p CO ₂ reconstructions and MAP. High δ ¹⁸ O values in specimens from the John Day (+ 21.8 ± 1.0‰; VSMOW) and Mascall (+ 21.3 ± 0.8‰) Formations could reflect lower elevations for the upwind Cascade Range prior to ~7 Ma, or proximity to the coast compared to younger, more inland sites (δ ¹⁸ O = + 17.7 ± 0.9 to + 19.6 ± 1.1‰). Unusually high δ ¹⁸ O values of Dromomeryx sp. from Red Basin (+ 27.4 ± 0.2‰) probably reflect drought tolerance. General circulation models commonly reproduce temperature but underestimate precipitation during the MMCO in the Pacific Northwest, suggesting that refinements are needed to predict more accurately how western North American precipitation will respond to increases in p CO ₂ during the next century.