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 delta C-13 values relative to the delta C-13 values of atmospheric CO2 (Delta C-13), increased with increasing partial pressure of atmospheric CO2 (pCO(2)). Analysis emphasized water-dependent equids, rhinos, and proboscideans, but several artiodactyls and Castor were also analyzed as available. Values of Delta C-13 > + 19.5 parts per thousand in the lower John Day (similar to 27 and similar to 18 Ma) and Mascall (15-16 Ma) localities imply relatively high mean annual precipitation (MAP = ca. 550-800 mm/yr). Values of Delta C-13 < + 18.5 parts per thousand at similar to 18 Ma and at four levels between 15 and 3 Ma imply low MAP (250 mm/yr), similar to modem climate. High vs. low MAP generally correlates with high vs. low pCO(2), implicating pCO(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 pCO(2) events alternated with cool-dry conditions during low pCO(2) events on timescales of 100 kyr. Because of poor time constraints associated with many of the specimens, changes in pCO(2) are mainly inferred from marine isotopic data, rather than direct comparison between terrestrial pCO(2) reconstructions and MAP. High delta O-18 values in specimens from the John Day (+ 21.8 +/- 1.0 parts per thousand; VSMOW) and Mascall (+ 21.3 +/- 0.8 parts per thousand) Formations could reflect lower elevations for the upwind Cascade Range prior to similar to 7 Ma, or proximity to the coast compared to younger, more inland sites (delta O-18 = + 17.7 +/- 0.9 to + 19.6 +/- 1.1 parts per thousand). Unusually high delta O-18 values of Dromomeryx sp. from Red Basin (+ 27.4 +/- 0.2 parts per thousand) 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 pCO(2) during the next century.