Oxygen isotope evidence for progressive uplift of the Cascade Range, Oregon

Oxygen isotope compositions of fossil equid teeth in the Cascade rainshadow reveal a similar to5parts per thousand decrease in mean delta(18)O since 27 Ma. Isotopic changes are inconsistent with expected effects from global climate change because: (a) the expected isotopic shift to tooth delta(18)O values due to global climate change (similar to1parts per thousand) is much smaller than the observed shift, (b) predicted and observed isotopic trends are opposite for Oligocene vs. Miocene samples, and (c) average compositions and ranges in compositions remained unchanged for samples from before and after major global cooling in the mid-Miocene. Accounting for a decrease in relative humidity of at least 15%, we infer a topographically driven secular shift in the delta(18)O value of rainwater of 6-8parts per thousand since the late Oligocene, which is approximately equivalent to the modem-day difference in delta(18)O values of precipitation and surface waters across the central Cascades. Rise of the central Cascades apparently occurred monotonically over the last 27 Ma, with a hiatus between similar to15.4 and 7.2 Ma, possibly related to eruption of the Columbia River Basalts. Progressive volcanic accumulation over tens of millions of years best explains the data, rather than a short-lived uplift event. Paleoseasonality, as inferred from isotope zoning and intertooth variability, decreased dramatically from 7-9parts per thousand at 15.4-7 Ma to similar to3parts per thousand at 3 Ma, then increased to 6-8parts per thousand today. The cause of the decrease in seasonality at 3 Ma may reflect either brief warming during the mid-Pliocene within the context of global tectonic reorganization, or consumption by equids of water from an isotopically buffered Lake Idaho. (C) 2002 Elsevier Science B.V. All rights reserved.