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

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

Nov 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002e%26psl.204..151k&link_type=abstract

Earth and Planetary Science Letters, Volume 204, Issue 1-2, p. 151-165.

Computer Science

12

Scientific paper

Oxygen isotope compositions of fossil equid teeth in the Cascade rainshadow reveal a ~5‰ decrease in mean δ18O since 27 Ma. Isotopic changes are inconsistent with expected effects from global climate change because: (a) the expected isotopic shift to tooth δ18O values due to global climate change (~1‰) 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 δ18O value of rainwater of 6-8‰ since the late Oligocene, which is approximately equivalent to the modern-day difference in δ18O 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 ~15.4 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 7-9‰ at 15.4-7 Ma to ~3‰ at 3 Ma, then increased to 6-8‰ 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.

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