Portland State University. Department of Geology
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
Paleoclimatology -- Pacific Northwest -- Cenozoic, Stable isotopes -- Analysis
1 online resource (vi, 61 pages)
The modern Cascades are a north-south striking mountain range that intercepts Pacific Ocean-derived moisture resulting in orographic precipitation on the western flank and a relatively arid rain shadow in the lee. Because of the strong coupling between topography and climate and consistent moisture source (Pacific Ocean) throughout the year, the region is ideally situated for paleoclimate studies that utilize the isotopic composition (δ18O and δD) of meteoric water to determine when uplifts occurred. Here, I synthesize the spatial and temporal evolution of stable isotopes in paleowater proxies across the Pacific Northwest during the Cenozoic. New volcanic glass (δD) and paleosol carbonate stable isotope (δ18O) results from central Oregon are presented, along with published proxy data, including fossil teeth, smectites, and carbonate concretions. I interpret a polygenetic history of Cascade Mountain topographic uplift along-strike, characterized by: 1) Steady uplift of the Washington Cascades through the Cenozoic due long-term arc rotation and shortening against a Canadian buttress, and 2) Uplift of the Oregon Cascades to similar-to-modern elevations by the late Oligocene, followed by topographic stagnation as extension developed into the Neogene. Since the Miocene, meteoric water δ18O values have decreased in Oregon, possibly due to emergence of the Coast Range and westward migration of the coastline. Estimates of ancient deuterium excess (d-excess) based on multiple, independent proxies suggest a trend of aridification in central Oregon during this time. Spatial variability in isotopic change across the Pacific Northwest suggests that secular global climate change is not the primary forcing mechanism behind isotopic trends, though Milankovitch cycles may be partly responsible for relatively short-term variation in isotopic results.
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McLean, Alex, "From Molecules to Mountains: A Multi-Proxy Investigation into Ancient Climate and Topography of the Pacific Northwest, USA" (2021). Dissertations and Theses. Paper 5673.
Compilation of stable isotope, location, and source data for all samples considered in this paper
788857_supp_0d52eaa2-0ad3-4a92-84af-6ceed918684f.xlsx (52 kB)
Modern water data for the Pacific Northwest