First Advisor

John Bershaw

Term of Graduation

Winter 2021

Date of Publication

4-22-2021

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Paleoclimatology -- Pacific Northwest -- Cenozoic, Stable isotopes -- Analysis

Physical Description

1 online resource (vi, 61 pages)

Abstract

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.

Rights

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Persistent Identifier

https://archives.pdx.edu/ds/psu/35546

788857_supp_3fbc4044-6466-4864-8535-01f03a5d611a.xlsx (331 kB)
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

Available for download on Friday, April 22, 2022

Included in

Geology Commons

Share

COinS