Tree Rings and Oxygen Isotopes as Climatic Indicators in the U.S. Pacific Northwest

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  • Schmidt-Simard, Karly
    • Affiliation: College of Arts and Sciences, Department of Geography
Abstract
  • The U.S. Pacific Northwest (PNW) relies on precipitation, much of which falls during the winter and is stored as snowpack until spring, for hydropower that provides over half the region’s electricity. Recently, record-breaking heat waves have increased demand for air conditioning while winter snowpack and summer precipitation have decreased, causing a mismatch between hydropower supply and demand. As this mismatch is projected to worsen under climate change, a more complex understanding of seasonal precipitation patterns in the region becomes crucial. Climate proxies provide a means of revealing the nuances of these patterns across space and time. This dissertation examines three climate proxies and their ability to capture seasonal climate to better understand climate-proxy relationships, identify each proxy’s advantages and limitations, and investigate how they may be combined to improve seasonal climate reconstructions in the PNW. I first examine the relationship between oxygen isotope ratios in precipitation at five sites in the U.S. PNW and air mass trajectories to characterize the ways in which these trajectories influence precipitation isotope ratios. I then establish the extent to which annual and subannual (earlywood, latewood, and adjusted latewood) ring-width measurements of trees located near three of the five sites capture seasonal variations in precipitation and temperature. Lastly, I determine the precipitation, temperature, and source water signals captured by the oxygen isotope ratios stored in the earlywood and latewood of those same trees, which theoretically reflect the precipitation examined as the first proxy. In this research, I show that oxygen isotope ratios in precipitation in the PNW largely reflect air mass trajectories, but this relationship is complicated by other factors. I demonstrate that subannual tree-ring widths capture temperature and precipitation during the current and prior growing seasons, and I also show that subannual tree-ring isotope ratios strengthen summer climate capture while providing promise for reconstructions of growing-season and winter source water, which are linked with atmospheric circulation. Taken together, these findings demonstrate that subannual tree-ring widths and oxygen isotope ratios capture unique and complementary climate signals, and the combination of these two proxies stands to provide nuanced seasonal climate histories in the U.S. PNW.
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  • In Copyright - Educational Use Permitted
Advisor
  • Wise, Erika K.
  • Csank, Adam Z.
  • Konrad, Charles E.
  • Riveros-Iregui, Diego A.
  • Surge, Donna M.
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2022
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