Publication Date

5-2025

Date of Final Oral Examination (Defense)

3-7-2025

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geoscience

Department

Geosciences

Supervisory Committee Chair

Mark D. Schmitz, Ph.D.

Supervisory Committee Member

Matthew J. Kohn, Ph.D.

Supervisory Committee Member

Linda M. Reynard, Ph.D.

Abstract

The Mid-Miocene Climatic Optimum (~17-14Ma) marks the last time that Earth’s atmospheric CO2 concentrations were as high as modern levels, correlating to a peak of significant warming during an otherwise cooling trend through the Cenozoic. To understand how modern ecological systems may respond to CO2 -induced climate change, paleontologists are studying how plant type and diversity changed with the rise and fall of CO2 concentrations over the ~3Ma interval, and utilize intercalated tephras amenable to radioisotope geochronology to bracket fossiliferous horizons in time. The legacy model for tephrostratigraphy in the Pacific Northwest utilized Fe and Ca concentrations to chemically correlate volcanic horizons across several basins and six directly-dated horizons to anchor the correlated horizons in time. In addition to the sparsity of directly-dated horizons, the previous dates’ uncertainties are large enough to be difficult to use in studies analyzing fine-time scale disturbances. This has spurred a need to reevaluate the age of these tephras with alternative, higher-precision techniques like U-Pb zircon geochronology, and to reevaluate previous chemical correlations with a wider suite of analytes and more advanced statistical techniques like singular value decomposition principal component analysis (PCA).

I present a collection of U-Pb zircon dates and geochemical analyses for volcanic horizons in the Sucker Creek (eastern OR/western ID), Mascall (central OR), and Bully Creek (eastern OR) formations that can be used to refine mid-Miocene proxy records relevant to climate forcing and ecosystem response. U-Pb zircon dates were obtained with chemical abrasion isotope dilution thermal ionization mass spectrometry, and geochemical analyses were measured with scanning electron microscope energy dispersive spectroscopy and laser ablation inductively coupled plasma mass spectrometry. Previous chemical correlations by Nash and Perkins (2012) and Streck et al. (2015) were reevaluated with major element compositions in PCA, and chronologically-similar U-Pb dates were hypothesized to be correlated and tested with major, rare earth, and incompatible elements in PCA.

These new ages significantly improve the temporal resolution of the MMCO in the three formations. Reevaluation of previous chemical correlations with PCA has both reaffirmed and refuted said correlations, thereby severing some inter-basin connections. Evaluation of this study’s chronologically-similar samples with chemical compositions in PCA has shown that many of these horizons should not be compositionally correlated, with the exception of a horizon connecting the Sucker Creek and a horizon at Bully Creek formations. These results allow for existing frameworks (Nash and Perkins 2012, Downing 1992, Maguire et al. 2018) to be updated and refined.

DOI

10.18122/td.2366.boisestate

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