RECENT RECTIFICATIONS IN THE TEPHROSTRATIGRAPHIC RECORD OF THE PACIFIC NORTHWEST

In an effort to understand how modern ecological systems may respond to increasing temperature due to rising atmospheric CO₂ concentrations, paleontologists are studying how plant type and diversity have changed through the mid-Miocene. The Mid Miocene Climatic Optimum (MMCO) marks the last time that 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 utilize the paleoclimate records of the MMCO, tight geochronological constraints are needed to evaluate past ecosystem responses to climate change which occurred over thousands to hundreds of thousands of years. Sedimentary basins in the Pacific Northwest contain fossils from the mid-Miocene intercalated with volcanic tephra amenable to radioisotope geochronology. Unfortunately, previous radioisotopic dates for tephras are sparse, have high uncertainties, and/or were attained with methods now obsolete. As such, there is a need to reevaluate the age of these tephras with modern, higher-precision techniques capable of sub-20ka age resolution, and correlate them chemically across basins in the Pacific Northwest.

Several tephras from four Pacific Northwest formations (Succor Creek, Mascall, Bully Creek, and Huntington Creek) were sampled and processed to obtain suitable glass shards for correlation and zircon grains for dating. Glass shards were analyzed for their major element compositions with scanning electron microscope electron dispersive spectroscopy and electroprobe microanalysis, and analyzed for their trace element compositions with laser ablation inductively coupled plasma mass spectrometry. Suitable zircons were processed with chemical abrasion isotope dilution thermal ionization mass spectrometry to determine the dates of a set of zircons (using U-Pb ratios), and therefore the age of the eruption and deposition of the tephras.

The integration of high-precision uranium-lead zircon geochronology and geochemical correlation of tephras constitutes a novel approach to studying basin tephrostratigraphy in the Pacific Northwest, allowing for existing models to be conditioned with new age constraints and correlations. This will allow us to rectify tephrochronology frameworks for Mid-Miocene strata.