New publication: On the eruption age and provenance of the Old Crow tephra

Seth Burgess sampling the Old Crow tephra at the Halfway House site, south of Fairbanks. August 30, 2014
Free download here until March 26, 2019.

The Old Crow tephra is the largest preserved Pleistocene ashfall event in eastern Beringia, and is a significant stratigraphic marker. This new publication gives a preferred eruption age of 202.9 ± 9.5 ka based on radiometric geochronology of zircon crystals, and provides new oxide geochemistry showing that the source of the Old Crow tephra is not the Emmons Lake Volcanic Center.

Abstract:Tephrochronology is used to correlate and reconstruct geographically disparate sedimentary records of changing environment, climate, and landscape throughout geologic time. Single tephra layers represent isochronous markers across broad regions, thus accurate and precise radiometric constraints on the timing of eruption are critical to their utility. The Old Crow tephra is found throughout eastern Beringia and represents the largest preserved Pleistocene ashfall event in the region. Despite its volume and significance as a stratigraphic marker, the provenance of this tephra is debated, and the interpreted eruption age of marine isotope stage (MIS) 5 at ∼125 ka has vacillated. To investigate provenance and eruption age, we develop a geochemical fingerprint for the Old Crow tephra via titanomagnetite geochemistry, and zircon crystallization/cooling age via coupled U/Pb, U/Th, and (UTh)/He zircon geochronology. Our results indicate that Old Crow oxides are geochemically distinct from the commonly assumed source-caldera system at the Emmons Lake Volcanic Center (ELVC). Zircon crystals from the Old Crow tephra range in age from Proterozoic to Pleistocene, with concordant zircon U/Pb, U/Th, and (UTh)/He dates on the youngest population of grains suggesting crystallization in their parent magma, and in turn eruption, at 202.9 ± 9.5 ka. We discuss strengths and shortcomings of our radiogenic datasets in light of this result and review the far-reaching implications of a change in Old Crow eruption age.

Burgess, S.D., Coble, M.A., Vazquez, J.A., Coombs, M.L., and Wallace, K.L., 2019, On the eruption age and provenance of the Old Crow tephra: Quaternary Science Reviews, v. 207, p. 64-79.