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USGS Mendenhall Postdoctoral  Research Fellowship Program

14-11. Dating the archives of explosive volcanism and climate change in Alaska using combined U-Th and (U-Th)/He geochronology

Widespread beds of volcanic ash, or tephra, occurring throughout Alaska and western Canada are critical geological markers because they record the frequency and magnitude of the largest explosive eruptions from Alaskan volcanoes and anchor the chronology of multiple paleoclimate archives spanning Pleistocene-Holocene time.  These tephras reflect explosive eruptions that, were they to occur today, would produce widespread social disruption and would threaten infrastructure. Accurate and precise ages for eruptive chronologies are critical to producing probabilistic hazard assessments for volcanoes (e.g., Nathenson et al., 2012).  Despite their importance, many voluminous (>5 km3) Alaskan tephras are undated or imprecisely dated by direct or indirect methods.  In most cases, direct dating of the tephras using K-Ar or Ar/Ar methods has been thwarted because the required potassium-rich minerals (e.g., sanidine feldspar) are absent or too small.  Moreover, many of the tephras are beyond the practical age limits (~50 ka) of radiocarbon (14C) dating, which yields an indirect age from associated organic matter.  Recently, the combination of the (U-Th)/He and 238U-230Th disequilibrium dating methods has been demonstrated to be powerful for resolving the eruption ages of young volcanic deposits that are problematic or effectively impossible to date by other methods (e.g., Schmitt et al., 2010; Danisik et al., 2012; Vazquez and Lidzbarski, 2012).

Important and widespread marker tephras bracketing loess and sedimentary sequences that record climate variability in Alaska and northwestern Canada include the voluminous Old Crow tephra that is thought to have erupted from an eastern Aleutian arc volcano (Preece et al., 2011a); the Sheep Creek tephra(s), erupted from Hayes volcano or the Wrangell volcanic field, which possibly represent multiple episodes of volcanism (Westgate et al., 2008); and the indirectly dated Variegated Tephra (Jensen et al., 2011).  Nearly all of these regionally distributed tephras are documented to contain zircon, making them amenable to combined (U-Th)/He and U-Th dating.  Numerous other Alaskan tephras require better numerical age control, and importantly, links to hazardous source volcanoes that generate voluminous explosive eruptions (e.g., Preece et al., 2011b).  Preliminary source-volcano studies by USGS-AVO scientists suggest that pyroclastic deposits generated by explosive silicic eruptions from Hayes and Augustine volcanoes in Cook Inlet are broadly latest Pleistocene in age, but await direct dating and integration into the regional tephrochronologic framework.

The goals of this Mendenhall Research Opportunity are to determine absolute ages and potential source volcanoes for key regionally distributed Alaskan tephras. This project will take advantage of the unique analytical partnership between Stanford University and USGS by combining the Stanford-USGS SHRIMP-RG ion microprobe laboratory ( and and Stanford’s Noble Gas Thermochronology Laboratory, both of which are located at Stanford.  The Mendenhall Fellow will combine 238U-230Th and (U-Th)/He geochronology by 1) dating the final pre-eruptive crystallization of the accessory mineral zircon using 238U-230Th disequilibrium (e.g., Vazquez and Lizdbarski, 2012), and 2) dating the timing of tephra cooling, and in turn eruption, using (U-Th)/He geochronology (e.g., Danisik et al., 2012). In addition the Fellow will develop an analytical strategy that leads to integrated and complimentary geochronologic and geochemical datasets to link regional tephras to their source volcanoes.

The Mendenhall Fellow should have a strong background in volcanology and/or geochronology, and will design a project that builds upon previous stratigraphic, tephrochronologic, and geochemical studies in order to maximize the refinement of chronologies and identification of sources for regionally extensive tephras in Alaska and western Canada.  Expected results include a refined chronostratigraphic framework for Pleistocene volcanic, paleomagnetic, and paleoclimate chronologies in Alaska and northwestern Canada.  The Fellow will have leeway to integrate complementary avenues of research related to geochronology and geochemistry, such as magma generation processes and timescales leading to voluminous explosive eruptions in Alaska, the interplay between eruption frequency and ice-cover change, or implications of refined chronologies for geoarcheology.


Coble, M.A., Grove, M., Calvert, A.T., 2011, Calibration of Nu-Instruments Noblesse multicollector mass spectrometers for argon isotopic measurements using a newly developed reference gas.  Chem. Geol., vol. 290, p 75–87.

Danišík, M, et al., 2012. Re-anchoring the late Pleistocene tephrochronology of New Zealand based on concordant radiocarbon ages and combined 238U/230Th disequilibrium and (U–Th)/He zircon ages. Earth and Planetary Science Letters, vol. 349, p. 240–250.

Jensen, B.J.L., Preece, S.J., Lamothe, M., Pearce, N., Froese, D.G., Westgate, J.A., Schaefer, J., and Beget, J., 2011. The variegated (VT) tephra: A new regional marker for middle to late marine isotope stage 5 across Yukon and Alaska. Quat. Intl., vol. 246, p. 312–323.

Nathenson, M., Clynne, M.A., and Muffler, P., 2012. Eruption probabilities for the Lassen Volcanic Center and regional volcanism, northern California, and Probabilities for large explosive eruptions in the Cascade range. USGS Sci. Invest. Rep. 2012–5176–B, 23 p.

Preece, S.J., Pearce, N.J.G., Westgate, J.A., Froese, D.G., Jensen, B.J.L., and Perkins, W.T., 2011a. Old Crow tephra across eastern Beringia: a single cataclysmic eruption at the close of marine isotope stage 6. Quat. Sci. Rev., vol. 30, p. 2069–2090.

Preece, S.J., Westgate, J.A., Froese, D.G., Pearce, N.J.G, and Perkins, W.T., 2011b. A catalogue of late Cenozoic tephra beds in the Klondike gold fields and adjacent areas, Yukon Territory. Can. J. Earth Sci., vol. 48, p. 1386–1418.

Schmitt, A.K., Stockli, D.F., Lindsay, J.M., Robertson, R., Lovera, O., and Kislitsyn, R., 2010. Episodic growth and homogenization of plutonic roots in arc volcanoes from combined U-Th and (U-Th)/He zircon dating.  Earth Plan. Sci. Lett., vol. 295, p. 91–103.

Vazquez, J., and Lidzbarski, M., 2012. High-resolution tephrochronology of the Wilson Creek Formation (Mono Lake, California) and Laschamp event using 238U-230Th SIMS dating of accessory mineral rims. Earth Plan. Sci. Lett., vol. 357–358, p. 54–67.

Westgate, J., Preece, S., Froese, D., Pearce, N., Roberts, R., Demuro, M., Hart, W., and Perkins, W., 2008. Changing idea son the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America. Quat. Intl., vol. 178, p. 183–209.

Proposed duty station: Menlo Park, CA

Areas of Ph.D.: Geochronology, geochemistry, volcanology, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications - Research Geologist, Research Chemist.

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant’s background, educations, and research proposal. The final classifications of theposition will be made by the Human Resources specialist).

Research Advisors: Jorge Vazquez, (650) 329-5240,; Michelle Coombs, (907) 786-7403,; Kristi Wallace (907) 786-7109,; Matt Coble (Stanford University), (650) 353-1768,; Marty Grove (Stanford University), (650) 721-5490,

Human Resources Office Contact: Robert Hosinski, (916) 278-9397,

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U.S. Department of the Interior, U.S. Geological Survey
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