GLACIAL HISTORY, SURFACE EXPOSURE AGES, AND PALEOELA'S OF THE YUKON TANANA UPLAND: PRELIMINARY RESULTS
MANLEY, WILLIAM F. INSTAAR, Univ. of Colorado.
Briner, Jason P. INSTAAR, Univ. of Colorado.
Lubinski, David J. INSTAAR, Univ. of Colorado.
Caffee, Marc W. PRIME Lab and Dept. of Physics, Purdue University.
A combination of field-based research, cosmogenic 10Be and 26Al surface-exposure dating, and spatial analysis of former Equilibrium Line Altitudes (paleo-ELA's) is yielding new insights into the Pleistocene glacial and climate history of the Yukon Tanana Upland (YTU), eastern Alaska (Fig. 1). Across this broad region of dispersed massifs, glaciers existed for at least five periods during the Pleistocene (Weber and Hamilton, 1984; Weber, 1986; cf. Hamilton, 1994). The type locality for these first-order glacial events was established at a nested moraine and drift sequence in and near the Ramshorn Creek valley, central YTU (Weber, 1986). From youngest to oldest, they were named the Ramshorn, Salcha, Eagle, Mt. Harper, and Charley River glaciations.
Field research during the summer of 2000 concentrated on sampling five granodiorite boulders from the stable crests of each of the four youngest terminal moraines at the type locality. Boulder samples were also taken from three moraines in the Mt. Prindle area, in the northwestern YTU, but were not analyzed due to budget constraints. Field research included measurements of moraine morphology and other assessments of relative age.
Sample preparation and Accelerator Mass Spectrometry (AMS) measurements over the following year yielded 23 cosmogenic exposure ages (for overview of the method and interpretations, see Gosse and Phillips, 2001). Preparation was done at the University of Colorado Cosmogenic Isotope Laboratory following the procedure of Kohl and Nishiizumi (1992). Isotopic ratio measurements were made by AMS at Lawrence Livermore National Laboratory. We used 10Be and 26Al production rates of 5.1 and 31.1 atoms/g (Gosse and Stone, 2001).
Seven of the 16 analyzed boulders yielded both Al and Be ages, which agree closely (r2 = 0.98). In addition to standard corrections for latitude, elevation, and shielding, the ages have been model-corrected for erosion and snow cover. Analytical errors average 4%. Boulder ages were averaged to calculate a mean and standard deviation for each of the two youngest moraines (Table 1). Each of the older moraines exhibits a scatter of boulder ages, as expected for moraine crests that have experienced significant morphologic degradation. For these moraines, we more heavily weight the older boulder ages on each moraine (cf. Putkonen, 2001).
Previous age assessments were based on a suite of relative age data (morphologic preservation, soil development, clast weathering, number of moraines within a glaciation, and downvalley extent), correlation to other glaciated ranges in Alaska, and two radiocarbon dates: a minimum age of 50 14C ka for the Mt. Harper glaciation (Weber and Ager, 1984), and a minimum age of 2.3 14C ka for the Ramshorn glaciation (Weber, 1986).
The new surface exposure dates are the basis for the following interpretations:
1) The Salcha moraine represents the maximum glacier extent during the Late Wisconsin glaciation, consistent with previous assessments.
2) However, the Ramshorn cirque moraine is older than previously believed, with an age statistically indistinguishable from the Late Wisconsin maximum. Inheritance of cosmogenic isotopes in all four boulders is unlikely. Most likely the Ramshorn moraine marks a late-glacial recessional position or readvance.
3) The age of the Eagle moraine has been under debate, dependent on correlations to the Delta and Late Reid glaciations in the Alaska Range and Yukon Territory, respectively (cf. Berger et al., 1996; Westgate et al., 2001). With one young outlier omitted, the cosmogenic ages range from 44 ka to 76 ka. Based on these ages, rather than regional correlation, our preliminary assessment is that the Eagle moraine marks the maximum of an Early Wisconsin glaciation (s.l., OIS 4 or 5).
4) The four boulder exposure ages for the Mt. Harper moraine display some scatter, with the oldest age, 190 ka, as the oldest to our knowledge for a cosmogenic age in Alaska. Given the limits of the technique for moraines of this apparent age, we conservatively conclude that the Mt. Harper moraine is OIS 6 or older (i.e., pre-last interglacial).
Thus, the first direct, quantitative age estimates for glaciation in the Yukon Tanana Upland provide a few surprises, and add fuel to the debate on the age (and spatial variability) of the penultimate glaciation in the region.
Glacier extents for the youngest three glacial periods were mapped in the field and from aerial photography, guided by previous mapping (Weber and Hamilton, 1984; Weber, 1986; Weber, unpub.). The glacier extents are currently being analyzed in a Geographic Information System (GIS). Preliminary ELA results for 32 Ramshorn, 79 Salcha, and 89 Eagle paleoglaciers will be presented at the workshop.
Berger, G. W., Peteet, T. L., Westgate, J. A., and Preece, S., 1996, Age of Sheep Creek tephra (Pleistocene) in central Alaska from thermoluminescence dating of bracketing loess: Quaternary Research, v. 45, p. 263-270.
Gosse, J. C., and Phillips, F. M., 2001, Terrestrial in situ cosmogenic nuclides: Theory and application: Quaternary Science Reviews, v. 20, p. 1475-1560.
Gosse, J. C., and Stone, J. O., 2001, Terrestrial cosmogenic nuclide methods passing milestones toward paleo-altimetry: Eos, Transactions of the American Geophysical Union, v. 82, p. 82, 86, 89.
Hamilton, T. D., 1994, Late Cenozoic glaciation of Alaska, in Plafker, G., and Berg, H. C., eds., The Geology of Alaska: The Geology of North America, v. G-1, Geological Society of America, p. 813-844.
Kohl, C. P., and Nishiizumi, K., 1992, Chemical isolation of quartz for measurement of in-situ-produced cosmogenic nuclides: Geochimica et Cosmochimica Acta, v. 56, p. 3583-3587.
Putkonen, J., 2001, Quantifying the moraine degradation by exposure age dating of surface boulders: Geological Society of America Abstracts with Programs, v. 33, no. 6, p. A285-A286.
Weber, F. R., 1986, Glacial geology of the Yukon-Tanana Upland, in Hamilton, T. D., Reed, K. M., and Thorson, R. M., eds., Glaciation in Alaska -- The Geologic Record: Anchorage, Alaska Geological Society, p. 79-98.
Weber, F. R., and Hamilton, T. D., 1984, Glacial geology of the Mt. Prindle area, Yukon-Tanana Upland, Alaska, Short Notes on Alaskan Geology 1982, Alaska Division of Geological and Geophysical Surveys Professional Report 86, p. 42-48.
Westgate, J. A., Preece, S. J., Froese, D. G., Walter, R. C., Sandhu, A. S., and Schweger, C. E., 2001, Dating Early and Middle (Reid) Pleistocene Glaciations in Central Yukon by Tephrochronology: Quaternary Research, v. 56, p. 335-356.
Figure 1. Fig. 1. Map of the central Yukon Tanana Upland, showing paleoglacier extents as mapped and entered into the Geographic Information System (GIS).
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