 2001 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; December 2001; v. 29; no. 12; p. 1095–1098; 2 figures; Data Repository item 2001127. 1095 Cosmogenic 3 He and 10 Be chronologies of the late Pinedale northern Yellowstone ice cap, Montana, USA Joseph M. Licciardi* Peter U. Clark    Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA Edward J. Brook Department of Geology, Washington State University, Vancouver, Washington 98686, USA Kenneth L. Pierce U.S. Geological Survey, Northern Rocky Mountain Science Center, Montana State University, Bozeman, Montana 59717, USA Mark D. Kurz Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA David Elmore Pankaj Sharma    Department of Physics, PRIME Lab, Purdue University, West Lafayette, Indiana 47907, USA ABSTRACT Cosmogenic 3 He and 10 Be ages measured on surface boulders from the moraine sequence deposited by the northern outlet glacier of the Yellowstone ice cap indicate that the outlet glacier reached its terminal position at 16.5  0.4 3 He ka and 16.2  0.3 10 Be ka, respectively. Concordance of these ages supports the scaled pro- duction rates used for 3 He (118.6  6.6 atoms · g 1 · yr 1 ) and 10 Be (5.1  0.3 atoms · g 1 · yr 1 )(2 at high latitudes at sea level). Two recessional moraines upvalley from the terminal moraine have mean ages of 15.7  0.5 10 Be ka and 14.0  0.4 10 Be ka, respec- tively, and a late-glacial flood bar was deposited at 13.7  0.5 10 Be ka. These cosmogenic chronologies identify a late Pinedale glacial maximum in northern Yellowstone that is significantly younger than previously thought, and they suggest deglaciation of the Yel- lowstone plateau by 14 10 Be ka. Keywords: cosmogenic elements, geochronology, glacial geology, Pinedale glaciation, Yellowstone. INTRODUCTION The late Pleistocene Yellowstone ice cap was the largest indepen- dent glacier system in the western United States. In the past 25 years, many innovative geochronological methods have been used to develop a record of fluctuations of this ice cap (Pierce et al., 1976; Pierce, 1979; Sturchio et al., 1994), and these have formed a cornerstone of under- standing the timing of the last (Pinedale) glaciation for much of the Rocky Mountains (Porter et al., 1983). None of the ages constraining the history of the Yellowstone ice cap, however, directly constrains the age of moraine-building events when the ice-cap margin was at or near its maximum size. Here, we use the cosmogenic nuclides 3 He and 10 Be to directly date a well-preserved moraine sequence deposited by the large outlet glacier that drained the northern Yellowstone ice cap (Fig. 1). Our exposure ages, combined with existing chronological data from this region, establish a new record of late Pinedale millennial-scale fluctuations of this outlet glacier. The high coherency and reproduc- ibility of the 3 He and 10 Be data demonstrate the utility of the surface exposure dating technique for developing high-resolution glacial chronologies. SAMPLING The sampled deposits have well-preserved morphology, large sur- face boulders of granitic lithologies and occasional basalt, and are cov- ered only by grasslands. All boulders selected for sampling appeared pristine and undisturbed, as evidenced by glacial polish, striae, or neg- *Present address: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA; e-mail: jlicciardi@whoi.edu. ligible surface pitting that indicated limited erosion or spalling. Sam- ples were collected from horizontal boulder surfaces on stable parts of the landforms. Only the largest boulders (1 m above the surface) were sampled, in order to reduce potential problems of soil and snow cover. We sampled ten granitic and eight basaltic boulders from the out- ermost set of end moraines (Eightmile terminal moraines) and associ- ated outwash fan that mark the limit of the northern Yellowstone outlet glacier at the last Pinedale glacial maximum (Pierce, 1979) (Fig. 1), providing a rare opportunity to measure cosmogenic 10 Be and 3 He concentrations on the same landform. We also sampled eight granitic boulders from the recessional Chico moraines that are 7 km upvalley (Fig. 1B) and ten granitic boulders from the recessional Deckard Flats moraines that were deposited another 47 km upvalley (Pierce, 1979) (Fig. 1C). Finally, we sampled seven granitic boulders from a late- glacial flood deposit in the Yellowstone River valley within the limit of the Deckard Flats event (Fig. 1C); this deposit probably formed during a large glacial outburst flood by release of an ice-dammed lake upvalley from Deckard Flats (Pierce, 1979). COSMOGENIC 3 He AND 10 Be MEASUREMENTS For the 3 He measurements, olivine phenocrysts were separated from basalt samples by crushing, sieving, magnetic separation, and handpicking. The cosmogenic 3 He content of the olivine was measured by gas mass spectrometry at the Woods Hole Oceanographic Institution following previously described methodology (see Kurz, 1986; Kurz et al., 1990, 1996; and Licciardi et al., 1999, for procedural details, re- producibility, and blanks). We measured 10 Be in quartz, using standard techniques of rock crushing and grinding, isolation of quartz by re- peated acid leaching (Kohl and Nishiizumi, 1992), and separation of beryllium by ion-exchange chemistry and selective precipitation tech- niques (see Licciardi, 2000, for full procedural details). All 10 Be con- centrations were determined by accelerator mass spectrometry (AMS) at the PRIME Lab facility at Purdue University (Sharma et al., 2000). Cosmogenic 3 He and 10 Be concentrations at each sample site were normalized to the rock surface by accounting for sample thickness and density and by using the estimated dependence of isotope production with depth (Kurz, 1986; Brown et al., 1992). Shielding by surrounding topography is 10° at all sites; thus, no corrections for this phenom- enon were necessary. SCALING METHODS AND PRODUCTION RATES We use the mean of the cosmogenic 3 He production rate calibra- tions derived from the well-dated Tabernacle Hill flow and Bonneville flood deposits (Cerling and Craig, 1994) for the helium age calculations because these calibration sites are very close in altitude and latitude to the Yellowstone sample locations and are nearly identical in age to the