Edward J. Brook

Variable responses of western U.S. glaciers during the last deglaciation

Cosmogenic 10 Be exposure ages from moraines in the Wallowa Mountains, Oregon, identify two maximal late Pleistocene glaciations at 21.1 6 0.4 ka and 17.0 6 0.3 ka and a minor glacial event at 10.2 6 0.6 ka. Our new high-resolution chronology, integrated with other well-dated glacial records from the western United States, demonstrates sub- stantial differences in the synoptic responses of western U.S. glaciers to climate forcing associated with the global Last Glacial Maximum and subsequent millennial-scale events originating in the North Atlantic region. These variable synoptic glacier responses identify large changes in the relative contributions of regional to global controls on the climate of the western United States that accompanied the deglaciation.

Calibration of cosmogenic 3He production rates from Holocene lava flows in Oregon, USA, and effects of the Earth's magnetic field

Abstract We have measured cosmogenic 3He production rates in olivine phenocrysts from four radiocarbon-dated Holocene lava flows in Oregon. The flows span the period between 2 and 7 ka when there were significant fluctuations in the intensity of the Earths dipole moment. Our individual 3He production rate determinations are consistent with previous estimates, and reinforce the feasibility of dating very young (late Holocene) surfaces with the cosmogenic 3He method. Integrated cosmogenic 3He production rates exhibit small temporal variations during the Holocene, supporting predictions that production rates at mid-latitudes are weakly affected by geomagnetic modulation of cosmic ray flux. However, the time-varying difference between geographic and geomagnetic latitude caused by secular variation of dipole axis position may represent an important source of error (as much as 5%) in Holocene surface exposure ages and production rate calibrations. The best value for the integrated Holocene production rate of cosmogenic 3He from calibration sites in this study is 116±3 atoms g−1 yr−1.

Cosmogenic 3He and 10Be chronologies of the late Pinedale northern Yellowstone ice cap, Montana, USA

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 6 0.4 3 He ka and 16.2 6 0.3 10 Be ka, respectively. Concordance of these ages supports the scaled production rates used for 3 He (118.6 6 6.6 atoms · g 21 ·y r 21 ) and 10 Be (5.1 6 0.3 atoms · g 21 ·y r 21 )( 62s at high latitudes at sea level). Two recessional moraines upvalley from the terminal moraine have mean ages of 15.7 6 0.5 10 Be ka and 14.0 6 0.4 10 Be ka, respectively, and a late-glacial flood bar was deposited at 13.7 6 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 Yellowstone plateau by ;14 10 Be ka.

Accretion of Interplanetary Dust in Polar Ice

Measurements of helium isotopes in particles separated from polar ice demonstrate that extraterrestrial ³He dominates the ³He flux at the GISP2 (Greenland) and Vostok (Antarctica) ice core sites. Replicate measurements of late Holocene ice samples yield ³He fluxes of 0.62±0.27×10−12 cm³ STP cm−2 ka−1 (GISP2) and 0.77 ± 0.25 × 10−12 cm³ STP cm−2 ka−1 (Vostok), similar to results from marine sediments. These are the first detailed measurements of ³He in particles from ice core samples, and they demonstrate the utility of the ice core record for evaluating the temporal history of the extraterrestrial dust flux. Results from Vostok samples from 1096–1403 m depth (75–97 ka B.P.) are similar to the late Holocene data, with the exception of two highly anomalous results from 1307 m. The latter probably indicate the presence of rare, large or gas-rich extraterrestrial particles.

Cosmogenic 10Be ages of the Saglek Moraines, Torngat Mountains, Labrador

Cosmogenic 10 Be ages on boulders from the Saglek Moraines of the Torngat Mountains, Labrador, suggest that the moraines were deposited in their type area 13.4 6 1.5 ka. The 10 Be ages on boulders from similar moraines in a valley system 100 km north of the type area yield a mean age of 12.0 6 2.1 10 Be ka. These data support the hypothesis that the Saglek Moraines were deposited by a regional system of outlet glaciers that drained the Laurentide Ice Sheet and left extensive areas of the Torngat Mountains ice free as nunataks. Weighted mean ages for erratic boulders and bedrock 5‐50 m higher in elevation than the Saglek Moraines at both field areas are indistinguishable at 1s from the weighted mean ages of moraines in each area. These data either indicate that thicker late Wisconsinan ice receded to the level of the Saglek Moraines at 12‐13 10 Be ka, or that the Saglek ice margin represents the maximum late Wisconsinan extent, but the ice margin fluctuated sufficiently to create a vertical zone of ;50 m with essentially the same exposure history.