Ackert Robert P.

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Ackert
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Robert P.
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  • Thesis
    Antarctic glacial chronology : new constraints from surface exposure dating
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2000-06) Ackert, Robert P.
    Surface exposure dating, using the concentration of cosmogenic nuclides (3He, 21Ne, and 36Cl) in moraine boulders, combined with mapping of glacial moraines from three key locations, is used to provide new constraints to Antarctic glacial chronology. The results are used to reconstruct past West Antarctic Ice Sheet (WAIS) geometry and test models of WAIS behavior. Mount Waesche is a volcanic nunatak near the dome of the WAIS in Marie Byrd Land. The Dominion Range is at the head of the Beardmore Glacier, an outlet glacier of the East Antarctic Ice Sheet in the Transantarctic Mountains. Dromedary Platform is a bench along the Koettlitz Glacier which flows into southern McMurdo Sound. In addition, a new 3He production rate calibration, and determination of initial 3He/4He in Ferrar Dolerite and Beacon Sandstone, substantially reduce uncertainties in Antarctic exposure ages. 3He production rates of 129 ± 4 atoms/g/yr (olivine) and 124 ± 4 atoms/g/yr (clinopyroxene) at sea level, high latitude, are determined from an independently dated 125 ka lava flow in Patagonia (46°S). Paired 3He and 36Cl measurements are consistent with negligible surface erosion which is inferred from flow morphology. These mid-latitude, long term, 3He production rates reduce uncertainties previously introduced when scaling production rates calibrated at lower latitudes to Antarctica. The results also confirm the compositional dependence of 3He production rates predicted by theoretical calculations and are used to scale the production rates to quartz. Determinations of initial 3He/4He in pyroxene from shielded dolerite and by incremental heating of quartz show that inherited (nucleogenic) 3He concentrations are very low (3He/4He <.010 R/Ra). For exposures longer than about 30,000 years the inherited component can be ignored. These results enable correction for inherited 3He in less than 30,000 year exposures of these common Antarctic lithologies. Exposure ages of less than 6000 years were obtained on moraine boulders in the Dominion Range. At Mt. Waesche, a moraine band on the flank of the volcano records past ice elevations up to 85 meters above the present ice elevation. 3He and 36Cl surface exposure ages of basalt boulders up to 45 meters above the present ice level indicate that the last highstand of the WAIS occurred about 10 ± 1 ka , several thousand years after maximum extent of the WAIS in the Ross Sea. These data, suggest that the WAIS was out of equilibrium during the last glacial maximum (LGM). This conclusion is supported by results from a time-dependent ice sheet model. The new chronology and model also place constraints on past WAIS ice volume and the WAIS contribution to sea level rise during the last deglaciation. Surface exposure ages from the upper part of the moraine band are scattered (10-282 ka), but consistently fall within glacial (low sea level) stages in the marine isotope record, including an interstadial during the last interglacial stage. Paired 3He and 36CI data are consistent with negligible erosion. 3He and 21Ne surface exposure ages from six drifts of Beardmore Glacier are presented. Ages on Beardmore 1 drift range from 6 to 13 ka and record the most recent damming of the Beardmore Glacier by an expanded WAIS. The exposure ages on the older Beardmore 2 drift are more scattered (6-56 ka). This drift probably records thickening of the upper Beardmore Glacier during the LGM. The Meyer drift records at least four expansions of the upper Beardmore Glacier. Because Meyer drift geometry is similar to that of the Beardmore drift, these drifts also likely record an expanded WAIS. 3He and 21Ne ages on Meyer 1 drift (~600 ka) clearly distinguish it from Meyer 4 drift (~1 Ma). 21Ne ages on the Dominion drift are ~2 Ma. The Meyer 4 drift and Dominion drift are correlated with Taylor Glacier moraines in Arena Valley on the basis of surface exposure ages and moraine morphology. These results indicate that expansions of the WAIS similar to that of the LGM have occurred throughout the Pleistocene. On Dromedary Platform, lateral moraines record advances of grounded ice from McMurdo Sound during stage 2, damming of the lower Koettlitz Glacier by grounded WAIS during stage 6 and thickening during the Holocene and stage 5. The 3He exposure ages are widely scattered, but independent stratigraphic control provided by 14C,U series, and surface exposure dates of correlative drifts in McMurdo Sound allows evaluation of prior exposure and cover. Older moraines up to 350 meters above the stage 2 ice limit record earlier thickening of lower Koettlitz Glacier. Stratigraphic control provided by 40Ar/39Ar dated lava flows show the surface exposure ages are probably effected by cover and erosion and provide only minimum ages for the moraines. The oldest moraines are older than 400 ka and require grounded ice in McMurdo Sound, implying mid-Pleistocene advances of the WAIS similar to or larger than that of Stage 2. Surface exposure ages on a 1.68 Ma volcanic cone require erosion rates of ~65 cm/Myr, an order of magnitude higher than those previously obtained using cosmogenic nuclides in Antarctica. The 40Ar/39Ar age limits uplift of Dromedary Platform to <450 m/Myr. These results show that surface exposure dating, combined with careful field observations, is a powerful tool for constraining Antarctic glacial history, and can be used to date moraines from 5 to 2,000 ka. The results are consistent with models in which the WAIS extent is largely determined by relative sea level and implies that the WAIS has fluctuated synchronously with the Northern Hemisphere ice sheets for at least the last 1 million years.