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To contrast continental and alpine subglacial weathering regimes and thereby assess the role of large ice masses in chemical weathering, borehole and outlet water samples were collected from multiple locations on a major, land-terminating outlet of the Greenland Ice Sheet. Boreholes, reaching ice depths to 824 m, were drilled to the bed with hot-water methods in four areas of the ice sheet ablation zone along a 45 km transect extending inland from the outlet terminus. The bulk chemical composition of these samples shows substantially less infl uence of sulfi des and carbonates than found in alpine glaciers, suggesting that the sediment under this region of the ice sheet has become depleted of accessory minerals. The waters show wide variability in chemical composition over both large and small temporalspatial scales, suggesting large ranges in length of subglacial water storage and in rates of abrasion and comminution of subglacial earth materials. The dissolved solids concentrations found in the Greenland Ice Sheet are comparable to and in some cases exceed those of alpine glaciers, suggesting that large ice masses are capable of generating substantial dissolved loads through silicate weathering mechanisms. INTRODUCTION AND FIELD CONTEXTThe effects of glaciation on chemical weathering are critical to explaining the role of ice ages in Earth's geochemical cycles. Studies of alpine glaciers suggest that dissolved loads in glacial outlet streams are comparable to nonglacial rivers (Anderson et al., 1997). However, the weathering mechanisms differ, with subaerial weathering dominated by the alteration of silicate minerals to clays, and subglacial weathering controlled chiefl y by the high abundance of reactive surfaces in freshly comminuted material (Anderson, 2005). In many glacial systems, carbonate dissolution and sulfi de oxidation produce most of the dissolved products in outlet waters. This can be true even in glaciers on silicate bedrock, as the abrasion and comminution of subglacial sediments allows fast-reacting accessory materials to be exposed (Hosein et al., 2004;Raiswell, 1984;Tranter et al., 2002).Most measurements of glacial water chemistry have been performed in alpine environments, so it remains to be seen whether the weathering of fast-reacting accessory minerals from freshly comminuted surfaces plays as important a role in continental ice sheets. The size of ice sheets allows for larger and more complex subglacial hydrologic pathways and for longer transport distances for subglacial sediment. Subglacial water and sediment may therefore spend more time in contact with each other underneath larger bodies of ice. Existing data show a positive correlation between ice body size and the proportion of dissolved load derived from silicate weathering (Wadham et al., 2010). Limited data suggest that saturation limits on carbonate dissolution allow sulfi de-mediated silicate dissolution to dominate the chemistry of Antarctic ice stream till pore waters . Here we present major ion and dissolved silica chemist...

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High-latitude landscape evolution processes have the potential to preserve old, relict surfaces through burial by cold-based, nonerosive glacial ice. To investigate landscape history and age in the high Arctic, we analyzed in situ cosmogenic 10 Be and 26 Al in 33 rocks from Upernavik, northwest Greenland. We sampled adjacent bedrock-boulder pairs along a 100 km transect at elevations up to 1000 m above sea level. Bedrock samples gave signifi cantly older apparent exposure ages than corresponding boulder samples, and minimum limiting ages increased with elevation. Two-isotope calculations ( 26 Al/ 10 Be) on 20 of the 33 samples yielded minimum limiting exposure durations up to 112 k.y., minimum limiting burial durations up to 900 k.y., and minimum limiting total histories up to 990 k.y. The prevalence of 10 Be and 26 Al inherited from previous periods of exposure, especially in bedrock samples at high elevation, indicates that these areas record long and complex surface exposure histories, including signifi cant periods of burial with little subglacial erosion. The long total histories suggest that these highelevation surfaces were largely preserved beneath cold-based, nonerosive ice or snowfi elds for at least the latter half of the Quaternary. Because of high concentrations of inherited nuclides, only the six youngest boulder samples appear to record the timing of ice retreat. These six samples suggest deglaciation of the Upernavik coast at 11.3 ± 0.5 ka (average ± 1 standard deviation). There is no difference in deglaciation age along the 100 km sample transect, indicating that the ice-marginal position retreated rapidly at rates of ~120 m yr −1 .

Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been nonerosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets.