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Differences in the timing of glacial advances, which are commonly attributed to climatic changes, can be due to variations in valley topography. Cosmogenic 10Be dates from 24 glacial moraine boulders in 5 valleys define two age populations, late-glacial and early Holocene. Moraine ages correlate with paleoglacier valley hypsometries. Moraines in valleys with lower maximum altitudes date to the late-glacial, whereas those in valleys with higher maximum altitudes are early Holocene. Two valleys with similar equilibrium-line altitudes (ELAs), but contrasting ages, are

Water plays a first-order role in basal sliding of glaciers and ice sheets and is often a key constituent of accelerated glacier motion (1-4). Subglacial water is known to occupy systems of cavities and conduits at the interface between ice and the underlying bed surface, depending upon the history of water input and the characteristics of the substrate (5). Full understanding of the extent and configuration of basal water is lacking, however, because direct observation is difficult. This limits our ability to simulate ice dynamics and the subsequent impacts on sea-level rise realistically. Here we show that the subglacial hydrological system can have a large volume of water occupying basal crevasses that extend upward from the bed into the overlying ice. Radar and seismic imaging combined with in situ borehole measurements collected on Bench Glacier, Alaska, reveal numerous water-filled basal crevasses with highly transmissive connections to the bed. Some crevasses extend many tens of metres above the bed and together they hold a volume of water equivalent to at least a decimetre layer covering the bed. Our results demonstrate that the basal hydrologic system can extend high into the overlying ice mass, where basal crevasses increase water-storage capacity and could potentially modulate basal water pressure. Because basal crevasses can form under commonly observed glaciological conditions, our findings have implications for interpreting and modelling subglacial hydrologic processes and related sliding accelerations of glaciers and ice sheets., Glacier sliding motion is frequently tied to the geometry and flow conditions of water at the ice-bed interface. The geometrical configuration of the basal hydrologic system has largely been inferred [...]

Although downstream fining of clasts is typical in modern and ancient river systems, the Hoh River, a cobble-and-boulder-bed river in Washington state, contains surprisingly little downstream fining of the coarsest tail of the grain-size distribution along its lower 63 km. Mean rate of fining of the coarsest size fraction is only 0.24 mm/km, barely significant given uncertainties in measurement. In addition, these same clasts have weathering-rind thicknesses that change very little along the length of the river (they decrease by 0.01 mm/km). Because weathering rinds are well developed in this setting and both tumbler studies and field observations show that abrasion of weathering rinds greatly accelerates clast diminution, there seems to be a paradox between predicted and observed downstream fining rates. Field study shows that detritus is constantly being resupplied to the river by erosion of late glacial materials along the river's cutbanks and tributaries. The clasts supplied range in size but include abundant coarse grains with very thick weathering rinds. The continuous resupply of grains strongly attenuates the rate of downstream fining, despite the fact that these weathered grains abrade relatively rapidly. Thus, the dominance of reworked glacial debris overwhelms any processes by which grains are reduced in size in the modern river. These results suggest that relatively infrequent glaciation can have a long-lived effect on river sedimentation. Keywords: gravel, Hob River, Olympic Mountains, Washington, fluvial, sedimentation, downstream fining, geomorphology.

Abrasion mill analysis of fluvial clast (> 4.0 mm) abrasion in a meandering reach of the Rio Grande, south-central Colorado, indicates that (1) abrasion rates of natural clasts from various locations in the channel and flood plain differ very little, (2) abrasion rates in the first few hours are high, then drop to much lower levels for these natural clasts, and (3) a previously experimentally abraded clast sample, otherwise identical to natural samples, had a much lower initial abrasion rate than any natural samples. The initial high abrasion rate of all natural clasts can be attributed to the presence of an easily abraded weathered layer. We interpret the relatively abrupt drop in abrasion rate after a few hours of abrasion to the complete removal of the weathered layer and the exposure of more resistant rock. The low initial abrasion rate of the previously experimentally abraded sample supports this interpretation and also indicates that none of the clasts in the river have been abraded as extensively as the experimentally abraded samples. From this we infer that clasts have not traveled very far in the channel. These results have led to the development of a weathering-controlled abrasion model to explain the effect of abrasion on downstream fining (size reduction over distance downstream). Weathering-controlled abrasion is essentially a step function in time, composed of the alternation of long periods of weathering with no abrasion (flood-plain storage) with very brief but intense periods of mechanical abrasion (channel transport). This alternation of weathering and abrasion can produce a net abrasion rate sufficient to generate the amount of downstream fining present along the Rio Grande and also can explain why many abrasion mill experiments (which simulate abrasion but not weathering) fail to reproduce natural abrasion rates. Changes in the amount of fluvial downstream fining in an aggrading basin margin (and thus, in the rock record) can result from variation in selective deposition (temporary or permanent flood-plain storage) or from changes in the abrasion rate. Experiments demonstrate, however, that the complex interaction between these two processes may in some cases obscure the tectonic signal that is commonly interpreted from basin margin depositional geometries.

Do glaciers enhance or inhibit chemical weathering rates relative to other environments? The importance of glaciers in the global carbon cycle and climate change hinges on the answer. We show that catchments occupied by active alpine glaciers yield cation denudation rates greater than the global mean rate but do not exceed rates in nonglacial catchments with similar water discharge. Silica denudation rates are distinctly lower in glacier-covered catchments than in their nonglacial counterparts. Because sediment yields are high from glaciers, this suggests that water flux, rather than physical erosion, exerts the primary control on chemical erosion by glaciers. Potassium and calcium concentrations are high relative to other cations in glacial water, probably due to dissolution of soluble trace phases, such as carbonates, exposed by comminution, and cation leaching from biotite. Preferential weathering of biotite may result in higher 87Sr/86Sr in glacial runoff than expected from whole-rock compositions. Thus, although glaciers do not influence total chemical denudation rates at a given runoff, they may yield compositionally distinctive chemical fluxes to the oceans. Disruption of mineral lattices by grinding increases dissolution rates; this and high surface area should make glacial sediments exceptionally weatherable. Weathering of glacial erosion products in environments beyond the glacier margin deserves attention because it may figure prominently in global chemical cycles.

Internally oscillating cycles of cutting and filling occur naturally in many coupled geomorphic systems. As a demonstration, we have modeled stream entrenchment and backfilling where alluvial systems couple to uplifting mountain blocks. This simple model tracks bedrock stream erosion and transport of alluvium to base level. Internal oscillations arise from coupling of a kinematic wave model for mountain erosion to a diffusion model for the alluvial basin. Decaying periodic cutting and filling occur on time scales of [10.sup.5] yr; the largest magnitudes are found at the coupling point. Model results suggest that cyclic sedimentation may result from the tintinnabulation of single perturbations applied to natural systems and need not record cyclic changes in climate, tectonics, or base level.

Contrary to the popular belief that stream power determines whether a river incises through or diverts around an evolving bedrock uplift, we propose that river sediment flux and tectonic uplift rate are the most influential variables. We construct a simplified theoretical model of an equilibrium alluvial river system responding to local bedrock uplift by coupling a diffusion model describing the alluvial river with a kinematic wave model describing the incising bedrock river. The initial uplift reduces local river gradient, causing the river to deposit sediment in the accommodation space created upstream. At the same time, the river incises into the uplifting bedrock, starting from a knickpoint at the downstream edge of the uplift. The sediment aggradation must keep pace with the uplift rate until the incision has propagated through to the upstream edge of the bedrock block; otherwise the river will divert to another course. We use geometric arguments to identify sediment flux and uplift rate as the controlling variables of the system. Keywords: fluvial, incision, diversion, modeling, long time scale, river dynamics, landscape evolution.

In order to constrain CO2 fluxes from biogeochemical processes in subglacial environments, we model the evolution of pH and alkalinity over a range of subglacial weathering conditions. We show that subglacial waters reach or exceed atmospheric pCO2 levels when atmospheric gases are able to partially access the subglacial environment. Subsequently, closed system oxidation of sulfides is capable of producing pCO2 levels well in excess of atmosphere levels without any input from the decay of organic matter. We compared this model to published pH and alkalinity measurements from 21 glaciers and ice sheets. Most subglacial waters are near atmospheric pCO2 values. The assumption of an initial period of open system weathering requires substantial organic carbon oxidation in only 4 of the 21 analyzed ice bodies. If the subglacial environment is assumed to be closed from any input of atmospheric gas, large organic carbon inputs are required in nearly all cases. These closed system assumptions imply that order of 10 g m−2 y−1 of organic carbon are removed from a typical subglacial environment-a rate too high to represent soil carbon built up over previous interglacial periods and far in excess of fluxes of surface deposited organic carbon. Partial open system input of atmospheric gases is therefore likely in most subglacial environments. The decay of organic carbon is still important to subglacial inorganic chemistry where substantial reserves of ancient organic carbon are found in bedrock. In glaciers and ice sheets on silicate bedrock, substantial long-term drawdown of atmospheric CO2 occurs. [ABSTRACT FROM AUTHOR]

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 influence of sulfides 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. [ABSTRACT FROM AUTHOR]

Examines the downstream fining and weathering-rind formation in the Hoh River in Washington. Mean rate of fining of the coarsest size fraction; Acceleration of clash diminution due to abrasion of weathering rinds; Correlation between the rate of downstream fining and the continuous resupply of grains.