The evolution of glaciers and ice sheets depends on processes in the subglacial environment. Shear seismicity along the ice–bed interface provides a window into these processes. Such seismicity requires a rapid loss of strength that is typically ascribed to rate-weakening friction, i.e., decreasing friction with sliding or sliding rate. Many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds. To our knowledge, however, these studies have all found rate-strengthening friction. Here, we investigate the possibility that rate-weakening rock-on-rock friction between sediments frozen to the bottom of the glacier and the underlying water-saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds. We test this ‘entrainment-seismicity hypothesis’ using targeted laboratory experiments and simple models of glacier sliding, seismicity and sediment entrainment. These models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity. We propose that stagnation at the Whillans Ice Stream, West Antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins. Our results suggest that basal shear seismicity may indicate geomorphic activity.

High-precision triple oxygen isotope analysis of water has given rise to a novel second-order parameter, 17 O-excess (often denoted as D 17 O), which describes the deviation from a reference relationship between d 18 O and d 17 O. This tracer, like deuterium excess (d-excess), is affected by kinetic fractionation (diffusion) during phase changes within the hydrologic cycle. However, unlike d-excess, 17 O-excess is present in paleowater proxy minerals and is not thought to vary significantly with temperature. This makes it a promising tool in paleoclimate research, particularly in relatively arid continental regions where traditional approaches have produced equivocal results. We present new d 18 O, d 17 O, and d 2 H data from stream waters along two east-west transects in the Pacific Northwest to explore the sensitivity of 17 O-excess to topography, climate, and moisture source. We find that discrepancies in d-excess and 17 O-excess between the Olympic Mountains and Coast Range are consistent with distinct moisture source meteorology, inferred from air-mass back trajectory analysis. We suggest that vapor d-excess is affected by relative humidity and temperature at its oceanic source, whereas 17 O-excess vapor is controlled by relative humidity at its oceanic source. Like dexcess, 17 O-excess is significantly affected by evaporation in the rain shadow of the Cascade Mountains, supporting its utility as an aridity indicator in paleoclimate studies where d 2 H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on dexcess and 17 O-excess along altitudinal transects. We find that subcloud evaporation explains much, but not all of observed increases in d-excess with elevation and a minor amount of 17 O-excess variation in the Olympic Mountains and Coast Range of Oregon. KEY POINTS 1. 17 O-excess correlates spatially with relative humidity across the Pacific Northwest, supporting its use as an aridity indicator in paleoclimate studies. 2. Discrepancies in d-excess and 17 O-excess between the Olympic Mountains and Oregon Coast Range suggest that their moisture source is different. 3. Subcloud evaporation explains most of observed increases in d-excess with elevation, and a minor amount of 17 O-excess variation in the Olympic Mountains and Oregon Coast Range.

Subglacial rock friction is an important control on the sliding dynamics and erosive potential of hard-bedded glaciers, yet it remains largely unconstrained. To explore the relative influence of basal melt rate, effective stress and ice temperature on frictional resistance, we conducted abrasion experiments in which limestone beds were slid beneath a fixed slab of ice laden with granitic rock fragments. Shear stress scales linearly with melt rate and cryostatic stress, confirming that both viscous drag and effective stress are first-order controls on the contact force in drained conditions. Furthermore, temperature gradients in the ice increase the contribution of viscous drag on basal shear stress. In all experiments, the relationship between melt rate and shear stress is best explained by a model that accounts for the effects of regelation and viscous creep on the bed-normal drag force. We interpret this to mean fluid flow around entrained clasts contributed to basal drag even at subfreezing temperatures. Incorporating premelting dynamics into the Watts/Hallet model for subglacial rock friction, we find that the predicted debris-bed drag decreases by approximately an order of magnitude, with a corresponding ~3.5 × increase in the transition radius. This is lower than we observe for ice slightly below the pressure melting point.

Deformation of subglacial sediment during basal slip shapes the beds of many fast‐flowing glaciers and ice streams. The resultant sediment flux impacts glacier dynamics and rates of subglacial erosion over a range of timescales, but its fundamental dependencies are not well understood. Using a cryogenic ring shear device, we conducted experiments to investigate the effects of both effective stress and slip speed on rates of till transport. Sediment fluxes were computed using digital image correlation from a photographic time series of the till bed. We find a near‐linear relationship between sediment flux and slip speed, but a non‐monotonic, double‐valued dependency of sediment flux on effective stress. Deformation primarily occurred in a thin shear band near the ice sole, the thickness of which could vary with both parameters. Coupling between ice and till increased at higher slip speeds and effective stresses and scaled the magnitude of the flow profile.

Subglacial till can deform when overriding ice exerts shear traction at the ice-till interface. This deformation leaves a strain signature in the till, aligning grains in the direction of ice flow and producing a range of diagnostic microstructures. Constraining the conditions that produce these kinematic indicators is key to interpreting the myriad of features found in basal till deposits. Here we use a cryogenic ring shear device with transparent sample chamber walls to slip a ring of temperate ice over a till bed from which we examine the strain signature in the till. We use cameras mounted to the side of the ring shear and bead strings inserted in the till to estimate the strain distribution within the till layer. Following completion of the experiment, we extract and analyze AMS samples and create thin sections of the till bed for microstructure analysis. We then compare the AMS and microstructures with the observed strain history to examine the relationship between kinematic indicators and strain in a setting where shear traction is supplied by ice. We find that AMS fabrics show a high degree of clustering in regions of high strain near the ice-till interface. In the upper most zone of till, k1 eigenvector azimuths are generally aligned with ice flow, and S1 eigenvalues are high. However, S1 eigenvalues and the alignment of the k1 eigenvector with ice flow decrease nonlinearly with distance from the ice-till interface. There is a high occurrence of microshears in the zone of increased deformation.