[1] We used time-lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbrae and its proglacial ice mélange. The mélange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the mélange matrix by binding iceberg clasts together, ultimately preventing the calving of full-glacier-thickness icebergs (the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the mélange weakens and the terminus retreats. The mélange remains strong enough, however, to be largely unaffected by ocean currents (except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the mélange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus (at $40 m d À1 ), whereas during calving events it can move in excess of 50 Â 10 3 m d À1 for more than 10 min. By influencing the timing of calving events, the mélange contributes to the glacier's several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large-scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice mélange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near-floatation criterion for calving.

ABSTRACT. Heat, fresh-and sea-water balances indicate that the late-summer rate of submarine melting at the terminus of tidewater LeConte Glacier, Alaska, U.S.A., in 2000 was about 12 m d^1 w.e., averaged over the submerged face. This is 57% of the estimated total ice loss at the terminus (calving plus melting) at this time. Submarine melting may thus provide a significant contribution to the overall ablation of a tidewater glacier. Oceanographic measurements (conductivity^temperature^depth) made 200^500m from the terminus identified an isohaline (27 ppt) and isothermal (7.2 C) layer extending from 130 m depth to the fjord floor. Capping this is a 40 m thick overflow plume, distinguished by high outflow rates, low salinity (22^25 ppt) and lower temperatures (5^6 C). Mixing models indicate that fresh water comprised about 11% of this plume; it originates mostly as subglacial discharge whose buoyancy drives convection at the terminus. Deep, warm saline waters are incorporated into the plume as it ascends, causing substantial melting of ice along the submarine face. The calving terminus undergoes seasonal changes that coincide with changes in subglacial discharge and fjord water temperatures, and we suggest that these fluctuations in terminus position are directly related to changes in submarine melting.

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[1] We use data from campaign and continuous GPS sites in southeast Alaska and the neighboring region of Canada to constrain a regional tectonic block model that estimates block angular velocities and derives a self-consistent set of fault slip rates from the block motions. Present-day tectonics in southeast Alaska is strongly influenced by the collision of the Yakutat block. Our model predicts a velocity of 50.3 ± 0.8 mm/a toward N22.9 ± 0.6°W for that block. Our results suggest that the eastern edge of the Yakutat block is deforming. Along this edge, the Fairweather fault accommodates a large portion of the Pacific-North America relative plate motion through 42.9 ± 0.9 mm/a of dextral slip. Further south along the Queen Charlotte fault, our model predicts an average of 43.9 ± 0.6 mm/a of dextral slip and a southward increasing amount of transpression. Strain from the Yakutat collision is transferred far to the east of the strike-slip system. This strain transfer causes the region north of Glacier Bay to undergo a clockwise rotation. South of Glacier Bay and inboard of the Queen Charlotte fault, a smaller but clearly defined clockwise rotation is observed. The heterogeneous block motion north and south of Glacier Bay may indicate the area is undergoing internal deformation and could explain regional patterns of diffuse seismicity. The Northern Cordillera of Canada displays a small northeasterly motion. Our block model suggests that the entire southeastern Alaskanorthwestern Canada margin is mobile.

[1] The recent loss of Jakobshavn Isbrae's extensive floating ice tongue has been accompanied by a change in near terminus behavior. Calving currently occurs primarily in summer from a grounded terminus, involves the detachment and overturning of several icebergs within 30-60 min, and produces long-lasting and far-reaching ocean waves and seismic signals, including ''glacial earthquakes''. Calving also increases near-terminus glacier velocities by $3% but does not cause episodic rapid glacier slip, thereby contradicting the originally proposed glacial earthquake mechanism. We propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving. Citation: Amundson, J. M., M. Truffer, M. P. Lüthi, M. Fahnestock, M. West, and R. J. Motyka (2008), Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbrae, Greenland, Geophys. Res. Lett., 35, L22501,

[1] The Greenland Ice Sheet releases large amounts of freshwater into the fjords around Greenland and many fjords are in direct contact with the ice sheet through tidewater outlet glaciers. Here we present the first seasonal hydrographic observations from the inner part of a sub-Arctic fjord, relatively close to and within 4-50 km of a fast-flowing tidewater outlet glacier. This region is characterized by a dense glacial and sea ice cover. Freshwater from runoff, subglacial freshwater (SgFW) discharge, glacial, and sea ice melt are observed above 50-90 m depth. During summer, SgFW and subsurface glacial melt mixed with ambient water are observed as a layered structure in the temperature profiles below the low-saline summer surface layer (<7 m). During winter, the upper water column is characterized by stepwise halo-and thermoclines formed by mixing between deeper layers and the surface layer influenced by ice melt. The warm (T > 1 C) intermediate water mass is a significant subsurface heat source for ice melt. We analyze the temperature and salinity profiles observed in late summer with a thermodynamic mixing model and determine the total freshwater content in the layer below the summer surface layer to be between 5% and 11%. The total freshwater contribution in this layer from melted glacial ice was estimated to be 1-2%, while the corresponding SgFW was estimated to be 3-10%. The winter measurements in the subsurface halocline layer showed a total freshwater content of about 1% and no significant contribution from SgFW.

[1] The digital elevation model (DEM) from the 2000 Shuttle Radar Topography Mission (SRTM) was differenced from a composite DEM based on air photos dating from 1948 to 1987 to determine glacier volume changes in southeast Alaska and adjoining Canada. SRTM accuracy was assessed at ±5 m through comparison with airborne laser altimetry and control locations measured with GPS. Glacier surface elevations lowered over 95% of the 14,580 km 2 glacier-covered area analyzed, with some glaciers thinning as much as 640 m. A combination of factors have contributed to this wastage, including calving retreats of tidewater and lacustrine glaciers and climate change. Many glaciers in this region are particularly sensitive to climate change, as they have large areas at low elevations. However, several tidewater glaciers that had historically undergone calving retreats are now expanding and appear to be in the advancing stage of the tidewater glacier cycle. The net average rate of ice loss is estimated at 16.7 ± 4.4 km 3 /yr, equivalent to a global sea level rise contribution of 0.04 ± 0.01 mm/yr.

[1] We show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers. This buoyant discharge draws in warm seawater, entraining it in a turbulent upwelling flow along the submarine face that melts glacier ice. To capture the effects of subglacial discharge on submarine melting, we conducted 4 days of hydrographic transects during late summer 2012 at LeConte Glacier, Alaska. A major rainstorm allowed us to document the influence of large changes in subglacial discharge. We found strong submarine melt fluxes that increased from 9.1 ± 1.0 to 16.8 ± 1.3 m d À1(ice face equivalent frontal ablation) as a result of the rainstorm. With projected continued global warming and increased glacial runoff, our results highlight the direct impact that increases in subglacial discharge will have on tidewater outlet systems. These effects must be considered when modeling glacier response to future warming and increased runoff. Citation: Motyka, R. J., W. P. Dryer, J. Amundson, M. Truffer, and M. Fahnestock (2013), Rapid submarine melting driven by subglacial discharge, LeConte Glacier, Alaska, Geophys. Res. Lett., 40,[5153][5154][5155][5156][5157][5158]