In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming a plume near the calving front. To elucidate the effects of this discharge on nutrient and dissolved inorganic carbon transport to the surface in these fjords, we conducted observational studies on Bowdoin Glacier and in its fjord in northwestern Greenland during the summer of 2016. Our results provide evidence of macronutrient and dissolved inorganic carbon transport from deep in the fjord to the surface in front of the glacier. This transport is driven by plume formation resulting from subglacial freshwater discharge and subsequent upwelling along the glacier calving front. The plume water is a mixture of subglacial freshwater and entrained fjord water. The fraction of glacial meltwater in the plume water is ~14% when it reaches the surface. The plume water is highly turbid because it contains substantial amounts of sediment derived from subglacial weathering. After reaching the surface, the plume water submerges and forms a turbid subsurface layer below fresher surface water at densities of 25.0 to 26.5 σθ. Phytoplankton blooms (~6.5 μg/L chlorophyll a) were observed near the boundary between the fresher surface and turbid subsurface layers. The bloom was associated with a strong upward NO3− + NO2− flux, which was caused by the subduction of plume water. Our study demonstrated that the subglacial discharge and plume formation at the front of Bowdoin Glacier play a key role in the availability of nutrients and the subsequent growth of phytoplankton in the glaciated fjord.

Patagonian icefields are losing volume, and their loss is due partly to rapid changes in their outlet glaciers that terminate in lakes or the ocean. Despite this key influence from outlet glaciers, relatively few of these calving glaciers have had high-frequency measurements on their frontal variations and ice speed changes. We describe here recent frontal variations and ice speed changes of all 28 major calving glaciers in the Southern Patagonia Icefield (SPI), including ice speed maps covering approximately half of the entire icefield. The analysis is based on satellite data from 1984 to 2011. Over this period, only the two termini of Glaciar Pío XI advanced. Of the remaining glacial fronts, 12 changed less than ±0.5 km, but 17 retreated at least 0.5 km. In the latter group, three glacial fronts (Glaciar Jorge Montt, HPS12, and Upsala) retreated over 6 km. Averaged over all 31 glacial fronts of the calving glaciers, the front positions retreated 1.56 km (median is 0.71 km). Along the flowline within 20 km of the front, the ice speeds up to 5900 ± 200 m a À1. Except for regions showing large acceleration or deceleration, the mean speed over the measured area decreased by 30 m a À1 from 1984 to 2011. The three most rapidly retreating glaciers showed much larger acceleration near the calving front, suggesting that ice dynamics drive their rapid retreat. Thus, we see retreat as a long-term trend for the calving glaciers in the SPI, with behavior that implies a dynamically controlled rapid recession that may explain the recently reported volume change of the SPI.

To better understand recent rapid recession of marine-terminating glaciers in Greenland, we performed satellite and field observations near the calving front of Bowdoin Glacier, a 3 km wide outlet glacier in northwestern Greenland. Satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for >20 years. Ice radar measurements showed that the glacier front is grounded, but very close to the floating condition. These results, in combination with the results of ocean depth soundings, suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat. Presumably, glacier thinning due to atmospheric and/or ocean warming triggered the initial retreat. In situ measurements showed complex short-term ice speed variations, which were correlated with air temperature, precipitation and ocean tides. Ice speed quickly responded to temperature rise and a heavy rain event, indicating rapid drainage of surface water to the bed. Semi-diurnal speed peaks coincided with low tides, suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance. These observations demonstrate that the dynamics of Bowdoin Glacier are sensitive to small perturbations occurring near the calving front

Abstract. In this paper, we analyse the calving activity of the Bowdoin Glacier, north-western Greenland, in 2015 by combining satellite images, UAV (unmanned aerial vehicle) photogrammetry and ice flow modelling. In particular, a highresolution displacement field is inferred from UAV orthoimages taken immediately before and after the initiation of a large fracture, which induced a major calving event. A detailed analysis of the strain rate field allows us to accurately map the path taken by the opening crack. Modelling results reveal (i) that the crack was more than half-thickness deep, filled with water and getting irreversibly deeper when it was captured by the UAV and (ii) that the crack initiated in an area of high horizontal shear caused by a local basal bump immediately behind the current calving front. The asymmetry of the bed at the front explains the systematic calving pattern observed in May and July-August 2015. As a corollary, we infer that the calving front of the Bowdoin Glacier is currently stabilized by this bedrock bump and might enter into an unstable mode and retreat rapidly if the glacier keeps thinning in the coming years. Beyond this outcome, our study demonstrates that the combination of UAV photogrammetry and ice flow modelling is a promising tool to horizontally and vertically track the propagation of fractures responsible for large calving events.

To study the glaciological processes controlling the mass budget of Greenland's peripheral glaciers and ice caps, field measurements were carried out on Qaanaaq ice cap, a 20 km long ice cap in northwestern Greenland. In the summer of 2012, we measured surface melt rate, ice flow velocity and ice thickness along a survey route spanning the ice margin (200 m a.s.l.) to the ice-cap summit (1110 m a.s.l.). Melt rates in the ablation area were clearly influenced by dark materials covering the ice surface, where degree-day factors varied from 5.44 mm w.e. K -1 d -1 on a clean surface to 8.26 mm w.e. K -1 d -1 in the dark regions. Ice velocity showed diurnal variations, indicating the presence of surface-meltwater induced basal sliding. Mean ice thickness along the survey route was 120 m, with a maximum thickness of 165 m. Ice velocity and temperature fields were computed using a thermomechanically coupled numerical glacier model. Modelled ice temperature, obtained by imposing estimated annual mean air temperature as the surface boundary condition, was substantially lower than implied by the observed ice velocity. This result suggests that the ice dynamics and thermodynamics of the ice cap are significantly influenced by heat transfer from meltwater and changing ice geometry.