Andreas B. Mikkelsen

Persistent flow acceleration within the interior of the Greenland ice sheet

We present surface velocity measurements from a high-elevation site located 140 km from the western margin of the Greenland ice sheet, and ~ 50 km into its accumulation area. Annual velocity increased each year from 51.78 ± 0.01 m yr−1 in 2009 to 52.92 ± 0.01 m yr−1 in 2012—a net increase of 2.2%. These data also reveal a strong seasonal velocity cycle of up to 8.1% above the winter mean, driven by seasonal melt and supraglacial lake drainage. Sole et al. (2013) recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer, faster, summers. Our data extend this analysis and reveal a year-on-year increase in annual velocity above the equilibrium line altitude, where despite surface melt increasing, it is still sufficiently low to hinder the development of efficient drainage under thick ice.

Subglacial water drainage, storage, and piracy beneath the Greenland ice sheet

Meltwater drainage across the surface of the Greenland Ice Sheet (GrIS) is well constrained by measurements and modeling, yet despite its critical role, knowledge of its transit through the subglac ...

Meltwater export of prokaryotic cells from the Greenland ice sheet

Microorganisms are flushed from the Greenland Ice Sheet (GrIS) where they may contribute towards the nutrient cycling and community compositions of downstream ecosystems. We investigate meltwater microbial assemblages as they exit the GrIS from a large outlet glacier, and as they enter a downstream river delta during the record melt year of 2012. Prokaryotic abundance, flux and community composition was studied, and factors affecting community structures were statistically considered. The mean concentration of cells exiting the ice sheet was 8.30 × 104 cells mL-1 and we estimate that ∼1.02 × 1021 cells were transported to the downstream fjord in 2012, equivalent to 30.95 Mg of carbon. Prokaryotic microbial assemblages were dominated by Proteobacteria, Bacteroidetes, and Actinobacteria. Cell concentrations and community compositions were stable throughout the sample period, and were statistically similar at both sample sites. Based on our observations, we argue that the subglacial environment is the primary source of the river-transported microbiota, and that cell export from the GrIS is dependent on discharge. We hypothesise that the release of subglacial microbiota to downstream ecosystems will increase as freshwater flux from the GrIS rises in a warming world.

Ice-dammed lake drainage evolution at Russell Glacier, west Greenland

Glaciological and hydraulic factors that control the timing and mechanisms of glacier lake outburst floods (GLOFs) remain poorly understood. This study used measurements of lake level at fifteen minute intervals and known lake bathymetry to calculate lake outflow during two GLOF events from the northern margin of Russell Glacier, west Greenland. We used measured ice surface elevation, interpolated subglacial topography and likely conduit geometry to inform a melt enlargement model of the outburst evolution. The model was tuned to best-fit the hydrograph’s rising limb and timing of peak discharge in both events; it achieved Mean Absolute Errors of < 5 %. About one third of the way through the rising limb, conduit melt enlargement became the dominant drainage mechanism. Lake water temperature, which strongly governed the enlargement rate, preconditioned the high peak discharge and short duration of these floods. We hypothesize that both GLOFs were triggered by ice dam flotation, and localised hydraulic jacking sustained most of their early-stage outflow, explaining the particularly rapid water egress in comparison to that recorded at other ice-marginal lakes. As ice overburden pressure relative to lake water hydraulic head diminished, flow became confined to a subglacial conduit. This study has emphasised the inter-play between ice dam thickness and lake level, drainage timing, lake water temperature and consequently rising stage lake outflow and flood evolution.

Observed sediment and solute transport from the Kangerlussuaq sector of the Greenland Ice Sheet (2006–2016)

ABSTRACT New measurements of Watson River sediment and solute concentrations and an extended river discharge record improved by acoustic Doppler current profiler (ADCP) measurements are used to calculate the total sediment and solute transport from a large ice-sheet sector in southern west Greenland. For the 2006–2016 period, the mean annual sediment and solute transport was 17.5 ± 7.2 × 106 t and 85 ± 30 × 103 t, respectively (standard deviation given). The highest annual transport occurred in 2010, attaining values of 29.6 × 106 t and 138 × 103 t, respectively. The corresponding annual average values of specific transport are 1.39 × 103 t km−2 a−1 for sediment and 6.7 t km−2 a−1 for solutes from the approximately 12,600 km2 (95% ice covered) catchment, yielding an area-average erosion rate of 0.5 mm a−1. The specific transport is likely several times higher under the ice sheet near the margin where all meltwater passes than it is in the interior where the ice sheet is frozen to the bed. We conclude that the Greenland Ice Sheet is a large supplier of sediment and solutes to the surrounding fjords and seas. We find that the proglacial area can be a net source of sediments during high floods and we confirm that an increased amount of meltwater-transported sediments can explain the expansion of deltas around Greenland, contradictory to delta erosion observed elsewhere in the Arctic in recent years.

Reconstructing Greenland Ice Sheet meltwater discharge through the Watson River (1949–2017)

ABSTRACT Ice-sheet melting is the primary water source for the proglacial Watson River in southern west Greenland. Discharge from the large, approximately 12,000 km2 ice-sheet catchment draining through the Watson River has been monitored since 2006. While this record is of respectable length for a Greenland monitoring effort, it is too short to resolve climate signals. Therefore, we use observed Tasersiaq lake discharge and Kangerlussuaq air temperature to reconstruct annual Watson River discharge back to 1949. The resulting sixty-five-year record shows that average ice-sheet runoff since 2003 has roughly increased by 46 percent relative to the 1949–2002 period. The time series suggests that the five top-ranking discharge years occurred since 2003. The three top-ranking discharge years (2010, 2012, and 2016) are characterized by melt seasons that were both long and intense. Interannual variability more than doubled since 2003, which we speculate to be because of hypsometric runoff amplification enhanced by albedo decrease and decreased firn permeability. The reconstructed time series proves to be a valuable tool for long-term evaluation of Greenland Ice Sheet surface mass balance models. A comparison with freshwater fluxes calculated by a downscaled version of the regional climate model RACMO2 reveals high correlation (r = 0.89), and also shows that the model possibly underestimates runoff by up to 26 percent in above-average melt years.