Investigating tidewater glacier and iceberg submarine melting in Greenland’s fjords
Moyer, Alexis Noelle
Accelerated mass loss from the Greenland Ice Sheet in recent decades has been concentrated around the coastal margins, where glacier ice fronts and the undersides of floating ice are in contact with warm ocean waters. The interaction of the ice sheet with a warming ocean leads to thinning of tidewater glaciers, which has been linked to increased glacier velocity, calving, retreat and subsequent mass loss. Our understanding of these ice-ocean interactions is limited, particularly regarding submarine melting of tidewater glacier ice fronts, which has been proposed as an initial trigger for glacier retreat and acceleration. Understanding the mechanisms promoting changes to tidewater glacier dynamics is critical, as they are currently absent from ice sheet models and present a large source of uncertainty in 21st century sea level rise predictions. This thesis develops two novel remote sensing techniques, to investigate both submarine melt rates under tidewater glacier floating ice tongues and iceberg freshwater fluxes from submarine melting, providing improved datasets and process understanding that can be used to constrain changes to the Greenland Ice Sheet in a warming world. The first result of this thesis is the development of a methodology to estimate submarine melting under floating ice tongues using satellite imagery. Submarine melt rates were derived by differencing along-flow ice tongue surface elevation, in combination with ice tongue velocity and changes in surface mass balance. Kangiata Nunaata Sermia (KNS), a large tidewater glacier in southwest Greenland, was used as a proof-of-concept study site. Mean submarine melt rates under the seasonal ice tongue at KNS in spring 2013 reach over 0.8 ± 0.3 m d−1, decreasing with distance down-fjord from the glacier grounding line and varying across-fjord. These variations in melt rate likely result from changes in ice tongue draft and fjord water temperature with depth, but may also reflect the strength of subglacial discharge plumes exiting beneath the glacier grounding line. Expanding upon these initial melt rate results, the thesis next explores the spatial and temporal variations in submarine melting at KNS from 2012 to 2014. Using the same methodology, derived melt rates vary significantly near the glacier grounding line, both spatially and temporally, with mean melt rates of 1.3 ± 0.6, 0.8 ± 0.3, and 1.0 ± 0.4 m d−1 across the ice tongue in 2012, 2013 and 2014, respectively. Areas of higher submarine melting correspond spatially with locations of subglacial discharge plumes and ice front calving activity observed during the melt season using time-lapse camera imagery, as well as to locations of modelled subglacial flow paths with large upstream catchment areas. These results suggest a dynamic subglacial hydrology system capable of rapidly re-routing subglacial discharge to different terminus outlets both within and between melt seasons. Furthermore, they provide an empirically-derived link between the presence of subglacial discharge plumes and areas of high spring submarine melting and calving along glacier termini. Moving to east Greenland, the final results chapter turns to Sermilik Fjord, where a new methodology for estimating freshwater fluxes from icebergs is developed. The amount, timing and location of meltwater produced via submarine melting of icebergs can significantly impact local fjord water circulation and heat budget, which has implications for glacier calving, retreat and acceleration in addition to nutrient fluxes and primary productivity. Previous methods for estimating iceberg meltwater fluxes have either been complex models, themselves reliant on limited field data and poorly constrained model parameters, or user-intensive, expensive remote sensing techniques. This thesis presents a simplified approach for deriving summer and autumn iceberg freshwater fluxes in 2017, using freely available Sentinel-2 satellite imagery to estimate iceberg velocity and seasonal changes in iceberg volume with distance down-fjord. Integrated 2-month full-fjord freshwater fluxes reach 1270 ± 650, 1200 ± 610, 3410 ± 1740, and 1150 ± 590 m3 s−1 for June-July, July-August, August-September and September-October, respectively. The estimated iceberg freshwater fluxes are highest across August and September, likely due to a combination of warming fjord water temperatures at depth as autumn approaches and increased calving into the fjord system in the months prior, and decrease with distance from the head of the fjord. The proportion of solid ice exiting the fjord averages 14% of the total ice volume calved into the fjord from Helheim Glacier, the largest tidewater glacier feeding into Sermilik Fjord, confirming that a significant volume of freshwater is released at depth along the length of the fjord. The volume of freshwater generated from iceberg melt is comparable to or greater than subglacial discharge volume throughout the year, and has important implications for fjord-scale circulation, submarine melt rates and primary productivity. In terms of future developments, this thesis presents two new methodologies focused on ice-ocean interactions: one to estimate submarine melt rates near tidewater glaciers and one to derive iceberg freshwater fluxes entering glacial fjords. These innovative methods have generated important new datasets in a challenging environment from which it has consistently proven difficult to derive estimates of submarine melt rates. The additional utility of these methods is their potential application to other fjord systems for constraining both fjord-scale and ice-sheet wide ice-ocean models, developments that are critical for understanding the sensitivity of the Greenland Ice Sheet and surrounding ocean basins to future climate change.