Ice-marginal and proglacial fluvial characteristics of a high-arctic glacier, Linnébreen, Svalbard
John Antonelli Whiting
May 14, 2013
Geology - Environmental Studies
Linnébreen, a 2 km² high-arctic primarily cold-based polythermal glacier in a valley of Carboniferous sedimentary rocks and Proterozoic phyllite and schist in southwest Spitsbergen, has retreated approximately 208 m since 2004, and 1.5 km from its Little Ice Age maximum (LIAM) at around 1936. Drift is continuously being reworked by meltwater as the area between the glacier and the LIAM moraine is both a sediment sink and source. Suspended sediment concentration (SSC) and particle size distribution analyses (PSD) help us to understand the current state of the ice-marginal and proglacial area. Samples were collected during the late 2012 melt season from 18 locations along supraglacial and two ice-marginal meltwater channels. These ice-marginal channels converge 150 m downvalley of the glacial terminus forming the proglacial meltwater-dominated stream, Linnéelva. SSC and discharge of Linnéelva were measured from July 24 to August 8 at two proglacial locations, 0.23 and 1.22 km downvalley of the glacial terminus. Data from the ice-marginal and proglacial study locations provide clues about where meltwater is transporting sediment from the glacier or reworking ice-marginal deposits, and a broad idea of how Linnéelva is reworking sediments in the proglacial area upvalley of the LIAM moraine. Linnébreen, like many other small polythermal and cold-based glaciers in Spitsbergen, does not have moulins or many crevasses that permit meltwater flow to the glacier base; therefore, supraglacial, ice-marginal, and sub-marginal channels play a significant role in the fluvial transport of drift. Sampling of these channels took place in the late melt season when diurnal solar radiation cycles dictated discharge. On August 4 discharge of the east and west ice-marginal channels totaled 0.3 m³/s, SSC of 16 supraglacial and ice-marginal sample sites ranged from 0.01 to 0.23 g/L and averaged 0.12 g/L, and particle size at the same sample sites ranged from 1.01 to 22.09 μm and averaged 8.73 μm (silt). The relationships between SSC, PSD, and channel characteristics reveal ice-marginal channels to be complicated sources and sinks for glaciofluvial sediment. In many places there is a direct correlation between change in slope, SSC, and PSD. Also, samples from a supraglacial channel suggested meltwater dilution based on a decrease in SSC but no significant change in PSD. SSC and PSD suggest that the eastern ice-marginal channel was acting as a sediment sink during the late melt season. At the two proglacial sampling locations on Linnéelva discharge from July 24 to August 8 averaged 0.77 m³/s with the upper site SSC averaging 0.135 g/L, and the lower site SSC averaging 0.212 g/L. From late July to early August approximately 6.6 x 10⁴ kg more sediment in suspension passed through the lower site than the upper site, suggesting Linnéelva is significantly eroding its banks in-between the two sample sites. However, data from the 2010 field season indicates net deposition in this same area. This shift from deposition to erosion can most likely be explained by Linnéelva downcutting through the LIAM moraine-dammed lake deposits before flowing past the lower sampling site. As Linnébreen retreats ice-marginal channels develop in areas of newly exposed basal drift, however the bulk of those sediments are most likely eroded during the high discharge events earlier in the melt season, such as the spring freshet. Data from the late melt season suggest that the bulk of material transported in suspension via the ice-marginal channels originates from supraglacial and englacial debris higher on the glacial surface. These larger particles sizes are then deposited in meltwater channels downvalley as the slope and water velocity decrease. Further downvalley the SSC data from July 24 to August 8 reveal a significant amount of erosion, possibly from downcutting into lacustrine deposits. The spatial and temporal inconsistencies of stream behavior between the ice-marginal and proglacial regions within the LIAM moraine, and between the 2012 and 2010 field seasons represent difficulties in correlating environmental variation with the proglacial sediment record along Linnéelva and the glaciolacustrine record downvalley in Linnévatnet.