DISCHARGE AND SUSPENDED SEDIMENT DYNAMICS OF HIGH-ARCTIC COLD AND POLYTHERMAL GLACIERS
IRVINE-FYNN, TRISTRAM D. L.. Department of Geography, University of Calgary.
Willis, Ian C.. Department of Geography, University of Cambridge.
Mumford, Paul N. . Department of Geography, University of Sheffield.
Hodson, Andrew J.. Department of Geography, University of Sheffield.
The discharge (Q) and suspended sediment (SS) dynamics of streams draining temperate glaciers has been more thoroughly researched than that of streams draining cold and polythermal glaciers. Recent studies undertaken have suggested glacier thermal regime exerts an important control on Q and SS dynamics in proglacial streams. Subglacial processes are dominant in temperate glaciers leading to high SS concentrations early in the ablation season that rapidly decline as discharge increases through the season as a hydraulically-inefficient subglacial drainage system is replaced by a more efficient one. Conversely, ice marginal processes are more important at cold and polythermal glaciers and SS concentrations tend to remain constant or increase during the ablation season.
A problem with previous comparative studies at non-temperate glaciers is that they have tended to rely on data collected from non-neighbouring glaciers (with differing sizes, climates and geologies) and in some cases over different time periods and with different sampling techniques.
This research was conducted using identical methods, simultaneously, at the adjacent and geometrically similarly Midre Lovenbreen (a polythermal glacier) and Austre Broggerbreen (a cold glacier) near Ny Alesund, Svalbard. From 22nd June to 10th August 2000, proglacial streams were monitored for Q and SS concentration variations. Air temperature and precipitation were also recorded at Midre Lovenbreen.
Data were analysed using bivariate and multi-variate statistical techniques. The discharge dynamics were strongly linked to the transition from snow-melt to ice-melt dominated glacier drainage at both glaciers. At Austre Broggeerbreen, peak Q lagged behind peak diurnal temperatures, with a decrease in the lag time over the course of the season. On Midre Lovenbreen, a similar lag was seen over the former half of the season. However, Q became 'decoupled' from air temperature forcing and there were substantial increases in proglacial SS concentrations when a subglacially-fed up-welling broke through the cold-based margin of this polythermal glacier on July 16th. These changes did not occur at the cold-based Austre Broggerbreen.
The Q and SS time-series from both glaciers were split into pre- and post-upwelling periods. At Austre Broggerbreen diurnal SS variations led Q variations during both sub-periods indicative of negative hysteresis, with rate of change in Q forcing SS variations. At Midre Lovenbreen, however, the lack of any apparent hysteresis was suggestive of more complex controls. Multivariate regression analysis suggests that SS dynamics are primarily forced by ice marginal discharge fluctuations throughout the summer at Austre Broggerbreen but that SS variations were also influenced by frequent, random fluctuations in subglacial sediment supply at Midre Lovenbreen in the post-upwelling period. This notion of the sub-glacial system's importance was supported by the respective seasonal SS yields. Furthermore, there was evidence of sediment exhaustion at Midre Lovenbreen during the post-upwelling period, suggesting the possibility of subglacial drainage system evolution, similar to that documented for temperate glaciers. Finally, multivariate and auto-regressive integrated moving average (ARIMA) analysis indicated that at both glaciers SS concentrations depended on previous SS values indicating temporally discontinuous 'flushes' and tapping of new sediment supplies, possibly derived from spatially variable paraglacial and cryogenic inputs.
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