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| Titel |
Surface-to-base transfer and subglacial hydrology controls on subglacial erosion evidenced from 7 years of hydro-sedimentary observations within Bossons glacier catchment (Mont-Blanc massif, France) |
| VerfasserIn |
Hervé Guillon, Jean-Louis Mugnier, Jean-François Buoncristiani |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250127816
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| Publikation (Nr.) |
 EGU/EGU2016-7733.pdf |
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| Zusammenfassung |
| Continuous monitoring of sediment load in proglacial rivers allows to quantify the
mass of sediment exported from a glaciated catchment toward the fluvial system.
However, supraglacial, subglacial and proglacial sources contribute to this export and
their specific roles are poorly understood. Our general intent in this study is to
understand their respective contribution to the total exported load of suspended
sediment. The catchment of the rapidly retreating Bossons glacier (Mont-Blanc massif,
France) provides an ideal field laboratory to infer about the intensity of present-day
erosion processes. On this glacier, two proglacial rivers were monitored : Crosette
stream and Bossons stream. For Bossons stream, completing the initial 2009-2010
data, discharge and sediment load have been measured from 2011 to 2015 at a
high time resolution (timestep from 15 to 2 min). Two stations acquired these data,
upstream and downstream, respectively, from a valley sandur located at ∼1 km
from the glacial tongue. Crosette stream was monitored directly at the exit of the
subglacial system during 2013 and 2014. The combined dataset currently available
from these 3 stations spans 7 years from 2009 to 2015, is not only limited to the
melt season duration and is appreciably rich (4.3 millions raw data points). This
tremendous data profusion allowed developping statistical methods. Multi-linear
models were used to investigate sediment transfer processes in the proglacial area
during the 2013 melt season. An original probabilistic approach provided critical
information for every other years. Uncertainties on sediment masses were assessed
by a careful propagation and by a Monte-Carlo method. Additionnaly, sediment
transfer from supraglacial to subglacial position has been estimated for Crosette
and Bossons streams. Applying the conclusions of these previous works on the 7
years dataset, we provide erosion rates for the supraglacial rockwalls, subglacial
bedrock and proglacial area. Finally, we focus on subglacial erosion processes by
comparing Bossons and Crosette catchments, and analyzing export distribution
throughout each melt season. Excluding the case of extreme event, the mean proglacial
erosion rate is significantly lower, 0.12±0.10 mm.yr−1, than the mean supraglacial
and subglacial erosion rates, 0.86±0.42 and 0.51±0.39 mm.yr−1, respectively.
Our results show peaks of sediment export in the middle of the ablation season.
Additionnally, uncommon export peaks took place during the late 2010 and 2012 melt
season and are not correlated with increase in temperature, melt water runoff or
precipitations. These results could indicate the presence of an heterogeneous sediment
storage beneath Bossons glacier and abrupt exports related to the evolution of the
subglacial drainage system. Furthermore, the calculated glacial erosion rates are
0.41±0.44 and 0.61±0.35 mm.yr−1 for Crosette and Bossons streams, respectively.
Such difference could be explained by moulins and bédières efficiently transferring
surface sediment and melt water towards glacier base in the lower ablation area
drained by Bossons stream but not present in the thickest and steepest parts of glacier
drained by Crosette stream. Surface-to-base sediment and water transfer as well as
subglacial drainage development may thus significantly control subglacial erosion. |
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