『Abstract
We analysed the composition of phyllosilicate minerals in sediments
deposited by the Rhone(oの頭に^) and Oberaar
glaciers (Swiss Alps), in order to identify processes and rates
of biogeochemical weathering in relation to glacial erosion. The
investigated sediments are part of chronosequences consisting
of (A) suspended, “fresh” sediment in melt water; (B) terminal
moraines from the Little Ice Age (LIA; approximately 1560-1850);
and (C) tills of the Younger Dryas interval (YD; approximately
11'500y BP). Secondary weathering products associated with the
suspended sediment have not been observed; we therefore exclude
intermittent subglacial storage and weathering of this material
and assume that the suspended sediment is directly derived from
mechanically abraded bedrock. This implies that biogeochemical
weathering processes started once the glacially-derived sediment
was deposited in the proglacial area. The combination of a developing
vegetation cover, the generally high permeability allowing the
percolation of precipitation, and the chemical reactivity related
to the dominance of fine-grained material (<63μm) drives the weathering
process and the initial Umbrepts present in LIA profiles undergo
podzolisation and lead to the formation of Humods observed in
YD profiles. Systematic XRD analyses of these chronosequences
show a progressive decrease in biotite contents and a concomitant
increase in pedogenically formed vermiculite with increasing sediment
age. Biotite contents decrease by 25-50% in the upper 30 cm of
the moraines after 145-275 yr in the proglacial environment.
Biotite weathering rates are calculated using the difference
in the biotite content between unweathered and weathered glacial
sediments within the investigated profiles. The reactive mineral
surface area is estimated geometrically, both with regards to
the total relative surface (WRT) as well
as to the relative edge surface (WRE). WRT Biotite weathering rates are estimated as 10-13-10-15
molbiotite m-2biotite
s-1. WRE Biotite weathering rates
are on the order of 10-13-10-14 molbiotite m-2biotite
s-1. Biotite weathering rates obtained by this study
are in the order of one magnitude higher in comparison to other
published field-based weathering rates. Using biotite as an indicator,
we therefore suggest that glacially-derived material in the area
of the Oberaar and Rhone(oの頭に^) glaciers
is generally subjected to enhanced biogeochemical weathering,
starting immediately after deposition in the proglacial zone and
subsequently continuing for thousands of years after glacier retreat.
Keywords: Biotite; Chemical weathering; Chronosequence; Oberaar
and Rhone(oの頭に^) glaciers; Switzerland』
1. Introduction
2. Sampling sites and methods
2.1. Sampling sites
2.2. Soil profiles
2.3. Samples and methods
3. Results
4. Biotite field weathering rates
5. Discussion
5.1. Differences in ice-covered lithologies of the catchments
during the YD and LIA
5.2. Biotite weathering
5.3. Biotite weathering rate
5.4. Possible mechanisms leading to the elevated biotite weathering
rates in the pro-glacial environments of the Oberaar and Rhone(oの頭に^) glaciers
6. Conclusions
Acknowledgements
References