5th Swiss Geoscience Meeting, Geneva 2007
The combined effects of Little Ice Age and human
impact on sediment input from Lake Brienz catchment
Girardclos Stéphanie*,** & Anselmetti Flavio S.*,***
*Geological Institute – ETH Zurich, Universitätsstrasse 16, CH-8092 Zurich
**present address: Départment de Géologie et Paléontologie, Rue des Maraîchers 13,
CH-1205 Genève (stephanie.girardclos@terre.unige.ch)
***present address: EAWAG, Überlandstrasse 133, CH-8600 Dübendorf
Swiss National Science Foundation Grant 620-066113
Modern lakes, located in areas where historic and instrumental data is
available, provide ideal basins to understand the sedimentological response to
climate change and human impact in the lake’s catchment, as well as to
constrain the causes of subaquatic mass-transport events.
A previous study of superficial sediments in Lake Brienz (Switzerland) showed
that a large sublacustrine mass-transport occurred in April 1996 and induced a
series of limnological events, including the release of an ancient corpse to
surface waters and the occurrence of a small ‘tsunami-like’ wave. Results
reveal that a large turbidite deposit (volume 2.72 * 106 m3) was generated by a
slope failure in the main river delta, remobilizing a mass equivalent to 14 years
of the lake’s annual input. The correlation with contemporary environmental
data indicates that no particular cause (i.e., earthquake, explosion, flood, wind
storm, lake-level change, seiche or sediment dredging) triggered this event.
Instead, it was due to a normal delta slope failure caused by normal sediment
accumulation (Girardclos et al. 2007).
Here, the deeper sediment record, studied on basinwide scale by combining
high-resolution seismic and sedimentary cores, reveals that three additional
large turbidite deposits have occurred in the lake’s history. These meter-thick
beds correlate to prominent semi-transparent onlaping units in the seismic
record, and alternate with hemi-pelagic sediment. The large turbidites consist of
normally graded sand to silt-sized sediment topped by a thin, white, clay-sized
layer and contain clasts of hemi-pelagic sediment. These beds are longitudinally
graded and thin out toward the end of the lake basin. The age model, inferred
from 137Cs activity, 14C datings, onset of steamboat activity (slag particles) and
detailed catalogue of flood events, indicates a very high sedimentation rate for
hemi-pelagic deposits (2.4 cm/yr) and attributes large turbidites to 1860, 1910,
1942 AD and 1996 AD respectively. The recurrence interval of these turbidites
is 32-54 years and expresses the Aare delta accumulation/failure cycle. The
seismic and sediment record show that no similar failure occurred before 1860
AD, and that sedimentation rate continuously increased from 1680 to 2005 AD.
The onset of Aare delta failure cycle is certainly due to climate warming and
onset of glacier retreat in the Swiss alps happening at the end of ‘Little Ice Age’.
The canalization of the Aare river combined with the draining of upstream
swamps, carried out from 1866 to 1876 AD, probably added to climate-induced
changes to further increase sediment river transport to the lacustrine delta,
5th Swiss Geoscience Meeting, Geneva 2007
ultimately leading to higher delta instability. The proportional decrease of
turbidites vs. hemi-pelagic deposits which occurred since ~1940 AD seems to
be caused by the construction of Grimsel hydropower dams (started in 1929,
completed in 1955) in the lake’s catchment, as 2/3 of the ‘natural’ Aare river
load, consisting mostly of coarse sediment, is trapped in the reservoirs
(Anselmetti et al. 2007).
Figure 1. Lake Brienz sediment core transect. Megaturbidites start at the end of
Little Ice Age, occurring in 1860, 1910, 1942 and 1996 AD.
REFERENCES
Girardclos, S., Sturm, M., Schmidt, O.T., Ariztegui, D., Pugin, A. & Anselmetti
F.S. 2007: The 1996 AD delta collapse and large turbidite in Lake Brienz.
Marine Geology, 241, 137-154.
Anselmetti, F.S., Bühler, R., Finger, D., Girardclos, S., Lancini, A., Rellstab, C.
& Sturm M. 2007: Effects of Alpine hydropower dams on particle transport and
lacustrine sedimentation. Aquatic Sciences, 69, 179-198.