@phdthesis{01H9JCCBQJGD7BPH2C6HPB9CBH,
  abstract     = {{Glaciers are currently retreating at an alarming rate worldwide as a consequence of global climate
change. Since glaciers respond rapidly to changes in temperature and winter precipitation,
reconstructing their behavior in the past can provide precious information regarding the evolution of
the climate system. Traditional geomorphic techniques used to reconstruct the evolution of glaciers,
however, provide records that are notoriously discontinuous. Proglacial sediments, on the other hand,
have the potential to hold continuous and high-resolution records of past glacier dynamics. How
changes in glacier dynamics are reflected in sediment physical and chemical properties remains
however relatively unknown.
With this in mind, the goal of this thesis is to understand how glacier dynamics are recorded in
proglacial lake and fjord sediments. We do so by comparing detailed sedimentological and
geochemical analyses of lake and fjord sediment cores collected in different proglacial settings with
existing and newly acquired data on historical glacier variability. Patagonia is particularly suited for this
project since (1) it contains many pristine proglacial lakes and fjords in all types of configurations, (2)
glacier variability over the last century is relatively well known or easy to ascertain, and (3) Patagonian
glaciers are among the most dynamic glaciers on Earth. Four different glacier – sedimentary basin
systems were specifically selected to address research questions related to how glacier variability is
recorded in proglacial lake and fjord sediments
The processes leading to the initial formation of proglacial lake sediment records were examined in
Calluqueo Lake by comparing the progressive deglaciation of the basin during the 20th century with its
lake floor morphology and sediment infill. Results indicate that the initial lake floor right after
deglaciation consisted of diamicton and exposed bedrock, which created an uneven surface. We
hypothesized that the first fine-grained sediments that reached the lake accumulated between the
coarse till material to sufficiently smoothen the uneven lake floor prior to the formation of a finegrained
stratigraphic record. Based on a varve-dated sediment core taken in a relatively small finegrained
sediment field in the lake, we concluded that a fine-grained stratigraphic record only starts
forming 20 – 50 years after deglaciation. This delay likely varies between proglacial basins depending
on the roughness of the basin floor and sedimentation rates after deglaciation. Our findings imply that
(1) the first few decades of glacier variability are not recorded in the proglacial fine-grained lake or
fjord sediments, and (2) deglaciation chronologies based on basal ages are not as accurate as
previously thought.
The influence of proglacial intermediate lakes on downstream sediment properties and yield was
investigated by comparing the detailed reconstruction of the growth of Steffen proglacial lake with
downstream fjord sediment cores. The 137Cs (Cesium-137) chronology of the most distal core combined
with sediment trap data suggest that sediment accumulation in the fjord remained relatively stable
through time, despite the accelerating growth of the intermediate lake. This suggests that the lake
trapping effectiveness was balanced by higher sediment production due to accelerating glacier
shrinkage and meltwater production. Our results also indicate that the growing intermediate lake
increasingly trapped coarse sediments, as demonstrated by the progressive fining of the sediments
deposited in Steffen Fjord. However, this filtering effect appears to reach a threshold above which lake
size no longer influences downstream sediment grain size. When exactly such a threshold is reached
likely varies between lakes, and depends on site-specific geomorphological factors. This study suggests
that glacier variability may not always be clearly recorded in downstream fjord sediments, if an
intermediate lake is present.
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The geomorphological, hydrological, and sedimentological impacts of a high-magnitude glacier lake
outburst flood (GLOF) on a river-fjord system were studied at the head of Baker Fjord. One of the
largest contemporary GLOFs in Patagonia is the 1945-10
+8 GLOF that originated from Bergues Lake due
to the retreat of Lucía Glacier. This ~256 × 106 m3 GLOF was identified through the analysis of historical
maps and recent satellite images. A sediment core collected in the downstream fjord showed that the
GLOF triggered a turbidity current, which was recorded as a 6 cm-thick turbidite on the levee of the
main submarine channel. The GLOF furthermore led to the abandonment of a subaerial river channel
and of the associated submarine channel, as demonstrated by historical maps, bathymetric data, and
a sudden switch in fjord sedimentology after the event. These results show that a single GLOF can
strongly shape subaerial river planform and submarine morphology, and alter fjord sedimentology in
the long term.
The sedimentary signature of a glacier transitioning from calving to land-based conditions was
investigated in Eyre Fjord through the study of satellite imagery, hydrographic profiles, and sediment
cores. In 2010, the transition of Pio XI Glacier from calving to land-based resulted in the formation of
an ice-contact delta at the southern terminus of the glacier. Concurrently, turbidity currents started
occurring in Eyre Fjord, as recorded in fjord sediment cores over 35 km offshore of the ice-contact
delta. We proposed that these plume-triggered turbidity currents started forming due to sediments
being concentrated at specific locations along the fjord head once the delta had formed. These results
suggest that (1) a strong change in a sediment record may reflect a relatively small-scale change at the
fjord head instead of large ice-front position fluctuations or strong climatic shifts, and (2) the sudden
appearance of turbidites in fjord sediment records could indicate a glacier transitioning from calving
to land-based, provided that such an interpretation is supported by possible turbidity current
pathways.
Taken together, these results show that glacier dynamics are generally not linearly recorded in
proglacial sediments. Although glacier variability can be linearly recorded in proglacial sediments in
simple settings and on short timescales, multiple processes acting on decadal timescales disrupt this
linear relationship (e.g. GLOFs, delta formation, intermediate lake growth, glacier surges, random
iceberg behavior). These processes might influence the sedimentary signature of long-term glacier
variability (e.g. millennial-scale glacier variations) but likely do not obscure or supersede them.
Overall, our findings indicate that grain size and sediment accumulation rates are the best indicators
of glacier variability in proglacial sediments. Grain size can represent either (1) the grain size of the
sediment reaching the sedimentary basin, which is influenced by proglacial plain geomorphology (e.g.
intermediate lakes and floodplains) and the grain size of the eroded sediment, or (2) hydrodynamic
variability, which is influenced by factors such as distance to the glacier, meltwater discharge, and
current pathways. These observations emphasize that thorough site reconnaissance is needed to
accurately interpret grain-size signals. By comparison, sediment accumulation rates in proglacial lakes
and fjords mostly reflect the amount of sediment brought to the proglacial basin predominantly by
glacial meltwater. Therefore, they can be used to determine meltwater production, hence glacier
variability, particularly on short (decadal) timescales. On longer timescales (centuries – millennia),
sediment accumulation rates increasingly represent glacial erosion rates. Given the difficulties in
obtaining accurate sediment chronologies in proglacial environments, efforts to develop new
chronological methods are urgently needed.
To conclude, this thesis supports the use of proglacial sediments to reconstruct glacier variability, but
it warns that the linear relationship between glacier variability and proglacial sediment properties may
be disrupted by several processes occurring on decadal timescales, such as proglacial delta formation,
growth of intermediate proglacial lakes, and high magnitude GLOFs.}},
  author       = {{Piret, Loïc}},
  language     = {{eng}},
  pages        = {{XII, 154}},
  publisher    = {{Ghent University. Faculty of Sciences}},
  school       = {{Ghent University}},
  title        = {{Understanding how glacier dynamics are recorded in lake and fjord sediments}},
  year         = {{2023}},
}