EGQSJE&G Quaternary Science JournalEGQSJE&G Quaternary Sci. J.2199-9090Copernicus PublicationsGöttingen, Germany10.5194/egqsj-67-37-2018Glacial history of the upper Drac Blanc catchment (Écrins massif, French Alps)Glacial history of the upper Drac Blanc catchmentHofmannFelix Martinfmhofmann9892@gmail.comDepartment of Geology, Lund University, Sölvegatan 12, 22362 Lund,
SwedenFelix Martin Hofmann (fmhofmann9892@gmail.com)23November20186723740This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://egqsj.copernicus.org/articles/67/37/2018/egqsj-67-37-2018.htmlThe full text article is available as a PDF file from https://egqsj.copernicus.org/articles/67/37/2018/egqsj-67-37-2018.pdfcitationstatementHofmann, F. M.: Glacial history of the upper Drac Blanc catchment (Écrins massif, French Alps), E&G Quaternary Sci. J., 67, 37–40, https://doi.org/10.5194/egqsj-67-37-2018, 2018.
Supervisors: Helena Alexanderson (Lund University) & Philippe Schoeneich
(Université Grenoble-Alpes)
Numerous studies dealing with Late Glacial and early Holocene glacier
variability have been carried out in the Austrian and Swiss Alps. The advent
of cosmic ray exposure (CRE) dating, based on the concentration of in
situ produced cosmogenic nuclides in the quartz fraction of boulders on
moraines, has, since then, enabled the chronology of glacier fluctuations in
these regions during this period to be refined (see references in Ivy-Ochs,
2015). However, this holds partly true for the French (Western) Alps. The few
available CRE ages from boulders on moraines in this region (e.g. Chenet et
al., 2016) indicate glacier fluctuations concomitant with the cooling at
12.7 ka b2k (kiloyears before ∼ 2000 CE; Heiri et al., 2014).
Further glacier fluctuations in the French Alps occurred until the early
Holocene (e.g. Le Roy, 2012), thereby suggesting a similar pattern as in the
Austrian and Swiss Alps where glaciers shaped moraines during the Egesen and
Kartell stadials (Ivy-Ochs, 2015). However, precise palaeogeographic and
palaeoclimatic reconstructions require additional chronological constraints
on Late Glacial and early Holocene glacier variability in the French Alps.
The latter application of glacier records is of particular interest, as the
lack of biological proxies from the Late Glacial hampers detailed
reconstructions of climatic variations in this region.
Moraines in the upper Drac Blanc catchment.
Considering that a sequence of well-preserved moraines in the Rougnoux
valley in the southern Écrins massif (Fig. 1; Di Costanzo and Hofmann,
2016) has been assigned to glacier fluctuations at the end of the Late
Glacial or in the early Holocene, this set of moraines and prominent
moraines at two nearby locations (Fig. 1) were considered suitable targets
to obtain additional chronological constraints on the related glacier
variability. Hence, this study not only contributes to a better
understanding of the chronology of Late Glacial and early Holocene glacier
fluctuations in the French Alps but also provides a solid base for future
palaeogeographic and palaeoclimatic reconstructions.
Based on freely available aerial photographs taken by the French National
Institute of Geographic and Forest Information (IGN), an orthophoto and a
high-resolution digital elevation model (DEM) were established using the
structure-from motion technique. The use of a combination of a DEM-based
hillshade, the orthophoto and photographs as well as extensive field surveys
enabled the establishment of geomorphological maps of the study area using
the mapping system of Lausanne University. The identification of pre-Little Ice Age
(LIA) moraines in the study area was based on a two-fold approach. Firstly, it was
assessed whether the ridges in question are characterised by an asymmetric shape
and a steeper proximal side. Secondly, it was verified whether the landforms
are composed of a diamicton, if outcrops were available. All moraines, even
small ones, which are indicative for the extent of the two palaeoglaciers in
the study area, were then assigned to former positions of the glaciers based
on their relative position in the field to finally establish a
morphostratigraphy.
Due to the lack of organic material in the moraines suitable for radiocarbon
dating, the measurement of the concentration of the in situ accumulated cosmogenic
nuclide 10Be in the quartz fraction of samples from boulders on
selected moraines was deemed the only solution to overcome this limitation
and to be able to constrain their ages. The 10Be production rate of
Young et al. (2013) was chosen for the determination of the CRE ages.
Considering that the CRE ages with a snow shielding correction are believed
to be most realistic, they are presented below. Given that, in most cases,
three CRE ages were obtained from each moraine, landform ages were computed
(Fig. 2). First, reduced χ2 statistics was applied. If reduced
χ2 turned out to be roughly 1, the average CRE age was chosen
as landform age to eliminate the scatter due to analytical uncertainties,
whereas the oldest CRE age from a moraine was considered the landform age if
reduced χ2 was significantly larger than 1, as most sampled
boulders were small, thereby increasing the likelihood for post-depositional
exhumation. The computed CRE ages were compared with previously published
10Be CRE ages from boulders on moraines at key sites in the Alps,
recalculated according to a recent 10Be production rate (Young et al.,
2013), and data from palaeoclimatic proxies other than glaciers.
10Be CRE ages from boulders on moraines in the (a) Rougnoux
valley, (b) at Pré de la Chaumette and in the (c) Prelles valley. See
Fig. 1 for the location.
Thirdly, DEMs of the palaeoglaciers during the deposition of their outermost
LIA moraines and the further down-valley moraines were
established using the GlaRe ArcGIS toolbox (Pellitero et al., 2016).
Generally, the basal shear stress as primary input of the GlaRe toolbox was
adjusted to fit the DEMs of the palaeoglaciers to the preserved moraines. A
second ArcGIS toolbox enabled the equilibrium line altitudes (ELAs) of the palaeoglaciers to be
determined, whereby the most common accumulation area ratio of 0.67 was
assumed. This ratio is believed to be appropriate for Alpine glaciers being
in equilibrium with the climate (Gross et al., 1977). Lastly, ELA
depressions with respect to the end of the LIA were computed to allow for
stratigraphical correlations between sampled and not-sampled moraines.
In total, 10 and 8 pre-LIA positions of the Rougnoux and Prelles
palaeoglaciers were identified, respectively. An enigmatic 10Be CRE age
of a boulder on the lowermost sampled moraine (PdC M10-11) indicates that it
may have stabilised after a period of stable ice margins at or before 17.2±1.8 ka b2k (Fig. 2). Given its location at the confluence of two
valleys, the moraine was probably deposited at the common margin of
the glaciers from both valleys. During this potential event, the ELA of the
palaeoglacier from the Rougnoux valley was depressed by about 210 m relative
to the LIA ELA, whereas the ELA of the formerly confluent two glaciers from
the northern valley must have been situated at a 500–600 m lower elevation
relative to the LIA. The moraine was certainly reached, or alternatively
re-occupied, by the glacier from the Rougnoux valley slightly at around 12.8±0.7 ka b2k when the ELA was 220 m lower than at the end of the LIA.
It can be excluded that the palaeoglaciers from the northern valley reached
the frontal moraine during this event, since the required ELA depression of
the order of 500–600 m to trigger a re-advance of both glaciers up to the
moraine can be considered unrealistically high for a glacial re-advance at
around 12.8±0.7 ka b2k. Considering that the required ELA lowering
(ca. 200 m) for an advance of the Prelles palaeoglacier up to its lowermost
pre-LIA moraine matches well the ELA depression during the deposition of the
latter moraine, both landforms can probably be correlated.
The 10Be CRE ages from the Rougnoux valley (Fig. 2) provide evidence
for multiple periods of stable ice margins at the end of the Late Glacial
and potentially in the earliest Holocene (Fig. 2). These events were
associated with ELA depressions between 220 and 160 m. One of the youngest
periods of stable ice margins in the Prelles valley that was associated with
an ELA depression of 150 m relative to the LIA occurred at or before 11.0±0.8 ka b2k (Fig. 1).
Considering that the inferred age of the event during which the lowermost
moraine was last reached by a glacier matches the cooling at around
12.7 ka b2k registered in regional and hemispheric palaeoclimatic archives, it can
be inferred that the PdC M10-11 moraine and the outermost arcuate pre-LIA
moraine in the Prelles valley were deposited during a glacial re-advance
concomitant with this climatic downturn. The new 10Be CRE ages agree
well with previously published 10Be CRE ages from moraines at different
sites in the Central and Eastern Alps that have been assigned to the Egesen
and Kartell stadials (e.g. Moran et al., 2016), thereby reinforcing the
hypothesis of a common climatic forcing of the corresponding glacier
fluctuations. Uncertainties associated with CRE dating as well as an
asynchronous response of the palaeoglaciers to the same climatic signal due
to local factors, such as topography, are invoked as explanations for the
slight variations in the 10Be CRE ages.
Overall, this study improves the knowledge about the chronology of Late
Glacial and early Holocene glacier fluctuations in the Écrins massif, as
the youngest periods of stable ice margins have been dated for the first
time. This study highlights that a holistic approach relying on converging
lines of evidence, such as geomorphological mapping, CRE dating and ELA
reconstructions, should be applied to decipher the glacial history of a
region with the greatest possible accuracy.
Details on the samples for 10Be CRE dating can be found in a
global database of glacial 10Be and 26Al data available at http://expage.github.io/data/txt/Hofmann-2018.txt
(last access: 16 November 2018). All other data are available from the author upon request.
This study was mostly undertaken during the master thesis project of FMH. PS carried out geomorphological field
mapping and supported FMH together with HA during the establishment of the geomorphological maps. The samples for 10Be CRE dating
were taken by FMH and PS. The preparation of the rock samples for 10Be CRE dating in the Laboratoire Nationale des Nucléides Cosmogéniques
in Aix-en-Provence (France) was performed by FMH. HA and PS provided FMH support during the interpretation of the results of
CRE dating and the ELA reconstructions. The manuscript was written by FMH.
The author declares that there is no conflict of interest.
Acknowledgements
Firstly, Helena Alexanderson and Philippe Schoeneich are thanked for their
excellent supervision. The project was financially supported by the German Academic Exchange Service (DAAD) through a one-year
scholarship for graduate students to Felix Martin Hofmann. The administration of the Écrins national park is acknowledged for the authorisation
for sampling (authorisation no. 472/2017). Sébastien Bolbenes and Melaine Le Roy are thanked for their dedication during field work.
Jordan R. Mertes is acknowledged for his help during the establishment of the DEM and the orthophoto.
This study would have been impossible without the unanimous support of Didier L. Bourlès, Laëtitia Léanni,
Régis Braucher and Valéry Guillou during the lab work. This paper benefitted from insightful discussions with Lena Håkansson,
Per Möller, Melaine Le Roy, Irene Schimmelpfennig, Max Boxleitner, Sven Lukas, Susan Ivy-Ochs
and
Jürgen M. Reitner and other colleagues. The comments of the reviewers are gratefully appreciated.
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