Forum Comment
doi: 10.1130/G36757C.1 Glacial cirques and the relationship between equilibrium line altitudes and
mountain range height
Jörg Robl1, Günther Prasicek2, Stefan Hergarten3, and
1
Bernhard Salcher
1
University of Salzburg, Geography and Geology, 5020 Salzburg,
Austria
University of Salzburg, Geoinformatics, Z_GIS, 5020 Salzburg, Austria
3
University of Freiburg, Earth and Environmental Sciences, 79085
Freiburg, Germany
2
The effects of climate variations on the evolution of orogens
(e.g., Champagnac et al., 2012; Molnar and England, 1990; Montgomery
et al., 2001) and, more specifically, of a climatically induced glacial
buzz-saw on mountain height and relief (e.g., Brozović et al., 1997;
Egholm et al., 2009; Mitchell and Montgomery, 2006) have been widely
discussed in the geosciences. Most recently, Mitchell and Humphries
(2014) presented a global dataset of 14,000 ice-free cirques. By
analyzing this compilation, the authors discovered that the elevations of
ice-free cirque floors worldwide correspond to the Quaternary average
east Pacific equilibrium line altitude (QAEP-ELA) and that the altitude
of the highest peak within the upstream drainage area of the cirque outlet
is closely related to cirque floor elevation. While such spatially extensive
investigations are much needed to feed the discussion about climatic and
tectonic controls on mountain height, and to improve the understanding
of the processes involved in the evolution of orogens, we believe that the
major conclusions by Mitchell and Humphries are weakened by some
important issues apparent in their analysis.
Mitchell and Humphries filter the cirque floor data based on
ELA constraints to show that cirque elevation correlates with the ELA.
This seems to be a circular argument. First, they sample only ice-free
cirques, omitting all cirques above the modern snowline. Second, they
define the QAEP-ELA as the midpoint between the Last Glacial
Maximum (LGM) ELA (approximated by the LGM East Pacific
snowline; Porter, 1989) and the modern ELA. As the highest ice-free
cirques have to be located below the modern ELA, and the lowest cirques
must be found at or above the lowest Pleistocene ELA (i.e., the LGM
ELA), all collected cirque floor elevations must fall into the range
between these two ELAs. This results in a data set of cirque floor
elevations bracketed by the modern ELA and the LGM ELA—the
reference levels used for defining the QAEP-ELA in the first place.
Consequently, the correlation between the cirque floor elevations and the
QAEP-ELA presented in Mitchell and Humphries’ figures 2A and 2B is
an inevitable artifact of the chosen approach. Furthermore, the methodology excludes all peaks within the drainage area of ice-covered cirques;
i.e., not only the highest peaks, but also extensive parts of the most
glaciated alpine areas worldwide (e.g., the Alaska Range and Saint Elias
Mountains [Alaska], the Tien-Shan [China], and the Himalaya). Hence, a
general relationship between ELAs and mountain-range height, as
stressed in the title, can hardly be inferred from this data set.
Peaks within the drainage area of the investigated cirques are
reported to be closely related to the QAEP-ELA as well, as they tower an
average of 346 ± 107 m above the respective cirque floors (Mitchell and
Humphries’ figure 2C). The relation between cirque and peak elevation
can be explained by either (1) glacial erosion limiting the post-glacial
relief, i.e., by glacially driven head-wall retreat limiting cirque relief
(Anders et al., 2010), or (2) glacial erosion being related to the preglacial relief in the sense that certain drainage areas and topographic
gradients are needed to enable ice flow sufficient for an erosive glacier
(e.g., Benn and Lehmkuhl, 2000; Egholm et al., 2011; Pedersen et al.,
2014). While both alternatives likely influence the evolution of glacial
cirques, Mitchell and Humphries prefer explanation (1) and neglect the
established understanding of the physical processes involved in glacial
erosion leading to alternative (2). They reason that, in the case of
explanation (2), precipitation rates would control the accumulation area
needed to develop an erosive glacier and hence must relate to cirque wall
relief. Such a correlation was not found in a previous study performed in
the Swiss Alps where one of the present authors was involved (Anders et
al., 2010), leading to the conjecture that glacial cirque erosion does not
depend on drainage area everywhere on Earth.
This conjecture is apparently confirmed by Mitchell and Humphries’ figure 3C, where only a weak correlation between cirque relief
and modern precipitation was observed. However, this result is obviously
biased by joining two different data sets. If the data are separated into
data from literature (mainly restricted to low precipitation) and original
data, the original data indicate a more distinct correlation between cirque
relief and precipitation. Such a negative correlation would rather support
hypothesis (2) that the pre-glacial relief has a strong influence on the
formation of glacial cirques.
The contradictions reported here raise considerable doubts
whether the conclusion that glacial erosion limits mountain-range height
is indeed supported by the undoubtedly great data set presented by
Mitchell and Humphries.
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