Introduction
Glaciers in high-mountain environments are able to archive climate signals in
regions and altitudes where other proxy records are scarce. Non-temperate
glaciers which archive snow on a quasi-continuous basis can hold past
climate and environmental signals that can be retrieved by drilling ice
cores. In a number of pioneering studies it has already been shown that,
given the climate conditions and altitude range of the European Alps, cold
firn and ice areas suitable for ice core studies are located in the uppermost
summit ranges (typically above 4000 m a.s.l.), mostly in the Western Alps
e.g.,. In comparison,
summit glaciers of lower altitudes (typically between 4000 and
3000 m a.s.l.) received less attention regarding their role as climate
archives, until proposed and performed initial
investigations of cold ice in detail for the European Alps. Among other
findings, the discovery of the Oetztal ice man at Tisenjoch (3210 m a.s.l.)
and the subsequent dating to more than 5000 years before present
demonstrated that old ice can be preserved
at comparatively lower altitudes under certain conditions. First and foremost
these conditions require little to no ice flow, as favored through locations
near ice divides, certain bedrock geometries (e.g., depressions) and, most
importantly, basal ice temperatures persistently below the pressure melting
point ensuring that the ice is frozen to the underlying permafrost bedrock.
A key concept here is that constraining the age of the (stagnant) lowermost
layer at the summit may indicate the maximum age of these ice bodies. In
contrast, warm-based conditions, e.g., comprising lubrication by meltwater
and basal melting, are generally unfavorable for preserving the oldest ice at
the base of a glacier. At warm-based sites it is therefore impossible to date
the onset of the most recent phase of glaciation. Current warming conditions
pose an immediate risk of cold-based sites becoming warm-based and thereby
losing this archive (see the discussion of
, regarding the strong impact of the 2003 summer on Piz
Murtèl). Attempting a systematic investigation of cold-based summit
glaciers promises important paleo-climatic information on warm periods
involving minimum ice extents in the Alps. This would be complementary
information to techniques such as the dendro-chronological analyses of trees
formerly buried by glacier advances that usually provide evidence of glacier
fluctuations at lower elevations (i.e., in the vicinity of the former glacier
tongue) . In addition, a systematic
investigation of cold-based summit glaciers could also provide additional
constraint to the question if today's glacier covered highest elevation have
been ice free during the Holocene. Such a systematic study essentially
requires (i) the identification of cold-based summit sites throughout the
Alpine region. Promising candidates for holding cold ice can be identified
based on glacio-meteorological parameters including mean annual air
temperature, aspect and snow accumulation. (ii) Evidently, obtaining access
to the lowermost ice parts is essential, i.e., for direct measurements of
englacial temperature and ice sampling. (iii) Constraining the age of the presumably oldest ice is needed. Since the
stratigraphy of the expected glacier types (usually mostly made from
congelation ice) cannot be expected to include layers of every single year,
conventional dating methods like annual layer counting are severely hampered
and, as a result, age constraints must be obtained mostly from radiometric
methods . Novel developments in adapting and refining
radiocarbon techniques for microscopic organic material from glacier ice
are now available to offer an
indispensable dating tool in this context. This is especially the case since
the expected glacier age falls within the age range for the application of
the radiocarbon technique, e.g., as indicated by the dating of the Oetztal
ice man. Combining glaciological surveying (e.g., mass balance,
ground-penetrating radar) with radiocarbon dating of ice samples limits the
glacier age, especially if direct access to the lowermost ice section can be
obtained for sampling at large volume, e.g., through an ice cave
. This is especially important for application of the
14C technique, which in ice core science is often hampered by limited
sample sizes and low organic carbon concentrations.
Here we report on an investigation designed as a pilot study to a systematic
investigation of cold-based summit glaciers in the Eastern Alps. For this
purpose we selected Chli Titlis glacier, located at 3030 m a.s.l. in the
central Swiss Alps (Fig. ) as the target site. This choice was
motivated by considering that, at Chli Titlis, (i) direct access to the
lowermost ice parts can be obtained at low logistical cost even for obtaining
large sample volumes, enabled through cable car access and an ice tunnel dug
along bedrock for touristic purposes; (ii) previous work has already demonstrated
cold ice conditions, albeit more than 25 years ago ; and (iii) in an ice cave, direct observations of the ice
stratigraphy offer a more detailed picture of settings and potential
processes than a small subsample obtained from an ice core.
The study site on Titlis glacier, central Swiss Alps. The ice tunnel
is adjacent to the cable car station on Chli Titlis summit (red square in
overview map). The enlargement corresponds to the red square, and the broken
black line shows the position of the ice tunnel under the glacier surface.
Note the lighter shading of the surface above the ice tunnel due to surface
covers. The orthophotos and elevation data have been kindly provided by
Swisstopo (Swissimage and DHM25/200; ). GPS
coordinates are reported in the Swiss grid system.
Site characteristics and previous work
Glaciological investigations of the summit glacier at Chli Titlis started
some 30 years ago in connection with the construction of a
telecommunication tower . The cornice-type summit holds
the glacier on its north-facing slope, the lee side (Fig. ). The
summit itself features a tunnel through the bedrock connecting the cable-car
station with the telecommunication tower. Another tunnel was dug for tourist
purposes around 100 m into the ice along bedrock starting at the cable car
station. Ice/firn thickness increases towards the end of the tunnel, where
meltwater percolation through small crevasses was reported by
. The authors also mention well-layered ice roughly 25 m
thick and that accumulation rates on the flat summit area generally seem to
be low. According to , the existence of old ice at depth
appears likely due to negative temperatures reaching far into the underlying
bedrock and basal flow velocity close to zero. From sampling ice at the
ice–bedrock interface, found a distinct shift towards
more negative values in the stable water isotopologues (δ18O and
δD). Although doubt an Ice Age origin as the cause
of this signal, the peculiar isotopic signature is nonetheless an important
marker of the basal ice section, also found in other alpine glacier ice
bodies . Notably the term “basal ice” commonly refers
to a thin ice layer with properties characterized primarily by processes
operating at the bed . We adopt this
terminology here and use the term “lowermost ice section” to refer to the
deepest ice sections in close vicinity to the bed, which includes parts still
unaffected by ice–bed interaction, however. In our study we made an attempt
to re-find the isotope anomaly, assuming that the isotope anomaly could only
be preserved under cold and stagnant ice conditions, i.e., becoming temperate
and/or enhanced ice flow would have likely erased this isotopic signature
over the last 25 years.
Methods
A total of three campaigns were conducted in January 2014, January 2015
and August 2015. Three profiles of ice blocks cut using a chainsaw were
obtained from three different locations in the cave. Two profiles were cut
out near the entrance of the cave, aiming to be close to the original
sampling location by . The third profile was located
about 20 m deeper in the cave (Fig. ). All profiles were cut down
to bedrock at locations where the ice–bedrock interface is clearly visible.
Individual blocks were cut around 20 cm deep into the wall and varied,
depending on the profile, between 10 and 20 cm and 8 and 17 cm in height and width,
respectively (Fig. 1).
Overview on ice sampling at the Chli Titlis glacier.
Panel (a) shows a schematic diagram with ice sampling locations
within the ice tunnel. Distinct near-horizontal layering is visible within
the tunnel (b). A total of three ice block profiles were cut by
chainsaw down to bedrock (c). Panels (d, e) present
schematic details on the ice block sampling. Note that two additional blocks
(2-7 and 2-8) were cut in parallel behind block 2-4.
Englacial temperature
Based on their earlier reconnaissance, reported sub-zero
bedrock temperature and temperatures around -1 ∘C in the ice
tunnel, with an increase towards the end of the tunnel. Nowadays, active
cooling is performed by ventilation of cold outside air into the ice tunnel,
especially due to the cave being highly frequented by tourists. Additional
anthropogenic technical measures include covering the glacier section hosting
the ice tunnel with fabric during the summer season. Today's conditions at
Chli Titlis are characterized by clear signs of a negative mass balance at
the summit. For example, the fabric-covered section substantially exceeds
uncovered neighboring glacier sections in local thickness
(Fig. ). The glacier has a remaining thickness of 7–8 m above
the ice cave (Christoph Bissig, Bergbahnen Titlis-Engelberg, personal
communication, 2017).
Only basic temperature measurements at about 10–15 cm depth in the ice wall
could be performed during the initial reconnaissance campaign in
January 2014. At the location of profile 1, a vertical profile of nine holes
was drilled for temperature measurements at 20 cm vertical intervals. In
the follow-up campaign in January 2015, two holes were drilled nearly
horizontal by means of a stream drill (drilling slightly upward to let
meltwater drain) just above bedrock roughly 2.6 m deep, at locations 1 and 3
(Fig. ). Temperature was measured by means of a negative
temperature coefficient thermistor chain attached to a solid probe that was
inserted into the boreholes. Calibration of the thermistors was done in an
ice–water bath at 0 ∘C. The quality of the temperature measurements
is further determined by the data logger and the physical properties of the
involved thermistors and cables. Based on earlier studies using the same
instruments we estimate the maximum total accuracy to be ±0.2 ∘C see. Temperature readings were taken
from the first thermistor at the end of the chain, i.e., at the deep end of
the boreholes. To reduce latent heat effects that stem from drilling the
borehole and to allow for the thermal adjustment of the thermistors, readings
were taken at intervals of 4–10 min and were logged over 40–60 min. For
all boreholes, this was sufficient to attain temperature fluctuations at
least 2 orders of magnitude smaller than the estimated measurement accuracy.
Stable water isotopes
All ice samples were stored in coolers with thermal packs and transported in
frozen condition to the Institute of Environmental Physics, Heidelberg
University (IUP-HD), for further analyses. The outermost 10 cm of each
block exposed to the tunnel was removed. The opposite, i.e., inside-facing,
side of each block was used to obtain samples for stable water isotope
analysis. Initially, each block was sampled at coarse resolution between
7 and 10 cm in distance along the vertical axis. To investigate lateral
variability within the blocks, each coarse sample was further divided along
its vertical axis (denoted samples A and B). The lowest 25 cm of profile 1
were cut at higher resolution (around 2 cm). Small uncertainty in assigning
a distance above bedrock may be caused by the slightly irregular block
shape (round edges) and was estimated as accumulating to a few cm at most.
All samples were analyzed using conventional mass spectrometry (δ18O only) at IUP-HD. In Fig. the average of samples A and B
is plotted against height above bedrock, using the absolute range in isotope
values to indicate the lateral isotope variability. No stable isotope data
are available for block 2-5 in profile 2 (see respective data gap in
Fig. ). In order to obtain data for both water isotopes, δ18O and δD, additional measurements were performed using a Picarro
cavity ring-down spectrometer at IUP-HD. Co-isotopic measurements comprised
all samples of profile 1 and, in addition, samples at high resolution (around
2 cm) of profile 2. Measurement uncertainties range within ±0.1 and
±0.4 ‰ for δ18O and δD, respectively. However,
due to technical difficulties with the instrument at the time of measurement
δD values of profile 1 are associated with larger uncertainty (up to
4 ‰) and hence were not further used in this study.
Radiocarbon dating
For radiocarbon dating five different ice blocks have been analyzed using the
microscopic particulate organic fraction (POC). Two blocks each were selected
of profiles 1 and 2, and an additional block of profile 3
(Table ). The ice samples were melted, the POC was filtered,
combusted into CO2 and the radiocarbon content was measured via an
accelerator mass spectrometer utilizing a gas ion source. Details on POC
extraction and 14C measurement can be found in .
Visible dark layers and sediment-contaminated parts were carefully avoided
during sub-sampling. Upon processing for 14C analysis, all samples
exhibited a thick layer of very black and highly organic material on the
filter surface after filtration. Samples 2-3 and 2-6 were combusted at
340 ∘C. Samples 1-2, 1-9 and 3-5 were combusted at 800 ∘C.
It is important to point out that due to different combustion temperatures
of multiple organic species and increasing influences of aged and decomposed
organic material (reservoir effect), higher combustion temperatures can lead
to higher 14C ages. In a separate investigation parallel to the work
presented here, a combustion temperature of 340 ∘C for POC was
determined as the best way to avoid reservoir effects caused by influences of
already aged material incorporated into the sample and hence yields the best
representation of the actual ice sample age .
Accordingly, the retrieved ages for the 800 ∘C combustion
temperature samples are regarded as upper age limits only.
POC 14C dating results for the samples from Titlis glacier
cave. The sample names denote the profile and the block number as indicated
in Fig. . The additional temperature in the sample name refers to
the POC combustion temperature. The F14C value is given according to the
convention stated in . The calibrated ages have been
calculated using OxCal version 2.4 . All calibrated ages
are reported as their 1σ ranges in years before present (1950 CE).
Block
Combustion temperature
POC mass
F14C
Calibrated age
number
(∘C)
(µgC)
(years BP)
1-2
800
96.5
0.848±0.008
1180–1305
1-9
800
47.7
0.702±0.007
2861–3070
2-3
340
43.6
0.610±0.009
4237–4615
2-6
340
20.1
0.754±0.009
2122–2378
3-5
800
56.2
0.568±0.009
5047–5319
Visual stratigraphy and physical ice properties
The macroscopic characteristics of the stratigraphy within the tunnel were
recorded visually, with special emphasis on the three sampling locations. The
ice generally has a high density in small bubbles, giving it the white
appearance typical for glacier ice. The walls show distinct thin
(millimeter–centimeter
thick) bubble-free layers and, less abundant, yellow-brown dust-like layers.
Towards the entrance of the tunnel, at the location of profile 1 and 2,
layering is near horizontal and parallel to bedrock. At this site the
stratigraphy also features a distinct layer rich in dust and sand-like
material, giving it a dark appearance with respect to the surrounding layers
(thus we refer to it as a “dark band” here). At the far end of the tunnel
(profile 3), the layers are substantially inclined. The deepest part of
profile 1 showed a basal section of very clear, i.e., bubble-free, ice. This
clear ice section at the base extended through a larger part of the cave, but
was present at neither profile 2 nor 3 (Fig. ). For further
description of the visual stratigraphy and macroscopic characteristics of the
sampling sites in the ice tunnel, we summarize the stratigraphic properties
in Table , adopting the scheme of for ice
facies classification.
Macroscopic characteristics of the ice facies at the locations of
the three profiles in the ice tunnel at Chli Titlis. The facies/ice type
classification is based on the scheme of . Height refers
to the distance above bedrock.
Height (cm)
Bubble content
Debris content
Internal layering
Ice type
Profile 1
190–79
Very high, small bubbles
Very low, sand
Thin, bubble-free, sand, near horizontal
Glacier ice
79–41
Very high, small bubbles
Low, mainly sand
None
Glacier ice
41–15
Very high, small bubbles
Very low, sand
None
Glacier ice
15–0
Very low
Low, silt and sand, some gravel
None
Clean
Profile 2
100–70
Very high, small bubbles
Very low, sand
Thin, bubble-free, sand, near horizontal
Glacier ice
70–63
Very high, small bubbles
Low, mainly sand
None
Glacier ice
63–3
Very high, small bubbles
Very low, sand
None
Glacier ice
3–0
Low
Low, silt and sand, some gravel
None
Clean
Profile 3
160–90
Very high, small bubbles
Very low sand
Some sand, inclined (ca. 30∘)
Glacier ice
90–70
Very high, small bubbles
Low, diamicton
Heterogenous debris
Dispersed
70–30
Very high, small bubbles
Low, some gravel
None
Glacier ice
30–0
Low
Low, diamicton
None
Dispersed
To obtain complementary information about the microscopic physical ice
properties, thick and thin section samples were prepared from four blocks of
profile 1, from a sample of the clear basal ice in the back of the tunnel,
and from one block of profile 2. The sections are analyzed using a large-area scanning macroscope (LASM, Schaefter+Kirchhoff GmbH) to obtain
microstructure maps of grain boundaries (GB) and bubbles, and with an
automated fabric analyzer (FA, Russell-Head Instruments), which provides the
crystallographic orientation of individual ice crystals in a thin section
sample. An overview of the samples is given in Table . The fabric
analyzer data were automatically processed and provide
estimates for grain size and crystal-preferred orientation (CPO). The
microstructure maps are qualitatively evaluated and discussed below.
Results and discussion
Englacial temperature
The initial temperature measurements (in 2014) in shallow horizontal
boreholes (mechanical drilling) showed little variability within the vertical
profile, with -2.3 ∘C at about 1.8 m above bedrock,
-2.2 ∘C just above bedrock and the average over nine different
measurements around -2.3 ∘C. Notably, even the shallow
measurements differed unambiguously from the air temperature in the ice
tunnel, which was measured at -1.5 ∘C. The subsequent measurements
in the deeper horizontal boreholes (2.6 m, thermal drilling) at locations of
the isotope profiles 2 and 3 revealed -2.9 and -2.6 ∘C,
respectively, in January 2015. For comparison, the temperature measurement
was repeated in high summer, showing -1.9 ∘C at profile 2 in
August 2015, which is still substantially below zero. It is worth noting that
(i) due to the limited ice thickness an influence of seasonal temperature
variability may be present at the bed, and (ii) the englacial temperatures
measured in this study are systematically below the values reported by
(e.g., -1 ∘C at the entrance of the tunnel),
potentially connected to today's artificial cooling of the tunnel. Our results
demonstrate that sub-zero englacial temperatures very likely prevail at our
sampling sites in the ice tunnel, although the values should be considered as
upper limit estimates. This is (i) due to the limited time for establishing
equilibrium and latent heat effects from drilling of the borehole and
(ii) because of the higher air temperature in the ice tunnel that affects air
temperature in the borehole and the temperature of the surrounding ice.
Overview of measurements of physical properties of ice samples from
Chli Titlis. Indicated are sample and measurement type, i.e., using a large-area
scanning macroscope (LASM) and fabric analyzer (FA).
Block number
Type of thin section
LASM
FA
1-9
2 vertical sections (6 × 7 cm)
yes
yes
1-10
2 vertical sections (7 × 9 cm)
yes
yes
1-11
1 vertical section, from clear ice (7 × 8 cm)
yes
no
1-B1
1 vertical sections from the side (5 × 10 cm)
yes
yes
3-2014
horizontal section, just above clear ice (6 × 9 cm)
yes
yes
2-7
2 sections in orthogonal planes:
yes
no
1 vertical section from the back (7 × 10 cm),
1 vertical section from the side (10 × 11 cm)
Stable water isotopes
As already stated above, the stable water isotope data primarily served as a
general stratigraphic marker and, more importantly, were specifically
investigated with respect to refinding the outstanding basal isotope
signature. The isotope data of the three profiles are presented against
height above bedrock in Fig. . The average levels in δ18O are -13.35, -14.45 and -14.54 ‰ for profiles 1, 2 and
3, respectively, thus broadly consistent with the values reported by
. Although the coarse resolution isotope levels are very
similar among the upper parts, the profiles differ significantly regarding
their basal section. While no outstanding signal with respect to the rest of
the profile is found in profile 3 (albeit available only at coarse
resolution) the lowermost 10–20 cm of profile 1 and 2 comprise more
enriched and depleted values, respectively. In profile 1, the lowermost
15 cm coincide with the basal layer visibly clear and free of bubbles (see
Fig. ). The isotopic values of this section show little
variability with respect to the rest of the profile, except for a stepwise
increase by roughly 3 ‰ within the last 5 cm of the profile.
Profile 2, on the other hand, features a gradual rise in δ18O
values between 40 and 20 cm above bed, followed by a sharp drop by about
2 ‰ within the lower 20–15 cm. The differences in basal isotope
signature between profile 1 and 2 are also reflected in different visual and
microstructural properties (see below). With values around -16 ‰
just before bedrock (clearly outside of the range of the rest of the
profile), the basal isotope signature of profile 2 is in near-perfect
agreement with the phenomenon described by . Sampling an
adjacent block of roughly the same depth at the location of profile 2
reproduces this anomaly, confirming its presence at this location (see grey
line in Fig. ). A basal isotope signature characterized by a
gradual enrichment followed by distinct depletion in isotope values has been
also observed at various other mountain ice core drilling sites, in
particular at Colle Gnifetti and Mont Blanc . The
signature is unlikely to be of pure atmospheric origin, but rather the result
of post-depositional effects being involved . An adequate
investigation of this intriguing phenomenon remains outside the scope of the
present work. However, the re-discovery of the isotope anomaly initially
described at Chli Titlis by is significant in the
following sense: its unchanged presence since the last 25 years strongly
indicates that the basal ice has not undergone substantial changes as would
be expected if having become temperate or under significant basal ice
movement. Using the δD values available for profile 2 we also
performed a co-isotopic analysis (Fig. ). Calculating ordinary
linear regression of δD against δ18O yields a slope of
(7.7±0.2, 1 standard error) at a correlation coefficient of r=0.99.
These values are nearly identical to the previous investigation
and do not show clear signs of isotopic change after ice
formation (e.g., by melting and refreezing) for this part of the stratigraphy.
Stable water isotope profile of the three sampling locations.
Profile 2 reproduces the basal anomaly previously described by
. The grey plot shows measurements of a neighboring
block.The data gap in profile 2 corresponds to block 2-5, for which isotope
measurements are not available. Note that the comparatively less negative
isotope values close to bedrock in profile 1 correspond to the layers with
clear ice (grey shading; see text). The basal isotope signal in
profile 3 does not show outstanding
values with respect to the rest of the profile. Error bars denote the range
in values from two adjacent samples that were analyzed and the results
averaged (profile 1 and 3).
Visual stratigraphy and physical ice properties
Except for the basal ice section, the visual stratigraphy at profiles 1 and 2
comprised bubble-rich glacier ice with distinct layers composed of
bubble-free and dust-rich ice (see Table ). We interpret the
layers as representing former surface conditions, e.g., soil or dust-like
material deposited on the surface. Likewise the bubble-free sections
represent layers of refrozen meltwater. Notably all layers are
near horizontal, parallel to the bed and do not show any signs of folds or
other stratigraphic disturbances. As a characteristic feature, we find the
above-mentioned distinct dark band running through profiles 1 and 2. The dark
band is thicker than the dust-like layer above, but does not feature
different types of debris, e.g., mainly sand and no gravel. The visual
stratigraphy at profile 3 also shows bubble-rich glacier ice but, in
comparison to the section near the entrance of the tunnel (profiles 1 and 2),
differs by showing inclined layers and generally a higher content of gravel
dispersed within the lower 90 cm above bed.
To obtain additional information regarding the lower and basal ice sections,
all microstructural samples were taken from the lower parts in profiles 1 and
2. The mean grain size (i.e., ice crystal size), derived from the FA images,
lies between 0.55 and 2.4 cm2 but the largest grains cover between 10
and 20 cm2. The grains show little indication of active deformation
(e.g., no abundant subgrain boundaries) and it can be assumed that the large
grains are a consequence of normal grain growth, which is the dominant
microstructural process in stagnant ice and strongly enhanced by warm
temperatures, or migration recrystallization at low strain rates
. While this finding holds for the entire basal section of
profile 2, including the lowermost 10 cm, the lowermost 10 cm of profile 1
(block 11) includes many smaller grains and are characterized by a laminar or
elongated grain structure with irregular grain boundaries and almost
bubble-free, corresponding to the basal sections visually identified as
clear ice (Fig. , Table ). Accordingly, the basal
microstructural pattern independently confirms the presence of congelation
ice at the base of profile 1. Additionally, in the basal sample of
profile 2, which was cut perpendicular to the tunnel wall, elongated bubbles (mean aspect
ratio of 1.8) inclined at approximately 45∘ were observed. This
implies that a moderate deformation is to be expected, due to the
lacking constraint of the tunnel and/or the decrease in viscosity due to the
warm temperatures. The crystal-preferred orientation (CPO) observed in the
large-grained samples can be described as a multiple-maxima pattern – i.e., the
c axes of the grains are oriented in several clusters around the vertical.
This pattern is often found in basal ice close to the melting point in
conjunction with coarse grains .
Co-isotopic analysis of profile 2 showing both δD and δ18O in grey and black, respectively (left side). Also shown is the linear
regression of δD vs. δ18O, revealing a slope of
(7.7±0.2), in agreement with co-isotopic results previously described by
.
Selected results from microstructural analysis. Two example
images of a large-area scanning macroscope (LASM) of ice samples of
profile 1 (block 1-11) and 2 (block 2-7) are shown in (a, b),
respectively. The basal ice of the two profiles looks distinctly different,
with large grains and abundant air bubbles dominating profile 2 (b).
In contrast, the lowest 10 cm of profile 1 is almost bubble-free and show
very small elongated grains (a), indicating refrozen meltwater (see
text). Schmidt diagrams shown in (c) illustrate the multiple-maxima
crystal-preferred orientation of the (non-meltwater) basal ice.
Cold-based ice at Chli Titlis
Based on our evidence from basal temperature measurements revealing
temperatures well below the pressure melting point, a preserved basal isotope
anomaly and analysis of physical properties, we conclude that almost stagnant
ice frozen to bedrock still exists at Chli Titlis today. This is not to be
expected a priori in view of ongoing warming conditions and an ice thickness
not large enough to fully dampen seasonal temperature variability. A
temperature of only -0.7 ∘C at 15 m depth was reported in
1979/1980 by at the summit firn of Chli Titlis, and a
temperature inside the ice tunnel of around -1 ∘C reported in 1990
. At the same time, atmospheric warming trends of the past
decades are reflected in rising englacial temperatures even at the highest
glaciers above 4000 m a.s.l. in the Western Alps . Titlis glacier is reported to show a negative mass balance for
the time period 1986–2010 , consistent with evidence for
negative mean geodetic mass balance of Swiss glaciers between 1980 and 2010,
extending to locations above 3500 m a.s.l. . No direct
mass balance measurements with stakes close to the ice cave have been carried
out. The glacier-wide mass balance data provide only limited information
regarding the mass balance at the ice cave, but this is enough to draw a
general picture. A comparison to other glaciers at similar elevations would
have to take into account the local climatological settings
and, in addition, stake measurements in summit locations
are generally sparse. In the Eastern Alps, mass balance is measured for the
full altitude range with stakes on Kesselwandferner in Oetztal Alps. The
stake L8, located at much the same altitude in a similar climatic setting to
Chli Titlis, changed from being close to ELA (equilibrium line altitude) to increasing mass losses
since the mid-1980s .
The above evidence suggests that the ongoing change may have also affected
the lowermost ice sections at Chli Titlis over the past decades. However,
making a straightforward connection between atmospheric warming trends and
conditions in the ice cave suffers from a great deal of complexity, connected
to the surface mass and energy balance and, in particular, anthropogenic
technical measures. Investigation of ice masses in karst caves pointed out
that mass and energy balance in cave systems is more intricate
than for a glacier without englacial
cavities as it would be the case when drilling an ice core. For instance,
this potential warming influence is counteracted by ongoing efforts to
actively control the air temperature in the ice tunnel and to protect the
surface from ablation, which may contribute to keeping the ice frozen to
bedrock. Since the tunnel is located in a ski area,
substantial reworking of the surface snow cover takes place in its vicinity.
Surface covers reduce ablation and
alter the surface energy balance mainly by reducing the direct incoming solar
radiation . The propagation of these changes in energy
balance into the glacier, and potential changes in the ice temperatures have
only been investigated in a depth of 3 m ice on Schaufelferner in the Stubai
Alps with temperatures close to 0 ∘C during summer. No significant
differences between minimum ice temperatures of covered and uncovered areas
had been evident then. Trying to disentangle these anthropogenic technical
measures from natural effects is difficult. However, the above considerations
illustrate the complexity of the situation, while raising doubts as to what
extent the cold-based ice conditions would have been preserved without the
current technical measures. Predicting the future fate of Chli Titlis glacier
would certainly require a separate dedicated investigation including a
multi-annual logging of a full englacial temperature profile at the site.
Constraining the maximum age of Chli Titlis ice
Calibrated radiocarbon ages of profiles 1 and 2 (in red,
a). (b) Collection of three pictures taken during the 2015
sampling campaign to illustrate the visual layering between profiles 1 and 2.
The lower end of the visible dark band (indicated by red arrows) is located
at about 41 and 63 cm above bedrock for profile 1 and 2, respectively.
Yellow diamonds indicate the location of radiocarbon-dated samples (see
text).
Basal melting and non-stagnant basal ice flow fundamentally hamper the
preservation of old ice at the bed of glaciers with warm-based conditions.
Consequently, a cold-based thermal regime is required to preserve ice of
substantial age near bedrock at small glaciers like Chli Titlis. Persistent
stagnant ice flow conditions may hold the original ice in place after glacier
formation. The results of the radiocarbon dating efforts reveal samples with
generally very large POC concentrations starting at
500 µgC kg-1 of up to almost 4 mgC kg-1 ice. This
includes samples without any visible inclusions of particulate material. The
POC concentrations are a factor of 10–100 higher than for other high alpine
glaciers (e.g., at Colle Gnifetti; ). The organic
material is assumed to be of eolian origin originally deposited on the
glacier surface. The basal layer, which may contain a substantial amount of
sediment from the bed, has been avoided for 14C analysis. The high POC
concentrations result in relatively small statistical 14C counting
errors of 1–2 % and also no significant influences of process blanks are
expected. All radiocarbon ages have been calibrated using OxCal 4.2
. Figure presents the calibrated ages reported
with their 1σ range and gives an overview of the location of the dated
ice blocks of profiles 1 and 2. We generally find younger ages at greater
distance above bedrock in each profile. This suggests a general chronological
order of the layers in the ice tunnel wall, which is also expected
considering its distinct visible layering free of folding. Tracing the
sharp lower edge of the dark band leads from about 63 cm at profile 2
(roughly at the center of block 2-5) to about 41 cm at profile 1
(incidentally right at the border between blocks 1-9 and 1-8). It remains
difficult from our data to precisely identify the physical cause of the dark
band, e.g., whether this layer could be indicating a former hiatus in glacier
growth. However, tracing the dark band between profile 1 and 2 suggests that
at least parts of the lower 63 cm of profile 2 are not present at profile 1.
Having found evidence of refrozen meltwater at the base of profile 1 suggests
basal melting has potentially played a role in causing this deficit. In this
view, it appears reasonable to find block 1-9 at least about 500 years older
than block 2-6 but more than 1000 years younger than block 2-3 (at a similar
distance above bedrock). However, sample 1-9 was combusted at
800 ∘C, which may partially contribute to slightly higher 14C
ages compared to the samples combusted at 340 ∘C (e.g., 2-3 and
2-6). It is worth noting in this context that the maximum ages found at
profiles 2 and 3 (both without the bubble-free basal layer) differ only by
about 450 years, based on combustion temperatures of 340 and 800 ∘C,
respectively. We took great care in sub-sampling the ice blocks to avoid any
age-biasing contaminants such as old cryoconite or organic sediment. For this
reason, the lowest and arguably the oldest samples (roughly the lowest
10–20 cm) have not been analyzed for 14C,
since they show abundant inclusions of small rocks and sediment (although
these samples may serve for a future dedicated investigation into age-biasing
processes in radiocarbon glacier dating). Taking all this into consideration,
the oldest sample (5047–5319 years BP, profile 3) is regarded as
representing an upper age limit for ice at Chli Titlis.
The large gradient in age (e.g., for profile 2, several thousand years' difference
within less than a 1 m) also deserves attention. Numerical flow modeling by
indicated the presence of substantial shear near
bedrock even close to the summit, making thinning of the lowermost layers a
possibility. Results of our ice microstructure analysis are consistent with
moderate ice deformation. Notably, deformation by simple shear would
primarily entail thinning of layers as opposed to layer folding or turbulent
ice flow. As an alternative, considering ice formation at the site, phases of
ablation (like observed today) followed by recurring accumulation could also
play a role in explaining the observed vertical age gradient. Reconstructing
the details of the glacier response to past (and future) climate conditions
deserves a separate thorough investigation, e.g., taking into account
numerical modeling of ice flow and energy balance. As a potential
contribution to such an effort, the results of the study presented here
already constrain the maximum age of the ice remaining at the summit of Chli
Titlis today.
Outlook
Based on the above results, promising future contributions can be expected
from extending the approach presented here in a more systematic investigation
to constrain the age of other summit glaciers, selected with respect to
comprising cold-based ice condition, access to lowermost ice parts and
geographic coverage (preferably also in the Eastern Alps). Taking as a
promising example in this regard, we have also started to investigate the ice
of the Schladminger glacier at Hoher Dachstein (Austria). This site offers
similar glaciological and ice sampling conditions to Chli Titlis:
Schladminger glacier is located at 2700 m a.s.l. on a cornice-type,
north-facing summit, and direct access to ice at bedrock is possible by means
of an ice tunnel for tourists. We have employed the same set of tools
described here, and found an englacial temperature of -1.3 ∘C at
190 cm inside the tunnel wall just above bedrock. As a preliminary result,
radiocarbon analysis of one of the obtained ice blocks indicates an age
around 4297–3715 years BP (1σ range), which is, interestingly, close
to the age range observed at Chli Titlis.
In their earlier study, had estimated, based on
their flow model considerations, that the oldest ice at Chli Titlis is likely
of Holocene origin with a maximum age of centuries to millennia. The results
of the present study show that this estimation was accurate, but they further
demonstrate that in fact the ice clearly reaches several millennia in age.
The presence of millennia-old ice in the lowermost sections at Chli Titlis,
and the preliminary evidence at Schladminger glacier, is significant from a
glaciological perspective, since demonstrating that cold-based summit
glaciers around 3000 m a.s.l. can (still) archive ice of substantial age.
In a paleoclimatological context, the maximum age constraints of the ice at
Chli Titlis suggest that this location has been ice-covered over roughly the
last 5000 years. This finding is in general agreement with widespread
evidence of a period of minimal glacier extent throughout the Alps at that
time e.g.,. This includes evidence from dating
archaeological artifacts recovered at other summit locations of comparable
altitude, such as possible ice-free conditions at Tisenjoch (3210 m a.s.l.,
the location of the Oetztal ice man) and, at greater
proximity to Titlis, Schnidejoch pass (2730 m a.s.l.) .
Notably the presumed onset of the recent phase of glaciation at Chli Titlis
follows the termination of the last “Holocene Optimum Event” (HOE) around
5650 years BP as defined by . The latter study investigated
Tschierva glacier, located in the Eastern Swiss Alps at similar altitude as
Chli Titlis, and reports the equilibrium line altitude rising by more than
220 m to above 3040 m a.s.l. during the HOE phases. In order to
investigate whether the evidence from Titlis and Schladminger glacier points in
fact towards a widespread phenomenon of reduced summit glaciers in the time
period around 4000–5000 years BP, it will be intriguing to continue our
analysis and to integrate additional Alpine summit glaciers at comparable
altitude.
Conclusions
We have successfully employed a combination of englacial temperature
measurements with ice analysis and radiocarbon dating to show that ice frozen
to bedrock still exists at Chli Titlis, and we were able to constrain the
maximum age of the ice remaining at the summit today. For this purpose we
utilized an existing ice cave to directly access, sample and investigate the
age, isotopic and physical properties of the lowermost ice layers.
Temperature measurements demonstrate basal temperatures that are well below
the pressure melting point throughout the year, albeit likely influenced by
the air temperature in the tunnel. This finding indicates close-to-stagnant
ice frozen to bedrock, substantiated by results from ice microstructure
analysis. In addition, the stable water isotope measurements obtained from
one profile reproduce a particular basal anomaly found in a study performing
the first ice sampling over 25 years ago. Our radiocarbon analysis of five
ice blocks suggests a chronological order of the visible ice layers and
gives a constraint of the maximum age of the lowermost sections of maximal
5000 years before present. Based on the success of our approach we have
already extended the investigation to similar sites in the Eastern Alps, with
promising first results suggesting the presence of millennia-old ice in an
ice tunnel at Schladminger glacier.
The results of the study presented here demonstrate that, even today,
cold-based ice still persists at summits of substantially lower altitude than
4000 m a.s.l., reaching several millennia in age. In a paleoclimatological
context, our results indicate that Chli Titlis has likely been ice-covered
for at least the last five millennia, although mass loss is ongoing today
even at the summit. This finding is consistent with existing evidence
suggesting for that time ice-free conditions at summit sites of comparable
climatic and glaciological setting. Ultimately this shows that
age constraints at cold-based summit glaciers constitute a significant, yet
almost untapped, paleoclimate proxy, making the further exploration of these
sites a worthwhile target.