TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-11-17-2017Climate change threatens archaeologically significant ice patches: insights
into their age, internal structure, mass balance and climate
sensitivityØdegårdRune Strandrune.oedegaard@ntnu.nohttps://orcid.org/0000-0002-8066-1698NesjeAtleIsaksenKetilhttps://orcid.org/0000-0003-2356-5330AndreassenLiss MarieEikenTrondSchwikowskiMargithttps://orcid.org/0000-0002-0856-5183UgliettiChiaraNorwegian University of Science and Technology, Gjøvik, NorwayUniversity of Bergen, Bergen, NorwayNorwegian Meteorological Institute, Oslo, NorwayNorwegian Water Resources and Energy Directorate, Oslo, NorwayDepartment of Geosciences, University of Oslo, Oslo, NorwayPaul Scherrer Institute, Villigen, SwitzerlandRune Strand Ødegård (rune.oedegaard@ntnu.no)2January2017111173219April201623May201621November201626November2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://tc.copernicus.org/articles/11/17/2017/tc-11-17-2017.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/11/17/2017/tc-11-17-2017.pdf
Despite numerous spectacular archaeological
discoveries worldwide related to melting ice patches and the emerging field
of glacial archaeology, governing processes related to ice patch development
during the Holocene and their sensitivity to climate change are still largely
unexplored. Here we present new results from an extensive 6-year (2009–2015)
field experiment at the Juvfonne ice patch in Jotunheimen in central southern
Norway. Our results show that the ice patch has existed continuously since the
late Mesolithic period. Organic-rich layers and carbonaceous aerosols
embedded in clear ice show ages spanning from modern at the surface to ca.
7600 cal years BP at the bottom. This is the oldest dating of ice in
mainland Norway. The expanding ice patch covered moss mats appearing along
the margin of Juvfonne about 2000 years ago. During the study period, the
mass balance record showed a strong negative balance, and the annual balance
is highly asymmetric over short distances. Snow accumulation is poorly
correlated with estimated winter precipitation, and single storm events may
contribute significantly to the total winter balance. Snow accumulation is
approx. 20 % higher in the frontal area compared to the upper central
part of the ice patch. There is sufficient meltwater to bring the permeable
snowpack to an isothermal state within a few weeks in early summer. Below the
seasonal snowpack, ice temperatures are between -2 and -4∘C.
Juvfonne has clear ice stratification of isochronic origin.
Introduction
The emergence of glacial archaeology is described by Andrews and
Mackay (2012) and Dixon et al. (2014). In archaeology, the term “glacial
archaeology” or “snow patch archaeology” refers to several alpine contexts
in different regions of the world (Callanan, 2010). The release of Ötzi's
5300-year-old body from the ice in northern Italy marked the beginning of a
number of remarkable archaeological discoveries world-wide connected to
melting ice and thawing permafrost in the high mountains (Spindler, 1994).
Discoveries are known from the Alps (Grosjean et al., 2007; Suter et al.,
2005), mummies in Greenland (Hansen et al., 1985) and the Andes Mountains
(Ceruti, 2004), and from archaeological finds at retreating ice patches in
North America (Brunswig, 2014; Dixon et al., 2005; Farnell et al., 2004; Hare
et al., 2012; Lee, 2012; Meulendyk et al., 2012). When analysing the number
of artefacts on a global scale during the Holocene, there is a negative
correlation between periods of glacial advance and the number of artefacts.
This is particularly the case in the Alps and North America (Reckin, 2013),
but a similar pattern is also found in Norway (Nesje et al., 2012). The
question is if this is caused by changes in climate-dependent preservation
conditions or decreased human use of these areas in periods of cold climate.
In Norway, there has been an increasing focus on ice patches since the
extreme melting in southern Norway in the autumn of 2006. There are about
3000 known artefact finds globally from ice patches. Most of these have
melted out during the last 3 decades. Approximately 2000 of these
archaeological finds are in central southern Norway, making it by far the
most find-rich region in the world (Curry, 2014; Lars Pilø, personal
communication, 2014).
Among the most spectacular finds are a Bronze Age leather shoe that melted out
in late autumn 2006 and a well-preserved tunic dated between 230 and 390
(Common Era) CE (Finstad and Vedeler, 2008; Vedeler and Jørgensen, 2013).
The shoe was dated to be around 3400 years old (1429–1257 Before Common Era,
BCE), and is by far the oldest shoe found in Norway. Dates are given in
calibrated ages (BCE or CE), including 1 sigma errors (σ) when referencing
archaeological finds in Norway. Radiocarbon dates from ice patches are
referenced as calibrated years Before Present (BP = 1950 CE).
The geoscience of old ice patches is still in its infancy and the geoscience
literature about ice patches is sparse compared to glacial archaeology.
Within the glaciological community it is commonly differentiated between
glaciers and snowfields and active or inactive ice (UNESCO, 1970). Snowfields
may be seasonal or perennial. Seasonal snowfields melt during the summer.
Perennial snowfields exist for 2 or more years. Smaller ice bodies without
significant movement may be remnants of a past active glacier or a perennial
snowfield and are commonly referred to as glacierets. In this paper, we use
ice patch for perennial snowfields and glacierets. Ice patches are, in
contrast to glaciers, mostly stagnant and therefore do not convey mass from
an accumulation towards an ablation area. In fact, ice patches often do not
exhibit distinct glacier facies such as a firn area. In the wet-snow zone,
the transformation of snow to ice occurs rapidly by metamorphism and
refreezing of meltwater (Kawasaki et al., 1993). Ice patches and surrounding
terrain are generally underlain by permafrost (Haeberli et al., 2004). There
are few studies related to their thermal regime, mass balance and dynamics
(Eveland et al., 2013; Fukui, 2003; Fukui and Iida, 2011; Sato et al., 1984).
Fujita et al. (2010) concluded that ice patches exist below the regional
equilibrium-line altitude (ELA) of glaciers. A study by Glazirin et
al. (2004) showed that they can modify the nearby wind field. The studies mentioned
documented feedbacks between ice patch size and both summer
ablation and winter snow accumulation. The spatial variability of the
turbulent fluxes in alpine terrain is of particular interest to ice
patches. Ice patches are influenced by advective heat transfer in summer
(Essery et al., 2006; Mott et al., 2015; Pohl et al., 2006).
Despite some progress in these studies, the state of knowledge is not at a
level to design reliable models of how ice patches have developed during the
Holocene and to evaluate their sensitivity to future climate changes. The
main objective of this study is to help fill this knowledge gap based on a
6-year field experiment at the Juvfonne ice patch (Figs. 1 and 2), located in
Jotunheimen in central southern Norway.
The field site Juvfonne in central southern Norway. Dark blue areas
are glaciers.
Overview picture from September 2008 towards SSW showing Juvfonne
and the Juvflye area including Kjelen, Juvvatnet, Juvvasshytta, Vesljuvbrea
and the P30/31 Permafrost and Climate in Europe (PACE) boreholes at
Juvvasshøe. Also visible is the highest mountain of Norway,
Galdhøpiggen (2469 m a.s.l.). Photo: Helge J. Standal.
The overall objective of this study is to better understand the governing
processes of ice patch mass balance and Holocene development. A
multi-disciplinary approach was chosen, combining a set of new geophysical
data, radiocarbon dating, mass balance measurements and visual observations
from two 30–70 m tunnels that were excavated into the central parts of the
ice patch to better understand (1) the age, (2) the mass balance, (3) the
thermal regime, (4) ice layering and deformation on Holocene timescale, and
finally (5) the physical processes relevant to artefact displacement and
preservation.
Field site and physical setting
In central southern Norway archaeologists have so far identified more
than 65 sites with finds related to ice patches, but many sites with
potential finds have not been checked in the field. The archaeological finds
are related to reindeer hunting. The snowfields are an important refuge for
reindeer during hot summer days, giving them relief from pestering
insects. The focus of this study is the ice patch Juvfonne
(61.676∘ N, 8.354∘ E) and the surrounding terrain (Figs. 1
and 2). This site is a well-preserved Iron Age hunting “station” documented
by more than 600 registered wooden artefacts and 50 hunting blinds.
Radiocarbon dating of artefacts shows ages in two separate time intervals,
246–534 and 804–898 CE (Nesje et al., 2012). The geoscience studies at
Juvfonne started in 2009 (Ødegård et al., 2011). Nesje et al. (2012)
gave a comprehensive presentation and discussion of archaeological finds in
central southern Norway related to late Holocene climate history.
The width of the ice patch is approx. 500 m, with an upslope length of 350 m.
Juvfonne had an area of 0.15 km2 and ranged in altitude from 1839 to
1993 m a.s.l. in 2010 (Andreassen, 2011). The mean surface slope is
17∘ and the ice patch has a north-easterly aspect.
Due to snowdrift by prevailing westerly winds during the accumulation season,
Juvfonne is below the regional temperature-precipitation equilibrium-line
altitude (TP-ELA). Annual surface mass balance measurements were
conducted on three glaciers (since 1949 at Storbreen and 1962 at
Hellstugubreen and Gråsubreen) in the Jotunheimen mountainous region
(Andreassen et al., 2005; Andreassen and Winsvold, 2012). The ELA increases
with distance from the coast from 1780 m a.s.l. at Storbreen to
2150 m a.s.l. at Gråsubreen (Kjøllmoen et al., 2011). Except for a
transient mass surplus from 1989 to 1995 due to increased winter
precipitation in this period, the glaciers have lost mass. Map surveys and
inventory data show a reduction in area of the glaciers in Jotunheimen of
about 10 % from the 1960s to 2003 (Andreassen et al., 2008).
Juvfonne is well within the mountainous permafrost zone. Present permafrost
thicknesses at elevations where we find perennial ice patches (∼> 1700 m a.s.l.) can be estimated to be more than 100 m.
Observations of ground thermal regimes (Farbrot et al., 2011; Harris et al.,
2009), bottom temperature of snow cover (BTS) (Farbrot et al., 2011; Isaksen
et al., 2002; Ødegård, 1993) and geophysical surveys to delineate the
altitudinal limit of the permafrost (Hauck et al., 2004; Isaksen et al.,
2011), along with spatial numerical equilibrium and transient permafrost
models (Gisnås et al., 2013, 2016; Hipp et al., 2012; Westermann et al.,
2013), indicate a lower limit of permafrost at 1450–1600 m a.s.l. in the
area.
Juvfonne is at a distance of 750 m and at the same elevation as the
permafrost boreholes (the P30 and 31 Permafrost and Climate in Europe (PACE)
boreholes) and climate monitoring site at Juvvasshøe (Sollid et al., 2000)
(see Fig. 2). The site has a record of ground temperatures and meteorological
observations since September 1999. Mean annual air temperature for the period
2000–2015 is -3.5 ∘C. At 15 m depth, the permafrost temperature
ranges from a minimum of -3.1∘C in 1999 to a maximum of
-2.5∘C recorded in 2008. The active layer thickness has varied
between 2.0 and 2.4 m and permafrost thickness is estimated to exceed 300 m
(Isaksen et al., 2011). In 2008, an altitudinal transect of boreholes and
adjacent air temperature sensors were installed at three sites ranging from
shallow seasonal frost to permafrost (Farbrot et al., 2011).
For the period 1961–1990, the mean annual precipitation is estimated to be
between 800 and 1000 mm a-1 at 1900 m a.s.l. at Juvfonne (Norwegian
Meteorological Institute, unpublished data).
Sediment cores in the nearby Juvvatnet were used to reconstruct the glacier
activity of Kjelebrea and Vesljuvbrea (Nesje et al., 2012) following the
methodology described by Bakke et al. (2010). The results indicate that the
late Holocene variations of these glaciers are largely in agreement with size
variations of other glaciers in the Jotunheimen area (Matthews and Dresser,
2008; Nesje, 2009). Lichenometry suggests that the margin of Juvfonne
extended ∼ 250 m from its present position during the “Little Ice
Age” (LIA) maximum extent in the mid-18th century (Nesje et al., 2012).
MethodsGeoradar
The ice patch was surveyed by a RAMAC georadar on 23 September 2009 and
1 March 2012, using a high-frequency antenna of 250 MHz. The dielectric
constant of ice was set to be 3.2, giving a phase velocity of
168 m µs-1. Georadar data and positioning data from the
Global Navigation Satellite System (GNSS) were manually digitized to obtain a
point dataset of ice thickness and bed topography. The point datasets were
interpolated to get an ice thickness map and a digital terrain model (DTM) of
the ice patch bed. Obvious artefacts in the resulting data caused by the
interpolation technique were manually removed. A total of 40 independent
control points gave an estimated standard deviation in bed elevation of
1.1 m and a maximum error of 2.6 m. The control points were obtained by
point measurements (GNSS) in the recently exposed area.
Laser scanning
The ice patch and surrounding terrain were scanned with an airborne laser
(Leica ALS70) on 17 September 2011. The company COWI A/S, on assignment from
the Norwegian Water Resources and Energy Directorate, carried out the laser
scanning and the processing of the data. The flight altitude was
3080–3600 m a.s.l. The area was scanned with 5 points per m2. Quality
controls and accuracy assessments revealed accuracy better than 0.1 m in
surface elevation. Aerial photos were taken on the same day. These data were
used to produce a high-quality DTM and orthophotos of the ice patch surface
and surrounding terrain. The DTM was resampled to a resolution of 1 m.
Mass balance and front measurements
Surface mass balance measurements of winter accumulation (snow depth at
20–60 sites and density at one site) and ablation (at one to four stakes) were made
following standard methods for the melting seasons of 2010–2015 (Andreassen,
2011). Distance to the terminus was measured from two points outside the ice
patch (Fig. 3a) in August or early September using a laser distance
metre.
The extent of the Juvfonne ice patch was surveyed by foot with GNSS with a
Topcon receiver mounted on a backpack and one reference receiver mounted in a
fixed base point (Fig. 3a, Table 1). The GNSS data were processed with Topcon
software TTOOLS version 8.
Maps of Juvfonne with orthophotos from September 2011 as background
(UTM coordinates zone 32∘ N): (a) ice margins, position of
front measurements (JF1 and JF2, see Fig. 9), position of mass balance stake
J2, position of thermistor for ice temperature measurements (Fig. 13) and
position of the oldest radiocarbon dating and position of snow depth
measurement station; (b) interpolated contours of bed topography
relative to ice thickness in September 2011 (grey markers are radar points
used in the interpolation) and position of the georadar track in Fig. 4,
white line; (c) grey markers are snow depth measurements
(2010–2015), while the raster map shows a first order polynomial fit to the
deviation from mean accumulation each year; and (d) height differences
along GNSS tracks in 2014 relative to ice surface from laser data in 2011 and
positions of the ice tunnel excavated in 2012.
Surveys were done annually in August or September from 2010 to 2015, but the
survey from 2012 was only done along the lower part due to snow conditions.
Areal extent was also determined by digitizing outlines from orthophotos from
2011 and from topographical maps in 1981 and 2004 (map product coded N50 from
the Norwegian mapping authorities). Furthermore, outlines from Landsat
inventories from 1997 and 2003 were used (Andreassen et al., 2008; Winsvold
et al., 2014). The accuracy of the differential GNSS is within 1 m, the
accuracy of the topographic maps within 5 m and the accuracy of the Landsat
mapping within 30 m. The standard deviation in height of the GNSS
measurements is in the range 0.1–0.2 m, giving ±2 standard deviations of
0.6 m.
Meteorological measurements
Hourly meteorological data were obtained from the automatic weather station
(AWS) at Juvvasshøe (1894 m a.s.l.). Juvvasshøe is the highest
official meteorological station in Norway and is freely exposed and
considered representative for this study. Due to the sheltered setting of
Juvfonne compared to Juvvasshøe, the wind speed at Juvvasshøe is
generally higher. However, wind data from Juvvasshøe give important
information on wind direction and wind speed relevant to snow accumulation
and ablation processes at Juvfonne. The first station was set up in 1999
(Isaksen et al., 2003) and a new official weather station was established at
the same site in June 2009. One additional station recording hourly snow
depth was set up in autumn 2011 in front of Juvfonne (95 m from the eastern
margin of the snowfield). Hourly data on snow depth are scarce in the high
mountains in Scandinavia. Observed air temperature and wind speed at
Juvvasshøe were compared with the 1971–2000 climatological normal
based on interpolated air temperature data from seNorge (Engeset et al.,
2004) and daily observations of wind speed from Fokstugu (973 m a.s.l.),
70 km NE of Juvvasshøe, which was the best nearby correlated
meteorological station that had long time series.
Areal extents of Juvfonne derived from topographic maps, Landsat
imagery, GNSS measurements by foot and digitizing from orthophotos.
YearDateSourceArea (km2)1981Map0.171198410 Aug 1984Orthophoto0.208199715 Aug 1997Landsat0.208200309 Aug 2013Landsat0.150200412 Aug 2004Map0.187201025 Aug 2010GNSS0.149201102 Aug 2011GNSS0.150201117 Sep 2011Orthophoto0.127201212 Sep 2012GNSS0.160201312 Aug 2013GNSS0.151201409 Sep 2014GNSS0.101201511 Sep 2015GNSS0.186*
* Seasonal snow remaining along the extent.
A thermistor cable was installed in a 10 m deep borehole in 2009 to record
ice temperatures. Temperatures were recorded every 3 h until late
September 2011 with an accuracy of 0.05 ∘C (1 standard deviation).
The entire thermistor cable melted out in September 2014. Additional
thermistor measurements were made in the snow and ice at the onset of thaw in
spring 2010.
Radiocarbon dating
In May 2010, a 30 m long ice tunnel was excavated in the Juvfonne ice patch.
During spring 2012, a new 70 m long tunnel was excavated into the central
parts of the ice patch. The tunnels were excavated with specially designed
ice axes, causing minimal disturbance to the surrounding ice. The tunnels gave
an excellent opportunity to verify the radar data and to collect organic
material and ice for radiocarbon dating. Dateable organic material is
available, but there are no continuous layers of organic material.
Radiocarbons dating prior to 2012 are published in Nesje et al., 2012; Zapf
et al., 2013 and Ødegård et al., 2011. Conventional 14C ages were
calibrated using OxCal v4.2.4 software (Bronk Ramsey and Lee, 2013) with the
IntCal13 calibration curve (Reimer et al., 2013).
AMS radiocarbon dates from the ice tunnels (clear ice samples and
organic remains) and ice samples from the ice patch surface. Ice samples
collected as blocks and subdivided in several sub-samples. Therefore, an
average value is shown for every block (JUV1, JUV2 and JUV3), except for JUV0
because JUV0_1 and JUV0_2 were taken adjacent to the plant fragment layer,
dated 6600 cal BP (Poz-56955), while samples from JUV0_3 to JUV0_8 were
collected at the bottom of the wall, a few centimetres below the plant fragment layer.
Thus, JUV0_A is the yielded average of JUV0_1 and JUV0_2, while the other
six samples were averaged as JUV0_B. Calibrated ages (cal BP) denote the 1
σ range.
The organic debris was collected from the walls, below the floor of
the ice tunnels (five samples from the tunnel excavated in 2010 and five samples
from the tunnel excavated in 2012, Table 2) and from organic debris melting out at
the front, of which two datings are reported in this paper. Nine additional
datings were published by Nesje et al. (2012).
The recently developed method for radiocarbon dating of ice utilizes the
organic carbon fraction of carbonaceous aerosols scavenged from the
atmosphere during snowfall and embedded into the ice matrix (Jenk et al.,
2009; Sigl et al., 2009; Uglietti et al., 2016). This method was tested with
11 samples from Juvfonne in 2011 by comparing for the first time 14C
ages determined from carbonaceous particles with 14C ages conventionally
obtained from organic remains found in the ice (Zapf et al., 2013). The 2011
samples are JUV1 and JUV2, adjacent to the dated organic-rich layers in the
2010 tunnel, and a surface sample JUV3 (Table 2). In summer 2015 five samples
of clear ice were collected adjacent to the plant fragment layer located just
above the bed in the tunnel excavated in 2012 (JUV0, Table 2). All blocks of
ice (∼20×15×10 cm) were extracted with a pre-cleaned
chainsaw and were subsequently divided into smaller pieces. All ice blocks
were transported frozen to the Paul Scherrer Institute (PSI, Switzerland),
decontaminated in a cold room by removing the outer layer (0.3 mm) with a
pre-cleaned stainless steel band saw and by rinsing the ice samples with
ultra-pure water in a class 100 clean room (Jenk et al., 2007).
Insoluble carbonaceous particles were filtered onto preheated quartz fibre
filters (Pallflex Tissuquartz, 2500QAO-UP) and combusted with a
thermo-optical organic carbon–elemental carbon (OC–EC) analyser
(Model4L, Sunset Laboratory Inc., USA), using a well-established protocol
(Swiss_4S) for OC–EC separation (Zhang et al., 2012). Analyses of
14C were conducted using the 200 kV compact radiocarbon system
“MICADAS” at the University of Bern (LARA laboratory), equipped with a gas
ion source coupled to the Sunset instrument, allowing direct measurement of 14C
in CO2 of 3–100 µg C, with an uncertainty level as
low as 1 % (Ruff et al., 2010). Dates are given in calibrated ages BP
(BP = 1950 CE) including 1 sigma errors (σ).
Example of 250 MHz georadar profile. The position of the track is
shown in Fig. 3b. The arrow shows the approximate minimum front position in
September 2014.
ResultsIce thickness and ice layering
The bed reflection was clearly seen in the radar plots (see example in
Fig. 4). In addition, the ice layering was detected on most of the plots,
probably due to density differences in the ice layers (air bubbles) (Hamran
et al., 2009) or organic layers. Georadar soundings from 2009 revealed a
maximum ice thickness of 17–19 m (Ødegård et al., 2011). The
near-surface reflection horizons are nearly parallel to the present surface.
At depth, curved reflection horizons are observed. In the tunnels the curved
layers can be directly observed, forming a distinct angular discontinuity with
the surface-parallel ice layers (Fig. 5). The surface-parallel layers have
melted away since 2009 in the central and southern parts of the ice patch
(Fig. 6). The DTM obtained from laser scanning combined with the bottom
topography from the georadar gave a volume of 710 000 m3 in late
August 2011 (mean thickness 5.6 m). The surface of Juvfonne in
September 2011 was the reference surface for the depth map (Fig. 3b). The
maximum depth was 16 m close to the inner part of ice tunnel excavated in
2012. In this area the surface slope is about 18∘.
Photo of angular discontinuity at the wall of the 2010 ice tunnel,
as also observed from the georadar data (Fig. 4). The upper layering is
parallel to the surface of Juvfonne. Radiocarbon dating of the upper part
showed modern age. Width of the picture is approximately 0.4 m.
Photos of Juvfonne on 17 September 2014 (a) and
10 September 2014 (b) showing the pre-LIA surface exposed in the central
and southern parts of the ice patch (left side). The area on Juvfonne in the
north-west (right side) is interpreted to be ice of modern age. The entrance
of the ice tunnel sits on a small ridge that might be ice cored (left
side, lower image). The collapsed 2010 tunnel is to the left of the entrance.
Photo: Glacier Archaeology Program, Oppland County Council (upper) and
L. M. Andreassen (lower).
Summer (a) and winter (b) balance plotted against
summer temperature (positive-degree days) and estimated precipitation as
snow respectively. For the summer balance, the black markers are calculated
melt using a degree-day model with typical values calibrated from nearby
glaciers (3.5 mm ∘C-1 day-1 for snow and
7.5 mm ∘C-1 day-1 for ice). Winter precipitation is
obtained from seNorge (Engeset et al., 2004).
Mass balance, front changes and areal extent
Only one of the mass balance stakes (J2) existed continuously from spring
2010 to spring 2015 (Figs. 7 and 8). Stake J2 is in the central part of the
ice patch (Fig. 3a).
Snow sounding measurements (N= 232) range from 0.6 to 4.8 m over the
period 2010–2015. Mean snow depth is 2.6 m (1.2 m w.e.). Some years show
a pattern where most snow accumulates on the leeward side of the prevailing
wind the previous winter, but this is not consistent. Interannual variation
accounts for 66 %. The accumulation was further investigated by analysing
the deviation from mean each year. This dataset contains a significant trend
with increased accumulation towards the front (Fig. 3c). The difference
between the upper central area and the front is 0.2 m w.e. (Fig. 3c), which
corresponds to an approx. 20 % increase in accumulation.
Mass balance measurements at stake J2 on Juvfonne: bw – balance
winter, bs – balance summer, ba – annual (net) balance (Fig. 3a for
position of stake).
Front position of Juvfonne measured at two locations relative to the
2010 front. Minima are observed in 2011 and 2014. The front retreat
2009–2014 was measured to 69 m. For position of measurements, see Fig. 3a.
Red – JF1, blue – JF2.
The total mass loss is measured to 10 m of ice at the site of the thermistor
measurements (Fig. 3a). The 10 m thermistor cable installed on
29 October 2009 melted out in mid-September 2014. The total mass loss at
stake J2 was 10.5 m w.e. during the same period. Elevation changes from
September 2011 to September 2014 are shown in Fig. 3d. These results are
based on the laser scanning in 2011 and differential GNSS tracking in 2014.
The measurements show a highly significant asymmetric pattern with close-to-zero surface elevation changes in the western part and surface lowering of
3–5 m in the eastern and central parts of the ice patch. This strong
gradient is measured over a distance of just 200 m at approximately the same
altitude. The part with the most negative change has more than average
accumulation.
Front change measurements started in 2009 at JF1 and in 2010 at JF2 (Fig. 9).
The measurements revealed that Juvfonne retreated in all years except in 2012
and 2015 where the ice patch increased its size due to excessive snow that
formed a thin ice and snow layer around the margin. The total retreat
2009–2014 is -52 m measured from JF1, and during 2010–2014 the mean change
is 44 m (-51 m from JF1 and -38 m from JF2).
The annual extent measurements (2010–2015) show area fluctuations of the
margin, varying from 0.101 km2 (9 September 2014) to a maximum of
0.186 km2 on 11 September 2015 (Table 1). The extent measurements show
that the ice patch shrinks and grows along the whole margin. Furthermore,
field observations show that the ice is very thin along the margins. In 2015,
seasonal snow covered the entire margin, and the measured area of
0.186 km2 is thus only to be considered a maximum extent, not the
actual ice patch area.
Meteorological data from the station at Juvvasshøe (750 m from
the front of Juvfonne) and Fokstugu 70 km NE (a) Juvvasshøe
June–September mean air temperature. The black dotted line denotes the
1971–2000 mean, obtained from the interpolated seNorge dataset (Engeset et
al. 2004). (b) Number of days for the period June–September with
strong breeze or higher (wind speed above 10.8 m s-1) at Juvvasshøe
(grey bars) and at Fokstugu (black line), the latter shown as anomaly (in
%, right axes) with respect to 1971–2000 mean.
Climate parameters
Air temperature and wind speed at Juvvasshøe for the period 2000–2015 are
outlined in Fig. 10a–b over the ablation season (June–September). The mean
June–September air temperature in this period is 3.2 ∘C
(1.0 ∘C above the 1971–2000 mean). Air temperatures, near-ground
surface temperatures and frequency of days with daily mean air temperature
above 0 ∘C (the two latter are not shown in Fig. 10) are high in
summers 2002, 2003, 2006, 2011 and 2014, and especially 2006. Observations
from nearby weather stations with long climate series reported
record-breaking temperatures in late summer and autumn 2006. In the
investigation period 2009–2015 the coldest summer was 2012, which was the
only summer below the 1971–2000 mean (Fig. 10).
Relative frequency (as percentage of all hourly observations) of
strong gale or more (≥ 20.8 m s-1) at Juvvasshøe during
winter (Oct–Apr) 2009–2015 for the wind sectors SE to NW. The values
inserted show the total frequency of strong gale or more.
Due to the sheltered setting of Juvfonne compared to the meteorological
stations, strong breeze (wind speed above 10.8 m s-1) was used as a
lower limit to obtain sufficiently high wind speeds for effective and enhanced
turbulent fluxes at Juvfonne. In general there was a high frequency
(35–58 days per season) of strong breeze during the period 2009–2015
(Fig. 10b). Comparing wind data from the AWS at Fokstugu indicates 2 to
3 times more frequent strong wind mean during the investigation period
than 1971–2000. Observed incoming short- and long-wave radiation from
Juvvasshøe (not shown) show no clear patterns related to single summers,
but 2011 stands out as the summer with the greatest incoming long-wave radiation.
For snow accumulation or abrasion on ice patches, wind speed and wind
direction is crucial (Dadic et al., 2010; Lehning et al., 2008). There are
great variations from year to year in respect to frequency of strong gale and
wind direction. During the two stormiest winters 2011–2012 and 2013–2014, the
frequency of strong gale was 15.7 and 17.3 % respectively (Fig. 11).
Snow measurements and modelling
The automatic snow depth observations in front of Juvfonne show great hourly
to daily variability and there is a distinctly different pattern of snow
accumulation between the four winter seasons (Fig. 12). The greatest increase
in snow depth during early and mid-winter in all years is related to storm
events. This is also the case for strong snow depth decrease events (mainly
due to wind scouring). Comparing the observed and estimated snow depths
(which do not take into account redistribution of snow by wind), it is clear
that much of the accumulation is not correlated with precipitation (Fig. 12).
The snow depth for Juvfonne was obtained from a precipitation and degree-day
model operating on 1×1 km2 developed for a web-based system
(http://senorge.no/) for producing daily snow maps for Norway (Engeset
et al., 2004; Saloranta, 2012). A similar poor correlation (r2=0.24) is
also found for very small glaciers in the Alps (Huss and Fischer, 2016).
Hourly snow depth measurements (black lines) from the station 95 m
from the front of Juvfonne (see Fig. 3a for position). Grey lines show
modelled daily snow depth from seNorge (Engeset et al. 2004).
The observed melt in central parts (J2) was compared with a degree-day model
using typical values calculated from nearby glaciers (Fig. 7a) (Laumann and
Reeh, 1993). This modelling shows a quite good fit, except for the 2010 season. In
this season, the summer balance was about twice the outcome of the degree-day
model.
Temperature of ice and permafrost
Temperature measurements in Juvfonne reveal 10 m depth ice temperature in
the range of -2 to -4∘C (Fig. 13). The ice and snow temperature
results show that the Juvfonne ice patch is cold-based and underlain by
permafrost. The measurements at 5–10 m depth in the ice are similar to the
measurements in the nearby permafrost borehole at Juvvasshøe (Fig. 13). In
spring, the meltwater percolates and refreezes in the snowpack until the
snow is isothermal at a temperature close to 0 ∘C (Fig. 14). There
is cold ice below the level of meltwater percolation, which means that there
is a heat flow into the ice that gradually decreases during the melt season.
Because of this heat flow, superimposed ice, generally less than
0.1 m year-1, forms at the level of impermeable ice.
Radiocarbon dating
The AMS radiocarbon dating obtained from organic-rich layers and from
carbonaceous aerosols embedded in clear ice in the Juvfonne ice patch shows
ages spanning from modern at the surface to ca. 7600 cal years BP at the
bottom (clear ice below the basal organic-rich layer), thus showing that
Juvfonne has existed continuously during the last ∼ 7500 years. So far,
the basal ice in Juvfonne is the oldest dated ice in mainland Norway
(Table 2).
Temperature for November 2009–September 2011 in a 10 m deep
borehole in the Juvfonne ice patch (see Fig. 3a for position). The red line
is the temperature at 10 m depth in the P31 permafrost borehole 750 m north
from the ice patch (see Fig. 2 for location). Arrow points to the time when
the sensor placed at 3 m depth in autumn 2009 melted out. The entire
thermistor string melted out in mid-September 2014.
Plot of temperature measurements in ice and snow at the onset of
thaw in May 2010 (position at the thermistor shown in Fig. 3a). The depth
reference is the ice surface the previous autumn. The red line is the snow
temperature 0.25 m from the base of the snow cover. The arrow points to the
first signal of surface meltwater refreezing close to the base of the snow
cover.
In the tunnel opened in 2010 the AMS radiocarbon dating of organic matter
embedded in the ice shows modern age in the top layer at the entrance and
ages ranging from 3065–3174 to 963–1052 cal years BP inside the tunnel.
These results were previously published in Nesje et al. (2012) and
recalibrated for this study (Table 2 and Fig. 15).
Photo from the old ice tunnel excavated in 2010 showing the layering
in the ice and position of two samples for radiocarbon dating. Photo:
Klimapark2469 AS.
In the tunnel opened in 2012 the AMS radiocarbon dating of five organic
layers embedded in the ice about 70 m from the margin of the ice patch,
yielded dates in chronological order from the base upwards, ranging from
6561–6656 cal years BP at the base to 1929–2002 cal years BP in the
ceiling of the ice tunnel, approximately 2.8 m above the tunnel floor. The
organic debris that yielded the oldest age was collected from the innermost
part of the ice tunnel, about 0.4 m above the bed. The layer where the
sample was retrieved could be followed close to the bed in the inner parts of
the tunnel. The carbon dates on carbonaceous aerosols were sampled at the
same location to the side and below the plant fragment layer. The oldest
dating is 7476–7785 cal years BP. The position of the sample site in the
2012 tunnel is marked in Fig. 3a.
In autumn 2014, two in situ Polytrichum moss mats melted out
along the margin of Juvfonne south of the ice tunnel excavated in 2010. AMS
radiocarbon dates of the two moss mats indicate that the moss was killed by
the expanding margin of the ice patch about 2000 years ago
(1951–1896 cal years BP – Poz-66166 and 1945–1882 cal years BP –
Poz-66167). Thus, the minimum extent of the south-eastern part of the ice
patch observed in September 2014 is most likely the smallest in 2000 years.
With the exception of one identified outlier, the results obtained from
dating of carbonaceous aerosol particles in the ice could reproduce the
expected ages very well (Zapf et al., 2013). This gives confidence that the
age of organic debris in the ice is similar to the surrounding ice. In
Fig. 16 radiocarbon datings from both ice tunnels are plotted according to
vertical distance from bed.
Plot of the samples in Table 2 except samples with modern age. In
the inner parts of the 2012 tunnel the bed is partly exposed, which gives
good distance to bed estimates. In the 2010 tunnel, the distance estimates
depend on the radar data (the old tunnel partly melted out). The horizontal
distance between the samples is up to 50 m.
Discussion
The discussion focuses on the value of this research in the context of the
long-term objective to develop models of mass balance and thermal regime on a
Holocene timescale at ice patches and surrounding terrain.
The discussion is organized in four sections: (1) the mass balance,
(2) thermal regime, (3) ice layering and deformation on Holocene timescale,
and (4) the environmental processes relevant to artefact displacement and
preservation.
The mass balance
Perennial ice patches are, due to their existence, located at sites with
close to long-term zero mass balance. The interannual variability in summer
and winter balance could be considerable, but the long-term changes in mass
must be close to zero as long as they do not disappear or develop into a
glacier. The 6-year record of mass balance gives some insight into the
spatial and temporal variability of the mass balance.
The snow accumulation during the 6-year period (2010–2015) shows increased
accumulation towards the front of the ice patch. This is probably a response
to increased melt, which will increase the snow accumulation at the leeward
side of prevailing westerly winds.
Along the outer rim of Juvfonne, the surface elevation changes (negative mass
balance) vary between less than 1 m and nearly 5 m within a 200 m distance
at the same altitude over a period of 3 years (Fig. 3d). Field data are consistent
with the interpretation of increased melting due to sensible and latent heat
fluxes. Micrometeorological investigations by Mott et al. (2011) of
processes driving snow ablation in an alpine catchment show that advection of
sensible heat causes locally increased ablation rates at the upwind edges of
snow patches.
The 2010 anomaly in the summer balance (Fig. 7a) is most likely related to
increased melt during periods with strong south and south-easterly winds
(unsheltered direction for Juvfonne), combined with relatively high air
temperatures and high relative humidity, causing enhanced turbulent fluxes.
Extreme melt was observed in early to mid-August. The warmest 10-day period in
2010 was 8–18 August. Median wind speed was 3.4 m s-1 from the SE and
humidity was 79.5 % at the meteorological station 750 m from the ice patch.
This 2010 anomaly is probably the reason for the asymmetric net balance of
Juvfonne (Fig. 6). Exceptionally large melt episodes have been reported from
the Central Cascade Mountains of Oregon, where snow melt was enhanced by
strong wind, high air temperature and high humidity (Marks et al., 1998). At
higher unsheltered sites, 60–90 % of the energy for snowmelt came from
sensible and latent heat exchanges, while it was only about 35 % at more
sheltered sites. Recently, similar extreme melt events were reported from
the southern and western parts of the Greenland ice sheet in July 2012, where
non-radiative energy fluxes dominated the ablation area surface energy budget
during multi-day episodes (Fausto et al., 2016).
The snow recording from the station in front of Juvfonne (95 m from the
front) clearly illustrates the complexity of snow accumulation in this
environment. In front of Juvfonne, abrupt changes in snow depth within hours
dominate the series, causing great day-to-day variability. These changes seem
to be mainly driven by wind speed and wind direction. Single storm events
with westerly winds could account for almost 50 % of the winter
accumulation in less than 24 h, like the storm on 7–8 February 2015
(Fig. 12, 2014–2015). Spring snow accumulation with insignificant wind drift
could also influence mass balance, like the period from early April to mid-May 2012, when more than 40 cm of snow accumulated (Fig. 12, 2011–2012).
Ground and ice thermal regime
The temperature measurements at Juvfonne show that there is sufficient meltwater to bring the permeable snowpack to an isothermal condition within a few
weeks in early summer (Fig. 13). Below the seasonal snowpack, the ice remains
cold during the summer, with temperatures in the range of -2 to
-4∘C at 5–10 m depth (Fig. 13). In Norway most glaciers are
considered temperate, although measurements are available for only a
few glaciers (Andreassen and Winsvold, 2012). Recent observations from nearby
glaciers in Jotunheimen, reveal that at the lower parts of Storbreen the
winter cold wave was removed during summer, but remained at Hellstugubreen and
Gråsubreen (Sørdal, 2013; Tachon, 2015). The temperatures measured
close to the equilibrium line at Hellstugubreen (-1∘C) and
Gråsubreen (-2∘C) were warmer than the temperature measured
at similar depths at Juvfonne (-3∘C).
Juvfonne consists of cold ice surrounded by permafrost terrain (Fig. 13).
Perennial ice patches can be used as indicators of local (mountain)
permafrost conditions (Imhof, 1996; Kneisel, 1998). The physical background
is that their ice cannot warm above 0 ∘C in summer but cool down
far below 0 ∘C during the cold season. Based on this argument, there
is good reason to suggest that long-term perennial ice patches like Juvfonne
indicate permafrost directly beneath them. Holocene permafrost modelling
(Lilleøren et al., 2012) suggests that permafrost survived the highest
areas of the Scandinavian mountains during the Holocene thermal maximum
(HTM), and thus permafrost ice could be of Pleistocene age. Radiocarbon dates
from Juvfonne show that the deepest central part of the ice patch contains
carbonaceous particles embedded in the ice 7476–7785 cal years BP
(JUV0_B, Table 2). This is a strong indication that Juvfonne has existed
continuously since mid-Holocene, and the dating of the ice could offer
testing data for Holocene permafrost models. Juvfonne could contain older
ice, and it is most likely that ice patches at higher elevation contain
older ice.
Ice layering and deformation on a Holocene timescale
The observed ice layers almost certainly represent surfaces of isochronic
deposition. Within both ice tunnels in Juvfonne there are several
organic and/or debris layers of uncertain origin. From the appearance of these
layers, it is probably wind- or water-transported material or reindeer
droppings. The organic layers are horizontally continuous over a few metres.
There is reasonable correlation between the age of the clear ice and the age
of the organic layers (Zapf et al., 2013). Contamination is not
likely in the clear ice samples, which gives confidence in the dating of the
ice stratigraphy. This is not necessarily the case at other ice patches,
where surface processes or microbial activity may contaminate organic
material exposed at the surface.
The ice deformation on a Holocene timescale is difficult to calculate based on
the available data. In the central parts of the ice patch, an estimate of
maximum basal shear stress is in the range of 30–40 kPa (surface slope
17∘, depth 12–16 m, laminar flow). Adding 5 m to the depth will
increase the basal shear stress to 45–55 kPa for the central part. The
latter is probably close to the range for the last decades. Calculation of
deformation based on a Glen-type flow law will be highly sensitive to the
chosen stress exponent (Glen, 1955). Using a softness parameter A=2.4×10-15 s-1 kPa-3 based on an ice temperature of
-2∘C from Table 5.2 in Paterson (1994) and a stress exponent of
n=2 (Duval et al., 2000) gives a surface velocity of
2.3 m 1000 years-1 (surface slope 17∘, depth 19 m, laminar
flow). A likely situation for the LIA (surface slope 15∘, depth
45–60 m) gives an estimate of 25–60 m 1000 years-1, assuming a cold-based glacier (Fig. 17). These calculations have high uncertainty but
suggest that a cumulative deformation of ice over millennia could explain the
observed curved layering in the basal layer of the ice patch (Fig. 4). The
possibility of cumulative ice deformation on a timescale of several
millennia makes it difficult to relate the present thickness and slope of
these layers to previous thicknesses of the ice patch.
Picture taken from Vesljuvbrea towards the NNW showing Juvfonne from
around 1900. The surface slope of Juvfonne is approximately 15∘.
Height and length estimate from map based on position in the picture. The
upper and northern part of Juvfonne cannot be seen in the picture.
Artefact displacement and preservation
From a cultural management perspective, there is particular interest in
developing methods to identify sites of interest (Rogers et al.,
2014) and a better understanding of the environmental threats (Callanan,
2015). The environmental threats are mainly related to subaerial exposure of
artefacts. Especially leather artefacts, textiles and steering feathers of
arrows are exposed to movement and decomposition only a short time after melt-out.
Wooden objects are more resistant.
The artefacts found at Juvfonne are in permafrost terrain surrounding the ice
patch; most of them are found in the front of the ice patch within a few tens
of metres. The wooden artefacts range from 250 to 900 CE. Even during the
extreme minimum in September 2014 (Fig. 6), there were no observations of
artefacts melting out from the ice.
The exposure time to subaerial processes and microbial activity is critical
to artefact decomposition. At Juvfonne, there is a gradual increase in the
ground exposure time depending on snow accumulation and melt over millennia.
The oldest ice found so far is 7476–7785 cal years BP (JUV0_B, Table 2).
At the eastern edge, AMS radiocarbon dates show that the moss mats were
covered (killed) by the expanding snowfield about 2000 years ago (Table 2,
Poz-56952). Lichenometry indicates that the front of Juvfonne extended
∼ 250 m from its present position during the LIA maximum in the
mid-18th century (Nesje et al., 2012). A photo of Juvfonne from around 1900
shows the front close to the expected LIA extent (Fig. 17). These results
constrain the extent of the ice patch since the mid-Holocene, but temporal
and spatial variability need to be considered to assess the actual exposure
time of artefacts.
Several radiocarbon dates of the top layer in 2010 (Table 2 and Fig. 4) show
modern age. This means that artefacts found outside the 2010 extent were
subaerially exposed after the LIA but prior to 2010. Thus, the dating and
position of artefacts cannot be used directly to reconstruct previous ice
patch extent.
Juvfonne and surrounding terrain is an active environment in terms of
geomorphological processes. In particular, during the extreme melting in
autumn 2014 several small accumulations of organic material and/or debris occurred
at the upper margin of the ice patch. Within a few days, meltwater moved
this material to the front of the ice patch. Downslope movement of artefacts
by meltwater is certainly possible at Juvfonne. Finds at other ice patches
in Jotunheimen support this interpretation, where different pieces of the
same artefact were found along the direction of the steepest slope. Textiles and
leather objects are more likely transported by wind, and preservation at its
original position is less likely. There are no finds of textiles or leather
objects at Juvfonne.
Conclusions and future perspectives
The exploratory analyses of field data from Juvfonne show the geoscience
research potential of ice patches in Scandinavia. The results give new
insights into their age, internal structure, mass balance and climate
sensitivity and have taken the state of knowledge to a level where models
can be designed.
These are the main conclusions from the analysis of field data:
Ice stratigraphic characteristics and radiocarbon dating strongly suggest
that the Juvfonne ice patch was small or absent during the Holocene thermal
maximum, but it has existed continuously since ca. 7600 cal years BP (the late
Mesolithic period) without disappearing. This is the oldest dating of ice in
mainland Norway.
Geophysical investigations show a clear stratification. The observed ice
layers almost certainly represent surfaces of isochronic deposition. Several
radiocarbon dates of the top layer (parallel to the surface) show modern age.
At depth, curved reflection horizons are observed consistent with cumulative
ice deformation over millennia.
A 6-year record of mass balance measurements shows a strongly negative
balance. The total mass loss at one site was 10.5 m w.e. Elevation changes
are highly asymmetric over short distances, from close to zero to a surface
lowering of several metres. There is a significant increase in snow
accumulation towards the front of approx. 20 % compared to the upper
central area. The winter balance is poorly correlated with winter
precipitation. One single storm event may contribute significantly to the
winter balance.
Temperature measurements of the ice in Juvfonne reveal colder ice than what
is found at similar depths close to the equilibrium line of nearby
polythermal glaciers. There is sufficient meltwater to bring the permeable
snowpack to an isothermal state within a few weeks in early summer. Below the
seasonal snowpack, at 5–10 m depth, the ice remains cold, with temperatures
between -2 and -4∘C. The cold ice is surrounded by permafrost
terrain that has similar ground temperatures.
Ice deformation and surface processes (i.e. wind and meltwater) may have
caused significant displacement of artefacts from their original position.
Since the surface ice shows modern age, artefacts melted out in front of
Juvfonne during the last decades must have been subaerially exposed on at
least one previous occasion after the LIA.
The radiocarbon datings show that Juvfonne is robust to climate change, even
on a Holocene timescale. The datings indicate a slow buildup over a period
of 8000 years. The survival of relatively thin ice over a long period is a
good documentation of the mass balance feedback mechanisms of ice patches.
The datings of moss mats appearing at the south-eastern edge of Juvfonne in
September 2014 suggest that it was the smallest ice patch in ∼ 2000 years. These
field data constrain the Holocene development of Juvfonne, but care should be
taken in the interpretation. Radiocarbon datings of the ice layers only show
the timing of minima in volume.
The possibility of cumulative deformation of ice on a Holocene timescale
makes it difficult to relate the present thicknesses and slope of these layers
to previous thickness of the ice patch. Maximum ice volume was reached
during LIA, when Juvfonne probably developed into a cold-based glacier with
significant internal deformation.
Perennial ice patches are, due to their existence, areas with close-to-zero
long-term mass balance similar to the zone close to the ELA of glaciers.
However, there are obvious differences between ice patches and glaciers. The
accumulation processes are to a variable degree dependent on surrounding
topography and the topography of the ice patch itself. One possible future
approach is field observations in combination with simulations of the wind
field to obtain the necessary spatial and temporal resolution to model the
snow accumulation during storm events.
The wind field with high spatial and temporal resolution is also needed to
calculate the turbulent fluxes. The melt anomaly in 2010 was probably related
to periods of strong south-easterly winds, high air temperatures and high
relative moisture boosting the turbulent fluxes at the upwind edge. The time
series of mass balance at Juvfonne is too short to study the long-term
effect of melt anomalies.
Data availability
The ice thickness and point mass balance data of Juvfonne were submitted to
the World Glacier Monitoring Service (WGMS) to their Glacier Thickness
Database (GlaThiDa) and Fluctuations of Glaciers Database (FoG). The snow
accumulation data are included in the supplement. Meteorological data for
the Juvvasshøe (15270) and Fokstugu (16610) stations are available for free
download from the climate database of the Norwegian Meteorological Institute,
eKlima (http://eklima.met.no/, Norwegian Meteorological Institute,
2016).
Acknowledgements
We thank the archaeologists Lars Pilø and
Espen Finstad for valuable comments and discussions related to artefact
displacements and Dag Inge Bakke at Mimisbrunnr Klimapark 2469 for support
in the field. Professor Emeritus Wilfried Haeberli and Professor Bernd
Etzelmüller gave useful comments on an earlier version of the manuscript
and are gratefully acknowledged. We thank the two anonymous reviewers for
precise feedback, which greatly improved the paper.
Edited by: S. Gruber
Reviewed by: two anonymous referees
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