TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-11-1733-2017Experimental observation of transient δ18O interaction between
snow and advective airflow under various temperature gradient conditionsEbnerPirmin PhilippSteen-LarsenHans ChristianStenniBarbarahttps://orcid.org/0000-0003-4950-3664SchneebeliMartinschneebeli@slf.chhttps://orcid.org/0000-0003-2872-4409SteinfeldAldoWSL Institute for Snow and Avalanche Research SLF, 7260 Davos Dorf, SwitzerlandLSCE Laboratoire des Sciences du Climat et de l'Environnement, Gif-Sur-Yvette CEDEX, FranceCenter for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, DenmarkDepartment of Environmental Sciences, Informatics and Statistics, University Ca' Foscari of Venice, Venice, ItalyDepartment of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, SwitzerlandMartin Schneebeli (schneebeli@slf.ch)25July2017114173317439February201720February201731May20179June2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://tc.copernicus.org/articles/11/1733/2017/tc-11-1733-2017.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/11/1733/2017/tc-11-1733-2017.pdf
Stable water isotopes (δ18O) obtained from snow and
ice samples of polar regions are used to reconstruct past climate
variability, but heat and mass transport processes can affect the
isotopic composition. Here we present an experimental study on the
effect of airflow on the snow isotopic composition through a snow
pack in controlled laboratory conditions. The influence of
isothermal and controlled temperature gradient conditions on the
δ18O content in the snow and interstitial water vapour
is elucidated. The observed disequilibrium between snow and vapour
isotopes led to the exchange of isotopes between snow and vapour
under non-equilibrium processes, significantly changing the
δ18O content of the snow. The type of metamorphism
of the snow had a significant influence on this process. These
findings are pertinent to the interpretation of the records of
stable isotopes of water from ice cores. These laboratory
measurements suggest that a highly resolved climate history is
relevant for the interpretation of the snow isotopic composition in
the field.
Introduction
Water stable isotopes in polar snow and ice have been used for
several decades as proxies for global and local temperatures
(e.g. Dansgaard, 1964; Lorius et al., 1979; Grootes et al., 1994;
Petit et al., 1999; Johnsen et al., 2001; EPICA Members,
2004). However, the processes that influence the isotopic
composition of precipitation at high latitudes are complex, making
direct inference of palaeotemperatures from the isotopic record
difficult (Cuffey et al., 1994; Jouzel et al., 1997, 2003;
Hendricks et al., 2000). Several factors affect the vapour and
snow isotopic composition, which give rise to ice-core isotopic
composition, starting from the process of evaporation in the
source region, transportation of the air mass to the top of the
ice sheet and post-depositional processes (Craig and Gordon,
1964; Merlivat and Jouzel, 1979; Johnsen et al., 2001; Ciais and
Jouzel, 1994; Jouzel and Merlivat, 1984; Jouzel et al., 2003;
Helsen et al., 2005, 2006, 2007; Cuffey and Steig, 1998; Krinner
and Werner, 2003). Mechanical processes such as mixing, seasonal
scouring or spatial redistribution of snow can alter seasonal
and annual records (Fisher et al., 1983; Hoshina et al.,
2014). Post-depositional processes associated with wind scouring
and snow redistribution are known to introduce a
“post-depositional noise” in the surface snow. Comparisons of
isotopic records obtained from closely located shallow ice cores
have allowed for estimations of a signal-to-noise ratio and
a common climate signal (Fisher and Koerner, 1988, 1994; White
et al., 1997; Steen-Larsen et al., 2011; Sjolte et al., 2011;
Masson-Delmotte et al., 2015). After deposition, interstitial
diffusion in the firn and ice affects the water-isotopic signal
but back-diffusion or deconvolution techniques have been used to
establish the original isotope signal (Johnsen, 1977; Johnsen
et al., 2000).
Snow is a bicontinuous material consisting of fully connected
ice crystals and pore space (air) (Löwe et al.,
2011). Because of the proximity to the melting point, the high
vapour pressure causes a continuous recrystallization of the snow
microstructure known as snow metamorphism, even under moderate
temperature gradients (Pinzer et al., 2012). The whole ice matrix
is continuously recrystallizing by sublimation and deposition,
with vapour diffusion as the dominant transport process. Pinzer
et al. (2012) showed that a typical half-life of the ice matrix
is a few days. The intensity of the recrystallization is dictated
by the temperature gradient and this can occur under midlatitude
or polar conditions. Temperature and geometrical factors
(porosity and specific surface area) also play a significant role
(Pinzer and Schneebeli, 2009; Pinzer et al., 2012).
The interpretation of ice-core data and the comparison with
atmospheric model results implicitly rely on the assumption that
the snowfall precipitation signal is preserved in the snow–ice
matrix (Werner et al., 2011). Classically, ice-core
stable-isotope records are interpreted as reflecting
precipitation-weighted signals and compared to observations and
atmospheric model results for precipitation, ignoring
exchanges between surface snow and atmospheric water vapour
(e.g. Persson et al., 2011). However, recent studies carried out
on top of the Greenland and Antarctic ice sheets combining
continuous atmospheric water-vapour-isotope observations with
daily snow surface sampling document a clear day-to-day variation
of isotopic composition of surface snow between precipitation
events as well as diurnal change in the snow isotopes
(Steen-Larsen et al., 2014a; Ritter et al., 2016; Casado et al.,
2016). This effect was interpreted as being caused by the uptake
of the synoptic-driven atmospheric water-vapour-isotope signal by
individual snow crystals undergoing snow metamorphism
(Steen-Larsen et al., 2014a) and the diurnal variation in
moisture flux (Ritter et al., 2016). However, the impact of this
process on the isotope-temperature reconstruction is not yet
sufficiently understood, but crucial to constrain. This process,
compared to interstitial diffusion (Johnsen, 1977; Johnsen
et al., 2000), will alter the isotope mean value. The field
observations challenge the previous assumption that sublimation
occurred layer-by-layer with no resulting isotopic
fractionation (Dansgaard, 1964; Friedman et al., 1991; Town
et al., 2008; Neumann and Waddington, 2004). It is assumed that
the solid undergoing sublimation would not be unduly enriched in
the heavier isotope species due to the preferential loss of
lighter isotopic species to the vapour (Dansgaard, 1964; Friedman
et al., 1991). Because self-diffusion in the ice is about 3
orders of magnitude slower than molecular diffusion in the vapour,
the amount of isotopic separation in snow is assumed to be
negligible.
Snow has a high permeability (Calonne et al., 2012; Zermatten
et al., 2014), which facilitates diffusion of gases and, under
appropriate conditions, airflow (Gjessing, 1977; Colbeck, 1989;
Sturm and Johnson, 1991; Waddington et al., 1996). In a typical
Antarctic and Greenland snow profile, strong interactions between
the atmosphere and snow occur, especially in the first
2 m (Neumann and Waddington, 2004; Town et al., 2008),
called the convective zone. In the convective zone, air can move
relatively freely and therefore exchange occurs between snow and the
atmospheric air. Air flowing into the snow reaches
saturation vapour pressure nearly instantly through sublimation
(Neumann et al., 2008; Ebner et al., 2015a). Models of the
influence of the so-called “wind pumping” effect (Fisher
et al., 1983; Neumann and Waddington, 2004), in which the
interstitial water vapour is replaced by atmospheric air pushed
through the upper metres of the snow pack by small-scale high and
low pressure areas caused by irregular grooves or ridges formed
on the snow surface (dunes and sastrugi), have assumed that the
snow grains would equilibrate with the interstitial water vapour
on timescales governed by ice self-diffusion. However, no
experimental data are available to support this assumption. With
this in mind the experimental study presented here is
specifically developed to investigate the effect of ventilation
inside the snow pack on the isotopic composition. Only conditions
deeper than 1 cm inside a snowpack are
considered. Previous work showed that (1) under isothermal
conditions, the Kelvin effect leads to a saturation of the pore
space in the snow but does not affect the structural change
(Ebner et al., 2015a), (2) applying a negative temperature
gradient along the flow direction leads to a change in the
microstructure due to deposition of water molecules on the ice
matrix (Ebner et al., 2015b), and (3) a positive temperature
gradient along the flow had a negligible total mass change of the
ice but a strong reposition effect of water molecules on the ice
grains (Ebner et al., 2016). Here, we continuously measured the
isotopic composition of an airflow containing water vapour through
a snow sample under both isothermal and temperature gradient
conditions. Microcomputed-tomography (µCT) was applied to
obtain the 3-D microstructure and morphological properties of
snow.
Experimental set-up
Morphological properties and flow characteristics of the
experimental runs: µCT measured snow density (ρ), porosity
(ε), specific surface area per unit mass (SSA), mean pore space
diameter (dmean), superficial velocity in snow (uD), corresponding
Reynolds number (Re=dmean⋅uD/νair),
average inlet temperature of the humidifier and at the inlet
(Tin, mean), average outlet temperature at the outlet (Tout, mean) and average temperature gradient (∇Tave). Experiment (1)
corresponds to the isothermal conditions, experiment (2) to air warming and
experiment (3) to air cooling in the snow sample.
Isothermal and temperature gradient experiments with fully
saturated airflow and defined isotopic composition were performed
in a cold laboratory at around Tlab≈-15∘C with small fluctuations of ±0.8 ∘C
(Ebner et al., 2014). Snow produced from de-ionized tap water in
a cold laboratory (water temperature: 30 ∘C; air
temperature: -20 ∘C) was used for the snow sample
preparation (Schleef et al., 2014). The snow was sieved with
a mesh size of 1.4 mm into a box and isothermally
sintered for 27 days at -5 ∘C to increase the
strength in order to prevent destruction of the snow sample due
to the airflow and to evaluate the effect of metamorphism of
snow. The morphological properties of the snow are listed in
Table 1. The sample holder (diameter 53 mm, height
30 mm, 0.066 L) was filled by a cylinder cut out from
the sintered snow. To prevent airflow between the snow sample and
the sample holder walls, the undisturbed snow disk was filled in
at a higher temperature (about -5 ∘C) and sintering
was allowed for about 1 h before cooling down at the
start
of the experiment. The set-up of Ebner et al. (2014) was modified
by additionally inserting a water-vapour-isotope analyser (model:
L1102-I Picarro, Inc., Santa Clara, CA, USA) to measure the
isotopic ratio δ18O of the water vapour contained
in the airflow at the inlet and outlet of the sample holder. The
experimental set-up consisted of three main components
(humidifier, sample holder and the Picarro analyser) connected
with insulated copper tubing and Swagelok fitting (Fig. 1). The
tubes to the Picarro analyser were heated to prevent deposition
of water vapour and thereby fractionation. The temperature was
monitored with thermistors inside the humidifier and at the inlet
and outlet of the snow sample. A dry air pressure tank controlled
by a mass flow controller (EL-Flow, Bronkhorst) generated the
airflow. A humidifier, consisting of a tube (diameter
60 mm, height 150 mm, 0.424 L volume) filled
with crushed ice particles (snow from Antarctica with low
δ18O composition), was used to saturate the dry
air entering the humidifier with water vapour at an almost
constant isotopic composition. The air temperature in the
humidifier and at the inlet of the snow sample was maintained at
the same value (accuracy ±0.2 K) to limit the
influence of variability in absolute vapour pressure and isotopic
composition. We measured the δ18O of the water
vapour produced by the humidifier before and after each
experimental run (δ18Ohum). The outlet
flow (δ18Oa) of the sample holder was
continuously measured during the experiment to analyse the
temporal evolution of the isotopic signal. All data from the
Picarro analyser were corrected to the humidity reference level
using the established instrument humidity-isotope response
(Steen-Larsen et al., 2013, 2014b). In addition, VSMOW-SLAP
correction and drift correction were performed. We followed the
calibration protocol and used the calibration system described in
detail by Steen-Larsen et al. (2013, 2014b).
Schematic of the experimental set-up. A thermocouple (TC)
and a humidity sensor (HS) inside the humidifier measured the mean
temperature and humidity of the airflow. Two thermistors (NTC) close to the
snow surface measured the inlet and outlet temperature of the airflow (Ebner
et al., 2014). The Picarro analyser measured the isotopic composition
δ18O of the outlet flow. Inset: 3-D structure of 110×42×110 voxels (2×0.75×2mm3) obtained
by the µCT.
δ18O is the vapour in the humidifier (δ18Ohum) and of the snow in the sample holder (δ18Os) at the beginning (t=0) and end (t=end) of each
experiment and the final δ18O content of the snow in the sample
holder at the inlet (z=0mm) and outlet (z=30mm). Experiment (1)
corresponds to the isothermal conditions, experiment (2) to air warming and
experiment (3) to air cooling in the snow sample.
The sample holder described by Ebner et al. (2014) was used to
analyse the snow by µCT. Tomography measurements were
performed with a modified µ-CT80 (Scanco Medical). The
equipment incorporated a microfocus X-ray source, operated at
70 kV acceleration voltage with a nominal resolution of
18 µm. The samples were scanned with 1000 projections
per 180∘ in high-resolution setting, with typical
adjustable integration time of 200 ms per projection. The
field of view of the scan area was 36.9 mm of the total
53 mm diameter and subsamples with a dimension of 7.2×7.2×7.2mm3 were
extracted for further processing. The reconstructed µCT
images were filtered using a 3×3×3 median filter
followed by a Gaussian filter (σ=1.4, support=3). The Otsu method (Otsu, 1979) was used to automatically
perform clustering-based image thresholding to segment the
grey-level images into ice and void phase. Morphological
properties in the two-phase system were determined based on the
exact geometry obtained by the µCT. Tetrahedrons
corresponding to the enclosed volume of the triangulated ice
matrix surface were applied to the segmented data to determine
porosity (ε) and specific surface area (SSA). The
mean pore size distribution was estimated using the
opening-size-distribution operation. This operation can be
imagined as virtual sieving with different mesh sizes (Haussener
et al., 2012).
Three experiments with saturated advective airflow through the
snow sample were performed to record the following parameters and
analyse their effects: (1) isothermal conditions to analyse the
influence of curvature effects (Kaempfer al et., 2007), (2)
positive temperature gradient applied to the snow sample where
cold air entering the sample is heated while flowing through the
sample in order to analyse the influence of sublimation, and (3)
negative temperature gradient applied to the snow sample where
warm air entering the sample is cooled while flowing across the
sample to analyse the influence of net deposition. During the
temperature gradient experiments, temperature differences of
1.4 and 1.8 ∘C were imposed, resulting in
gradients of +47 and -60 Km-1,
respectively. The runs were performed at atmospheric pressure and
with a volume flow rate of 3.0 Lmin-1 corresponding
to an average flow speed in the pores of uD≈30mms-1. We performed the experiments with airflow
velocities in the snow sample at uD≈30mms-1, which is a factor of 3 higher than
calculated by Neumann (2003) for a natural snow pack. However, when looking
at the Reynolds number and describing the flow regime inside the
pores, our experiments (Re≈0.7) were in the feasible
flow regime (laminar flow) of a natural snow pack (Re≈0.65). The outlet temperature in experiment (2) and the inlet and the humidifier temperature in
experiment (3) were
actively controlled using thermo-electric elements. Variations
in temperature of up to ±0.8 ∘C were due to
temperature fluctuations inside the cold laboratory, leading to
slightly variable temperature gradients and mean temperature in
experiment (2) and (3). Table 1 presents a summary of the
experimental conditions and the morphological properties of the
snow samples. All snow samples were taken from the same snow
block with an average density of ≈210kgm-3. The density given in Table 1 was the
density of the snow sample in each experiment measured by
µCT. At the end of each experiment, the snow sample was
cut into five layers of 6 mm height and the isotopic
composition of each layer was analysed to examine the spatial
δ18O gradient in the isotopic composition of the
snow sample.
A slight increase with a maximum of 0.7 ‰ of
δ18O in the water vapour produced by the humidifier
was observed in experiment (1), with lower increases during
experiments (2) and (3) (Table 2). This change of ∼0.7 ‰ is not significant compared to the difference
between the isotopic composition of the water vapour and the snow
sample in the sample holder of ∼53 ‰ and the
temporal change of the water-vapour isotopes on the back side of
the snow sample.
In approximately the first 30 min, the isotopic
composition of the measured outflow air δ18Oa
increased from a low δ18O to a starting value of
around -29 ‰ in each experiment. This was due to a
memory effect and another possible effect might be condensed water
left in the tubes from a prior experiment which had no further
impact on the experiments (Penna et al., 2012).
ResultsIsothermal condition
Experiment (1) was performed for 24 h at a mean
temperature of Tmean=-15.5∘C. δ18Oa decreased
exponentially in the outlet flow observed throughout the
experimental run as shown in Fig. 2. Initially, the
δ18Oa content in the flow was
-27.7 ‰ and exponentially decreased to
-47.6 ‰ after 24 h. The small fluctuations in
the δ18Oa signal at t≈7, 17 and
23 h were due to small temperature changes in the cold
laboratory.
We observed a strong interaction between the airflow and the snow
as manifest by the isotopic composition of the snow. The
δ18Os signal in the snow decreased by
4.75–7.78 ‰ and an isotopic gradient in the snow was
observed after the experimental run, shown in Fig. 3. Initially,
the snow had a homogeneous isotopic composition of
δ18Os=-10.97 ‰ but post-experiment
sampling showed a decrease in the snow δ18O at the
inlet side to -17.75 ‰ and at the outlet side to
-15.72 ‰. The spatial δ18Os
gradient of the snow had an approximate slope of
0.68 ‰mm-1 at the end of the experimental
run. Table 2 shows the δ18O value in snow at the
beginning (t=0) and end (t=24h) of the
experiment.
Temporal isotopic composition of δ18O of the
outflow for each of the experimental runs. The spikes in the δ18O were due to small temperature changes in the cold laboratory
(Ebner et al., 2014). Exp. (1) corresponds to the isothermal conditions,
Exp. (2) to air warming and Exp. (3) to air cooling in the snow sample. The
higher the recrystallization rate of the snow the slower the adaption of
δ18O of the outlet air to the inlet air. The illustration in
the lower right corner shows the relation between δ18O of the
initial snow, inlet and outlet of the air.
Air warming by a positive temperature gradient
along the airflow
Experiment (2) was performed over a period of 24 h
with an average temperature gradient of approximatively
+47 Km-1 (warmer temperatures at the outlet of
the snow) and an average mean temperature of
-14.7 ∘C. As in the isothermal experiment (1), we
observed a relaxing exponential decrease of
δ18Oa in the outlet flow throughout the
measurement period as shown in Fig. 2, but the decrease was
slower compared to the isothermal run. Initially, the
δ18Oa content in the flow coming through the
snow disk was -29.8 ‰ and exponentially decreased
to -41.9 ‰ after 24 h. The small
fluctuations in the δ18Oa signal at t≈2.7 and 12.7 h were due to small
temperature changes in the cold laboratory.
The δ18Os signal in the snow decreased by
4.66–7.66 ‰ and a gradient in the isotopic
composition of the snow was observed after the experimental
run, shown in Fig. 3. Initially, the snow had a homogeneous
isotopic composition of δ18Os=-11.94 ‰, but post-experiment sampling showed
a decrease at the inlet side to -19.6 ‰ and at the
outlet side to -16.6 ‰. The spatial
δ18Os gradient of the snow had an
approximate slope of 1.0 ‰mm-1 at the end
of the experimental run. Table 2 shows the
δ18Os values in snow at the beginning (t=0) and end (t=24h) of the experiment.
Air cooling by a negative temperature gradient
along the airflow
Experiment (3) was performed for 84 h instead of
24 h to better estimate the trend in
δ18Oa in the outlet flow. An average
temperature gradient of approximately -60 Km-1
(colder temperatures at the outlet of the snow) and an average
mean temperature of -13.2 ∘C were observed during
the experiment. As in the previous experiments, a relaxing
exponential decrease of δ18Oa in the outlet
flow was observed throughout the experimental run as shown in
Fig. 2. The decrease was slower compared to experiments (1) and (2). Initially,
the δ18Oa content in the flow was
-29.8 ‰ and exponentially decreased to
-37.7 ‰ after 84 h. The small fluctuations
in the δ18Oa signal at t≈7.3,
21.3, 31.3, 45.3, 55.3, 69.3 and
79.3 h were due to small temperature changes in the
cold laboratory.
Spatial isotopic composition of δ18O of the snow
sample at the beginning (t=0) and at the end (t=end) for each
experiment. The air entered at z=0mm and exited at z=30mm. Exp. (1) corresponds to the isothermal conditions Exp. (2)
to air warming and Exp. (3) to air cooling in the snow sample.
The δ18Os signal in the snow decreased by
4.46–15.09 ‰ and a gradient in the isotopic
composition of the snow was observed after the experimental
run, shown in Fig. 3. Initially, the snow had an isotopic
composition of δ18Os=-10.44 ‰
but post-experiment sampling showed a decrease at the inlet
side to -25.53 ‰ and at the outlet side to
-15.00 ‰. The spatial δ18Os
gradient of the snow had an approximate slope of
3.5 ‰mm-1 at the end of the experimental
run. Table 2 shows the δ18Os value in snow at
the beginning (t=0) and end (t=84h) of the
experiment.
Discussion
All experiments showed a strong exchange in δ18O
between the snow and water-vapour-saturated air, resulting in
a significant change in the values of the stable isotopes in the
snow. The advective conditions in the experiments were comparable
with surface snow layers in Antarctica and Greenland, but at higher
temperatures, especially compared to the interior of Antarctica.
The results also showed strong interactions in δ18O
between snow and air depending on the different temperature
gradient conditions. The experiments indicate that temperature
variation and airflow above and through the snow structures (Sturm
and Johnson, 1991; Colbeck, 1989; Albert and Hardy, 1995) seem to
be dominant processes affecting water stable isotopes of surface
snow. The results also support the statement that an interplay occurs
between theoretically expected layer-by-layer sublimation and
deposition at the ice-matrix surface and the isotopic content
evolution of snow cover due to mass exchange between the snow cover
and the atmosphere (Sokratov and Golubev, 2009). The
specific surface area of snow exposed to mass exchange (Horita
et al., 2008) and by the depth of the snow layer exposed to the
mass exchange with the atmosphere (He and Smith, 1999) plays an
important role. Our results support the interpretation that changes
in surface snow isotopic composition are expected to be significant
if large day-to-day surface changes in water vapour occur in between
precipitation events, wind pumping is efficient and snow
metamorphism is enhanced by temperature gradients in the upper
first centimetres of the snow (Steen-Larsen et al., 2014a).
We expect that our findings will lead to an improvement of the
interpretation of the water stable-isotope records from ice
cores. Classically, ice-core stable-isotope records are interpreted
as palaeotemperature, reflecting precipitation-weighted
signals. When comparing observations and atmospheric model results
for precipitation with ice-core records, such snow–vapour exchanges
are normally ignored (e.g. Persson et al., 2011; Fujita and Abe,
2006). However, snow–vapour exchange enhanced by recrystallization
rate seems to be an important factor for the high variation in the
snow surface δ18O signal as supported by our
experiments. It was hypothesized that the changes in the
snow-surface δ18O reported by Steen-Larsen
et al. (2014a) are caused by changes in large-scale wind and
moisture advection of the atmospheric water-vapour signal and snow
metamorphism. The strong interaction between atmosphere and
near-surface snow can modify the ice-core water stable-isotope
records.
The rate-limiting step for isotopic exchange in the snow is
isotopic equilibration between the pore-space vapour and surrounding
ice grains. The relaxing exponential decrease of
δ18O in the outflow of our experiments predicted
that full isotopic equilibrium between snow and atmospheric vapour
will not be reached at any depth (Waddington et al., 2002; Neumann
and Waddington, 2004) but changes move towards equilibrium with the
atmospheric state (Steen-Larsen et al., 2013, 2014a).
As snow accumulates, the upper 2 m are advected through the
ventilated zone (Neumann and Waddington, 2004; Town et al.,
2008). In areas with high accumulation rate (e.g. South Greenland),
snow is advected for a short time through the ventilated zone. The
snow exposed for a relatively short time to vapour snow exchange would
result in higher spatial variability compared to longer
exposure. However, the effects of snow ventilation on isotopic
composition may become more important as the accumulation rate of
the snow decreases (<50mma-1), such that snow
remains in the near-surface ventilated zone for many years
(Waddington et al., 2002; Hoshina et al., 2014, 2016). As the snow
remains for a longer time in the near-surface ventilated zone, a larger
δ18O exchange will occur between snow and atmospheric vapour.
Consequently, the isotopic content of layers at sites
with high and low accumulation rates can evolve differently, even
if the initial snow composition had been equal, and the sites had
been subjected to the same histories of air-mass vapour.
Despite a relatively small change in the difference between the
isotopic composition of the incoming vapour and the snow, large
differences in the isotopic composition of the water vapour at the
outlet flow exist for the three different experimental
set-ups. Based on the difference in the outlet water-vapour isotopic
composition, we hypothesized that different processes are at play
for the different experiments. It is obvious that there is a fast
isotopic exchange with the surface of the ice crystals and a much
slower timescale on which the interior of the ice crystals is
altered. Due to the low diffusivity of H216O and
H218O in ice (DH218O≈DH216O=∼10-15m2s-1
(Ramseier, 1967; Johnsen et al., 2000), we assumed that the
interior of the ice crystals is not altered on the timescale of the
experiment. This explained why the net isotopic change of the bulk
sample is relatively small compared to the changes in the outlet
water-vapour isotopes. The effective “ice-diffusion depth” of the
isotopic exchange during the experiments is given as
LD=D⋅t, where D is the diffusion coefficient of
H216O and H218O in ice and
t is the experimental time. The calculated ice-diffusion depth
LD, is ∼9.3µm for experiments (1) and (2),
and ∼17.4µm for experiment (3), respectively,
indicating an expected a minimal change of the interior of the ice
crystal. However, snow has a large specific surface area and
therefore a high exchange area. This has an effect on the
δ18O snow concentration. The fraction of the total
volume Vtot of ice that is close enough to the ice
surface to be affected by diffusion in time t is then
ρice⋅SSA⋅LD, where SSA is the
specific surface area (area per unit mass), and LD is the
diffusion depth, defined above, for time t. For t≈24 h, a large fraction (24 to 43 %) of the total volume
Vtot of the ice matrix can be accessed through
diffusion. It is quite hard to see the total δ18O
snow difference between experiments (1) and (2) after the
experiment compared to the δ18O of the vapour in the
air at the outlet. There is a small but notable difference in the
total δ18O of the snow between experiments (1) and
(2). Due to the higher recrystallization rate of experiment (2) the
spatial δ18Os gradient of the snow
(1.0 ‰mm-1) is higher than for experiment (1)
(0.68 ‰mm-1). Increasing the experimental time,
the δ18O change in the snow increases (experiment
3). In general, the calculated ice-diffusion depth is
realistic under isothermal conditions where diffusion processes are
the main factors (Kaempfer and Schneebeli, 2007; Ebner et al.,
2015). By applying a temperature gradient, the impact of diffusion is
suppressed due to the high recrystallization rate by sublimation
and deposition. Due to the low half-life of a few
days of the ice matrix, the growth rates are typically of the order of
100 µmday-1 (Pinzer et al., 2012). Therefore, this
redistribution of ice caused by temperature gradient counteracts
the diffusion into the solid ice.
By comparing similarities and differences between the outcomes of
the three experimental set-ups we will now discuss the physical
processes influencing the interaction and exchange processes within
the snowpack between the snow and the advected vapour. We first
notice that the final snow isotopic profiles of experiments (1)
(isothermal) and (2) (positive temperature gradient along the
direction of the flow) are comparable to each other. Despite this
similarity, the evolution in the outlet water vapour of experiment
(1) showed a significantly stronger depletion compared to
experiment (2). For experiment (3) (negative temperature gradient
along the direction of the flow) we observed the smallest change in
outlet water-vapour isotopes but the largest snowpack isotope
gradient after the experiment. However, this change was caused by
84 h flow instead of 24 h.
Curvature effects, temperature gradients and therefore the
recrystallization rate influence the mass transfer of
H216O/H218O molecules. The higher the
recrystallization rate of the snow the slower the adaption of the
outlet air concentration to the inlet air concentration (see in
experiments 2 and 3). Under isothermal conditions (experiment
1) the only effect influencing the recrystallization rate is the
curvature effect (Kaempfer and Schneebeli, 2007). However, based on
the experimental observations (Kaempfer and Schneebeli, 2007) this
effect decreases with decreasing temperature and increasing
experimental time. Applying an additional temperature gradient to
a snow sample causes complex interplays between local sublimation
and deposition on surfaces and the interaction of water molecules
in the air with the ice matrix due to changing saturation
conditions of the airflow. Therefore, the recrystallization rate
increases and causes the change in the δ18O of the
air. For experiment (2) there is a complex interplay between
sublimation and deposition of water molecules into the interstitial
flow (Ebner et al., 2015c), while for experiment (3) there is
deposition of molecules carried by the interstitial flow onto the
snow crystals (Ebner et al., 2015b). Furthermore, in the beginning
of each experiment there is a tendency to sublimate from edges of
the individual snow crystals due to the higher curvature. As the
edges were sublimated and deposition occurred in the concavities,
the individual snow crystals became more rounded, slowing down the
transfer of water molecules into the interstitial airflow. We
noticed, for all three experiments, that within the uncertainty of
the isotopic composition of the snow, the initial isotopic
composition of the vapour was the same and in isotopic equilibrium
with the snow. The difference between experiments (1) and (2) lies
in the fact that due to the temperature gradient in experiment (2)
there is an increased transfer of water vapour with the isotopic
composition of the snow into the airflow. Hence the depleted air
from the humidifier advected through the snow disk is mixed with
a relatively larger vapour flux from the snow
crystals. Additionally, we also expected less deposition into the
concavities in experiment (2) compared to experiment (1). However,
it is interesting to note that the final isotopic profile of the
snow disk is similar in experiment (1) and experiment (2). We
interpreted this as being a result of two processes acting in
opposite directions: although relatively isotope-depleted vapour from
the humidifier was deposited onto the ice matrix there was also
a higher amount of sublimation of relatively isotope-enriched vapour
from the snow disk in experiment (2). Experiment (3) separates
itself from the other two experiments in that as the water
vapour from the humidifier is advected through the snow disk there
is a continuous deposition of very depleted air due to the negative
temperature gradient. As for experiments (1) and (2)
there was also a constant sublimation of the
convexities into the vapour stream in experiment (3). We notice that, despite the fact
that experiment (3) ran for 84 h, the snow at the outlet side of
the snow disk did not become more isotopically depleted compared to
experiments (1) and (2). However, the snow on the inlet side became
significantly more isotopically depleted. This observation,
together with the fact that the vapour of the outlet of the
snow disk is less depleted compared to experiments (1) and (2),
leads us to hypothesize that there is a relatively larger
deposition of isotopically depleted vapour from the humidifier as
the vapour is advected through the snow disk. This means that
a relatively larger component of the isotopic composition of the
vapour is originating by sublimation from the convexities of the
snow disk and a smaller one from the isotopically depleted vapour from the
humidifier.
Our results and conclusions indicate that there is a need for
additional validation. Specifically, it would be crucial to know
the mass balance of the snow disk more precisely, which could be
done by reconstructing the entire snow disk following the change in
density and morphological properties over the entire
height. Ideally, the entire sample would be tomographically
measured with a resolution of 4×4×4mm3,
each cube corresponding to the representative volume. Insights
would also be achieved with experiments using snow of the same
isotopic composition, but different SSA, as a more precise
calculation of the different observed exchange rates would be
allowed. Additionally, different and colder background temperatures
should be tested to better understand the inland Antarctic environment
and the effect of the quasi-liquid layer, which is necessary for
the development of a numerical model. Isotopically different
combinations of vapour and snow should be performed. In the present
paper, vapour with low δ18O isotopic composition
was transported through snow with relative high δ18O
isotopic composition. It would be interesting to reverse the
combination and perform experiments with different combinations to
provide more insights on mass and isotope exchanges between vapour
and snow. Experiments with longer running time help to understand
the change in the ice matrix better under low accumulation
conditions.
Summary and conclusion
Laboratory experimental runs were performed where a transient
δ18O interaction between snow and air was
observed. The airflow altered the isotopic composition of the
snowpack and supports an improved climatic interpretation of ice
core stable water-isotope records. The water-vapour-saturated
airflow with an isotopic difference of up to 55 ‰ changed
the original δ18O isotope
signal in the snow by up to 7.64 and
15.06 ‰ within 24 and 84 h. The disequilibrium between snow and air isotopes
led to the observed exchange of isotopes, the rate depending on the
temperature gradient conditions. To conclude, increasing the
recrystallization rate in the ice matrix causes the temporal change
of the δ18O concentration at the outflow to decrease
(experiment 2 and 3). Decreasing the recrystallization rate
causes the temporal curve of the outlet concentration to become
steeper, reaching the δ18O inlet concentration of the
air faster (experiment 1).
Additionally, the complex interplay of simultaneous diffusion,
sublimation and deposition due to the geometrical complexity of
snow has a strong effect on the δ18O signal in the
snow and cannot be neglected. A temporal signal can be
superimposed on the precipitation signal, (a) if the snow remains
near the surface for a long time, i.e. in a low-accumulation area,
and (b) if it is exposed to a history of air masses carrying vapour with
a significantly different isotopic signature than the precipitated
snow.
These are novel measurements and will therefore be important as the
basis for further research and experiments. Our results represent
direct experimental observations of the interaction between the
water isotopic composition of the snow, the water vapour in the air
and recrystallization due to temperature gradients. Our results
demonstrate that recrystallization and bulk mass exchange must be
incorporated into future models of snow and firn evolution. Further
studies are required on the influence of temperature and airflow as
well as on snow microstructure on the mass transfer phenomena for
validating the implementation of stable water isotopes in snow
models.
All data are available at 10.16904/20 (EnviDat.ch data
portal).
The authors declare that they have no conflict of
interest.
Acknowledgements
The Swiss National Science Foundation granted financial support under
project Nr. 200020-146540. Hans Christian Steen-Larsen was supported by the
AXA Research Fund. The authors thank Koji Fujita, Edwin Waddington and an
anonymous reviewer for the suggestions and critical review. Matthias Jaggi,
Sascha Grimm, Alessandro Schlumpf and Sarah Berben gave technical support.
The data for this paper are available by contacting the corresponding
author. Edited by: Benjamin Smith
Reviewed by: two anonymous referees
References
Albert, M. R. and Hardy, J. P.: Ventilation experiments in a seasonal snow cover, in: Biogeochemistry of Seasonally Snow-Covered Catchments, IAHS Publ. 228, edited by: Tonnessen, K. A., Williams, M. W., and Tranter, M., IAHS Press, Wallingford, UK, 41–49, 1995.Calonne, N., Geindreau, C., Flin, F., Morin, S., Lesaffre, B., Rolland du Roscoat, S., and Charrier, P.: 3-D image-based numerical computations of snow permeability: links to specific surface area, density, and microstructural anisotropy, The Cryosphere, 6, 939–951, 10.5194/tc-6-939-2012, 2012.Casado, M., Landais, A., Masson-Delmotte, V., Genthon, C., Kerstel, E., Kassi, S., Arnaud, L., Picard, G., Prie, F., Cattani, O., Steen-Larsen, H.-C., Vignon, E., and Cermak, P.: Continuous measurements of isotopic composition of water vapour on the East Antarctic Plateau, Atmos. Chem. Phys., 16, 8521–8538, 10.5194/acp-16-8521-2016,
2016.Ciais, P. and Jouzel, J.: Deuterium and oxygen 18 in precipitation: Isotopic model, including mixed-cloud processes, J. Geophys. Res., 99, 16793–16803, 10.1029/94JD00412, 1994.
Colbeck, S. C.: Air movement in snow due to windpumping, J. Glaciol., 35, 209–213, 1989.
Craig, H. and Gordon, L. I.: Deuterium and oxygen 18 variations in the
ocean and marine atmosphere, in: Proc. Stable Isotopes in
Oceanographic Studies and Paleotemperatures, edited by: Toniorgi, E., Spoleto, Italy, 9–130, 1964.
Cuffey, K. M. and Steig, E. J.: Isotopic diffusion in polar firn: implications for interpretation of seasonal climate parameters in ice-core records, with emphasis on central Greenland, J. Glaciol., 44, 273–284, 1998.
Cuffey, K. M., Alley, R. B., Grootes, P. M., Bolzan, J. M., and Anandakrishnan, S.: Calibration of the Delta-O-18 isotopic paleothermometer for central Greenland, using borehole temperatures, J. Glaciol., 40, 341–349, 1994.
Dansgaard, W.: Stable isotopes in precipitation, Tellus, 16, 436–468, 1964.Ebner, P. P., Grimm, S., Schneebeli, M., and Steinfeld, A.: An
instrumented sample holder for time-lapse micro-tomography
measurements of snow under advective conditions, Geosci. Instrum. Meth., 3, 179–185, 10.5194/gi-3-179-2014, 2014.Ebner, P. P., Schneebeli, M., and Steinfeld, A.: Tomography-based monitoring of isothermal snow metamorphism under advective conditions, The Cryosphere, 9, 1363–1371, 10.5194/tc-9-1363-2015, 2015a.Ebner, P. P., Andreoli, C., Schneebeli, M., and Steinfeld, A.:
Tomography-based characterization of ice–air interface dynamics of
temperature gradient snow metamorphism under advective conditions, J.
Geophys. Res.-Earth, 120, 2437–2451, 10.1002/2015JF003648, 2015b.Ebner, P. P., Schneebeli, M., and Steinfeld, A.: Metamorphism during
temperature gradient with undersaturated advective airflow in a snow sample,
The Cryosphere, 10, 791–797, 10.5194/tc-10-791-2016, 2016.EPICA Members: Eight glacial cycles from an Antarctic ice core, Nature, 429,
623–628, 10.1038/nature02599, 2004.
Gjessing, Y. T.: The filtering effect of snow, in: Isotopes and Impurities in
Snow and Ice Symposium, edited by: Oeschger, H., Ambach, W., Junge, C. E.,
Lorius, C., and Serebryanny, L., IASHAISH Publication, Dorking, 118,
199–203, 1977.
Grootes, P. M., Steig, E., and Stuiver, M.: Taylor Ice Dome study 1993–1994:
an ice core to bedrock, Antarct. J. US, 29, 79–81, 1994.
Fisher, D. A. and Koerner, R.: The effect of wind on d(18O) and accumulation
given an inferred record of seasonal d amplitude from the Agassiz ice cap,
Ellesmere island, Canada, Ann. Glaciol., 10, 34–37, 1988.Fisher, D. A. and Koerner, R.: Signal and noise in four ice-core records from
the Agassiz ice cap, Ellesmere Island, Canada: details of the last millennium
for stable isotopes, melt and solid conductivity, Holocene, 4, 113–120,
10.1177/095968369400400201, 1994.Fisher, D. A., Koerner, R. M., Paterson, W. S. B., Dansgaard, W.,
Gundestrup, N., and Reeh, N.: Effect of wind scouring on climatic records
from ice-core oxygen-isotope profiles, Nature, 301, 205–209,
10.1038/301205a0, 1983.
Friedman, I., Benson, C., and Gleason, J.: Isotopic changes during snow
metamorphism, in: Stable Isotope Geochemistry: A Tribute to Samuel Epstein,
edited by: O'Neill, J. R. and Kaplan, I. R., Geochemical Society,
Washington, D. C., 211–221, 1991.Haussener, S., Gergely, M., Schneebeli, M., and Steinfeld, A.: Determination
of the macroscopic optical properties of snow based on exact morphology and
direct pore-level heat transfer modeling, J. Geophys. Res., 117, 1–20,
10.1029/2012JF002332, 2012.He, H. and Smith, R. B.: An advective-diffusive isotopic
evaporation-condensation model, J. Geophys. Res., 104, 18619–18630,
10.1029/1999JD900335, 1999.
Helsen, M. M., van de Wal, R. S. W., van den Broeke, M. R., van As, D.,
Meijer, H. A. J., and Reijmer, C. H.: Oxygen isotope variability in snow from
western Dronning Maud Land, Antarctica and its relation to temperature,
Tellus, 57, 423–435, 2005.Helsen, M. M., van de Wal, R. S. W., van den Broeke, M. R., Masson-Delmotte,
V., Meijer, H. A. J., Scheele, M. P., and Werner, M.: Modeling the isotopic
composition of Antarctic snow using backward trajectories: Simulation of snow
pit records,J. Geophys. Res., 111, D15109, 10.1029/2005JD006524, 2006.
Helsen, M. M., van de Wal, R. S. W., and van den Broeke, M. R.: The isotopic
composition of present-day Antartic snow in a Lagrangian simulation, J.
Climate, 20, 739–756, 2007.Hendricks, M. B., DePaolo, D. J., and Cohen, R. C.: Space and time variation
of δ18O and δD in precipitation: can paleotemperature
be estimated from ice cores?, Global Biogeochem. Cy., 14, 851–861,
10.1029/1999GB001198, 2000.Horita, J., Rozanski, K., and Cohen, S.: Isotope effects in the evaporation
of water: a status report of the Craig-Gordon model, Isot. Environ. Health
Sci., 44, 23–49, 10.1080/10256010801887174, 2008.Hoshina, Y., Fujita, K., Nakazawa, F., Iizuka, Y., Miyake, T.,
Hirabayashi, M., Kuramoto, T., Fujita, S., and Motoyama, H.: Effect of
accumulation rate on water stable isotopes of near-surface snow in inland
Antarctica. J. Geophys. Res.-Atmos., 119, 274–283,
10.1002/2013JD020771, 2014.Hoshina, Y., Fujita, K., Iizuka, Y., and Motoyama, H.: Inconsistent relations
among major ions and water stable isotopes in Antartica snow under different
accumulation environments, Polar Sci., 10, 1–10,
10.1016/j.polar.2015.12.003, 2016.
Johnsen, S. J.: Stable isotope homogenization of polar firn and ice, in:
Isotopes and Impurities in Snow and Ice, Proceeding of the Grenoble
Symposium, August/September 1975, IAHS AISH Publication, 118, Grenoble,
France, 210–219, 1997.
Johnsen, S. J., Clausen, H. B., Cuffey, K. M., Hoffman, G., Schwander, J.,
and Creyts, T.: Diffusion of stable isotopes in polar firn and ice: the
isotope effect in firn diffusion, in: Physics of Ice Core Records, edited by:
Hondoh, T., Hokkaido University Press, Sapporo, Japan, 121–140, 2000.Johnsen, S. J., Dahl-Jensen, D., Gundestrup, N., Steffensen, J. P.,
Clausen, H. B., Miller, H., Masson-Delmotte, V., Sveinbjörnsdottir A. E.,
and White, J.: Oxygen isotope and palaeotemperature records from six
Greenland ice-core stations: Camp Century, DYE-3, GRIP, GISP2, Renland and
NorthGRIP, J. Quaternary Sci., 16, 299–307, 10.1002/jqs.622, 2001.Jouzel, J. and Merlivat, L.: Deuterium and oxygen 18 in precipitation:
modeling of the isotopic effects during snow formation, J. Geophys. Res., 89,
11749–11757, 10.1029/JD089iD07p11749, 1984.Jouzel, J., Merlivat, L., Petit, J. R., and Lorius, C.: Climatic information
over the last century deduced from a detailed isotopic record in the South
Pole snow, J. Geophys. Res., 88, 2693–2703, 10.1029/JC088iC04p02693,
1983.Jouzel, J., Alley, R. B., Cuffey, K. M., Dansgaard, W., Grootes, P., Hoffmann, G., Johnsen, S. J., Koster, R. D., Peel, D., Shuman, C. A., Stievenard, M., Stuiver, M., and White, J.: Validity of the
temperature reconstruction from water isotopes in ice cores, J. Geophys.
Res., 102, 26471–26487, 10.1029/97JC01283, 1997.Jouzel, J., Vimeux, F., Caillon, N., Delaygue, G., Hoffman, G.,
Masson-Delmotte, V., and Parrenin, F.: Magnitude of isotope/temperature
scaling for interpretation of central Antarctic ice cores, J. Geophys. Res.,
108, 1–6, 10.1029/2002JD002677, 2003.Kaempfer, T. U. and Schneebeli, M. Observation of isothermal metamorphism of
new snow and interpretation as a sintering process, J. Geophys. Res., 112,
1–10, 10.1029/2007JD009047, 2007.Krinner, G. and Werner, M.: Impact of precipitation seasonality changes on
isotopic signals in polar ice cores: A multi-model analysis, Earth Planet.
Sc. Lett., 216, 525–538, 10.1016/S0012-821X(03)00550-8, 2003.Lorius, C., Merlivat, L., Jouzel, J., and Pourchet, M.: A 30,000-yr isotope
climatic record from Antarctica ice, Nature, 280, 644–648,
10.1038/280644a0, 1979.
Löwe H., Spiegel, J. K., and Schneebeli, M.: Interfacial and structural
relaxations of snow under isothermal conditions, J. Glaciol., 57, 499–510,
2011.Masson-Delmotte, V., Steen-Larsen, H. C., Ortega, P., Swingedouw, D., Popp,
T., Vinther, B. M., Oerter, H., Sveinbjornsdottir, A. E., Gudlaugsdottir, H.,
Box, J. E., Falourd, S., Fettweis, X., Gallée, H., Garnier, E., Gkinis,
V., Jouzel, J., Landais, A., Minster, B., Paradis, N., Orsi, A., Risi, C.,
Werner, M., and White, J. W. C.: Recent changes in north-west Greenland
climate documented by NEEM shallow ice core data and simulations, and
implications for past-temperature reconstructions, The Cryosphere, 9,
1481–1504, 10.5194/tc-9-1481-2015, 2015.Merlivat, L. and Jouzel, J.: Global climatic interpretation of the
deuterium-oxygen 18 relationship for precipitation, J. Geophys. Res., 84,
5029–5033, 10.1029/JC084iC08p05029, 1979.
Neumann, T. A.: Effects of firn ventilation on geochemistry of polar snow,
PhD thesis, University of Washington, Washington, USA, 2003.
Neumann, T. A. and Waddington, E. D.: Effects of firn ventilation on isotopic
exchange, J. Glaciol., 50, 183–194, 2004.
Neumann, T. A., Albert, M. R., Lomonaco, R., Engel, C., Courville, Z.,
and Perron, F.: Experimental determination of snow sublimation rate
and stable-isotopic exchange, Ann. Glaciol., 49, 1–6,
2008.
Otsu, N.: A threshold selection method from gray-level histograms, IEEE T. Syst. Man Cyb., 9, 62–66, 1979.Penna, D., Stenni, B., Šanda, M., Wrede, S., Bogaard, T. A., Michelini,
M., Fischer, B. M. C., Gobbi, A., Mantese, N., Zuecco, G., Borga, M.,
Bonazza, M., Sobotková, M., Čejková, B., and Wassenaar, L. I.:
Technical Note: Evaluation of between-sample memory effects in the analysis
of δ2H and δ18O of water samples measured by laser
spectroscopes, Hydrol. Earth Syst. Sci., 16, 3925–3933,
10.5194/hess-16-3925-2012, 2012.Persson, A., Langen, P. L., Ditlevsen, P., and Vinther, B. M.: The influence
of precipitation weighting on interannual variability of stable water
isotopes in Greenland, J. Geophys. Res.-Atmos., 116, 1–13,
10.1029/2010JD015517, 2011.Petit, J. R., Jouzel, J., Raynaud, D., Barkov, N. I., Barnola, J.-M., Basile,
I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M.,
Kotlyakov, V. M., Legrand, M., Lorius, C., Pépin, L., Ritz, C., Saltzman,
E. S., and Stievenard, M.: Climate and atmospheric history of the past
420,000 years from the Vostok ice core, Antarctica, Nature, 399,
429–436, 10.1038/20859, 1999.Pinzer, B. R. and Schneebeli, M.: Snow metamorphism under alternating
temperature gradients: morphology and recrystallization in surface snow,
Geophys. Res. Lett., 36, L23503, 10.1029/2009GL039618, 2009.Pinzer, B. R., Schneebeli, M., and Kaempfer, T. U.: Vapor flux and
recrystallization during dry snow metamorphism under a steady temperature
gradient as observed by time-lapse micro-tomography, The Cryosphere, 6,
1141–1155, 10.5194/tc-6-1141-2012, 2012.
Ramseier, R. O: Self-diffusion of tritium in natural and synthetic ice monocrystals, J. Appl. Phys., 38, 2553–2556, 1967.Ritter, F., Steen-Larsen, H. C., Werner, M., Masson-Delmotte, V., Orsi, A.,
Behrens, M., Birnbaum, G., Freitag, J., Risi, C., and Kipfstuhl, S.: Isotopic
exchange on the diurnal scale between near-surface snow and lower atmospheric
water vapor at Kohnen station, East Antarctica, The Cryosphere, 10,
1647–1663, 10.5194/tc-10-1647-2016, 2016.
Schleef, S., Jaggi, M., Löwe H., and Schneebeli, M.: Instruments and
methods: an improved machine to produce nature-identical snow in the
laboratory, J. Glaciol., 60, 94–102, 2014.Sjolte, J., Hoffmann, G., Johnsen, S. J., Vinther, B. M.,
Masson-Delmotte, V., and Sturm, C.: Modeling the water isotopes in Greenland
precipitation 1959–2001 with the meso-scale model remo-iso, J. Geophys.
Res., 116, 1–22, 10.1029/2010JD015287, 2011.
Sokratov, S. A. and Golubev, V. N.: Snow isotopic content change by
sublimation. J. Glaciol., 55, 823–828,
2009.Steen-Larsen, H. C., Masson-Delmotte, V., Sjolte, J., Johnsen, S. J.,
Vinther, B. M., Breon, F. M., Clausen, H. B., Dahl-Jensen, D., Falourd, S.,
Fettweis, X., Gallee, H., Jouzel, J., Kageyama, M., Lerche, H., Minster, B.,
Picard, G., Punge, H. J., Risi, C., Salas, D., Schwander, J., Steffen, K.,
Sveinbjornsdottir, A. E., Svensson, A., and White, J.: Understanding the
climatic signal in the water stable isotope records from the neem shallow
firn/ice cores in northwest Greenland, J. Geophys. Res.-Atmos., 116, 1–20,
10.1029/2010JD014311, 2011.Steen-Larsen, H. C., Johnsen, S. J., Masson-Delmotte, V., Stenni, B., Risi,
C., Sodemann, H., Balslev-Clausen, D., Blunier, T., Dahl-Jensen, D.,
Ellehøj, M. D., Falourd, S., Grindsted, A., Gkinis, V., Jouzel, J., Popp,
T., Sheldon, S., Simonsen, S. B., Sjolte, J., Steffensen, J. P., Sperlich,
P., Sveinbjörnsdóttir, A. E., Vinther, B. M., and White, J. W. C.:
Continuous monitoring of summer surface water vapor isotopic composition
above the Greenland Ice Sheet, Atmos. Chem. Phys., 13, 4815–4828,
10.5194/acp-13-4815-2013, 2013.Steen-Larsen, H. C., Masson-Delmotte, V., Hirabayashi, M., Winkler, R.,
Satow, K., Prié, F., Bayou, N., Brun, E., Cuffey, K. M., Dahl-Jensen, D.,
Dumont, M., Guillevic, M., Kipfstuhl, S., Landais, A., Popp, T., Risi, C.,
Steffen, K., Stenni, B., and Sveinbjörnsdottír, A. E.: What controls
the isotopic composition of Greenland surface snow?, Clim. Past, 10,
377–392, 10.5194/cp-10-377-2014, 2014a.Steen-Larsen, H. C., Sveinbjörnsdottir, A. E., Peters, A. J.,
Masson-Delmotte, V., Guishard, M. P., Hsiao, G., Jouzel, J., Noone, D.,
Warren, J. K., and White, J. W. C.: Climatic controls on water vapor
deuterium excess in the marine boundary layer of the North Atlantic based on
500 days of in situ, continuous measurements, Atmos. Chem. Phys., 14,
7741–7756, 10.5194/acp-14-7741-2014, 2014b.Sturm, M. and Johnson, J. B.: Natural convection in the subarctic snow cover, J. Geophys. Res., 96, 11657–11671, 10.1029/91JB00895, 1991.Town, M. S., Warren, S. G., Walden, V. P., and Waddington, E. D.: Effect of
atmospheric water vapor on modification of stable isotopes in near-surface
snow on ice sheets, J. Geophys. Res.-Atmos., 113, 1–16,
10.1029/2008JD009852, 2008.van der Wel, G., Fischer, H., Oerter, H., Meyer, H., and Meijer, H. A. J.:
Estimation and calibration of the water isotope differential diffusion length
in ice core records, The Cryosphere, 9, 1601–1616,
10.5194/tc-9-1601-2015, 2015.
Waddington, E. D., Cunningham, J., and Harder, S. L.: The effects of snow
ventilation on chemical concentrations, in: Chemical Exchange Between the
Atmosphere and Polar Snow, edited by: Wolff, E. W. and Bales, R. C.,
Springer, Berlin, NATO ASI Series, 43, 403–452, 1996.Waddington, E. D., Steig, E. J., and Neumann, T. A.: Using characteristic
times to assess whether stable isotopes in polar snow can be reversibly
deposited, Ann. Glaciol., 35, 118–124, 2002. Werner, M., Langebroek, P. M., Carlsen, T., Herold, M., and Lohmann, G.:
Stable water isotopes in the ECHAM5 general circulation model: Toward
high-resolution isotope modeling on a global scale, J. Geophys. Res.-Atmos.,
116, D15109, 10.1029/2011JD015681, 2011.White, J. W., Barlow, L. K., Fisher, D., Grootes, P., Jouzel, J.,
Johnsen, S. J., Stuiver, M., and Clausen, H.: The climate signal in the
stable isotopes of snow from Summit, Greenland: Results of comparisons with
modern climate observations, J. Geophys. Res., 102, 26425–26439,
10.1029/97JC00162, 1997.Zermatten, E., Schneebeli, M., Arakawa, H., and Steinfeld, A.:
Tomography-based determination of porosity, specific area and permeability of
snow and comparison with measurements, Cold Reg. Sci. Technol., 97, 33–40,
10.1016/j.coldregions.2013.09.013, 2014.