Firn air provides plenty of old air from the near past, and can therefore be
useful for understanding human impact on the recent history of the
atmospheric composition. Most of the existing firn air records cover only
the last several decades (typically 40 to 55 years) and are insufficient to
understand the early part of anthropogenic impacts on the atmosphere. In
contrast, a few firn air records from inland sites, where temperatures and
snow accumulation rates are very low, go back in time about a century. In
this study, we report an unusually old firn air effective CO2 age of 93 years from Styx Glacier, near the Ross Sea coast in Antarctica. This is the
first report of such an old firn air age (>55 years) from a warm
coastal site. The lock-in zone thickness of 12.4 m is larger than at other
sites where snow accumulation rates and air temperature are similar.
High-resolution X-ray density measurements demonstrate a high variability of
the vertical snow density at Styx Glacier. The CH4 mole fraction and
total air content of the closed pores also indicate large variations in
centimeter-scale depth intervals, indicative of layering. We hypothesize that the
large density variations in the firn increase the thickness of the lock-in
zone and, consequently, increase the firn air ages because the age of firn
air increases more rapidly with depth in the lock-in zone than in the
diffusive zone. Our study demonstrates that all else being equal, sites
where weather conditions are favorable for the formation of large density
variations at the lock-in zone preserve older air within their open
porosity, making them ideal places for firn air sampling.
Introduction
Bubbles trapped in ice cores preserve ancient air and allow direct
measurements of the atmospheric composition in the past (e.g., Petit et al.,
1999). However, it is difficult to obtain air samples over the past several
decades from ice cores since the more recent air has not yet been completely
captured into bubbles closed off from the atmosphere. In contrast, we can
obtain recent records from the interstitial air in the porous,
unconsolidated snow layer (firn) on top of glaciers and ice sheets
(Schwander, 1989; Schwander et al., 1993). In addition, we can take advantage of the
very large amount of firn air because it allows us to accurately analyze
isotopic ratios of greenhouse gases and many trace gases such as synthetic
chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and SF6 (Buizert et al., 2012; Laube et al., 2012).
However, reported firn air ages date back only several decades at the sites
where snow accumulation rates are relatively high (Table 1). Old firn air
(>55 years) was observed only at sites where surface
temperatures and snow accumulation rates are low such as the South Pole (Battle
et al., 1996) and inland Antarctic Megadunes (Severinghaus et al., 2010)
(Table 1); however, even under such circumstances very old firn air is not
guaranteed, as demonstrated by Dome C (Table 1).
Glaciological characteristics of Styx Glacier and other
firn air sampling sites.
SiteTA (centimetersEffective CO2LIDCODLIZ thicknessReferences(∘)of ice per year)age (year)(m)(m)(m)Styx-31.7109352.464.812.4This study, Yang et al. (2018)Megadunes-49∼012964.568.54Severinghaus et al. (2010)South Pole-51.089111512510Severinghaus et al. (2001)Siple Dome-25.4135949589Severinghaus et al. (2001)Dome C-54.52.733971003Landais et al. (2006)WAIS Divide-312239∼6776.59.5Battle et al. (2011)NEEM-28.92250637815Buizert et al. (2012)NGRIP-31.1194567.57811.5Kawamura et al. (2006)Summit-3223267080.810.8Witrant et al. (2012)DE-08-191201371.888.516.8Etheridge et al. (1996)
In the firn layer, air moves through the open pores and is occluded into the
adjacent ice at total porosity of ∼0.1 (Schaller et al.,
2017). Firn air moves downward with the adjacent ice (advection), but is
furthermore mixed by diffusion and affected by thermal and gravitational
fractionation (Craig et al., 1988; Johnsen et al., 2000; Severinghaus et
al., 2001; Goujon et al., 2003). In addition, gradual bubble trapping in
the firn affects the movement of the air. As a result, at each depth there
is a gas age distribution (Schwander et al., 1993; Trudinger et al., 1997),
rather than a single gas age. Therefore, studying firn air is also important
for interpreting the record of ancient air trapped in ice cores.
The firn column is generally divided into three zones: convective, diffusive,
and lock-in, depending on the mechanisms of firn air movement (Sowers et
al., 1992). The gravitational enrichment in 15N of N2 is
traditionally used to define the boundaries between these zones. The
convective zone is the upper part of the firn where the air can ventilate
with the overlying atmosphere. With stronger wind pumping, there can be a
deeper convective zone (Kawamura et al., 2013). This zone has the same
δ15N of N2 value as that of the atmosphere. The
diffusive zone is located under the convective zone, where molecular
diffusion is the dominant mechanism of trace gas transport in interstitial
air (Blunier and Schwander, 2000). The age of the firn air increases slowly
with depth in the diffusive zone because of continued gas exchange with
atmospheric air via diffusion. Heavier isotopes are enriched with depth due
to the gravitational fractionation in the diffusive layer. Thus,
δ15N of N2 gradually increases with depth in the
diffusive zone. In the lock-in zone (LIZ) below the diffusive zone, gas
diffusion is strongly impeded, although the bubbles are not entirely closed.
The top of the lock-in zone is called lock-in depth (LID), where the
gravitational fractionation ceases, so that the δ15N
of N2 becomes constant. The bottom of the LIZ is defined as the full
close-off depth (zCOD), where all air bubbles are closed off and firn
becomes sealed ice. The zCOD can be estimated in two different ways.
First, we can calculate the zCOD from firn densification models.
Typically, the close-off occurs when the density of ice reaches about 830 kg m-3 (Blunier and Schwander, 2000), equivalent to a critical porosity
of around 0.1 (Schaller et al., 2017). Also, if temperature is known, the
average density at close-off can be estimated from empirical relations
(Martinerie et al., 1992). Second, the deepest position where air can be
sampled from the firn column is commonly considered (just above) the
zCOD. In theory, the zCOD is the depth at which all pores are
closed, but it can be ambiguous to specify the zCOD in the field
because firn air can be sampled at a slightly deeper depth than that of the
shallowest impermeable snow layer due to the existence of permeable layers
at deeper depths – this effect is due to density layering (Mitchell et al.,
2015).
The gas ages in the LIZ increase with depth faster than in the diffusive
zone. In the LIZ, firn air moves downward at nearly the same rate as the
surrounding ice, and therefore the age of the air increases with depth at
nearly the same rate as the age of ice increases.
The age of the firn air is directly related to the movement of the firn air.
Firn air models help calculate the firn air age using some parameters such
as temperature and accumulation rate. However, several studies found that
layering also affects the movement of firn air (e.g., Mitchell et al., 2015;
Schaller et al., 2017). This implies that physical properties of the ice may
affect the age of the firn air as well.
With regard to the lock-in and close-off processes, recent studies have
focused on snow layers and microstructure of the firn (Hörhold et al.,
2011; Gregory et al., 2014; Mitchell et al., 2015; Schaller et al., 2017).
Density variability on scales of millimeters to tens of centimeters is observed at all
polar sites. Hörhold et al. (2011) demonstrate that density variability
is caused by physical snow properties in the firn column. Several studies
have dealt with how snow density variations affect the transport of firn air
(Hörhold et al., 2011; Mitchell et al., 2015). Mitchell et al. (2015)
showed that the firn layering can affect the closure of pores and the
thickness of LIZ, but the relation between snow density variations and range
of firn air ages was not quantitatively examined.
In this study, we present firn air composition and δ15N-N2
from Styx Glacier, East Antarctica, to better understand the role of snow
density variations in the age of firn air. We also present X-ray density
data with millimeter resolution and compare them with δ18Oice and the closed-pore air composition in the LIZ.
We hypothesize that large snow density variations make the LIZ thicker and
facilitate preservation of old firn air at the Styx Glacier. This study will
help us better understand how the snow density layers of firn column affect
movement and preservation of firn air, and provide guidance on selecting
good sites for future firn air studies.
Materials and methodsFirn air sampling and gas mole fractions analysis
The firn air and ice core were collected at the Styx Glacier, East
Antarctica (73∘51.10′ S, 163∘41.22′ E, 1623 m a.s.l.), in
December of 2014 (Fig. 1). This site is located 85 km north of the Korean
Jang Bogo Station in the Southern Cross Mountains near the Ross Sea (Han et
al., 2015). The snow accumulation rate is ∼10 cm ice per year, calculated from the Styx16b ice chronology based on methane
correlation and tephra age tie-point and thinning functions (Yang et al.,
2018). The mean annual surface temperature was measured as -31.7∘
by borehole temperature logging at 15 m depth, 2 years after the ice core
drilling (Yang et al., 2018). Table 1 lists the characteristics of the Styx
Glacier and other firn air sampling sites. A total of 13 samples from the
surface to 64.8 m depth were collected. The firn air sampling device was
constructed, following the design of the University of Bern, Switzerland
(Schwander et al., 1993). Three vacuum pumps (two diaphragm pumps and one
metal bellows pump), several pressure gauges, stainless steel lines, and
vacuum valves were housed in an aluminum case to transfer to the polar site.
The pump system plays four major roles: (1) purging modern air from the
bottom of a borehole, (2) inflating the bladder to block the deep firn
layers from the atmosphere, (3) removing the contaminated air and extracting
the firn air, and (4) transporting firn air to a CO2 analyzer for
measurements of gas mole fractions and store it in firn air containers. The
bladder system is designed to be lowered into the borehole to seal the deep
firn layer(s) being sampled from the atmosphere. The bladder consists of a 4 m long rubber tube and metal caps on the top and bottom of the rubber tube. The
bladder's external diameter is 119.5 mm and the internal diameter is 114.5 mm.
The material of the tube is butyl rubber (BIIR), which can endure low
temperatures, providing no risk of sample contamination.
(a) Location map of study site, Styx Glacier, Antarctica, and (b) a photo of surface snow density layers. The thickness of the snow density layers varies horizontally. The top boundaries of the high-density layers are sharp (horizontal red dashed line). A hole on a high-density layer surface is indicated by a red dashed circle. The length of the black sharp pencil in panel (b) is 14.3 cm.
The firn air samples were collected in 3 L glass flasks at all
collection depths. However, to test preservation ability of the sample air
containers, SilcoCan canisters were also used at four depths (0, 35.36, 43.42,
53.95 m). Accurate mole fractions of CO2, CH4, and SF6 were
measured at the US National Oceanic and Atmospheric Administration (NOAA;
https://www.esrl.noaa.gov/, last access: 30 August 2015). The results for the two types of containers
show good agreement. δ15N of N2 was analyzed at Scripps
Institution of Oceanography for correcting gravitational fractionation
effect (Severinghaus et al., 2010).
Firn air transport model
We used the Center for Ice and Climate (CIC) firn air model which is a
one-dimensional advection–diffusion model to simulate how the air moves in
Styx firn column. In this model, there are four types of air transport in
the open porosity: (1) molecular diffusion, (2) vigorous mixing in the
convective zone, (3) advection, and (4) dispersion in the deep firn
(Buizert, 2012; Buizert and Severinghaus, 2016). The model uses the
stochastic bubble trapping formulation described by Mitchell et al. (2015).
CH4 in closed bubbles and total air content
measurements
CH4 mole fraction in the (closed) air bubbles in the firn was measured
at Seoul National University using a melt–refreeze air extraction method
(Yang et al., 2017). A total of 124 discrete firn samples (cross section of 8.5 cm × 3 cm, length of 3 cm, ∼35 g) were prepared from four
different depth intervals in the lock-in zone (54.59–55.34, 58.11–59.05,
59.86–60.55, 64.02–65.25 m). All ice samples were cut and trimmed by
∼2.5 mm with a band saw to remove contaminants on the surface
ice. Then, the ice samples were inserted into the glass flasks attached to
the gas extraction line. The pump system evacuated air in the flask placed
in a cooled ethanol bath at -70∘ for 20 min. The evacuation time
was limited to 20 min to prevent gas loss due to pore openings by
sublimation. After the pressure dropped below 0.027 Pa, the ice samples in
the glass flask were melted and air in the bubbles was extracted. After the
melting was finished, we refroze the ice using a cooled ethanol bath to
release the gas dissolved in the ice melt. Finally, the extracted air was
injected into the sample loop of the gas chromatograph equipped with a flame
ionization detector (FID). The calibration curve of the gas chromatography FID was
calculated by the standard air prepared at NOAA with a CH4 mole
fraction of 895 ppb on the NOAA04 scale (Dlugokencky et al., 2005).
Total air content of the firn ice samples was analyzed simultaneously with
CH4 mole fraction using the wet extraction system at Seoul National University (SNU). The total air
content was expressed as the volume of air trapped in the closed pores of
unit mass of firn ice sample (in units of milliliters per gram of ice at STP conditions).
The volume of air extracted from a firn ice sample was calculated by the
ideal gas law with the internal pressure, volume, and temperatures of the
sample flasks and vacuum lines. The pressure of extracted air was measured
by a pressure manometer connected to the sample loop of the gas chromatography FID. As no
direct measure of temperature was available, the temperature of extracted
air was assumed to be identical to the surrounding temperatures; the ethanol
temperature was used for the sample flasks, room temperature for vacuum
lines, and valve box temperature (50 ∘C) for the sample loop. In
this study, the corrections for bubble-cut effect and thermal gradient
within vacuum line were not considered. A more detailed description of the
protocols of total air content measurements is described in Yang (2019).
Analysis for stable isotopes of ice
After completing the measurements of the CH4 mole fraction in air, the
meltwater was put into cleaned 125 mL bottles and analyzed for water stable
isotope ratios at the Korea Polar Research Institute (KOPRI) using a cavity
ring-down spectroscopy (CRDS, L1102-i, Picarro, USA) system. We performed
the same analysis for the snow pit samples, but without CH4 analysis.
The data are presented here as δ-notations:
δ18O=18O/16Osample/18O/16OVSMOW-1.
The firn ice melt was filled into a 400 µL insert in a 2 mL glass vial
using a syringe filter. The autosampler transported the ice melt samples in
the insert to the vaporizer about 180 nL at a time. The samples with the
liquid state were transferred to the cavity after being converted into the
water vapor in a vaporizer at 110∘. The measurement precision
evaluated by measuring an in-house standard repeatedly (n=12) was
0.08 ‰ (1σ standard deviation).
X-ray firn density measurement
We obtained high-resolution density data using the X-ray transmission method
reported by Hori et al. (1999) for the firn ice at various depth intervals.
This method is advantageous because it can measure continuously and
nondestructively. The X-ray beam penetrates the ice samples, and the
detector on the opposite side analyzes the intensity of the beam. To make
equal thickness for each core section, upper and side parts of the half-circle-shaped core were shaved by a microtome. After putting the precut ice
core on a rack, we set the rate of measurement at 50 mm min-1, and
finally obtained 1 mm resolution density data.
ResultsLayered stratigraphy
We examined a snow pit, located 10 m away from the main ice core borehole, 2 years after drilling to understand the physical properties such as layers,
density, and ice grain size of the upper firn at the Styx site. We scratched the
snow wall by hand to remove soft layers and enhance the visibility of hard
layers (Fig. 2a). The soft layers are presumed to be depth hoar, and the
hard ones are wind crusts (Fig. 2b). The alternating layers repeat with
intervals of a few centimeters to 20 cm. The top boundaries of the
hard layers are sharp and extend horizontally about a meter, but the bottom
boundaries are not well defined due to gradual density changes. 10 cm resolution density data were obtained by a density cutter (Proksch et
al., 2016). The soft layers are coarsely grained, while the hard ones are
finely grained (Fig. 2b–d).
Snow pit photos at Styx Glacier. (a) The snow pit with dimensions of 280 cm × 65 cm × 220 cm (length × width
× height). (b) The illustration of qualitatively defined hard (dark blue) and soft (pale blue) layers observed in the top 180 cm depth interval. Progressive blue color changes indicate a gradual density decrease with depth. The red line is a 10 cm resolution density profile. (c) Coarse grains observed in a soft layer. The grains were placed on a black glove. (d) Enlarged snow layers. Dashed red lines indicate top boundaries of fine-grained hard layers. (e, f) Stable isotope ratio (δ18O) of snow profiles at the main core and a snow pit 100 m away from the main ice core borehole, respectively.
Firn gas sampling and the age of firn air
We calibrate the depth–diffusivity profile in the model using trace gases
with a well-known atmospheric history (Buizert et al., 2012; Trudinger et
al., 2013; Witrant et al., 2012). The atmospheric time series from
well-dated firn air (MacFarling Meure et al., 2006) and instrument
measurement records (NOAA; https://www.esrl.noaa.gov/) were used for
calibration. The simulated mole fraction profiles match well with the
observations (Fig. 3). CO2, CH4, SF6, and δ15N-N2 distributions in firn air were modeled. The model does not
include thermal fractionation, and therefore provides a poor fit to the
δ15N-N2 data in the upper firn where seasonal temperature
gradients fractionate the gases. Fitting the barometric equation to the
δ15N data of the upper diffusive zone suggests a convective
zone thickness of approximately 3 m. This is within the typical range of
observed convective zones, but perhaps lower than expected for a very windy
site (Kawamura et al., 2006). The firn air age (black curves in Fig. 3)
slowly increases with depth at the diffusive zone because it mixes with
fresh atmospheric air on the surface mostly by molecular diffusion (Blunier
and Schwander, 2000). In contrast, the firn air age rapidly increases within
the LIZ at a rate similar to that of the ice age. The gas age distribution
of Styx ice at zCOD is narrower than the other sites where old firn air
is reported (Fig. 4); we simulate a spectral width of 15.9, 22.8, and 45.5 years at Styx, South Pole, and Megadunes, respectively. This means that the
past atmospheric history of trace gases can in principle be reconstructed
with higher resolution at Styx than at the other old-air firn sites.
CO2, CH4, and SF6 mole fractions and δ15N of N2 measurements
(circles), as well as model results (solid line) for the Styx firn air (air in open
porosity). Black lines are modeled ages for the gas species.
Comparison of model-simulated CO2 age distributions at Styx (this study), South Pole (Battle et al., 1996),
and Megadunes (Severinghaus et al., 2010).
We estimate the age of samples in two ways. First, after calibrating the
firn air model, we can derive the mean sample age from the simulated gas age
distribution. At the deepest Styx sampling depth (64.8 m) we simulate a mean
CO2 age of 102 years and a mean CH4 age of 97 years; the CH4
age is younger than the CO2 age due to the higher diffusivity of
CH4. Second, we can estimate the sample ages by comparing the measured
trace gas concentrations directly to the atmospheric histories of these
gases – this age has been called the “effective age” (Trudinger et al.,
2013). The lowest CO2 mole fraction of 305.18 ppm at a depth of 64.8 m
(304 ppm after correcting for gravitational enrichment) corresponds to the
year 1921 and an effective age of 93 years (relative to sampling year 2014) on
the Law Dome ice core record (MacFarling Meure et al., 2006; Rubino et al.,
2019). The CH4 mole fraction of 943.36 ppb at the same depth (946.5 ppb
after gravitational correction) corresponds to an effective age of 96 years
(MacFarling Meure et al., 2006) (Fig. 3a, b). The second method provides
younger ages because the growth rate in the atmospheric mixing ratios of
these gases has increased over time, biasing the effective ages towards
younger values (Trudinger et al., 2002). Table 1 lists effective CO2
ages in the deepest firn air sample for several sites; we here compare the
effective CO2 age between sites rather than the modeled mean age, as it
is purely empirical and does not rely on model assumptions.
Only a few firn air sites have effective CO2 ages around 93 years or
older: 91 years from the South Pole (Battle et al., 1996) and 129 years from
Megadunes (Severinghaus et al., 2010; Table 1). These sites are
located in interior Antarctica and have low annual mean temperatures and low
snow accumulation rates (Table 1). Firn densification takes a long time if
snow accumulation and/or temperature are low, therefore firn air can be
preserved for a long time without being trapped. In contrast, the Styx site
is located near the coast and has relatively high snowfall, and therefore
the age of 93 years is very unusual. Sites of comparable climate
characteristics typically have an oldest firn air age of around 40 years.
This indicates that there may be other factors that can permit preservation
of the old firn air at Styx Glacier.
Density layering and its influence on bubble trapping
Firn density is the primary control on the bubble close-off process. Density
layering leads to staggered bubble trapping, with high-density layers
closing off before low-density ones (Stauffer et al., 1985; Etheridge et al.,
1992; Mitchell et al., 2015; Rhodes et al., 2016).
(a–d)CH4 mole fraction in closed
pores ([CH4]cl) (red line) and total air content (air
volume per ice weight) (blue line) in the lock-in zone of Styx Glacier. (e) Comparison of density with [CH4]cl and total air content near zCOD. A small dashed-box in panel (d) indicates the depth interval of Fig. 5e.
Because the mole fractions of atmospheric greenhouse gases (CO2,
CH4, N2O) have increased during the last century, we may obtain
information on the timing of the closure of the bubbles from the greenhouse
gas mole fractions of the air trapped in closed bubbles. In this study, we
used the CH4 concentration in closed bubbles ([CH4]cl) and
the total air content of the firn ice as indicators of the close-off
process. The density and [CH4]cl show an anticorrelation (Fig. 5). Our results confirm the CH4 concentration-total air content
relation observed in West Antarctic Ice Sheet (WAIS) Divide firn ice
(Mitchell et al., 2015). High-density layers reach the lock-in and close-off
densities at shallower depths than low-density layers. Thus, air bubbles are
trapped at shallower depths in high-density layers. Early trapped bubbles
preserve older air with lower greenhouse gas mole fractions. Higher air
content is expected in the high-density layers, in which open porosity is
small and closed porosity is large (Fig. 5). However, we cannot entirely
exclude the possibility of some post-coring bubble close-off (Aydin et al.,
2010). High open porosity in low-density layers may have more chances to
trap modern ice storage air, which has a higher mole fraction of CH4 than
atmospheric background levels.
Figure 5a shows [CH4]cl and total air content in the LIZ of the
Styx firn. [CH4]cl generally decreases with depth and the centimeter-scale
variability is reduced in the deep layers, while the total air content
generally increases with depth. The [CH4]cl greater than CH4
mole fraction in neighboring firn air (green line in Fig. 5a–d) indicates
part of bubbles formed after coring and increased the [CH4]cl, as
previous studies also observed (Mitchell et al., 2015; Rhodes et al., 2013).
Most [CH4]cl data show large centimeter-scale variations (Fig. 5). The
highs and lows of [CH4]cl repeat with cycles of 6 to 24 cm (Fig. 5e). Note that the layering observed in the snow pit likewise shows
irregular intervals (Fig. 2b). From the layer spacing, we conclude that
bubble trapping at Styx is not controlled by annual layers (Sect. 4), as
was observed at Law Dome (Etheridge et al., 1992).
The evolution of CH4 in the closed porosity may give information on how
the snow layers can induce inhomogeneous records and help constrain the gas
age distribution in ice (Fourteau et al., 2017). However, the details are
beyond the scope of this study and we will focus on the firn air age in the
open porosity.
High-resolution firn density measurements
The X-ray measurements show highly variable density on centimeter scales. We
converted the high-resolution density to total porosity using the following
equation:
Φtotal=1-ρρice,
where ρ is density of porous ice, ρice is density of
bubble-free ice (919 kg m-3), and Φ is porosity. We test the idea that
the lock-in zone corresponds to the depth range bounded by the first closed
layer (porosity below 0.1) on the shallow side, and the last open layer
(porosity above 0.1) on the deep side.
High-resolution X-ray density data obtained from the
lock-in zone. Panels (b) and (c) are enlarged portions of panel (a). Black lines show
individual density data, while the red lines are 1 cm running means. Blue
and orange lines represent the boundaries of the LIZ estimated from the gas
compositions (between two vertical blue lines) and the critical porosity
thresholds (between two orange vertical lines), respectively (see Sect. 3.4).
At Styx Glacier, the shallowest depth where the running mean of total
porosity with a 1 cm thick window reaches below 0.1 is 48.1 m (Fig. 6a and
b). It is approximately 4.3 m shallower than the LID of 52.4 m defined by
the modeled firn air δ15N-N2 profile. Meanwhile, the
deepest point, where the running mean (with a 1 cm thick window) exceeds
0.1, is at 63.7 m (Fig. 6a and c), which is shallower than the zCOD
of 64.8 m defined by the deepest successful firn pumping depth. Although the
LID and zCOD from the density data are different from those defined by
firn air data, the thickness of LIZ from density data (between the two
orange lines in Fig. 6a) is comparable to that from firn air analysis
(between two blue lines in Fig. 6) (15.6 vs. 12.4 m). The offsets of the LIZ
about 1–4 m between those from total porosity and the firn air measurement
may be due to, for example, small calibration offsets in the density data set or
the fact that actual critical porosity may be variable and depend on the
study site or on horizontal snow density variations and the horizontal
extent of diffusion-impeding layers. The similarity in the LIZ thicknesses
from the two methods supports the idea that the large variations in density
can increase the LIZ thickness by shallowing LID and/or deepening the
zCOD. The thick LIZ eventually permits storing old firn air at Styx
(Table 1). Usually, the LIZ thickness increases with a snow accumulation
rate (Witrant et al., 2012), presumably because at high-accumulation sites
density variability in the lock-in zone tends to increase (Hörhold et al.,
2011). Refrozen melt layers may also act as high density, diffusion-impeding
layers allowing for older firn air to be sampled as observed on Devon Island
(Witrant et al., 2012). We demonstrate here that the snow density
variability is an important factor in determining the firn air age. We
suggest that sites with higher density variations at the LIZ have a high
possibility of a thick LIZ and therefore old firn air, even in warm,
relatively high-precipitation coastal climates.
Discussions
To quantitatively compare density variability of Styx snow with that at
other glacier sites, we may use the standard deviation of densities (σρ) near the mean air-isolation density (Hörhold et al., 2011;
Martinerie et al., 1992). The mean density at the mean air-isolation depth
(ρcrit) can be related to mean annual temperature (T in kelvin)
using the following equation, which is empirically obtained from air content
measurements (Martinerie et al., 1992):
ρcrit=1ρice+7.6××10-4×T-0.057-1,
where ρice is the density of bubble-free pure ice.
Comparison of standard deviation of density (σρ) at critical density (ρcrit). For
data from all other sites, except Styx, refer to Hörhold et al. (2011).
Although this equation cannot provide exact ρcrit, we can take
advantage by estimating the density at LIZ without gas chemistry data
(Hörhold et al., 2011). We note that Martinerie et al. (1994) suggested
slightly different coefficients for the equation based on a different set of
data; however, the results do not significantly change our conclusions. We
also note that Bréant et al. (2017) used an equation relating ice
density at LID to snow accumulation rate; however, we prefer to use the
relation of temperature–ice density at LIZ by Martinerie et al. (1992)
because the latter is more relevant to the ice density at LIZ. Using the
Styx high-resolution X-ray density data at the depth interval of 43.13–66.97 m,
we calculated the standard deviation of densities (σρ). For
each σρ, we used 1000 density data points (Fig. 7) as
Hörhold et al. (2011) did for the other sites listed in Table 2. At
Styx, ρcrit is 821.68 kg m-3 according to Eq. (4), and
the standard deviation of densities at ρcrit (σρ, ρcrit) is 19.33±1.87 kg m-3, which is greater
than those at the other previously studied sites (Hörhold et al., 2011;
Fig. 7, Table 2). The high σρ and ρcrit at Styx
likely facilitate the thick LIZ and old firn air.
Density variability calculated from 1000 depth points and
their average density. The standard deviation at the critical density
(821.68 kg m-3) calculated from the approximate
second-order polynomial (R=0.84) is 19.33±1.87 kg m-3. The blue and red areas are the density ranges
near the LID (52.38–52.48 m) and the zCOD (64.91–65.01 m), respectively.
A high-density (low-density) layer at the surface may become a low-density
(high-density) layer (Freitag et al., 2004; Fujita et al., 2009) at density
of 600–650 kg m-3, which occurs at shallower depths than LIZ
(Hörhold et al., 2011). Thus, vertical snow layering at the surface may not
directly give information about density variability at LIZ (Hörhold et
al., 2011). However, conditions for snow layering at the surface still may
give us clues on the density variability at LIZ. The conditions may include
redistribution of snow by wind and formation of wind and/or radiation crusts
(Martinerie et al., 1992; Hörhold et al., 2011). To test the possibility
of seasonal causes, we analyzed stable isotopes of surface snow (δ18O) because the surface δ18O generally follows seasonal
variation (depleted in winter and enriched in summer). Figure 2e and f
show the stable isotope profiles of snow (δ18O) at Styx
Glacier, which are ∼100 m apart; one is from a snow pit
made in 2014 and the other is from the main ice core drilled in 2014. The
δ18O profiles commonly show cycles with intervals of
∼40 cm yr-1, given that local maxima of δ18O
indicate summer, and minima winter layers. Meanwhile, the repetition of the
density layers has 20 cycles (high- and low-density layer pairs) in the
top 180 cm at the snow pit (Fig. 2b). Using a snow accumulation rate
of ∼40 cm yr-1 in recent years, the density layers have
four to five cycles per year, indicating that the formation of snow
density layers is mainly controlled by nonseasonal factors.
A blizzard occurred during the ice coring campaign in December of 2014. We
observed that the blizzard strongly reworked the surface snow. The automatic
weather system (AWS) installed within 10 m from the borehole site shows that
blizzard events (wind speed >15 m s-1) took place on
29 December in 2015 and 23 May, 26 June, 17 August, and 7 September in 2016
(Fig. S1 in the Supplement). The number of blizzard events in a year is similar to the mean
density layer cycle of four to five per year. Although Blizzards occur
more frequently in winter, the frequency of five per year is comparable to the
number of the density layer cycles of four to five per year. During
the blizzard events, westerly wind prevailed, and snow particles may have
been redeposited with a sorted size distribution (large grains in the bottom
and small grains on the top) similar to winnowing seen in sedimentary
records (Sepp Kipfstuhl, personal communication, 2016). Between the blizzards, the
solar radiation and temperature gradient may have facilitated the diagenesis
of the snow layers (Alley, 1988; Fegyveresi et al., 2018). During the
diagenesis processes, fine and coarse flake layers may form high-density and
low-density layers, respectively. In summary, blizzard events may have
played a major role in forming snow density layers
Conclusions and implications
About 93-year-old firn air (effective CO2 age) was found at Styx
Glacier, East Antarctica, located near the Ross Sea coast. This is of great
scientific interest because such old firn air is commonly only found in the
inland sites such as the South Pole and Megadunes. The thickness of Styx LIZ
is relatively greater than that at other sites where snow accumulation and
temperature are similar. The thicker LIZ made the Styx firn layer preserve
old firn air because the age of stagnant firn air rapidly increases with
depth in the LIZ as air exchange with the atmosphere has stopped. We
hypothesized that the high snow density variations in the LIZ of Styx
Glacier made the thick LIZ and old firn air. To test the hypothesis, we
conducted high-resolution X-ray density measurements. We argue that the
thick LIZ is related to the high density variations at Styx Glacier. We also
examined why high snow density variability developed at the Styx site. The
effect of strong wind (e.g., blizzards) may facilitate the density layer
formation. It is likely that old firn air (>55 years) can be
found in areas where climatological conditions are favorable for high snow
density variations at LIZ even when the sites are located near the coast. We
may take advantage by sampling and transportation from the coastal sites
because logistics is easier for those sites. Theoretically, the oldest firn
air should be available at a site that has both strong layering and a low
accumulation rate. Older firn air, perhaps as old as 150 years, may still
be found under such suitable conditions on the Antarctic continent.
Data availability
The firn air composition data will be available at the NOAA Paleoclimatology dataset portal in the near future and can be accessed in the meantime by contacting the corresponding author. Other ice chemistry and density data are available upon request as well.
The supplement related to this article is available online at: https://doi.org/10.5194/tc-13-2407-2019-supplement.
Author contributions
JA and YJ designed and led the research; YJ and CB performed firn air diffusion modeling; SBH, HL, SH, JY, YI, AH, YH, SJJ, PT, TC, and SH produced analytical data; CB, SK, JA, and all the other co-authors participated in data interpretation.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank Jeff Severinghaus and Ross Beaudette at Scripps
Institution of Oceanography for accurate δ15N-N2 analysis,
and Jacob Schwander at University of Bern for kind advice in constructing
the SNU firn air sampling device. We also thank Mauro Rubino and the
anonymous reviewer for their constructive comments.
Financial support
This research has been supported by the Korea Polar Research Institute (grant no. PE 18040) and the National Research Foundation of Korea (grant no. NRF-2018R1A2B3003256).
Review statement
This paper was edited by Joel Savarino and reviewed by Mauro Rubino and one anonymous referee.
ReferencesAlley, R. B.: Concerning the Deposition and Diagenesis of Strata in Polar
Firn, J. Glaciol., 34, 283–290,
10.3189/s0022143000007024, 1988.Aydin, M., Montzka, S. A., Battle, M. O., Williams, M. B., De Bruyn, W. J., Butler, J. H., Verhulst, K. R., Tatum, C., Gun, B. K., Plotkin, D. A., Hall, B. D., and Saltzman, E. S.: Post-coring entrapment of modern air in some shallow ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and ice cores, Atmos. Chem. Phys., 10, 5135–5144, 10.5194/acp-10-5135-2010, 2010.
Battle, M., Bender, M., Sowers, T., Tans, P. P., Butler, J. H., Elkins, J. W.,
Ellis, J. T., Conway, T., Zhang, N., Lang, P., and Clarke, A. D.: Atmospheric
gas concentrations over the past century measured in air from firn at the
South Pole, Nature, 383, 231–235, 1996.Battle, M. O., Severinghaus, J. P., Sofen, E. D., Plotkin, D., Orsi, A. J., Aydin, M., Montzka, S. A., Sowers, T., and Tans, P. P.: Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide, Atmos. Chem. Phys., 11, 11007–11021, 10.5194/acp-11-11007-2011, 2011.
Blunier, T. and Schwander, J.: Gas enclosure in ice: age difference and
fractionation, in: Physics of Ice Core Records, edited by: Hondoh, T.,
Hokkaido University Press, Sapporo, Japan, 307–326, 2000.Bréant, C., Martinerie, P., Orsi, A., Arnaud, L., and Landais, A.: Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities, Clim. Past, 13, 833–853, 10.5194/cp-13-833-2017, 2017.
Buizert, C.: The influence of firn air transport processes and radiocarbon
production on gas records from polar firn and ice, PhD, Faculty of Science,
University of Copenhagen, Denmark, Copenhagen, 175 pp., 2012.Buizert, C. and Severinghaus, J. P.: Dispersion in deep polar firn driven by synoptic-scale surface pressure variability, The Cryosphere, 10, 2099–2111, 10.5194/tc-10-2099-2016, 2016.Buizert, C., Martinerie, P., Petrenko, V. V., Severinghaus, J. P., Trudinger, C. M., Witrant, E., Rosen, J. L., Orsi, A. J., Rubino, M., Etheridge, D. M., Steele, L. P., Hogan, C., Laube, J. C., Sturges, W. T., Levchenko, V. A., Smith, A. M., Levin, I., Conway, T. J., Dlugokencky, E. J., Lang, P. M., Kawamura, K., Jenk, T. M., White, J. W. C., Sowers, T., Schwander, J., and Blunier, T.: Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland, Atmos. Chem. Phys., 12, 4259–4277, 10.5194/acp-12-4259-2012, 2012.
Craig, H., Horibe, Y., and Sowers, T.: Gravitational separation of gases and
isotopes in polar ice caps, Science, 242, 1675–1678, 1988.Dlugokencky, E. J., Myers, R. C., Lang, P. M., Masarie, K. A., Crotwell, A.
M., Thoning, K. W., Hall, B. D., Elkins, J. W., and Steele, L. P.:
Conversion of NOAA atmospheric dry air CH4 mole fractions to a
gravimetrically prepared standard scale, J. Geophys. Res., 110, D18306,
10.1029/2005JD006035, 2005.Etheridge, D. M., Pearman, G. I., and Fraser, P. J.: Changes in tropospheric
methane between 1841 and 1978 from a high accumulation-rate Antarctic ice
core, Tellus B, 44, 282–294, 10.1034/j.1600-0889.1992.t01-3-00006.x,
1992.Etheridge, D. M., Steele, L. P., Langenfelds, R. L., Francey, R. J.,
Barnola, J. M., and Morgan, V. I.: Natural and anthropogenic changes in
atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn,
J. Geophys. Res., 101, 4115–4128, 10.1029/95jd03410, 1996.Fegyveresi, J. M., Alley, R. B., Muto, A., Orsi, A. J., and Spencer, M. K.: Surface formation, preservation, and history of low-porosity crusts at the WAIS Divide site, West Antarctica, The Cryosphere, 12, 325-341, 10.5194/tc-12-325-2018, 2018.Fourteau, K., Faïn, X., Martinerie, P., Landais, A., Ekaykin, A. A., Lipenkov, V. Ya., and Chappellaz, J.: Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions, Clim. Past, 13, 1815–1830, 10.5194/cp-13-1815-2017, 2017.
Freitag, J., Wilhelms, F., and Kipfstuhl, S.: Microstructure dependent
densification of polar firn derived from X-ray microtomography, J. Glaciol.,
50, 243–250, 2004.Fujita, S., Okuyama, J., Hori, A., and Hondoh, T.: Metamorphism of
stratified firn at dome fuji, antarctica: A mechanism for local insolation
modulation of gas transport conditions during bubble close off, J. Geophys.
Res., 114, F03023, 10.1029/2008JF001143, 2009.Goujon, C., Barnola, J.
M., and Ritz, C.: Modeling the densification of polar firn including heat
diffusion: Application to closeoff characteristics and gas isotopic
fractionation for Antarctica and Greenland sites, J. Geophys. Res.-Atmos.,
108, 4792, 10.1029/2002JD003319, 2003.Gregory, S. A., Albert, M. R., and Baker, I.: Impact of physical properties and accumulation rate on pore close-off in layered firn, The Cryosphere, 8, 91–105, 10.5194/tc-8-91-2014, 2014.
Han, Y., Jun, S. J., Miyahara, M., Lee, H.-G., Ahn, J., Chung, J. W., Hur,
S. D., and Hong, S. B.: Shallow ice-core drilling on Styx glacier, northern
Victoria Land, Antarctica in the 2014–2015 summer, Journal of the Geological
Society of Korea, 51, 343–355, 2015Hörhold, M. W., Kipfstuhl, S., Wilhelms, F., Freitag, J., and Frenzel,
A.: The densification of layered polar firn, J. Geophys.
Res.-Earth, 116, F01001, 10.1029/2009jf001630, 2011.Hori, A., Tayuki, K., Narita, H., Hondoh, T., Fujita, S., Kameda, T., Shoji,
H., Azuma, N., Kamiyama, K., Fujii, Y., Motoyama, H., and Watanabe, O.: A
detailed density profile of the Dome Fuji (Antarctica) shallow ice core by
X-ray transmission method, Ann. Glaciol., 29, 211–214,
10.3189/172756499781821157, 1999.
Johnsen, S. J., Clausen, H. B., Cuffey, K. M., Hoffmann, 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., vol. 159, 121–140, Hokkaido University Press, Sapporo,
Japan, 2000.
Kawamura, K., Severinghaus, J. P., Ishidoya, S., Sugawara, S., Hashida, G., Motoyama, H., Fujii, Y., Aoki, S., and Nakazawa, T.: Convective mixing of air in firn at four polar sites, Earth Planet. Sc. Lett., 244, 672–282, 2006.Kawamura, K., Severinghaus, J. P., Albert, M. R., Courville, Z. R., Fahnestock, M. A., Scambos, T., Shields, E., and Shuman, C. A.: Kinetic fractionation of gases by deep air convection in polar firn, Atmos. Chem. Phys., 13, 11141–11155, 10.5194/acp-13-11141-2013, 2013.Landais, A., Barnola, J. M., Kawamura, K., Caillon, N., Delmotte, M., Van
Ommen, T., Dreyfus, G., Jouzel, J., Masson-Delmotte, V., Minster, B.,
Freitag, J., Leuenberger, M., Schwander, J., Huber, C., Etheridge, D., and
Morgan, V.: Firn-air δ15N in modern polar sites and
glacial–interglacial ice: a model-data mismatch during glacial periods in
Antarctica?, Quaternary Sci. Rev., 25, 49–62,
10.1016/j.quascirev.2005.06.007, 2006.Laube, J. C., Hogan, C., Newland, M. J., Mani, F. S., Fraser, P. J., Brenninkmeijer, C. A. M., Martinerie, P., Oram, D. E., Röckmann, T., Schwander, J., Witrant, E., Mills, G. P., Reeves, C. E., and Sturges, W. T.: Distributions, long term trends and emissions of four perfluorocarbons in remote parts of the atmosphere and firn air, Atmos. Chem. Phys., 12, 4081–4090, 10.5194/acp-12-4081-2012, 2012.MacFarling Meure, C., Etheridge, D., Trudinger, C., Steele, P., Langenfelds,
R., van Ommen, T., Smith, A., and Elkins, J.: Law Dome CO2, CH4 and N2O ice
core records extended to 2000 years BP, Geophys. Res. Lett., 33, L14810,
10.1029/2006GL026152, 2006.Martinerie, P., Raynaud, D., Etheridge, D. M., Barnola, J. M., and
Mazaudier, D.: Physical and Climatic Parameters which Influence the Air
Content in Polar Ice, Earth Planet. Sc. Lett., 112, 1–13,
10.1016/0012-821X(92)90002-D, 1992.Martinerie, P., Lipenkov, V. Y., Raynaud, D., Chappellaz, J., Barkov, N. I.,
and Lorius, C.: Air content paleo record in the Vostok ice core
(Antarctica): A mixed record of climatic and glaciological parameters, J.
Geophys. Res., 99, 10565–10576, 10.1029/93JD03223, 1994.Mitchell,
L. E., Buizert, C., Brook, E. J., Breton, D. J., Fegyveresi, J., Baggenstos,
D., Orsi, A., Severinghaus, J., Alley, R. B., Albert, M., Rhodes, R. H.,
McConnell, J. R., Sigl, M., Maselli, O., Gregory, S., and Ahn, J.: Observing
and modeling the influence of layering on bubble trapping in polar firn, J.
Geophys. Res.-Atmos., 120, 2558–2574, 10.1002/2014JD022766,
2015.
Petit, J. R., Jouzel, J., Raynnaud, D., Barkov, N. I., Barnola, J.-M.,
Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte,
M., Kotlyakov, V. M., Legrand, M., Lipenkov, V. Y., Lorius, C., Pepin, L.,
Ritz, C., Saltzman, E., and Stievenard, M.: Climate and atmospheric history
of the past 420 000 years from the Vostok ice core, Antarctica, Nature, 399,
429–436, 1999.Proksch, M., Rutter, N., Fierz, C., and Schneebeli, M.: Intercomparison of snow density measurements: bias, precision, and vertical resolution, The Cryosphere, 10, 371–384, 10.5194/tc-10-371-2016, 2016.
Rhodes, R. H., Fain, X., Stowasser, C., Blunier, T., Chappellaz, J.,
McConnell, J. R., Romanini, D., Mitchell, L. E., and Brook, E. J.:
Continuous methane measurements from a late Holocene Greenland ice core:
atmospheric and in-situ signals, Earth Planet. Sc. Lett., 368, 9–19, 2013.Rhodes, R. H., Faïn, X., Brook, E. J., McConnell, J. R., Maselli, O. J., Sigl, M., Edwards, J., Buizert, C., Blunier, T., Chappellaz, J., and Freitag, J.: Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation, Clim. Past, 12, 1061–1077, 10.5194/cp-12-1061-2016, 2016.Rubino, M., Etheridge, D. M., Thornton, D. P., Howden, R., Allison, C. E., Francey, R. J., Langenfelds, R. L., Steele, L. P., Trudinger, C. M., Spencer, D. A., Curran, M. A. J., van Ommen, T. D., and Smith, A. M.: Revised records of atmospheric trace gases CO2, CH4, N2O, and δ13C-CO2 over the last 2000 years from Law Dome, Antarctica, Earth Syst. Sci. Data, 11, 473–492, 10.5194/essd-11-473-2019, 2019.Schaller, C. F., Freitag, J., and Eisen, O.: Critical porosity of gas enclosure in polar firn independent of climate, Clim. Past, 13, 1685–1693, 10.5194/cp-13-1685-2017, 2017.
Schwander, J.: The transformation of snow to ice and the occlusion of gases,
Environ. Rec. Glaciers Ice Sheets, 8, 53–67, 1989.Schwander, J., Barnola, J.-M., Andrié, C., Leuenberger, M., Ludin, A.,
Raynaud, D., and Stauffer, B.: The age of the air in the firn and the ice at
Summit, Greenland, J. Geophys. Res.-Atmos., 98,
2831–2838, 10.1029/92jd02383, 1993.Severinghaus, J. P., Grachev, A., and Battle, M.: Thermal fractionation of
air in polar firn by seasonal temperature gradients, Geochem. Geophy.
Geosy., 2, 1048, 10.1029/2000GC000146, 2001.Severinghaus, J. P., Albert, M. R., Courville, Z. R., Fahnestock, M. A.,
Kawamura, K., Montzka, S. A., Mühle, J., Scambos, T. A., Shields, E.,
Shuman, C. A., Suwa, M., Tans, P., and Weiss, R. F.: Deep air convection in
the firn at a zero-accumulation site, central Antarctica, Earth Planet. Sc.
Lett., 293, 359–367, 10.1016/j.epsl.2010.03.003, 2010.
Sowers, T., Bender, M., Raynaud, D., and Korotkevich, Y. S.: Delta n-15 of
n2 in air trapped in polar ice – a tracer of gas-transport in the firn and
a possible constraint on ice age-gas age-differences, J. Geophys.
Res.-Atmos., 97, 15683–15697, 1992.Stauffer, B., Schwander, J., and Oeschger, H.: Enclosure of air during
metamorphosis of dry firn to ice, Ann. Glaciol., 6, 108–112,
10.3189/1985AoG6-1-108-112, 1985.Trudinger, C. M., Enting, I. G., Etheridge, D. M., Francey, R. J.,
Levchenko, V. A., Steele, L. P., Raynaud, D., and Arnaud, L.: Modeling air
movement and bubble trapping in firn, J. Geophys. Res.-Atmos., 102,
6747–6763, 10.1029/96JD03382, 1997.Trudinger, C. M., Enting, I. G., Rayner, P. J., and Francey, R. J.: Kalman filter analysis of ice core data
2. Double deconvolution of CO2 and δ13C measurements, J. Geophys. Res., 107, 4423, 10.1029/2001JD001112, 2002.Trudinger, C. M., Enting, I. G., Rayner, P. J., Etheridge, D. M., Buizert, C., Rubino, M., Krummel, P. B., and Blunier, T.: How well do different tracers constrain the firn diffusivity profile?, Atmos. Chem. Phys., 13, 1485–1510, 10.5194/acp-13-1485-2013, 2013.Witrant, E., Martinerie, P., Hogan, C., Laube, J. C., Kawamura, K., Capron, E., Montzka, S. A., Dlugokencky, E. J., Etheridge, D., Blunier, T., and Sturges, W. T.: A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behaviour at eleven polar sites, Atmos. Chem. Phys., 12, 11465–11483, 10.5194/acp-12-11465-2012, 2012.
Yang, J. W.: Paleoclimate reconstructions from greenhouse gas and borehole
temperature of polar ice cores, and study on the origin of greenhouse gas in
permafrost ice wedges, PhD thesis, Department of Earth and Environmental
Sciences, Seoul National University, Seoul, Republic of Korea, 188 pp., 2019.Yang, J.-W., Ahn, J., Brook, E. J., and Ryu, Y.: Atmospheric methane control mechanisms during the early Holocene, Clim. Past, 13, 1227–1242, 10.5194/cp-13-1227-2017, 2017.Yang, J. W., Han, Y., Orsi, A. J., Kim, S. J., Han, H., Ryu, Y., Jang, Y.,
Moon, J., Choi, T., Hur, S. D., and Ahn, J.: Surface temperature in
twentieth century at the Styx Glacier, northern Victoria Land, Antarctica,
from borehole thermometry, Geophys. Res. Lett., 45, 9834–9842,
10.1029/2018GL078770, 2018.