CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-13-437-2017A glaciochemical study of the 120 m ice core from Mill Island, East AntarcticaInoueManamanainoue@gmail.comCurranMark A. J.MoyAndrew D.https://orcid.org/0000-0002-7664-9960van OmmenTas D.FraserAlexander D.PhillipsHelen E.https://orcid.org/0000-0002-2941-7577GoodwinIan D.Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart, Tasmania 7005, AustraliaInstitute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania 7001, AustraliaAustralian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, AustraliaInstitute of Low Temperature Science, Hokkaido University, N19, W8, Kita-ku Sapporo 060-0819, JapanMarine Climate Risk Group, Department of Environment and Geography, Macquarie University, Eastern Road, New South Wales 2109, AustraliaMana Inoue (manainoue@gmail.com)9May201713543745327June201615September20168December201619February2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/articles/13/437/2017/cp-13-437-2017.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/13/437/2017/cp-13-437-2017.pdf
A 120 m ice core was drilled on Mill Island, East Antarctica
(65∘30′ S, 100∘40′ E) during the 2009/2010 Australian
Antarctic field season. Contiguous discrete 5 cm samples were measured for
hydrogen peroxide, water stable isotopes, and trace ion chemistry. The ice
core was annually dated using a combination of chemical species and water
stable isotopes. The Mill Island ice core preserves a climate record covering
97 years from 1913 to 2009 CE, with a mean snow accumulation of 1.35 m
(ice-equivalent) per year (mIE yr-1). This northernmost East Antarctic
coastal ice core site displays trace ion concentrations that are generally
higher than other Antarctic ice core sites (e.g. mean sodium levels were
254 µEq L-1). The trace ion record at Mill Island is
characterised by a unique and complex chemistry record with three distinct
regimes identified. The trace ion record in regime A displays clear
seasonality from 2000 to 2009 CE; regime B displays elevated concentrations
with no seasonality from 1934 to 2000 CE; and regime C displays relatively
low concentrations with seasonality from 1913 to 1934 CE. Sea salts were
compared with instrumental data, including atmospheric models and
satellite-derived sea-ice concentration, to investigate influences on the
Mill Island ice core record. The mean annual sea salt record does not
correlate with wind speed. Instead, sea-ice concentration to the east of Mill
Island likely influences the annual mean sea salt record. A mechanism
involving formation of frost flowers on sea ice is proposed to explain the
extremely high sea salt concentration. The Mill Island ice core records are
unexpectedly complex, with strong modulation of the trace chemistry on long
timescales.
Introduction
The IPCC Fifth Assessment Report states that there are insufficient southern
hemispheric climate records to adequately assess climate change in much of
this region. Ice cores provide excellent archives of past climate, as they
contain a rich record of past environmental tracers archived in trapped air
and precipitation. However, Antarctic ice cores, especially those from East
Antarctica, are limited in quantity and spatial coverage. To help address
this, a 120 m ice core was drilled on Mill Island, East Antarctica
(65∘30′ S, 100∘45′ E). Hydrogen peroxide (H2O2),
water stable isotopes (δ18O and δD), and trace ion chemistry
were measured from the 120 m Mill Island ice core. This study presents these
measurement results.
Mill Island is a small island (∼ 45 × 35 km), rising
∼ 500 m above sea level, located in East Antarctica. It is connected
to the Antarctic continent by the Shackleton Ice Shelf. The relatively low
elevation and close distance to the ocean suggests the potential for
significant input of maritime air to the snow falling at Mill Island. Mill
Island is located approximately 500 km west of Law Dome (LD), 350 km east of
Mirny Station, and 60 km north of the exposed rock formation known as Bunger
Hills and lies at the northern edge of the Shackleton Ice Shelf in Queen
Mary Land (Fig. ).
Mill Island is the most northerly Antarctic ice core site outside of the
Antarctic Peninsula, and therefore the Mill Island ice core comprises the
most northerly climate record for East Antarctica . Mill
Island experiences a polar maritime climate and high precipitation,
particularly on its eastern flank, due to moist and warm air masses from the
Southern Ocean brought onshore by low pressure systems. The site also
experiences dry and cold air masses associated with strong katabatic winds
from the continent and low-level cloud, fog, and rime formation over the
summit caused by localised summer sea breezes associated with nearby sea-ice
breakout . Mill Island record is mainly influenced by
local conditions rather than wider-scale atmospheric conditions. Records from
Mirny Station show that the monthly mean temperature is below zero throughout
the year , suggesting that at its ∼ 500 m summit
elevation Mill Island likely experiences minimal melt. The high
precipitation rate and minimal melt makes Mill Island an ideal site from
which to extract high-resolution climate records for the Southern Hemisphere.
General glaciology information of Mill Island is summarised in Table
.
General glaciology information of Mill Island ice core (MI0910).
Latitude65∘33′10′′ SLongitude100∘47′06′′ ESurface elevation503 mBorehole temperature-13.86 ∘C(at a depth of 19.07 mfrom the 2011 CE surface)Annual snow accumulation1.35 mIE yr-1Mean wind speed7.6 m s-1Firn-ice transition83.4 mVertical strain rate-0.6 × 10-2
Early studies attributed the main source of sea salts in ice cores to sea
spray from the open ocean, transported by strong winds associated with storm
events e.g..
More recently, and reported the
importance of frost flowers (sea salt crystals which form on new sea ice) as
a sea salt source. Frost flowers have a sea salt concentration 3 times
higher than sea water . Hence
frost-flower-originated aerosols contain a higher concentration of sea salt
than aerosols originating from sea water. also suggested the
sublimation of salty blowing snow on sea ice as a potential unfractionated
sea salt source. It is likely that different sea salt sources dominate and
contribute to the sea salt records at different sites .
The aims of this paper are to present well-dated high-resolution records of
water stable isotopes (δ18O, δD) and trace ion chemistry
(sea salts, sulfate, methanesulfonic acid (MSA)) at Mill Island and to
investigate the seasonal and interannual variability of sea salt in order to
reveal the climate factors that influence the Mill Island ice core record.
This was completed by investigating the characteristics of the trace
chemistry record and by examining the environmental factors that influence
these records, e.g. wind speed and direction, sea-ice configuration, and
deposition processes. The sodium (Na+) and sulfate (SO42-) records
were determined to represent sea salt contribution to the Mill Island site.
MethodIce core drilling
In the 2008/2009 austral summer, one shallow core (MIp0809) was recovered
during a reconnaissance expedition. The main ice core drilling campaign was
carried out during the 2009/2010 Australian Antarctic program. The team spent
3 weeks in the field and drilled one 120 m main ice core (MI0910) and
seven shallow (from ∼ 5 to 10 m) firn cores. This paper focuses on the
main (MI0910) 120 m ice core record that is supplemented by two shallow firn
cores, MIp0910 and MIp0809 (Table ). The 120 m ice core was
drilled using the intermediate-depth ice core drill (ECLIPSE ice coring
drill, Icefield Instruments, Inc.). A ∼ 2 m trench was excavated prior
to drilling. Thus, the top 2 m of the full record presented here are obtained
from the MIp0910 core.
Ninety-seven-year record of annual snow accumulation (a),
H2O2(b), δ18O (c), δD (d),
and D-ex (e). All data except snow accumulation record were
resampled to a 0.1-year grid and smoothed with a Gaussian filter of width
σ=1 point.
The Mill Island firn and ice cores were processed in a clean freezer
laboratory using similar techniques to those described by .
The density of the cores was computed using core diameter, length, and weight
measurements. Visual observation was also completed for stratigraphy studies.
The cores were then transversely divided into three sticks using a clean
bandsaw. The sticks were used for hydrogen peroxide, stable water isotopes,
and trace ion chemistry measurements. The sticks for hydrogen peroxide and
water stable isotope measurements were then cut into 4 cm length samples.
The central sticks for trace ion chemistry were cleaned to avoid
contamination and sampled every 4 cm (i.e. approx. 25 samples per
∼ 1 m core segment). Cleaning was achieved by removing
∼ 3 mm of each surface with a microtome under a laminar airflow
hood. Chemistry samples were stored in a Coulter cup (Kartell brand), melted
in a refrigerator, and then refrozen again to minimise MSA loss . The refrozen samples were melted prior to
analysis. All tools used for processing ice cores were carefully precleaned
with deionised ultra-clean Milli-Q water (resistivity
> 18 MΩ cm), and polyethylene gloves were worn during
the ice core processing to minimise contamination.
Sample measurement
Hydrogen peroxide (H2O2) measurements were carried out using a
fluorescence detector as detailed by . Samples of 4 cm length were analysed at 8 cm resolution from the surface to a depth of
25 m and then at a sample resolution of 12 cm for the remainder of the
120 m ice core.
Water stable isotopes (δ18O and δD) were measured using a
Eurovector EuroPyrOH HT elemental analyser interfaced in continuous flow mode
to an Isoprime isotope ratio mass spectrometer. Samples at 4 cm resolution
were melted in a refrigerated unit prior to analysis. Liquid samples were
sampled by a Eurovector liquid autosampler (LAS EuroAS300). Analytical
precision for δD is < 0.5 and for δ18O is
< 0.1 ‰, and values are expressed relative to the Vienna
Standard Mean Ocean Water 2 (VSMOW2). Deuterium excess (D-ex) was then
calculated from the measured δD and δ18O using the following
equation :
D-ex=δD-8×δ18O.
Trace ion chemical measurements were carried out using a suppressed ion
chromatograph (IC) as detailed by . Samples were melted
overnight in a refrigerator prior to analysis. Due to the high sea salt
concentration, the melted samples were diluted at a ratio of 50:1 in
autosampler polyvials using a micropipette within a laminar flow hood.
Further dilutions (5 to 100 times) were completed according to the sea salt
concentrations, depending on the initial results.
Samples were then analysed using a Dionex™ AS18 ICS-3000 (2 mm) microbore
ion chromatograph. The major ion species measured in this study were
methanesulfonic acid (CH3SO3- [MSA]), chloride (Cl-), nitrate
(NO3-), sulfate (SO42-), sodium (Na+), magnesium (Mg2+),
and calcium (Ca2+). Anions (i.e. MSA, Cl-, SO42-, and
NO3-) were analysed using an IonPac® AS18
separation column and AG18 guard column. Cation (i.e. Na+, Mg2+, and
Ca2+) analysis was performed using CS12A separation columns. The system
performed anion and cation analysis simultaneously using dual isocratic
pumps. The non-sea-salt sulfate (nssSO42-) record was then calculated
using the formula
[nssSO42-]=[SO42-]-kNa×[Na+],
where kNa is sea salt ratio of SO42- to Na+, 0.120
. All trace ions were calibrated using diluted standards
expressed in concentrations of micro equivalents per litre
(µEq L-1).
Annual snow accumulation
The Mill Island annual snow accumulation record was obtained from the
thickness of annual layers after ice core dating was completed. The thickness
of annual layers was corrected using the density profile and vertical strain
rate, assuming that ice thinning is only caused by vertical strain. According
to the method and density profile in , the vertical strain
rate was estimated by least squares fitting from the density-corrected
thickness of each annual layer. The flow correction is broadly consistent
with the estimated ice thickness and a uniform vertical strain rate.
Datasets
Due to a lack of in situ meteorological observation data available at Mill
Island, atmospheric model outputs were used to investigate the differences
between regimes. Wind data were derived from National Centers for
Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR)
. CFSR provides high-resolution atmospheric reanalysis data
(∼ 0.313∘× 0.31∘). The closest grid point to
Mill Island was chosen for this analysis (65∘24′42.84′′ S,
100∘56′15′′ E; ∼ 17 km east of the exact MI0190 drilling
site). CFSR data are available from 1979.
Sea-ice concentration (SIC) data were provided by the National Snow and Ice Data
Center. SIC was derived from the passive microwave Scanning
Multichannel Microwave Radiometer (SMMR) instrument on the Nimbus-7
satellite, and from the Special Sensor Microwave/Imager (SSM/I) instruments
on the Defense Meteorological Satellite Program's (DMSP) F8, F11, and F13
satellites, using the bootstrap algorithm . The data are provided
at a monthly time step and have a spatial resolution of 25 km. SIC data are available from 1979.
Comparison of (a)δ18O, (b) D-ex,
(c) Na+, (d) MSA, and (e) SO42- records
from MI0910 (black solid line), MIp0910 (green dashed line), and MIp0809 (red
dotted line).
ResultsIce core dating
MI0910 and MIp0910 were dated by counting annual layers using H2O2,
water isotopes (δ18O, δD), and D-ex
according to the methods presented in . The layer counting
method using this multi-proxy approach was subsequently confirmed by the non-sea-salt sulfate (nssSO42-) record, which matches the timing of
volcanic eruptions (Pinatubo (1991), El Chichón (1982), and Agung (1963)) at LD and at other ice core sites
. Figure presents annual
snow accumulation, H2O2, δ18O, δD, and the D-ex
records. The MI910 annual snow accumulation rate averages
1.35 mIE yr-1 for the period from 1913 to 2009, with a minimum of
0.79 mIE yr-1 in 1969 and a maximum of 2.04 mIE yr-1 in 1934
(Fig. a). The H2O2 record generally shows a strong
annual cycle, except for the late 1970s and early 1950s where there is an
observed loss of H2O2 seasonality (Fig. b). H2O2
seasonality loss is generally attributed to transient melt events
. However, this is not necessarily the case at Mill
Island. Further discussion of this loss of H2O2 is presented later. The
H2O2 record shows a baseline drift prior to 1935 CE, which is
attributed to calibration problems. Despite this, the data show strong
seasonal variations which are sufficient to assist annual layer counting
throughout most of the record.
The δ18O, δD, and the D-ex records also show annual cycles
throughout the core (Fig. c, d, and e).
A water vapour diffusion correction was computed using the method adopted in
and , with specific Mill Island
parameters (density profile, mean temperature, and atmospheric pressure). As
a result, the diffusion length reached 6.7 cm at a depth of 43 m in the
firn. With the high snow accumulation rate in the Mill Island ice core
1.430 mIE yr-1, this is small enough to
ignore.
The shallow cores MIp0910 and MIp0809 were also annually dated using the
layer counting technique to supplement the top of the MI0910 core
(Table ), and to verify the MI0910 dating.
MIp0910 covers 4 years, from 2006 to 2009 CE (Fig. ), and
there is good agreement with δ18O, D-ex, Na+, and
SO42- where MIp0910 overlaps with the top of the MI0910.
MIp0809 was dated by counting annual layers of δ18O and D-ex
(H2O2 measurements were not available for MIp0809). MIp0809 covers
15 years, from 1994 to 2008 CE (Fig. ). The overlap of
δ18O and D-ex for MIp0809 is in agreement with MI0910. These
comparable and overlapping records provide confidence in a continuous ice
record from surface to a depth of 120 m.
Moreover, the fact that these two individual nearby ice core records (10 km
between MIp0809 and MI0910 sites) acquired in different years and processed
independently show similar records provides confidence in the ice core dating
methodology used.
The ambiguities in the seasonal cycles of H2O2 and δ18O give
rise to potential dating errors. Such errors are statistically independent,
as the decision of counting a seasonal cycle as a year marker is not affected
by other errors. Thus instead of adding each error linearly, the errors can
be combined in quadrature e.g. error =error12+error22+error32…;. Although
nssSO42- peaks do not stand out for the major volcanic eruption (e.g.
Pinatubo (1991), El Chichón (1982), and Agung (1963)), the timing of some
nssSO42- peaks in MI0910 records match with the eruption years. Thus,
the errors are periodically set to zero at the timing of each major eruption
year. As a result, MI0910 dating error is within the range from +2.4 to
-3.5 years.
Trace ion chemistry data: (a) Na+, (b) Cl-,
(c) MSA, (d) SO42-, (e) nssSO42-,
(f) Mg, (g) Ca, and (h) NO3. All data were
resampled to a 0.1-year grid and smoothed with a Gaussian filter of σ=1 point. The Na+ and Cl- records can be partitioned into three
regimes (regime A, B, and C). Arrows A, B, and C in the nssSO42- record
correspond to major volcanic eruptions (Pinatubo (1991), El Chichón (1982),
and Agung (1963), respectively).
Trace chemistry record
Figure shows concentrations of (a) Na+, (b) Cl-, (c)
MSA, (d) SO42-, (e) nssSO42-, (f) Mg2+, (g) Ca2+, and
(h) NO3- for the entire ice core. The nssSO42- in this figure is
calculated with k′=0.049 (Supplement Sect. 1) because the nssSO42-
record calculated with k=0.12 shows negative mean concentration. This
indicates that the nssSO42- at Mill Island is highly fractionated by
sea salt. Thus, a different k value is needed to correctly derive
nssSO42-.
Average seasonal cycles of (a) Na+, (b) Cl-,
(c) MSA, d) SO42-, (e) nssSO42-,
(f) Mg+, (g) Ca+, and (h) NO3-. The error
bars show the standard error of the mean.
Typically, these trace ion species show strong seasonal variations: Na+ and
Cl- have a winter peak, and MSA has a summer peak
e.g.. However, the results for the trace ion chemistry
at Mill Island show clear seasonality only in the top 10 years of the ice
core. The seasonality in trace chemistry either disappears or shows
incoherent peaks prior to 2000 CE (Fig. ). The baselines of
Na+ and Cl- are also higher from 1934 to 2000 CE.
The observed seasonality for Na+ and Cl- for the period 2009 to 2001 is
not present for the period of 2000 to 1934 and where there is a significant
elevated baseline values. Prior to 1934 the seasonality is present again and
values are lower in concentration than in the other period. Similarly, MSA,
SO42-, Mg2+, and Ca2+ show clear seasonality only for the
period 2009–2001. These periods (2009–2001, 2000–1934, 1933–1913 CE) are
shown in Fig. and henceforth termed regimes A, B and C,
respectively. Further discussion about these regime changes is presented
later.
Time series of (a) Na+, (b) SO42- concentrations, and (c)δ18O, and (d) D-ex ratios over the period from 1913 to 2009.
Each top panel: data were interpolated to 24 points per year, then smoothed with a Gaussian filter of width σ=1 point.
The x axis is year, the y axis is month, and colour scale is shown in each
bottom panel. Each bottom panel: time series for each species from
Figs. and . The background colour indicates
the colour used in the top panel. y axis is the concentration/ratio.
Regime B (2000–1934) is shown using a grey panel to delineate the regime
changes.
Average seasonal cycles for the period 1913–2009 CE are displayed in
Fig. . The monthly mean was computed by linearly dividing
each year into 12 portions.
Despite the unclear seasonality prior to 2001 CE (Fig. ),
the average seasonal cycles of Na+, Cl-, Mg2+, and Ca2+ show
clear seasonal variability with a peak in winter (May), and a trough in
summer (December) (Fig. a, b, f, and g, respectively).
The MSA average seasonal cycle shows low concentration during winter (May–October), then peaks in spring (November) and autumn (April)
(Fig. c). However, during summer (December–March)
concentrations are relatively low. There is a reversed phase observed between
Cl- and MSA during the latest 10 years, but they are synchronised in
older parts of the record (e.g. ∼ 1965 to ∼ 1975 CE). This is
likely due to post-deposition MSA movement .
SO42- also shows clear seasonal variability with a peak in April and a
trough in November (Fig. d). The wintertime maximum in
the SO42- record indicates that sea salt is the dominant source of the
SO42- at this site.
DiscussionSea salt regimes at Mill Island
Time series of Na+, SO42-, δ18O, and D-ex are shown over
the period from 1913 to 2009 CE (Fig. ).
Average seasonal cycles of (a) Na+, (b)
SO42-, (c)δ18O, and (d) D-ex for each
regime. Regime A: 2001–2009 (blue); regime B: 1934–2000 (green);
regime C: 1913–1933 (magenta). The x axis shows the month, and the y axis shows the concentration/ratio. Note that the Na+ concentration is shown with a different scale for regime A
(left y axis) and regimes B and C (right y axis).
Na+ shows clear differences between the regimes. The Na+ winter (April
to October) peak during regime A is less pronounced in regime B. Instead,
Na+ in regime B shows lengthy “plateau” periods
(∼ 300 µEq L-1) and “valley” periods with a
relatively low concentration (< 100 µEq L-1). Na+
in regime C shows lower concentrations (∼ 30 µEq L-1)
with observed seasonality (except for 1917–1920 CE).
The SO42- record shows peaks in winter during regime A and
seasonally incoherent high concentration peaks
(≳ 40 µEq L-1) in regime B. Regime C, however,
differs to regimes A and B, with low concentrations of SO42-
(∼ 5 µEq L-1). In regime B, SO42- shows
occasional winter peaks, e.g. between 1934 and 1940 CE, 1950 and 1957 CE,
and 1977 and 1987 CE. The winter peaks in SO42- in regime B suggest
that the main source of SO42- is sea salt. When the SO42- record
shows a high winter concentration
(≳ 40 µEq L-1), the Na+
record plateaus.
For δ18O, other than enriched values during summer (December–January) after 2000 CE, there appears to be little difference between
regimes. D-ex also does not show any differences associated with the regimes, although lower values are observed during winter before 1950 CE than after
1950 CE.
It appears that the regime shifts are only evident in the sea salt trace ion
record. Chloride (Cl-, not shown) also shows features similar to the
Na+ record, i.e. clear seasonality in regime A, mix of “plateau” and
“valley” regions in regime B, and lower concentrations with observable
seasonality in regime C.
Average seasonal cycles of Na+, SO42-, δ18O, and D-ex for
each regime are shown in Fig. . Na+ and SO42-
both show seasonality with a winter peak in regimes A (blue line) and B
(green line). The variability of the Na+ concentration is lower in regime
B (minimum 214 µEq L-1 in November, maximum
293 µEq L-1 in April) compared with regime A (minimum
92 µEq L-1 in December, maximum 1222 µEq L-1
in May), and the seasonality is not as clear (Fig. a). The
Na+ variation and concentration was lowest during regime C (minimum
17 µEq L-1 in January, maximum 55 µEq L-1 in
July), and seasonality is still evident, with a peak in winter. However,
SO42- seasonality is not evident in regime C.
δ18O (Fig. c) shows enriched values
(∼- 11 ‰) during summer in regime A compared to
regimes B and C (Fig. c). This suggests that the
controlling influence on δ18O (i.e. temperature) at the coring
site has increased during summer since ∼ 2000 CE. The D-ex seasonal
cycle shows the most depleted values (∼ 5 ‰) during summer in
regime A. In regime C, the D-ex variability within a year is smaller than in
other regimes (Fig. d). This may indicate that the
moisture source has changed or that some changes have happened in the moisture
source region since regime C (or since 1950 CE, according to
Fig. ).
In summary, in regime A, clear seasonality with a winter peak is observed for Na+ and SO42-.
The mean concentrations of Na+ and SO42- are high
(451 and 30.2 µEq L-1, respectively)
(Figs. , ).
In regime B, Na+ shows “plateaus” of ∼ 300 µEq L-1 and “valleys” (shorter periods of lower concentration,
< 100 µEq L-1). SO42- shows
seasonally unaligned peaks (≳ 40 µEq L-1)
during which Na+ “plateaus” (Fig. ).
In regime C, Mean concentrations of Na+ and SO42- are low (32.5 and 5.6 µEq L-1, respectively).
Na+ shows wintertime peak, but no seasonality is observed in the
SO42- record (Figs. , ).
Sea salt regime changes and the stratigraphy of MI0910
The regime changes only influence the trace ion record. δ18O
variability shows no detectable changes during the observed regime changes
other than enriched values in summer after 2001 CE, and D-ex shows lower
values prior to 1950 CE. The possible reasons for the trace ion record
features include analytical error in measurement or methodology, snow/firn
melt, or a true environmental signal.
Repeat trace ion chemistry analysis was completed using different dilutions
and this yielded the same results, thus discounting the possibility of errors
due to analytical measurement. Additionally, the two shallow cores were
analysed independently using the same instrument and method. These trace ion
measurements from both shallow ice cores agree with the MI0910 record
(Fig. ). Thus analytical error in measurement or methodology is
not the cause of these features.
The stratigraphy of the MI0910 ice core shows ∼ 1 to 5 mm thickness
of higher density layers distributed occasionally throughout the entire ice
core . These layers may be due to melt, but the cause
of each layer is difficult to explicitly investigate by close inspection
alone . Thus all such layers observed in MI0910 are here
termed “crust layers” for convenience. Visual stratigraphy observation was
achieved by counting and logging the crust layers.
Figure shows the distribution of crust layers observed in the
ice core (blue vertical lines) along with the full records of H2O2,
Na+, and SO42-. A total of 172 crust layers were recorded.
The crust layers appear not to correspond with the periods of observed loss
of H2O2 or reduced seasonality in the trace ion record. For example,
the early 2000s (indicated with a grey ellipse labelled “a”) includes
multiple crust layers, but all three species (H2O2, Na+, and
SO42-) show clear seasonality. Between the late 1970s and early 1980s
(grey ellipse “b”) there is a loss of H2O2 seasonality, but this
period includes occurrences of both few crust layers (late 1970s) and many
crust layers (early 1980s). Another period of observed loss of H2O2
seasonality occurs during the early 1950s (grey ellipse “c”) yet this
period shows few crust layers. In the trace ion record, grey ellipses “b” and
“c” show similar characteristics for each species, whereby there is a high
concentration of Na+ with a muted seasonal signal and a high concentration
of SO42-. However, there is no relationship with crust occurrence
frequency and the defined regimes.
The observed H2O2 loss may be related to SO42- concentration.
H2O2 is believed to be the most efficient oxidising agent of SO2,
producing SO42-. In the grey ellipses “b” and “c”
(Fig. ), large peaks of SO42- are associated with the
depletion of H2O2. However, not all SO42- peaks are associated
with H2O2 loss.
Nitrogen oxides also tend to reduce the concentration of peroxide
. However, there are no associated nitrate features observed
in the record (see Fig. ). The reason for the absence of the
H2O2 peaks in these two regions is unknown.
Crust layers recorded in MI0910 ice core (blue vertical lines) with
97 years of H2O2, Na+, and SO42- record. The thickness of the
blue lines has been exaggerated, relative to the ice core thickness, in order
to enhance visibility. Grey ellipses indicate regions discussed in the text.
The firn/ice density is unrelated to the occurrence of crust layers.
In the polar snowpack percolation zone, melt events occur generally during
summer . Assuming that all summer crust layers are caused
by melt events, there is an implication that these events are associated with
temperature (hence δ18O). The crust layers during the summer period
(October–March) and summer mean δ18O were compared.
Thirty-four summers out of the 97-year record had crust layers, and the data
points were not normally distributed (not shown); thus a Spearman's rank
correlation was used to asses the correlation. There was no significant
correlation between the number of melt layers and the associated summer mean
δ18O (ρ=-0.08, p=0.62, n=34). This indicates that
these crust layers may not be melt layers. Furthermore, the density profile
(Fig. ) showed no sign of strong melt.
Strong wind may cause of the crust layers . Generally, high
wind speed (exceeding 15 m s-1) is more frequent during winter at Mill
Island. Both the monthly number of crust layers and the number of high wind
periods peaks in June and July (Fig. S1 in the Supplement). This indicates
that the crust layers identified during winter may have been formed by strong
wind events .
Additionally, fog events could be a cause of the crust layers. Fog and rime
accretion associated with the fog events were observed during the field
season at Mill Island (M. Curran, personal communication, 2014). This rime
deposition may appear as low-density crust layers . However,
fog and low cloud events are difficult to accurately retrieve from
atmospheric model output data . An automatic weather station (AWS) instrumented with
shortwave and longwave radiometers (in addition to standard components,
relative humidity sensor, wind speed/direction/mean sea level
pressure/precipitation) would provide an ideal tool to assess the occurrence
of fog events at the Mill Island site.
Fine 1 cm sample resolution isotope measurements from the Mill Island
shallow core show no apparent influence of crust layers on the record (not
shown). Thus the crust layers probably have a minimal impact on the chemical
interpretation of the Mill Island ice core records.
In addition, analysis of annual snow accumulation record and vertical
velocity profile revealed no link with the regime
changes.
Since both analytical errors in measurement or methodology and snow/firn melt
were discounted as the cause of the ambiguous sea salt seasonality, the three
different regimes identified may indicate the recording of different
environmental signals at Mill Island. The influence of true environmental
signals on the chemistry record at Mill Island is explored in the next
section.
Density profiles of MIp0910 (red circle) and MI0910 (blue square).
Influence of environmental signals on the sea salt record
Sea-ice and atmospheric reanalysis data were compared with the Mill Island
sea salt record to investigate the possibility of true environmental signals
in the regime change. Both sea-ice concentration and atmospheric reanalysis
data are available only since ∼ 1979. Thus only regimes A and B (after
1979) are investigated in the next section.
Many ice core studies suggest that sea salt is a proxy for wind and
storminess e.g., because
salt is transported by air mass movement. Thus, wind direction and speed are
investigated in this section to determine the Mill Island sea salt transport
mechanism.
The Mill Island wind rose climatology, created from the 6-hourly wind speed
and direction data, was generated using data from 1979 to 2009 CE. At Mill
Island, the wind direction is predominantly from the east, and the mean wind
speed over the period is 7.6 m s-1.
The relationship between wind speed and Na+ and SO42- concentration
was investigated by correlating annual, summer (October–March), and winter
(April–September) means of Na+ and SO42- concentration against the
number of data points, where the wind speed was < 5, 5–15, and
> 15 m s-1 in the associated period (Table S1 in the
Supplement).
There is no significant correlation between annual mean Na+ concentration
and annual mean wind speed. The number of data points per year where the wind
exceeds 15 m s-1 or less than 5 m s-1 also shows no
significant correlation with annual mean Na+ concentration.
There is a significant negative correlation between Na+ concentration and
number of data points with wind speed between 5 and 15 m s-1
(r=-0.51, p<0.01). However, this correlation is
strongly influenced by two data point outliers in 2006 and 2007 CE where the
Na+ concentration is extremely high. The regression slope of the Na+
concentration versus medium wind speed is low, indicating that this
correlation displays little predictive power. This negative correlation
between Na+ and wind speed 5–15 m s-1 is likely coincidental and
thus disregarded. To confirm that this relation is coincidental, the number
of data points per year with wind speed less than or more than 7 m s-1
also shows no correlation with Na+ concentration. Thus, the wind speed is
unlikely related to the Mill Island sea salt regime changes, at least
post-1979 CE. Correlations between SO42- concentration and wind speeds
show almost the same results, except the outliers occur in years 2002 and
2006 CE.
Sixty percent of the wind at Mill Island comes from the easterly quadrant
(wind direction between 45 and 135∘). Ninety-nine percent of wind
with speed greater than 15 m s-1 wind also blows from the east.
This indicates that the sea salt source at Mill Island is predominantly from
the east. Therefore, the next section focuses on the environment to the east
of Mill Island.
Time series of mean SIC-m (blue, right y axis) and Na+
(orange, left y axis) over the period from 1913 to 2009. Note that the right
y axis is reversed to highlight the high degree of anti-correlation.
Bowman Island (65∘12′ S, 103∘00′ E) is located
∼ 100 km east of Mill Island (Fig. S2). During the observation record,
the ocean between Mill Island and Bowman Island is typically free of ice
during summer and covered with sea ice during winter. The sea-ice cover in
this area could possibly influence the sea salt record.
Monthly SIC was investigated for the region between
Mill Island and Bowman Island for the period between January 1979 and
December 2009. At the 25 km resolution of the SIC dataset, there are five
SIC pixels between Mill Island and Bowman Island (their coordinates and pixel
names are shown in Fig. S2). Annual mean SIC in these pixels was compared
with annual mean concentrations of Na+ and SO42-.
Annual mean SIC is negatively correlated with mean annual Na+
concentration for all pixels except SIC-W. The highest correlation with
Na+ is at SIC-SE (r=-0.57, p<0.01). The annual
mean concentration of SO42- is also significantly anti-correlated with
annual mean sSIC for all five SIC pixels. SIC-S shows the highest negative
correlation (r=-0.58, p<0.01) (Table S2). Thus, SIC
values from SIC-S and SIC-SE were averaged to form a single record, termed
SIC-m.
Time series of annual mean SIC-m (blue, right y axis), SIC-W (red,
right y axis), Na+ (orange, left y axis), and SO42- (green, left
y axis) over the period from 1979 to 2009. The horizontal dashed blue line
indicates the mean sea-ice concentration in SIC-W, and dotted blue lines indicate
the 1σ standard deviation of the sea-ice concentration in SIC-W. Note
that the right y axis is reversed to highlight the high degree of
anti-correlation.
SIC-m is significantly anti-correlated with both Na+ and SO42- in
all three periods (Table S3).
This indicates that the time series of annual mean sea salt record from Mill
Island may represent sea-ice concentration variability at the local area
(Fig. ).
Figure shows time series of SIC-m, SIC-W, Na+, and
SO42- for the period between 1979 and 2009 CE, covering all of regime
A and approximately one-third of regime B. The horizontal dashed blue line
indicates the mean value of SIC-W, and dotted blue lines indicate the
1σ standard deviation of SIC-W. This figure clearly shows the negative
correlation between SIC-m and Na+, SO42-.
It has been shown that the Mill Island site shows high sea salt
concentrations, the prevailing wind is from the east, and the sea ice to the
east frequently shows low concentration. A mechanism to explain the
relationship between wind direction, sea-ice concentration, and high sea salt
concentration is proposed here:
Areas of open water between Mill and Bowman Islands (i.e. the SIC-m area) freeze to form new sea ice.
Frost flowers are produced on newly formed sea ice, then fragments are transported to Mill Island by the prevailing easterly wind
.
Otherwise, sea-salt-enriched brine migrates upward through sea-ice brine channels to the snow on sea ice. Then the salty snow is blown to Mill Island by the prevailing easterly wind.
However, SIC-m is not particularly low in 2002, 2006, and 2007 CE when
Na+ and SO42- concentrations are high. The lowest SIC-m years are in
1993 and 1994 (45.1 and 46.4 %, respectively). In these years, Na+ and
SO42- show a mid-range concentration (weak anti-correlation between SIC
and Na+, SO42-).
Differences between 1993–1994 and 2006–2007 are found in SIC-W. In
2006–2007, SIC-W concentration is within 1σ of the mean sea-ice
concentration (73.9 % in 2006 and 69.6 % in 2007), whereas in 1993–1994
SIC is more than 1σ below the mean (58.0 and 62.2 %, respectively).
This implies that the SIC-W may affect the sea salt transport process.
However, in 2002, high levels of Na+ are not clearly explained by this
hypothesis alone.
Figure shows a schematic diagram of a hypothetical sea
salt transport mechanism.
Schematic diagram of a hypothetical sea salt transport mechanism at
Mill Island, including formation of a snow ramp. (a) No landfast
sea-ice case: large sea salt particles cannot reach the Mill Island summit.
(b) Landfast sea-ice case: large sea salt particles can now reach
the Mill Island summit.
The edges of large ice-covered islands such as Mill Island typically exhibit a
vertical discontinuity on the order of > 10 m, which may block
the direct transport of sea spray and sea water aerosol particles onto the
island (Fig. a). If stable landfast sea-ice cover exists,
it facilitates formation of a snow ramp, which effectively bridges the
vertical gap between the landfast sea ice and the ice sheet
(Fig. b).
For example, when SIC-m is low and SIC-W is high (2006 CE), Mill Island
records extremely high sea salt concentrations because of an abundance of
available sea salt as frost flowers in SIC-m area and as salty brine snow in
SIC-m and SIC-W areas, as well as an effective mechanism to transport sea
salt to Mill Island. When both SIC-m and SIC-W are relatively low (e.g. 2002
CE), Mill Island records still high Na+ but not as high as the first case
because, although there is an abundance of sea salt, the snow ramp is not
present. When both SIC-m and SIC-W are high (2001, 1996, 1987 CE), Mill
Island records low sea salt concentrations, suggesting that frost flowers are
a more important sea salt source than the briny snow. This hypothesis is
strengthened by noting that the ratio of SO42- to Na+ in 2001 CE
(0.121) and 1996 CE (0.122) is close to the sea water ratio of 0.12. When
both sea-ice concentration at SIC-m and SIC-W and sea salt are high (e.g.
1991), the source of sea salt could be the briny snow on sea ice and/or
nearby open water with storm events (the ratio of SO42- to Na+ in
1991 CE is 0.114). This snow ramp theory works well for the vast majority of
years.
An aerial photograph was taken on the 11 February 1947 over Bowman Island
(Fig. S3). The photo shows an ice-capped island edge, adjacent to landfast
sea ice, covered with a well-formed snow ramp.
The same feature is expected to form at Mill Island.
demonstrated the presence (and, at times, absence) of multi-year landfast ice
to the east of Mill Island, which would facilitate snow ramp formation.
Figure shows annual variations of SIC-m, SIC-W, Na+, and
SO42- concentrations for the period between 1979 and 2009 CE. Both
SIC-m and SIC-W are generally high in early summer (December and January)
and low in late summer (February and March). The negative correlation between
SIC-m and trace ions can be seen on a monthly basis here. For example, SIC-m
in early 1995 CE shows low concentrations (≲ 70 %) later in the year. Na+ shows high concentrations
(≳ 300 µEq L-1) in early 1995, then low
concentrations (≲ 100 µEq L-1) later in the
year. Similar features are also seen in 2000. A case showing high SIC-m and
SIC-W but low sea salt is observed from 1985 to 1987 CE. Some years show high
sea-ice coverage in SIC-W throughout the period, which suggests the existence
of multi-year landfast ice (e.g. 1986–1991, 1995–1997, and 2003–2006
CE).
Annual variations in SIC-m, SIC-W, Na+, and SO42- over the
period from 1979 to 2009. The x axis is year, y axis is month, and the
colour shows sea-ice/trace ion concentration. Each pixel shows the monthly
mean concentration of associated species. Chemical data were interpolated to
12 data points per year. No other filtering was used.
The local SIC changes may be related to corresponding changes in the local
ice shelf configuration . Using NASA Moderate Resolution
Imaging Spectroradiometer (MODIS) satellite imagery, two major configuration
changes were observed between 2000 and 2009 (Figs. S4 and S5).
The first event was the calving of the Scott Glacier in 2002 between
Chugunov Island and Mill Island (Fig. S4). The event formed an iceberg named
C20 (not shown), which drifted westward and continued to break up
.
This change occurred entirely to the west of Mill Island (i.e. downstream
both oceanographically and atmospherically), and thus is unlikely to have
influenced the Mill Island record.
The next event was the export of Pobeda Ice Island (C5) from the
north-north-west of Mill Island in 2003 or 2004 (Fig. S5). In the image from
the 6 March 2003, a large tabular iceberg, Pobeda Ice Island, is grounded to
the north-west of Mill Island. This iceberg is not present in the
15 September 2004 image.
The presence of such an ice island presents a strong dynamical barrier to
mobile pack ice being advected westward in the coastal current, leading to
higher pack ice concentration to the east of this barrier .
These changes all occurred to the north or west of Mill Island. Considering
that the Shackleton Ice Shelf, Pobeda Ice Island, and Scott Glacier are all
downstream of Mill Island, changes in these icescape elements are unlikely
to have any strong influence on the Mill Island record, which is strongly
influenced by changes to the east. Whereas the ice shelf configuration to the
immediate west of Mill Island varies on decadal (or longer) timescales, the
icescape to the immediate east (i.e. the region of ocean between Mill
Island and Bowman Island) likely varies on much shorter timescales due to
interannual variations in SIC.
Sea salt source
Understanding the mechanism behind the observed high sea salt concentration
is the key to further interpretation of the Mill Island record. Since wind
speed does not strongly relate to the sea salt record here, sea spray from
the open ocean is unlikely to be the main sea salt source. The presence of
negative nssSO42- values in the Mill Island ice core record
(Fig. ) indicates the occurrence of sea salt fractionation
i.e. a depletion of sulfate relative to sodium;.
The Mill Island sulfate record is highly fractionated (Supplement 1), which
indicates that frost flowers are likely to be an important sea salt source at
Mill Island. Combined with the low altitude at the site and proximity to the
sea salt source, frost-flower-enriched aerosols may explain the high sea salt
concentration at Mill Island.
Another factor contributing to the observed high sea salt levels may be rime
accretion associated with fog events. When supercooled fog droplets deposit
onto a surface, they form rime. Rime deposits generally have greater
concentrations of all trace elements than fresh snow samples
. An example of accumulated rime was observed in Roosevelt
Island (79∘25′ S, 162∘00′ W), within the Ross Ice
Shelf. Roosevelt Island is the ice core drilling site of the Roosevelt Island
Climate Evolution (RICE) project . Roosevelt Island has a
similar geographical setting to Mill Island; i.e. the distance from coast is
∼ 20 km and the altitude of the summit is ∼ 560 m. The field
team found ∼ 0.5 m of rime ice on the AWS
when they returned to the site after an interval of 1 year (Fig. S6). The
team experienced frequent fog and growth of rime ice associated with the
fog. The team also collected and analysed surface snow precipitation samples
from the site. They found complex chemical signals such as multiple peaks of
most measured trace elements within a single annual layer (A. Tuohy, personal
communication, 2015).
With this in mind, the hypothetical snow ramp scenario proposed earlier,
which explains the extremely high observed sea salt concentration, is
developed further here.
New sea ice forms between Mill and Bowman Islands. Frost flowers form on the
new sea ice. Also, upward migration of sea-salt-enriched brine through sea
ice produces salty snow on the sea ice. Frost flowers and salty snow are
aerosolised and then transported west in the prevailing easterly wind. The
coastal easterly wind also creates a coastal polynya in the lee of Bowman
Island, allowing formation of more new sea ice, and so a constant supply of
frost flowers can be produced. The easterly wind also facilitates the formation
of stable landfast ice immediately east of Mill Island. Precipitation and
drifting snow create a snow ramp which bridges the vertical discontinuity
between the landfast ice and the ice cap at the edge of Mill Island.
Transport of frost flower and sublimed salty snow aerosols to the summit of
Mill Island is facilitated by the presence of the snow ramp. Alternatively,
fog events may lead to rime accretion at the Mill Island summit.
Given the lack of in situ chemical and physical observations at the eastern
base of Mill Island, it is difficult to prove this hypothesis. For further
study, a high-resolution snow pit study and AWS will be crucial to verify
this hypothesis.
Conclusions
The Mill Island ice core was dated by counting annual layers of
δ18O, with support of the H2O2 and D-ex records as required.
The ice core contains 97 years of climate record from 1913 to 2009 CE. The
dating uncertainty is +2.4, -3.5 years.
The trace ion chemistry record of the Mill Island ice core was investigated
by comparison with other nearby ice cores and instrumental data. The mean
concentration of all major ion species except nitrate is much higher than in
other nearby ice core records, e.g. Law Dome Summit South. In
particular, sea salt concentration (Na+ and Cl-) is remarkably high
(254 and 290 µEq L-1, respectively).
The Mill Island ice core record is characterised by a unique chemistry record
in which there are periods of clear seasonality, periods where seasonality is
lost, and periods where there is high and low trace ion concentration with regime
changes in 1934 and 2000 CE. The stratigraphy shows crust layers throughout
the ice core. The cause of the crust layers is likely wind related, and there
is no evidence that the crust layers are caused by melt events. Furthermore,
these layers are not the cause of the ambiguous trace ion seasonality.
Sea salt ions (particularly Na+, SO42-, and Mg2+) were
investigated in conjunction with records of environmental conditions around
Mill Island. It was found that the dominant wind direction is from the east,
but wind speed was unlikely to influence the Na+ and SO42- records
at Mill Island. Instead, the Na+ and SO42- records were found to
correlate well with sea-ice concentration between Mill and Bowman Islands.
Based on current knowledge, no documented historical ice configuration
changes have been noted that might affect the Mill Island ice core record.
However, the abrupt change in the sea salt record in 1934 may indicate a
significant, unknown ice configuration change east of Mill Island.
A hypothetical mechanism for high sea salt concentration deposition was
proposed, including sea-ice concentration, snow ramp formation, and rime
(associated with fog) deposition. Further studies, including installation of
AWS at Mill Island and a high-resolution snow pit study, are required to prove
this hypothesis.
The trace ion chemistry data are available in Australian Antarctic Data Centre (10.4225/15/590147625b975) (Inoue et al., 2017).
The Supplement related to this article is available online at doi:10.5194/cp-13-437-2017-supplement.
Mana Inoue did the trace ion chemical measurement, led the analysis, and wrote the manuscript.
Mark A. J. Curran, Andrew D. Moy, and Tas D. van Ommen provided expertise on
ice core data interpretation and performed proofreading of the manuscript.
Alexander D. Fraser provided expertise on sea ice, atmospheric interpretation, and performed extensive proofreading of the manuscript. Helen E. Phillips did proofreading of the manuscript. Ian D. Goodwin led the field work.
The authors declare that they have no conflict of
interest.
Acknowledgements
The Australian Antarctic Division provided funding and logistical support
(AAS1236). This work was supported by the Japan Society for the Promotion of
Science Grant-in-Aid for Scientific Research (KAKENHI) no. 25.03748, and
by the Australian Government's Cooperative Research Centre program through
the Antarctic Climate and Ecosystems Cooperative Research Centre. The authors
would like to thank to Meredith Nation and Sam Poynter for their assistance
in laboratory and Indi Hodgson-Johnston for creating
Fig. . Edited by:
K. Goto-Azuma Reviewed by: E. Isaksson and J. Simões
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