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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, C09011, doi:10.1029/2008JC005088, 2009
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Increased CO2 uptake due to sea ice growth and decay
in the Nordic Seas
S. Rysgaard,1 J. Bendtsen,2,3 L. T. Pedersen,4 H. Ramløv,5 and R. N. Glud6,7
Received 20 August 2008; revised 2 March 2009; accepted 10 April 2009; published 16 September 2009.
[1] The uptake rates of atmospheric CO2 in the Nordic Seas are among the highest in the
world’s oceans. This has been ascribed mainly to a strong biological drawdown, but
chemical processes within the sea ice itself have also been suggested to play a role. The
importance of sea ice for the carbon uptake in the Nordic Seas is currently unknown. We
present evidence from 50 localities in the Arctic Ocean that dissolved inorganic carbon
is rejected together with brine from growing sea ice and that sea ice melting during
summer is rich in carbonates. Model calculations show that melting of sea ice exported
from the Arctic Ocean into the East Greenland current and the Nordic Seas plays an
important and overlooked role in regulating the surface water partial pressure of CO2 and
increases the seasonal CO2 uptake in the area by approximately 50%.
Citation: Rysgaard, S., J. Bendtsen, L. T. Pedersen, H. Ramløv, and R. N. Glud (2009), Increased CO2 uptake due to sea ice growth
and decay in the Nordic Seas, J. Geophys. Res., 114, C09011, doi:10.1029/2008JC005088.
1. Introduction
[2] The difference between surface water partial pressure
(pCO2) and that in the overlying air represents the thermodynamic driving potential for CO2 gas transfer across the
sea surface. As total alkalinity (TA) is closely connected to
buffer capacity, this quantity expresses the capacity of the
water to take up CO2, that is, high TA values reflect
seawater with large uptake potential [Stumm and Morgan,
1996]. The surface pCO2 in the Nordic Seas is generally
characterized by undersaturation during the year, but because of narrowing of the mixed layer and biological carbon
uptake, the surface pCO2 becomes increasingly undersaturated during the spring and summer seasons [Miller et al.,
1999; Takahashi et al., 2002]. Here in the sub-Arctic and
Arctic Atlantic Ocean, poleward flowing warm waters meet
and mix with nutrient-rich cold subpolar waters. Total
drawdown of CO2 in the Nordic Seas is estimated at
90 Tg C yr1 of which 50 Tg C yr1 is attributed to the
biological pump [Skjelvan et al., 2005]. Because of limited
data, however, the biological pump is not well quantified.
1
Greenland Climate Research Centre, Greenland Institute of Natural
Resources, Nuuk, Greenland.
2
Department of Marine Ecology, National Environmental Research
Institute, University of Aarhus, Roskilde, Denmark.
3
Centre for Ice and Climate, University of Copenhagen, Copenhagen,
Denmark.
4
Centre for Ocean and Ice, Danish Meteorological Institute, Copenhagen, Denmark.
5
Department of Science, Systems and Models, Roskilde University,
Roskilde, Denmark.
6
Biogeochemistry and Earth Science Department, Scottish Association
of Marine Sciences, Oban, UK.
7
Marine Biological Laboratory, University of Copenhagen, Helsingør,
Denmark.
Copyright 2009 by the American Geophysical Union.
0148-0227/09/2008JC005088$09.00
Recent studies of coastal first-year Arctic sea ice showed
that dissolved inorganic carbon (TCO2) is released from
growing sea ice into the water column [Anderson et al.,
2004; Rysgaard et al., 2007] and may be transported into
the bottom water together with the high-density brine,
eventually flowing off the fjord shelves to intermediate
and deep waters, as recently reported from Storfjorden,
Svalbard [Skogseth et al., 2004], the Okhotsk Sea
[Shcherbina et al., 2003], and the Canada Basin in the
Arctic Ocean [Yamamoto-Kawai et al., 2008]. CO2 is
rejected more efficiently than TA during ice formation
[Rysgaard et al., 2007], causing sea ice to be enriched in
TA relative to TCO2 compared with the ratio in seawater.
The observation of TA:TCO2 ratios as high as 2 indicates
that calcium carbonate (CaCO3) is formed in natural sea ice
[Rysgaard et al., 2007], which supports observations from
ice-tank studies [Assur, 1958; Papadimitriou et al., 2003]. It
has been hypothesized that while CO2-enriched brines are
expelled from growing sea ice, carbonate minerals remain
trapped in the brine tubes and channels until spring and
summer, when they dissolve within the sea ice or surface
waters. Thus melting of pure ice crystals and CaCO3
dissolution leads to marked CO2 undersaturation, and thus
constitutes a potential sink for atmospheric CO2 in areas
such as the Greenland Sea, where large amounts of sea ice
from the Arctic Ocean enters through the Fram Strait and
subsequently melts during its southward transport toward
the Denmark Strait [Vinje, 2001]. Quantification of the
effect of this sea ice-driven carbon pump on surface water
pCO2 in polar seas and the potential sequestration of CO2 to
the deep ocean requires extensive data on TA and TCO2
conditions in various sea ice types. In this study, measurements of the carbonate system from 50 sea ice cores in the
Arctic Ocean (77 – 88°N), representing areas with first-year
and multiyear sea ice, refrozen leads and pressure ridge ice,
and surface water, together with a simple mixed layer model
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Figure 1. Map of the study area in which a cross () indicates sea ice coring stations. The inserted
shaded area represents our model area (15°W –0°E, 74°N–80°N).
of the area form the basis of a new estimate of the sea icedriven carbon uptake cycle in the East Greenland current
system (EGC) and in the Nordic Seas.
2. Methods
[3] Sampling stations are presented in Figure 1. Two field
campaigns were performed, one in April 2006 and one in
August – September 2007. In addition, data on temperature,
salinity, TCO2 and TA during July –August (2003– 2006) in
the EGC were provided by the long-term monitoring
program Zackenberg Basic (www.zackenberg.dk). Access
to the sampling stations on the sea ice was provided by
Twin Otter planes, helicopters and an icebreaker (Oden). On
arrival to the sampling sites, sea ice was drilled with a
MARK III coring system (Kovacs Enterprises, Lebanon,
NH). At each sampling site, snow and sea ice thickness
were recorded. Furthermore, vertical temperature profiles
were measured with a thermometer (Testo, Lemzkirch,
Germany, Accuracy 0.1°C) at 10-cm intervals in the snow
and at the center of the cores through 3-mm holes drilled
immediately after coring. Sea ice was then cut into 10-cm
sections, and each section transferred to a 1-liter polyethylene jar and kept cold until further processing in the field
laboratory, which was reached within few hours. Cores were
processed at the military station Alert (Canada) or on board
Oden. At several locations, water samples from 1 m below
the sea ice were collected for salinity, temperature, TA and
TCO2 determinations with a Niskin bottle sampler through a
hole in the sea ice made by a Stihl power-head auger.
[4] In the laboratory, sea ice density was determined by
shaping 4- to 5-cm segments of the ice core into rectangular
pieces with planar sides and then measuring the volume and
weight of each segment. The segments were then cut in two.
One half was melted within 2 h and subsamples GF/F
filtered for chlorophyll a determination. Filters were
extracted for 24 h in 96% ethanol and analyzed fluorometrically for chlorophyll (10-AU Turner design fluorometer). Salinity of the melted sections (bulk salinity) was
determined with a sonde (Knick Konductometer) calibrated
to a PORTASAL salinometer. The other half of each sea ice
section was used to determine TA and TCO2 concentrations
in the following way: The ice segment was placed immediately in a gas-tight laminated (Nylon, ethylene vinyl
alcohol, and polyethylene) plastic bag [Hansen et al.,
2000] fitted with a 50-cm gas-tight Tygon tube and a valve
for sampling. The weight of the bag containing the sea ice
sample was recorded. Artificial seawater [Grasshoff et al.,
1983] (25 – 50 ml) of known weight and TA and TCO2
concentration was added together with 50 ml HgCl2. The
plastic bag was then closed immediately and excess air
quickly removed through the valve. The sea ice was melted
in the artificial seawater (at 0°C), and the meltwater mixture
transferred to a gas-tight vial (12 ml Exetainer1, Labco
High Wycombe, UK). Gas bubbles released from the
melting sea ice were likewise transferred to Exetainers.
Standard methods of analysis were used: TCO2 concentrations were measured on a coulometer [Johnson et al., 1987],
TA by potentiometric titration [Haraldsson et al., 1997],
and CO2 by gas chromatography. Routine analysis of
Certified Reference Materials (provided by A.G. Dickson,
Scripps Institution of Oceanography) verified that the accuracy of the TCO2 and TA measurements was 0.5 mmol
kg1 and 2 mmol kg1, respectively. Bulk concentrations of
TA and TCO2 in the sea ice (Ci) were calculated as;
Ci ¼ ððCm Wm Þ ðCa Wa ÞÞ=Wi
where Cm is the TA or TCO2 concentration in the meltwater
mixture (CO2 in gas bubble was added to the TCO2
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Figure 2. Exemplative profiles of temperature, bulk salinity, Chl. a, TA, and TCO2 conditions in sea
ice. (a) Refrozen lead. (b) First-year sea ice. (c) Multiyear sea ice. Horizontal dotted lines in each image
represent snow cover (upper two lines) and the interface between the sea ice and water column (lower
lines). (left) Conditions of temperature (open circles) and salinity (closed circles). (middle) Chl. a. (right)
TA (open bars), TCO2 (closed bars), and the ratio between TA and TCO2 (closed circles).
concentration), Wm the weight of the meltwater mixture, Ca
the TA or TCO2 concentration in the artificial seawater, Wa
the weight of the artificial seawater, and Wi the weight of the
sea ice.
3. Results
[5] Several different snow thicknesses (3 – 30 cm), sea ice
thicknesses (60 – 470 cm), temperatures (0.2 to 26°C)
and bulk salinity ranges (0.1– 9.6) are represented in this
study. Examples of the vertical temperature, salinity, Chl. a,
TA and TCO2 conditions in refrozen leads, first-year sea ice
and multiyear sea ice are presented in Figure 2. Lower
temperatures were observed in thick multiyear sea ice as
compared with first-year sea ice and sea ice in refrozen
leads. Furthermore, bulk salinities were lower in thicker and
older sea ice. Sea ice Chl. a values were low at all
locations investigated (0.01 –1.8 mg kg1 melted sea ice,
mean 0.12 mg kg1). TCO2 and TA concentrations in sea
ice varied between cores (23 – 482 mmol kg1 and 48–
612 mmol kg1 melted sea ice, respectively), corresponding
to less than 20% of concentrations in the underlying water. In
addition, the TA:TCO2 ratio in sea ice ranges from 1.1 in
young sea ice to 2.0 in the upper layers of thicker multiyear
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Figure 3. Salinity and carbonate conditions in Arctic sea ice. (a) Total alkalinity (TA, closed symbols)
and dissolved inorganic carbon (TCO2, open symbols) concentrations as a function of bulk salinity in sea
ice cores at the investigated sites shown in Figure 1. (b) Relation between TA and TCO2 (symbols). The
two lines show the TA:TCO2 ratio of 1.1 as in the underlying water and the TA:TCO2 ratio of 2.
sea ice and in pressure ridge ice exposed to low air temperatures.
[6] A plot across all ice layers and localities showed that
sea ice TCO2 and TA concentrations during both campaigns
(April and August – September) were highly correlated with
bulk salinity (r2 > 0.95, p < 0.0001), indicating release of
these species together with high-density brine to the underlying water during sea ice growth (Figure 3). Furthermore,
the TA:TCO2 ratio in sea ice was close to 1.1 when sea ice
concentrations of TA and TCO2 were high (new ice) and
close to 2.0 when sea ice concentrations of the two species
were low (older ice).
including the effect of melting sea ice on the surface pCO2
(Figure 5). The model covers a mixed layer, 50 m deep, in
the northern EGC system (15°W – 0°E, 74°N – 80°N, equalling 0.25106 km2) and considers the water-equivalent flux
4. A Simple Model of Melted Sea Ice in the Mixed
Layer
[7] Sea ice from the Arctic Ocean is mainly exported
southward in the East Greenland Current system where
most of it melts before it reaches Denmark Strait. The mean
sea ice transport is about 2900 km3 yr1 [Vinje, 2001] and it
constitutes the largest freshwater source for the Nordic Seas
[Aagaard and Carmack, 1989]. Together with advection of
low-saline Arctic surface water, the melting sea ice
decreases the salinity significantly during the summer
period, corresponding to an increase of about 1 – 2 m in
the freshwater content in the upper 50 m of the water
column north of 74°N (Figure 4). Relatively few measurements have been made in the EGC region but interpolation
of a multiyear data set resolved the spatial and temporal
changes of the surface pCO2 in the area [Nakaoka et al.,
2006]. In particular, it shows a significant spatial gradient
with a decrease toward the Fram Strait and the EGC region
during the summer months, in general accordance with the
spatial gradient in the freshwater content (Figure 4).
4.1. Model Description
[8] The carbon dynamics in the northern part of the
Nordic Seas are analyzed by a simple model of the area
Figure 4. Freshwater and pCO2 conditions in the Nordic
Seas. The filled contours show the difference between
freshwater content [m] in the upper 50 m in the Nordic Seas
between summer mean (July– August) and winter mean
values (January– March) calculated from the study by Boyer
et al. [2005] WOA01 using a reference salinity of 34.93
[Aagaard and Carmack, 1989]. The line contours show the
difference between the temperature-corrected surface field of
climatological pCO2 in winter (mean of January–March) and
summer (mean of July– August), which is based on a monthly
linear regression between observed pCO2 and sea surface
temperature [Nakaoka et al., 2006] and SST from WOA01.
The pCO2 fields were temperature corrected [Takahashi et
al., 2002] to the annual mean SST in the model domain of
0.5°C using the monthly SST from WOA01.
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Figure 5. Mass balance model describing the concentration of a substance F (S, TCO2 and TA) in a 50-m-deep
mixed layer in the East Greenland Current system,
extending from 74°N – 80°N and 15°W – 0°E. The model
considers the water-equivalent flux of the melted sea ice in
the region (qi) with an associated concentration Fi, the airsea flux (Fair), and the biological uptake (Fbio). The total
outflow from the region (qd) is the sum of melted sea ice (qi)
and the transport (qa) associated with mixing and advection
from neighboring water masses (i.e., the Arctic Ocean and
the Greenland Sea).
of the area-averaged melted sea ice in the region (qi), the airsea flux (F air) and the biological uptake (F bio). The total
outflow from the region (qd) is the sum of melted sea ice (qi)
and the transport (qa) associated with mixing and advection
from neighboring water masses (i.e., the Arctic Ocean and
the Greenland Sea). For quantifying the relative influence of
melting sea ice on surface pCO2, only sea ice fluxes, air-sea
exchange and biological CO2 uptake are considered, such
that the influence from qa is neglected. Thereby the model
solutions are only representative of the dynamics during
periods with significant melting of sea ice, i.e., the spring
and summer seasons. The area-averaged conservation equation for a tracer (F) is then given as:
hð@F=@tÞ ¼ qi ðFi FÞ þ F air þ F bio
where h is the depth of the mixed layer in the area and Fi is
the concentration in the sea ice. The model is solved for
salinity, TCO2 and TA. The surface pCO2 is then calculated
from the solubility and dissociation constants of seawater
[Zeebe and Wolf-Gladrow, 2001]. The model is initiated in
January and integrated for a single year. There are relatively
few winter measurements of TCO2 and TA in the area and
we found no representative data in the EGC region.
Therefore the average measurements made in July – August
2003 in Young Sound (74°N) are used as initial conditions
(TCO2 = 1810 mM and TA = 1928 mM). These values are
representative of conditions 25– 50 km off the coast [Rasch
and Caning, 2004]. The influence of sea ice melt is thus
analyzed as a perturbation around these initial values. Initial
salinity is taken as the maximum climatological surface
value averaged in the model domain (34.32 in February)
and the atmospheric pCO2 level is assumed to be 360 ppm
to be consistent with previous data [Nakaoka et al., 2006].
Sea ice concentrations are determined by averaging all ice
core values from the present study, yielding TCO2(i) =
278 mM and Si = 3.85, while TAi is determined from the
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mean ratio TAi:TCO2(i) = 1.8 found in August, which is
representative for the conditions in melting multiyear sea
ice. Bulk calculations based on satellite ice drift from [Kwok
et al., 2004] and passive microwave ice concentrations from
NSIDC [Comiso, 1999] show that the area of multiyear ice
in the EGC/GS area is >3 times the area of first-year ice. In
addition, measurements north of the Fram Strait showed that
approx. 80% of the ice exported from the Arctic through
Fram Strait is multiyear ice [Vinje and Finnekåsa, 1986].
Sea ice export through the Fram Strait and the corresponding volume flux qi were determined from a sea ice model
[Pedersen and Coon, 2004] which was updated with a
larger domain and extended to include 2007 for this study.
The air-sea exchange (Fair) is considered for TCO2 only.
The flux, as determined by solubility [Weiss, 1974] and
piston velocity [Wanninkhof, 1992], is calculated from the
model salinity, the climatological sea surface temperature
[Boyer et al., 2005], long-term mean wind speed [NCEP
Reanalysis Data, 1948 – 2008] and estimated sea ice cover
for the area using the method of Toudal [1999].
4.2. Model Solutions
[9] The monthly averaged surface fields of surface salinity, pCO2, sea ice cover, melt volume, wind speed and
temperature in the East Greenland Current region (74°N–
80°N, 15°W – 0°E) are presented along with model solutions
of surface water pCO2, salinity and CO2 uptake (Figure 6).
Observed surface water salinities above 34 occur from
January to May and decrease to 32 in August. Surface
values of pCO2 are approximately 300 ppm during winter
and approximately 225 ppm during summer (Figure 6a). A
sensitivity study using a lower value of TAi:TCO2(i) = 1.3 to
represent first-year sea ice, resulted in a slightly higher
minimum surface pCO2 of 235 during summer, but, still, a
significant pCO2 drop occurred because of sea ice melting.
[10] The transport of sea ice (meltwater volume) through
the Fram Strait increase from 100 km3 per month in January
to 450 km3 in June– July with values close to 0 during
autumn (Figure 6b). Sea ice cover decreases from 0.6 during
maximum cover in January to 0.2 in September –October.
The monthly averaged wind speed varies from 8.5 m s1
during winter to 5 m s1 during summer (Figure 6c).
Surface temperatures below 1°C are observed during
winter, while average summer temperatures reach 1.5°C.
[11] Model solutions in four cases are shown in Figure 6:
(Exp. 1) with sea ice melt, (Exp. 2) without sea ice
melt, (Exp. 3) without sea ice melt + biological uptake,
and (Exp. 4) with sea ice melt + biological uptake and the
results are summarized in Table 1. Model solutions of
surface pCO2 are shown in cases with (Exp. 1) and without
(Exp. 2) the influence of melting sea ice (Figure 6d). In the
case without melting, the pCO2 increases during the winter
and spring seasons and the mixed layer reaches atmospheric
saturation in mid-June (363 ppm) because of oceanic CO2
uptake and increasing temperatures, whereas the model
solution which includes the influence from melting sea ice
has reached only 324 ppm at this point. Analysis of a case in
which the concentration of TA and TCO2 in the melting sea
ice was assumed to equal the concentration in the surface
water yielded a surface pCO2 value of 351 by mid-June,
showing that the influence from the reduced salinity only
explained a decrease of 9 ppm, whereas the influence from
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Figure 6. Monthly averaged surface fields (bullets) and model solution (green and orange) in the East
Greenland Current region (74°N–80°N, 15°W – 0°E). (a) Spatial averaged climatological [Nakaoka et al.,
2006] pCO2 field (black) and salinity (red). (b) Volume of meltwater due to transport of sea ice through
the Fram Strait (black) and sea ice cover (red). (c) Monthly averaged wind speed (black) and sea surface
temperature (red). (d) Model solutions of surface pCO2 with (dashed green line) and without (dashed
orange line) sea ice melt. Corresponding solutions including biological uptake are shown as solid lines.
(e) Model solutions of salinity with (green) and without (orange) sea ice melt and corresponding changes
in surface TCO2 in the presence of sea ice melt (red). (f) The accumulated air-sea carbon flux in the four
cases. Vertical dashed lines indicate the maximum sea ice melt period of May– September.
the TA and TCO2 in the ice caused a further decrease of
29 ppm. Analysis of a case in which the ratio of TA:TCO2
was equal to 1.3 yielded a corresponding surface pCO2 of
336 ppm, showing that even at relatively low TA:TCO2 ratios
the influence from melting sea ice becomes significant.
[12] These model solutions only include part of the carbon
sinks in the area, as can be seen by comparing the climatology
of the seasonal evolution of pCO2 in the EGC area with an
open-ocean location in the eastern Greenland Sea (not
shown). In the eastern Greenland Sea the onset of the spring
bloom causes a significant drop of about 50 ppm in May–
June and this is followed by a gradual increase in surface
pCO2 until mid summer. The conditions in the EGC region
show a larger change in the surface pCO2 of about 100 ppm in
May– June, which is twice as high as the open-ocean pCO2
change of about 50 ppm. Correcting the seasonal pCO2
change for the difference in surface temperatures [Takahashi
et al., 2002] increases this difference between the two
Table 1. Four Sensitivity Experiments Analyzed in the Simple Modela
Experiment
Melting
Sea Ice
Biological Uptake
(Fbio)
Spring pCO2
1 May (ppm)
Summer pCO2
15 June (ppm)
Accumulated C Uptake Between
1 May to 1 September (mol C m2)
1
2
3
4
Yes
No
Yes
No
No
No
Yes
Yes
308
332
308
332
324
363
240
272
0.51
0
1.50
1.03
a
Surface pCO2 concentrations are determined prior the biological CO2 uptake (1 May), in summer (15 June) and the accumulated carbon uptake during
the summer season (1 May to 1 September).
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locations by about 7 ppm. The large decrease in pCO2
seen in both the EGC region and the Greenland Sea can
partly be explained by rapid biological uptake in the spring
season when inorganic carbon is used for the production of
organic matter. This effect can be included, by prescribing
a constant biological sink (F bio = 5 107 mol C m2 s1)
during May– June. This sink is added to the model to
simulate the combined effect of biological uptake and sea
ice melt (Exp. 3 & 4), and the size of the sink is calibrated
so that the model simulates the observed decrease of
pCO2. Thus the total biological uptake ascribable to F bio
in the 2-month period corresponds to 31 g C m2 yr1,
which is slightly lower than previous estimates of new
production in the area [Anderson et al., 2000]. Model
solutions including the biological uptake cause the pCO2
to decrease both in cases with and without sea ice melt
(Figure 6d). However, the model solution with sea ice melt
(Exp. 3) yields the largest decrease. The difference between
the two cases from 1 May preceding the biological uptake is
due to melted sea ice, and it shows that sea ice melt processes
in early spring before the onset of the spring bloom can affect
the pCO2 significantly in the area. Thus the influence of sea
ice melt on the surface pCO2 can explain the concurrent
low pCO2 and salinity levels during the spring-summer
season, and also explain the difference between the EGC
region and other parts of the Nordic Seas less influenced
by sea ice meltwater. Increasing the amount of sea ice
melted in the model domain would lower the pCO2
accordingly, but this would also cause an unrealistically
low model salinity, whereas the applied meltwater volume
flux of 41% of the exported sea ice flux through the Fram
Strait results in a realistic salinity distribution during the
spring and summer seasons (Figure 6e). The corresponding
change in TCO2 during the summer period of about 100 mM
is in accordance with changes observed elsewhere in the
region [e.g., Anderson et al., 2000]. During autumn and
winter, conditions in the area are less influenced by freshwater from melted sea ice and the dynamics are primarily
regulated by convection and mixing with neighboring water
masses, which cause the salinity to increase in the region.
These processes are not included in the model, and,
therefore, the salinity remains low during the autumn and
winter periods.
[13] The accumulated air-sea carbon flux in the four cases
is illustrated in Figure 6f. The highest accumulated air-sea
carbon flux occurs in the presence of sea ice melt +
biological uptake, whereas the lowest uptake occurs in the
absence of sea ice melt and biological uptake.
5. Discussion
[14] The Nordic Seas are among the few regions in the
world’s oceans that take up substantial amounts of atmospheric CO2 throughout the year. The ocean uptake rates of
atmospheric CO2 range from 20 to 85 g C m2 yr1 and the
uptake per area therefore is among the highest in the world’s
oceans [Anderson et al., 2000; Takahashi et al., 2002;
Skjelvan et al., 2005]. The high uptake has been ascribed
to the solubility pump and the biological pump, but recent
evidence of TCO2 rejection from sea ice [Rysgaard et al.,
2007] and CaCO3 in sea ice [Jones and Coote, 1981;
Dieckmann et al., 2008] suggests that a so-called sea ice
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pump may also be of importance in this region. We found
no significant correlation between sea ice chlorophyll a
concentrations and TA (r2 < 0.001, p 0.0001), TCO2 (r2 <
0.0006, p 0.0001) or TA:TCO2 ratio (r2 < 0.003, p 0.0001), showing that sea ice-algal production had a minor
influence on the inorganic carbon species in sea ice during
the present study. However, the high TA:TCO2 ratios
observed in the present study fit in well with laboratory
observations of CaCO3 being formed by the chemical
reaction: Ca2+ + 2HCO
3 ! CaCO3 + CO2 + H2O when
sufficiently high concentrations of Ca2+ and HCO
3 are
reached in sea ice, as temperature and brine volume decrease.
Further support is provided by recent evidence of ikaite
(CaCO3 6H2O) occurrence in Antarctic sea ice [Dieckmann
et al., 2008]. The CO2 generated during CaCO3 precipitation
can escape the sea ice system dissolved in the brine
provided that the sea ice system is open to brine drainage.
The long winter with low temperatures results in low
permeability in the upper parts of the sea ice and increasing
permeability toward the sea ice – water interface [Eicken,
2003]. Transport by diffusion within the brine system into a
brine channel representing a route out of the sea ice through
brine drainage will enable a higher net gaseous CO2 escape
to the underlying water compared with the solid CaCO3
[Rysgaard et al., 2007]. Thus melting of CaCO3-enriched
sea ice with a high TA:TCO2 ratio during summer results in
pCO2-depleted surface waters. This becomes especially
evident in areas such as the Greenland Sea/East Greenland
Current, where large quantities of sea ice exported from the
Arctic Ocean melt during summer.
[15] Previous studies have suggested that the presence of
CaCO3 in ice might also originate from calcareous components of rock flour and wind-blown dust [Killawee et al.,
1998, and references therein]. Atmospheric transport and
transformations to the Arctic were quantified in the Danish
AMAP program, and although CaCO3 was not specifically
included in this study, Ca2+ only occurred in very low
concentrations and originated partly from sea spay and soil
[Heidam et al., 2004]. Alternatively, impurities can be
incorporated into landfast ice and a small fraction transported out into the Arctic Ocean [Eicken et al., 2005].
However, the coring sites in the present study are far
offshore and did not contain any visible impurities when
melted in our bags. Another argument is that most of the
offshore sea ice was produced offshore and has never been
in contact with landfast ice. In order for impurities in
landfast sea ice to affect the TA:TCO2 ratio they should
contain CaCO3. Quantification of sediment transport in sea
ice presents a considerable challenge due to the patchy
distribution of sediments in sea ice [Eicken, 2004]. It is
correct that high coastal concentrations of suspended particulate matter (SPM) have been observed in dirty ice (10 –
1000 mg l1) and that even clean ice from shallow water
depths (<30 meter) can contain 2 – 20 mg l1 of which half
can be considered litogenic sediments rather than particulates of biogenic origin (above reference). However, we
would expect only a fraction of this to be CaCO3. A
comprehensive study of the sediments in the Russian Arctic
[Kosheleva, 2002] showed low concentrations of CaCO3
(<2.4%). Also, surface sediments of the Greenland Sea/
Nordic Seas are extremely poor in carbonates [Huber et al.,
2000; Hebbeln et al., 1998; Rysgaard and Glud, 2007]. If
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landfast ice containing sediment particles is transported into
the central Arctic ocean, one would also expect TA:TCO2
ratios measured in multiyear sea ice to have a peak of
approximately 2 where the sediment layer is situated. In the
many vertical profiles we have made so far we did not see
any distinct layer with high TA:TCO2 ratios. Thus it seems
likely that ice-rafted sediments play a minor role in the
CaCO3 signal in sea ice compared with CaCO3 being
produced in the brine system of the sea ice itself.
[16] In the model solution including sea ice melt and
biological uptake, accumulated carbon uptake increases by
1.50 mol C m2 from May to September, corresponding to a
mean value of 55 g C m2 yr1 (Figure 6f), which is within
current estimates of oceanic uptake in the Greenland Sea
area during spring and summer [Nakaoka et al., 2006]. This
is significantly larger than the corresponding increase of
1.03 mol C m2 from May to September in the absence of
sea ice melt, implying that sea ice melt increases the
seasonal uptake in the area by approximately 50%. The
relative importance of the carbon uptake ascribable to sea
ice melt increases greatly toward the EGC and in areas with
melting sea ice (Figure 4). Furthermore, our model results
consider only the uptake in the Nordic Seas, but meltinduced CO2 uptake also occurs in the Arctic Ocean and
continues beyond our model area.
[17] Our results reveal the short-term effects of sea ice
melt in the Nordic Seas and emphasizes the important and
hitherto overlooked role of sea ice in shaping the regional
distribution of TCO2 and TA and, consequently, oceanic
CO2 uptake in Polar Regions. As the deep thermohaline
circulation of the world’s oceans, which is regulated from
these cold areas, carries with it large amounts of dissolved
CO2 to the ocean interior, sea ice driven carbon transport
has the potential to alter the global carbon cycle and thereby
the atmospheric pCO2 level through changes in sea ice
formation in the Polar Regions. During glacial periods,
when the Antarctic winter sea ice extent increased by
100% [Gersonde et al., 2005], the sea ice driven carbon
transport would have been significantly larger than today.
Correspondingly, reductions in sea ice extent due to warmer
climatic conditions would imply reduced CO2 uptake in
Polar Regions.
[18] Acknowledgments. We thank A. Haxen and M. Blicher for their
technical assistance in the field and laboratory. The Greenland Geological
Survey of Denmark and Greenland (GEUS) is acknowledged for helping
with logistics. The study received funds from the Danish Natural Science
Research Council, DANCEA, the Danish Cooperation in the Environment,
Danish Ministry of the Environment, the Aage V. Jensen Charity Foundation and the Danish National Research Foundation and the European
Commission (IP-CarboOcean).
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