Chen, L. and Z. Gao (2007) Spatial variability in the partial pressures

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Deep-Sea Research II 54 (2007) 2619–2629
www.elsevier.com/locate/dsr2
Spatial variability in the partial pressures of CO2 in the northern
Bering and Chukchi seas
Liqi Chena,b,c,, Zhongyong Gaoa,b,d
a
Key Laboratory of Global Change and Marine-Atmospheric Chemistry, State Oceanic Administration (SOA), Xiamen 361005, China
b
Third Institute of Oceanography, SOA, Xiamen 361005, China
c
Chinese Arctic and Antarctic Administration, Beijing 100860, China
d
Key Laboratory for Polar Science, SOA, Shanghai 200136, China
Received in revised form 1 March 2007; accepted 4 August 2007
Available online 31 October 2007
Abstract
In the summers of 1999 and 2003, the 1st and 2nd Chinese National Arctic Research Expeditions measured the partial
pressure of CO2 in the air and surface waters (pCO2) of the Bering Sea and the western Arctic Ocean. The lowest pCO2
values were found in continental shelf waters, increased values over the Bering Sea shelf slope, and the highest values in the
waters of the Bering Abyssal Plain (BAP) and the Canadian Basin. These differences arise from a combination of various
source waters, biological uptake, and seasonal warming. The Chukchi Sea was found to be a carbon dioxide sink, a result
of the increased open water due to rapid sea-ice melting, high primary production over the shelf and in marginal ice zones
(MIZ), and transport of low pCO2 waters from the Bering Sea. As a consequence of differences in inflow water masses,
relatively low pCO2 concentrations occurred in the Anadyr waters that dominate the western Bering Strait, and relatively
high values in the waters of the Alaskan Coastal Current (ACC) in the eastern strait. The generally lower pCO2 values
found in mid-August compared to at the end of July in the Bering Strait region (66–691N) are attributed to the presence of
phytoplankton blooms. In August, higher pCO2 than in July between 68.5 and 691N along 1691W was associated with
higher sea-surface temperatures (SST), possibly as an influence of the ACC. In August in the MIZ, pCO2 was observed to
increase along with the temperature, indicating that SST plays an important role when the pack ice melts and recedes.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: CO2; Ecosystem; Global change; Bering Sea; Chukchi Sea; Chlorophyll
1. Introduction
In recent decades, the rate of release of CO2 from
anthropogenic sources has increased, with this CO2
Corresponding author. Key Laboratory of Global Change
and Marine-Atmospheric Chemistry, State Oceanic Administration (SOA), Xiamen 361005, China. Tel.: +86 592 2195351;
fax:+86 592 2195982.
E-mail address: Lqchen@soa.gov.cn (L. Chen).
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dsr2.2007.08.010
being distributed among atmospheric, oceanic and
terrestrial reservoirs. With increasing CO2, all
General Climate Models (GCMs) suggest a maximum annual warming in high latitudes (IPCC,
2001). The Arctic has long seemed to be especially
sensitive to global change; polar amplification and
feedbacks are recurrent themes in numerical climate
modeling (Manabe et al., 1991; Manabe and Stouffer,
1993; Dickson, 1999). During the last 30 years, the
Arctic Ocean, in fact, has been undergoing rapid
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change with major decreases in the area and volume
of the sea ice (e.g., Stroeve et al., 2005).
Because CO2 sequestrated in the ocean is
removed from the atmosphere and thus does not
contribute to global warming, considerable effort
has been devoted to investigating oceanic CO2
uptake. Essential for carbon sequestration by the
ocean is the transport of carbon from surface to
depth by one of two mechanisms: the sinking of cold
water rich in carbon (the ‘‘solubility pump’’) and the
sinking of organic matter (the ‘‘biological pump’’).
In most areas of the ocean, the strength of the
biological pump is controlled by the availability in
the upper layers of the ocean of macronutrients
such as nitrate, phosphate and silicate. This is not
the case, however, in the subarctic Pacific Ocean,
the equatorial Pacific, and the Southern Ocean.
These regions, often characterized as ‘‘high-nutrient
low-chlorophyll (HNLC)’’ waters, comprise about
30% of the global ocean and are considered to have
surface waters deficient in iron. An alteration in the
magnitude of the biological pump in these HNLC
regions could significantly affect the ocean’s capacity to take up CO2 (Behrenfeld et al., 1996; Boyd
et al., 2000; Lam et al., 2006).
The Bering Strait is the only connection between
the Arctic and Pacific oceans separating the
Chukchi Sea to the north from the Bering Sea
(Fig. 1). The Bering Strait plays a significant role in
the transport of heat, freshwater and nutrients, as
well as carbon fluxes to the Arctic (Goosse et al.,
1997; Wadley and Bigg, 2002). It is thus important
to understand the transformations and fate of the
Pacific waters that enter the Arctic via this conduit
(Berger, 1987; Martin et al., 1989; Wheeler, 1997;
Harrison et al., 1999; PICES, 1999; Caldeira and
Duffy, 2000; Kawaguchi, 2001).
The Bering Sea and the Chukchi Sea are also
recognized as sites of high biological productivity
(Sambrotto et al., 1984; Banse and English, 1999).
Grebmeier et al. (2006) found that the northern
Bering Sea ecosystem has been undergoing a major
shift. Changes in biological communities are contemporaneous with shifts in regional atmospheric
and hydrographic forcing. In the past decade,
geographic displacement of marine mammal population distributions has coincided with a reduction
of benthic prey populations, an increase in pelagic
fish, a reduction in sea ice, and an increase in air and
ocean temperatures. These changes, now observed
on the shallow shelf of the northern Bering Sea,
should be expected to affect a much broader portion
of the Pacific-influenced sector of the Arctic Ocean.
Because the Bering/Chukchi region is the only
northern site for exchange between the Pacific and
Arctic oceans, because there is likely to be some net
carbon sequestration, and because the region has
experienced rapid change, it is important to improve
our understanding of carbon cycling in this region.
Recently, several international programs have
been conducted to improve understanding of
carbon cycling in the Bering and Chukchi seas.
These include SBI Phase I (Shelf-Basin Interactions,
1997–2001) and SBI Phase II (2002–2006), led by
the United States, and the Canadian and Japanese
cooperative WACS (Western Arctic Climate Study,
2002–2006). In addition, in 1994, the United States
and Canada jointly pursued a multidisciplinary
investigation of the Canadian Basin. Here, we
report the results of the 1st and 2nd Chinese
National Arctic Research Expeditions that, in the
summers of 1999 and 2003, surveyed the Bering Sea
and the western Arctic Ocean, and measured the
partial pressure of CO2 in the air and in surface
waters (pCO2).
2. Characteristics of the study region
The Bering Sea is one of the largest marginal seas
in the world ocean. The eastern Bering Sea shelf is
wide and extends from Bristol Bay northwards to
Bering Strait along the western coast of Alaska.
Primary production in this region has been estimated to be as high as 85–400 gC m 2 y 1 and is
among the high production waters in the world
ocean (Iverson et al., 1979; Goering and Iverson,
1981; Springer et al., 1996). Because of the effects of
the biological productivity and biological pump,
pCO2 in Bering Sea surface waters displays widespread undersaturation in summer, with the lowest
pCO2 values occurring in Anadyr Bay of the
northwestern continental shelf (Park et al., 1974).
During PROBES (Processes and Resources of the
Bering Eastern Shelf), Codispoti et al. (1982, 1986)
also found low values over the eastern shelf.
Nutrient concentrations are enriched in surface
water by winter mixing, which provides the potential to support the undersaturated state of pCO2 in
the coming summer.
3. Measurements and experiments
During the Chinese National Arctic Research
Expeditions of 1999 and 2003, transects were
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Fig. 1. (A) Cruise tracks and sampling stations for CHINARE-1 (solid line and open circle) and CHINARE-II (dashed line and plus
symbols). The gray lines marked with ‘‘ 50, 200, 1000’’ are the isobaths. (B) Schematic diagram of major currents in the Bering Sea
(after Stabeno et al., 1999). ACC: Alaska Coastal Current, ANSC: Aleutian North Slope Current, BSC: Bering Slope Current.
located in both the northern Bering Abyssal Plain
(BAP) and Chukchi Sea, as well as in the southern
Bering Strait (Fig. 1A). In addition to hydrography,
nutrients and chlorophyll-a data, partial pressures
of CO2 in the air and surface waters (pCO2) were
collected. The latter were obtained using a ship-
board US-made Li6262 CO2/H2O infrared analyzer.
Standard carbon dioxide gases from the National
Research Center of Standard Materials of China
were used for calibration, with concentrations of
285, 348 and 401 ppmv [CO2/air]. The accuracy of
the system was 1.0 matm. The procedures for
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sampling and analyzing equilibrium CO2 in surface
seawater, as well as data treatments, are described in
more detail in Wang et al. (2003) and Chen et al.
(2004).
We took 1000 cm3 water samples, respectively,
from 0, 25, 50, and 100 m depths using a CTD
(conductivity–temperature–depth) rosette. Concentrations of nitrate were determined by a colorimetric
technique (Jin, 2002). Chlorophyll-a was determined
by filtering 500-ml subsamples on Whatman GF/F
filters and extracting them in 90% acetone for 24 h
before measuring the concentration using a Turner
Model II Fluorometer (Zhu et al., 1994). The pH
was measured using a Chinese-made pH meter,
Model b-4, with a precision of 0.02 stipulated by the
‘‘Standards of Marine Survey’’ (The State Technical
Supervision Administration, 1991).
4. Results and discussion
The distribution of pCO2 in surface water of the
western Bering Sea is shown in Fig. 2. The lowest
pCO2 appeared over the continental shelf, which is a
pattern similar to that over the eastern continental
shelf. We attribute this pattern to higher biological
production over the shelves. Codispoti et al. (1982)
also found an undersaturated state of pCO2 over the
eastern continental shelf and suggested that the
main reason was the biological net production of
organic matter there. The higher values over the
BAP are in accord with the lower values of
chlorophyll observed there (Fig. 3). Except for a
0.85 mg m 3 value observed in the surface water at a
station over the western BAP, low (0.2–0.5 mg l 1)
chlorophyll values were observed over this region of
deep water (Fig. 3). Nitrate throughout the BAP
was relatively high (Fig. 4) and could theoretically
support a primary production rate an order of
magnitude higher than the observed chlorophyll
values. In summertime, the average nitrate concentration was about 8 mmol l 1 and the highest value
was 15 mmol l 1. It is apparent, therefore, that the
BAP is a typical HNLC area (Figs. 3 and 4). Banse
and English (1999) recognized both the BAP and
the subarctic northern Pacific Ocean south of the
Aleutian Islands to be HNLC areas with chlorophyll values far below those that would be inferred
from the observed nutrient concentrations.
Fig. 2. pCO2 (matm) in surface waters of the Bering Sea during the summers of 1999. The contour lines (solid line) represent the pCO2 data
that were measured in July 1999, and the gray dotted lines are the cruise tracks at that time.
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Fig. 3. Distribution of chlorophyll-a (mg m 3) in surface water over the Bering Abyssal Plain (BAP) in July 1999.
Fig. 4. Distribution of NO3 (mmol m 3) in surface water in the BAP in July 1999.
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4.1. The marginal Bering Abyssal Plain
In the marginal BAP, the pCO2 values were
higher than that over the continental shelves, but
lower than in the central region of the BAP (Fig. 2).
The Bering Slope Current flows along the 200-m
isobath and forms the Bering Gyre in the marginal
BAP (see Fig. 1B). Over the eastern continental
shelf at depths less than 200 m, there is high
biological productivity, and this region is considered
to be part of the Bering Sea green belt (Springer
et al., 1996). Markina and Khen (1990) also
described a high production area in the continental
shelf margin associated with the Anadyr Current.
About 2–4 mg l 1 chlorophyll concentrations could
be calculated based on the presence of phytoplankton concentrations of 500–1000 mg m 3. In summertime, the higher chlorophyll concentrations
extend over the slope, although the highest concentrations are along the marginal of the continental shelf and in the Anadyr Current area (Iverson
et al., 1979; Goering and Iverson, 1981; Springer
et al., 1996). Conkwright et al. (1994) suggested
that in summertime very high nutrient values could
be found in the southwestern Bering Sea as well as
its adjacent waters, which could be attributed to the
eddies generated from flow through the passages of
the Aleutian Islands. Therefore, there are considerable differences between the waters of the BAP and
those of the marginal continental shelf (the Bering
Slope). The former have low chlorophyll and pH
(Fig. 6), and high nitrate and pCO2, but the latter
have the reverse with high chlorophyll and pH, low
nitrate and pCO2 in surface waters in summer.
4.2. Bering Slope Current impacts
It is obvious that transport of the Bering Slope
Current can affect the distribution of properties in
Bering Sea. This can be seen in the pH isolines in
surface waters of the Bering Sea (Fig. 5). In the
center of the BAP, pH distribution was clearly
distinct from that in the surrounding shelves and the
Bering Slope. However, for the Bering Sea as a
whole, the distribution of pH isolines represents the
Bering circulation structure. Currents with relatively
higher pH flow along the Bering Slope northwestward and to some extent, southwestward to
form the Bering Gyre. Another branch of this
current continues northwestward to join the Anadyr
Current and enters the Arctic Ocean through Bering
Strait. The Bering Slope Current also carries high
Fig. 5. Isolines of pH in surface water in the Bering Sea in July 1999.
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nutrients from the marginal BAP to the Arctic
Ocean, impacting the carbon sink and nutrients
there, as well as the ecosystem structure in the
Chukchi Sea.
4.3. Bering Sea outflow to the Arctic Ocean
The only Bering Sea gateway to the Arctic Ocean
is the narrow (85 km wide), shallow (50 m deep)
Bering Strait. To know whether nutrients would be
depleted before reaching the Arctic Ocean, we
placed stations transversely across the northern
portion of the Chirikov Basin Strait to investigate
the hydrography and nitrate concentrations. Fig. 6
2625
shows chlorophyll-a and nitrate in surface waters
along the transect. Nitrate concentrations as high as
16 and 7.2 mg l 1 were present in the western
Chirikov Basin, whereas in the eastern Chirikov
Basin values were as low as 0.3 and 0.5 mg l 1.
However, chlorophyll-a values in the Chirikov
Basin appeared high, even in the eastern side where
a bloom had greatly decreased nitrate concentrations. High nitrate concentrations in the western
Chirikov Basin may be maintained because the
Aynadyr Current waters are well mixed there. The
Current therefore can transport its high nutrient
load through Bering Strait into the Chukchi Sea to
sustain the high primary production. In the eastern
pCO2 (µatm)
300
250
200
150
100
pCO2 (µatm)
300
250
200
150
NO3- (mmolm-3)
100
0.4
6
chl-a (mgm-3)
2
0
172
171
170
169
168
167
0.1
chl-a (mgm-3)
1.5
1
August
July
15
10
5
0
5
2
chl-a (mg m-3)
July
August
0.2
0
20
4
-
NO3 (mmol m-3)
8
0.3
August
July
4
3
2
1
0
0.5
66
67
68
69
70
71
Latitude (°N)
0
172
171
170
169
168
167
°
Longitude ( W)
Fig. 6. Distributions of NO3 and chlorophyll-a in surface water
in the northern Chirikov Basin during July 2003.
Fig. 7. Distributions of pCO2, (matm), nitrate (mmol m 3), and
chlorophyll-a (mg m 3) in surface water along transects at 1701W
(1999) in the Chukchi Sea. (A) July-pCO2. (B) August-pCO2.
(C) July and August NO3. (D) July and August Chl-a. (E) July
and August Chl-a (without bloom).
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Chirikov Basin, the dominant Alaskan Coastal
Current (ACC) waters are characterized by a low
pH and relatively high pCO2 due to river runoff
from the Yukon River (Chen, 1993). Therefore, the
mixture of waters in the eastern Bering Strait
display higher pCO2 values with concentrations as
high as 300 matm (see Fig. 1A, at Station BS11 at
65.51N, 168.91W), compared to an average of
200 matm in the Chukchi Sea (Fig. 7A and B).
350
2003. 7. 29-8. 2
300
pCO2 (µatm)
2626
250
200
150
100
65
66
67
68
69
70
71
72
73
74
75
76
350
4.4. The Chukchi Sea
2003. 8. 8-8. 12
250
200
150
100
65
66
67
68
69
70
71
72
1.6
73
74
75
76
July 30 to Aug. 2
NO3- (mmol m-3)
1.4
1.2
1
0.8
0.6
0.4
0.2
65
66
67
68
69
70
71
72
73
74
75
76
71
72
73
74
75
76
5.0
chl-a (mg m-3)
The outflow from Bering Strait separates into two
branches (Stabeno et al., 1999), one of which flows
northeastward to the deep Arctic Ocean, while the
other flows around Cape Barrow into the Beaufort
Sea, before branching into two parts, one of which
becomes the Beaufort undercurrent and the other
flows northwestward. We selected transects at
1691W and 1701W in the Chukchi Sea to assess
the impact of outflows from Bering Strait on the
Chukchi ecosystem.
Distributions of pCO2, nutrients (NO3 ), and
chlorophyll in July and August are presented in
Figs. 7 and 8, respectively, along the 1701W transect
in 1999 and along the 1691W transect in 2003. Most
of the pCO2 concentrations were less than 200 matm,
as shown in Fig. 7A and B. These low values could
be attributed to high production in summer in the
Chukchi Sea when nutrients such as NO3 were
almost depleted in the surface layer (Fig. 7C).
However, at 69.5–701N in the 1701W transect, the
NO3 indicates an extra replenishment (Fig. 7C)
such to increase chlorophyll-a levels (Fig. 7E). At
the same time, the pCO2 value increased to 250 matm
(Fig. 7A and B) and an anomaly in pH also was
observed (Fig. 9). The low pH value and increase in
pCO2 is believed to be caused by effects of Bering
inflow water. Based on the circulation in the Bering
Sea, the Alaska Coast Current would pass through
the eastern Bering Strait with lower pH and higher
pCO2; the Anadyr Current through the western
Bering Strait (Coachman, 1993; Stabeno et al.,
2001; Hermann et al., 2002).
A similar situation was found along 1691W, in
July and August, 2003. Increased nitrate appeared
between 67–681N and 72–731N along with increased
pCO2 (Fig. 8A). Chlorophyll-a concentrations
rapidly increased between the four transects taken
in July and August, respectively, in 1999 or 2003,
suggesting that there were biological blooms that
depleted most of the local nutrients. Therefore, a
pCO2 (µatm)
300
4.0
July 30 to Aug. 2
3.0
Aug. 8 to 10
2.0
1.0
0.0
65
66
67
68
69
70
°
Latitude ( N)
Fig. 8. Distributions of pCO2, (matm), nitrate (mmol m 3), and
chlorophyll-a (mg m 3) in surface water along transects at 1691W
(2003) in the Chukchi Sea. (A) July and August pCO2. (B) August
pCO2. (C) July and August NO3. (D) July and August Chl-a.
supply for nutrients could be traced to the Bering
inflow water. The transportation of the Bering
inflow water would provide additional nutrients to
the western Arctic Ocean and impact the ecosystem
structure there. For example, at 69.51N, a strong
bloom was found with chlorophyll-a concentrations
of up to 18.171 mg m 3, almost 10 times higher than
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75°N
8.5
8.45
8.4
8.35
8.3
70°N
8.25
8.2
8.15
8.1
8.05
65°N
180° W
8
°
175 W
°
170 W
°
165 W
°
160 W
°
155 W
Fig. 9. Distribution of pH in surface water in surface layer in the Chukchi Sea.
the average value. This unusually high chlorophyll-a
value may be an indication of the transport of high
nutrient concentrations from the Bering Sea.
5. Conclusions
Our study suggests that transport of subarctic
northern Pacific waters from the Bering Sea to the
Arctic Ocean enhances carbon sequestration in
the western Arctic Ocean by supporting high levels
of production there, and thus provides a potential
negative feedback locally to global warming. Forecasts of global warming indicate that because
high biological production appears in the eastern
Siberia and the Chukchi Sea, global biogeochemistry of carbon will decrease albedo and accelerate
the melting of snow and ice (Walsh, 1989). If climate
change causes the ice-free season to be of
longer duration in Bering Strait and the Chukchi
Sea, it could lead to stronger flows (Johannessen
et al., 2004) and accordingly more transport of
nutrients through Bering Strait to the Arctic. This
could lead to greater production and carbon
sequestration in the Chukchi and a negative feedback to global warming. Better understanding
of carbon cycling in the northern Bering and
Chukchi Seas and its relationship to ecosystem
structure is critical to our ability to predict the
consequences of global climate change on arctic
marine ecosystems with respect to biological productivity on arctic shelves.
Acknowledgments
These projects were supported by the National
Natural Science Foundation of China (NSFC) from
the key program (40531007) and general programs
(40406014 and 40276001), from SOA Youth Foundation Grant (2004606), from Youth Technological
Innovative Grant (2004J056), as well as Key
Laboratory for Polar Science Grants (KP2005003)
and Natural Science Foundation Grant of Fujian
Province (Z0513027). Thanks also to Chaolun Li,
Zilin Liu, and Mingming Jing for providing
chlorophyll and nutrient data as well as to the
crews from the Xuelong Vessel for their sincere help.
We thank Lou Codispoti and Ray Sambrotto for
their comments and helpful suggestions.
This paper was first presented in the GLOBECESSAS Symposium on ‘‘Effects of climate variability on sub-arctic marine ecosystems,’’ hosted by
PICES in Victoria, BC, May 2005.
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