Journal of Oceanography
Vol. 49, pp. 257 to 270. 1993
Anthropogenic CO2 Distribution in the North Pacific Ocean
CHEN-TUNG ARTHUR CHEN
Institute of Marine Geology, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China
(Received 26 May 1992; in revised form 28 September 1992; accepted 5 October 1992)
The depth of penetration of anthropogenic CO2 in the North Pacific Ocean based
on carbonate data in the literature is discussed. The results indicate that the
deepest penetration (over 2000 m) is found in the northwest North Pacific. The
shallowest penetration (to less than 400 m) is found in the eastern equatorial
Pacific. Depth of penetration of anthropogenic CO2 appears to have been controlled by such factors as deep water formation in the Northwest Pacific; upwelling
in the equatorial Pacific and; vertical mixing in the western boundary areas. These
results compare well with results implied from tritium, C-14, and freons distributions. The total inventory of excess carbon in the North Pacific was 14.7 ± 4 ×
1015 g around 1980.
1. Introduction
It has been known for decades that the concentration of carbon dioxide in the atmosphere
has been increasing due to burning of fossil fuels and clearing of forests over the last 140 years.
The matter is of great concern because the so called “excess” CO2 in the atmosphere enhances
the greenhouse effect which may significantly change the climate on earth. The rate of the climate
change depends on the rate of CO2 increase in the atmosphere. How much CO2 is left in the
atmosphere is in turn controlled by the amount taken up by the oceans (Chen and Drake, 1986).
The estimates of oceanic uptake of CO2 come primarily from models with varying complexity, ranging from the simple one-dimensional box-diffusion and outcrop-diffusion models
to the sophisticated 3-dimensional ocean general circulation models (Maier-Reimer and
Hasselmann, 1987; Sarmiento et al., 1992). Recently it has been shown that the oceanic penetration of excess CO2 can be calculated directly using carbonate data (Brewer, 1978; Chen and
Millero, 1978; Chen and Pytkowicz, 1979; Kanamori and Ikegami, 1982; Chen, 1982a, b, 1987;
Cline et al., 1985; Chen et al., 1986; Poisson and Chen, 1987; Krumgalz et al., 1990; Anderson
and Jones, 1991; Ono et al., 1992). Because the method is subject to large uncertainties, the
accuracy of the results is not known (Chen and Millero, 1979; Shiller, 1981; Chen et al., 1982;
Broecker et al., 1985). However, the precision of the method is adequate to show the excess CO2
signal. The abundant carbonate data in the literature can thus be used to supplement the tracer
data in showing oceanic mixing features for waters formed in the last 140 years.
2. Method of Calculation
The method of computation and its limitations have been described in detail elsewhere
(Chen and Millero, 1979; Chen and Pytkowicz, 1979; Chen et al., 1982, 1990; Chen, 1985). The
following equations summarize the method.
∆NTCO2 ° = NTCO2° (old) – NTCO2 ° (present)
= NTCO2 (measured) – 0.5 NTA (measured) – 0.78 AOU
(1)
+0.5 NTA° (present) – NTCO2 ° (present)
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C. T. A. Chen
Fig. 1. Correlation of [∆NTCO2 – 0.5∆NTA] with AOU for selected GEOSECS Pacific stations.
where ∆NTCO2 ° is the difference of the preformed total CO2 values for a water formed some time
ago and for a water formed recently. NTCO2 and NTA are the normalized total CO2 and total
alkalinity respectively (NTCO2 = total CO2 × 35/S; NTA = alkalinity × 35/S), node indicates
preformed values, AOU is the apparent oxygen utilization.
The above equation is rearranged:
[NTCO2 (measured) – NTCO2° (present)] – 0.5[NTA (measured)
– NTA° (present)] = 0.78 AOU + ∆NTCO2°
(2)
∆NTCO2 – 0.5 ∆NTA = 0.78 AOU + ∆NTCO2°.
(3)
Hence, if (∆NTCO2 – 0.5 ∆NTA) is plotted against AOU a slope of 0.78 should be obtained and
the intercept is ∆NTCO2° observed in old waters formed prior to industrialization (Chen and
Pytkowicz, 1979). Results based on selected GEOSECS data below the AOU maximum layer is
shown in Fig. 1. The slope is 0.78 ± 0.05 and the intercept is –40 µmol/kg. This indicates that old
seawaters contain 40 µmol/kg less total CO2 than present day seawaters. We now add 40
µmol/kg to the calculated ∆NTCO2°, we obtain the excess CO2 signal. For surface waters, ∆NTCO2°
approaches zero and the excess CO2 concentration is thus 40 µmol/kg. For deep waters formed
prior to industrialization ∆NTCO2° ⯝ –40 and thus the excess CO2 concentration approaches
zero. Different interception is obtained for different data sets depending on the year samples were
collected.
3. Sources of Data
YALOC 69 (Yaloc, 1969) 31 stations (Wyatt et al., 1970);
KH-71-5 (Hakuho Maru, 1971) 13 stations (Tsubota, 1973);
GEOSECS (Melville, 1973) 72 stations (Takahashi et al., 1980);
KH-75-4 (Hakuho Maru 1975) 12 stations (Hattori, 1977);
Anthropogenic CO2 Distribution in the North Pacific Ocean
259
INDOPAC (Melville, 1976) 36 station (INDOPAC, 1978);
KH-78-3 (Hakuho Maru, 1978) 7 stations (Hattori, 1979);
KH-80-2 (Hakuho Maru, 1980) 3 stations (Horibe, 1983);
CO2 Dynamics Cruise ENP (Miller Freeman, 1981) 10 stations (Chen et al., 1986);
CO2 Dynamics Cruise WNP (Discoverer, 1982) 10 stations (Chen et al., 1986);
Outer Continental Shelf Environmental Assessment Program (Polar Sea, 1983) 2 stations
(Chen et al., 1985);
OR 1 (Ocean Researcher 1, 1987, 1988) 8 stations (Chen, unpublished).
4. Penetration Depth along 150°W and 165°E Cross-Sections
The lower boundary of the excess CO2 penetration is defined as the depth at which the excess
CO2 concentration is less than 5 µmol/kg, approximately 12% of the magnitude of the signal.
Because of the uncertainty of the method and the large vertical spacing of the samples, the lower
boundary may be up to 150 m too deep or up to 300 m too shallow.
Figure 2(a) shows the results of fossil-fuel CO2 penetration along the 150°W cross-section
(CO2 Dynamics, Eastern North Pacific; ENP). The waters underlying the 0 µmol/kg excess CO2
contour are free from fossil fuel CO2. The increasing excess CO2 from the deep to the surface
layer indicates that the fossil-fuel CO2 content is greater in the shallow water than in the deep
water. Chen (1982a) has discussed the penetration of excess CO2 along the ENP section. He
proposed that the deepening of penetration depth at 30°N (Fig. 2(a)) is not a local phenomenon
but is an advected feature derived from the northwest North Pacific. Local convergence in the
winter only brings surface water to approximately 300 m (Niiler and Reynolds, 1984). Tsunogai
et al. (1992) reported that the Intermediate Waters in the North Pacific are an important excess
CO2 sink.
The values of excess CO2 plotted vs σθ in Fig. 3 cannot be contoured in detail but nevertheless provide supporting information (Chen et al., 1986). Chen (1982a) gives detailed discussion of the excess CO2 along this section. The isopleths less than 10 µmol/kg are relatively
flat and the 0 µmol/kg contour which is indicative of old water is at or below ~27.2 σθ throughout
the section. At values greater than 10 the distribution is very non-isopycnal, with a value greater
than 20 centered in the S-min at the intermediate depth, indicating that younger water is
associated with these strata. Since this water originates in the northwest Pacific as a result of near
surface processes, this higher excess CO2 indicates more recent surface origin.
The shallow excess CO2 values (i.e. at values greater than 10) are more closely aligned with
the AOU and salinity isopleths than the other parameters (Chen et al., 1986). In the vicinity of
the subarctic front near 45°N the strongest north-south gradient exists and extends to depths of
about 600 m and σθ of 27.00. Between 50°N and 40°N the 20 µmol/kg isopleth shallows from
the 26.8 σθ isopleth to the 25.5 σθ isopleth, suggesting that vertical processes are able to rejuvenate the water north of the subarctic front more effectively than south of this front.
The 26.8 σθ density surface was chosen by Fine et al. (1981, 1983) as the lower boundary
of the intermediate layer containing the most tritium in the North Pacific. Evidently excess CO2
has penetrated much deeper than this level. The 26.8 σθ density surface is just below the
maximum outcrop density for the winter North Pacific and thus diffusion downward from the
mixed layer and lateral ventilation along outcrops provided a direct atmospheric link. The depth
of this surface was from 800 m at 30°N in the western North Pacific to 150 m further north and
to 300 m in the equatorial regions. For the 150°W section presented here the depth of this
isopycnal is generally near 500 m and somewhat deeper near 30°N as part of the geostrophic
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C. T. A. Chen
(a)
(b)
Fig. 2. Cross-section of excess CO2 along 150°W from (a) carbonate data (taken from Chen (1982a)) and
from (b) a 3-D model (taken from Sarmiento et al. (1992)).
Anthropogenic CO2 Distribution in the North Pacific Ocean
261
Fig. 3. Excess CO2 vs σθ along 150°W.
response to the subtropical gyre. There is also some indication that the center of the gyre shifts
northward due to the northward progression of deepest penetration for each isopycnal. It is
important to note that significant tritium is found below this surface despite no direct atmospheric
links having been established (Van Scoy et al., 1991). Van Scoy et al. also reported that tritium
measured during the Long Lines cruise (1983–1985) penetrated 100–200 m deeper compared
with the GEOSECS data (1973–74), particularly in the subtropical gyre and in the tropics.
The excess CO2 vs salinity distribution (Fig. 4) for the 150°W section shows a scatter which
changes its slope in the vicinity of S = 34.5. Less excess CO2 is associated with waters with a
salinity between 34.5 and 34.6. The shallow waters at salinities higher than approximately 34.6,
or lower than approximately 34.4 are younger. These waters are believed to be coming from the
Northwest Pacific where intermediate water formation occurs and where vertical mixing, mode
water formation and thermocline ventilation appear to be most enhanced (Chen et al., 1986). The
salinity where the deep water θ/S curves converge is near or less than 34.6. Deep waters with
salinity higher than 34.6 do not contain excess CO2 .
The CO2 Dynamics: Western North Pacific (WNP) cross-section (Fig. 5(a)) indicates that
the excess CO2 has penetrated to the deepest depth of approximately 1500 m (σθ = 27.5) at 25–
30°N where the contours of the other physical and chemical properties also exhibit a downward
dip (Chen et al., 1986).
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C. T. A. Chen
Fig. 4. Excess CO2 vs salinity along 150°W.
Figure 5(a) shows a similar form as the ENP section but with a generally deeper penetration
depth. The shoaling of the penetration from west to east is also seen in tritium distributions. The
east-west vertical section along 35°N of tritium distribution (Fine et al., 1981) indicates that the
0.5 TU contour shoals from 900 m at 165°E to 600 m at 150°W. If the zero TU contour is assumed
to be 100 m deeper than the 0.5 TU contour, then the penetration depth from the tritium data is
1000 m at 165°E and 700 m at 150°W. This result is consistent with our results for excess CO2
(Figs. 2(a) and 5(a)).
The vertical section of tritium distribution has been produced by Fine et al. (1981) from selected
GEOSECS stations between 15°–55°N between 165°W and 170°E. This section is located
between our two sections. Similar to our excess CO2 pattern, the 0.2 TU contour shows an overall
concave structure with the deepest penetration at 900 m or deeper at 38°N, and the shallowest
penetration at 400 m at 10°N. It is significant to note that the tritium data are from the GEOSECS
expedition which occurred about ten years earlier than our ENP and WNP expeditions. The
penetration depth obtained from the tritium data, therefore, may be shallower than today’s depth
of penetration, since vertical diffusion and continued advection are still bringing the fossil fuel
CO2 signal downward over the last decade. An increasing penetration depth of tritium has been
observed in several regions of the world oceans (Van Scoy et al., 1991). Gamo et al. (1987) also
measured carbon-14 between 12°N and 30°N along approximately 170°E. They found that
carbon-14 values have increased above the σθ = 27.5 layer between 1973 and 1982. This increase
indicates a rapid penetration of surface water, and hence excess CO2, to such depth.
Recently Sarmiento et al. (1992) simulated the uptake of excess CO2 by the ocean using a
perturbation approach in a 3-dimensional global general circulation model. Figures 2(b) and 5(b)
Anthropogenic CO2 Distribution in the North Pacific Ocean
263
(a)
(b)
Fig. 5. Cross-section of excess CO2 along 165°E from (a) carbonate data and from (b) a 3-D model (taken
from Sarmiento et al. (1992)).
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C. T. A. Chen
show the comparison of the 3-D model-predicted excess CO2 with estimations based on carbonate data along our two north-south sections. The 3-D model results in a penetration pattern
similar to that obtained by the direct approach. The structure, however is somewhat different. The
carbonate-derived results show a deepening of the penetration depth at 30°N while this feature
is lacking in the 3-D model. These differences may reflect problems in the 3-D model’s
thermocline ventilation (Sarmiento et al., 1992).
5. Lower Boundary of Excess CO2 Penetration in the Bering Sea
Figure 6 shows the estimated penetration depth in the North Pacific Ocean using all available
data sets.
On the Bering Sea shelf the excess CO2 penetration is limited by the depth of the shelf. In
the Aleutian Basin the penetration is the deepest, to approximately 1000 m, in the eastern and
southern regions. The penetration is slightly shallower off the Kamchatka Peninsula (Fig. 6).
Overall the Bering Sea contains 0.19 ± 0.05 × 1015 g excess carbon which is a small amount
compared with 149 × 1015 g of biospheric carbon released between 1600 and 1975 or with the
135 × 1015 g of carbon generated by fossil fule (Stuiver et al., 1984). Because of the small volume
of the seawater in the Bering Sea, its capacity as an excess-CO2 sink is limited (Chen, 1993). The
carbonate deposits on the vast Bering Sea shelf, however, could provide a major sink for excess
CO2 in the near future. This will be discussed in the later section.
Fig. 6. Penetration depth of fossil fuel CO2 in the North Pacific Ocean.
6. Lower Boundary of Excess CO2 Penetration in the North Pacific Ocean
The overall pattern in Fig. 6 shows a similarity with major physical oceanographic features.
The penetration depth is generally deeper for areas of high advection and is shallower for more
stagnant regions or upwelling areas.
Several prominant features can be found: (1) A general shoaling trend from west to east is
observed (Chen, 1987; Chen et al., 1986); (2) a deepening of penetration depth between 30 and
40°N which is coincident with the subtropical convergence; (3) a deep pool (to below 2000 m)
at the western North Pacific off Japan, is observed. This is the region off Japan near the area of
circulation of the North Pacific variety of the Subtropical Mode Water. The deepening is
probably related to the interaction of Kuroshio and Oyashio currents. The mixing of these
Anthropogenic CO2 Distribution in the North Pacific Ocean
265
currents enhances the penetration of fossil-fuel CO2 into the ocean; (4) the penetration depth in
the equatorial upwelling region is the shallowest in the North Pacific Ocean. The penetration
depth is less than 300 m, especially in the eastern equatorial area.
The major difference between the Pacific and the Atlantic Ocean (Chen and Millero, 1979;
Chen 1982b, 1987) is that excess CO2 does not penetrate below the thermocline everywhere in
the Pacific Ocean because there is no bottom water formation in the North Pacific. The shallowest
penetration outside of the Southern Ocean occurs in the eastern equatorial region where the
excess CO2 only penetrates to 300 m, or shallower. Because few data are available for the
complex oceanic region in the western equatorial Pacific, results there are less reliable. The
general trend, however, indicates a deeper penetration (800 m) in the western Pacific and in the
South China Sea. Overall, the excess CO2 penetrates to a shallower depth in the equatorial Pacific
than in the Atlantic, perhaps reflecting the higher equatorial upwelling rate in the Pacific and less
influence of the newly formed water advected from the north.
These results agree qualitatively with the recent freons, tritium and C-14 data. Freons are
found as deep as 1800 m at the western boundary, rising to about 900 m at 160°E and continuing
Fig. 7. Comparison of the excess CO2 profiles based on carbonate data (dots) and those modeled using
freon-11 data (thick lines; taken from Cline et al. (1985)).
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C. T. A. Chen
to rise to 650 m at the eastern boundary along 47°N. Along 24°N, the freons penetrate to 900 m
throughout most of the section but deepen to 1200 m at the western boundary (Warner and Weiss,
1985).
Freons data have also been used to quantitatively model the excess CO2 penetration in the
North Pacific Gyre (Cline et al., 1985). The results (Fig. 7) show that excess CO2 signal derived
from the freon-11 distribution agrees reasonably well with the signal calculated from the
carbonate data. Figure 8 is an isogram map of the depth where tritium equals to 0.1 TU. The
similarity in distribution with the excess CO2 results is striking.
The preindustrial ∆C-14 concentration for low and mid-latitude waters in the Pacific Ocean
is probably about –50‰, similar to the value in the Atlantic Ocean (Toggweiler et al., 1989). Figure
9 shows an isogram map of the depth where the ∆C-14 concentration equals to this value. The
agreement with the excess CO2 results is rather good between Equator and 30°N. At higher
latitudes the agreement is poor because the subsurface waters in the Subarctic and Antarctic
regions have a pre-industrial ∆C-14 concentration below –110‰ (Toggweiler et al., 1989; Chen
and Rodman, 1990). Figure 10 shows an isogram map of the depth at which ∆C-14 equals –110‰.
Fig. 8. Isogram of the lower boundary of tritium penetration in the Pacific Ocean. Data taken from Ostlund
et al. (1979), Fine et al. (1981), and Van Scoy et al. (1991).
Fig. 9. Isogram of the depth in the Pacific Ocean where the ∆C-14 concentration equals –50‰. Data taken
from Ostlund et al. (1979) and Ostlund and Stuiver (1980).
Anthropogenic CO2 Distribution in the North Pacific Ocean
267
Fig. 10. Isogram of the depth in the Pacific Ocean where the ∆C-14 concentration equals –110‰. Data
taken from Ostlund et al. (1979) and Ostlund and Stuiver (1980).
The results north of 30° agree better with the excess CO2 results shown in Fig. 6, but the excess
CO2 still penetrates much deeper than the bomb-produced ∆C-14 except in the equatorial region.
Overall the North Pacific (including the Bering Sea) contains 14.7 ± 4 × 1015 g excess carbon
around 1980, with approximately 75% of the total occurring in the equatorial and subtropical
regions below 40°N and the remaining 25% occurring in the subarctic region above 40°N.
7. Discussion
The existence of data and their use are fundamental to future progress. Higher resolution
oceanic models must be built upon extensive sets of consistent oceanographic data. Few high
quality carbonate data, however, exist in the Sea of Okhotsk, the Japan Sea, the Yellow Sea, the
East China Sea, the South China Sea, and the Philippine Sea. Countries adjacent to these seas
must collaborate in order to study these regions cost-effectively.
Winter data in the western Bering Sea near the Kamchatka Peninsula and in the Sea of
Okhotsk are particularly important. The highest effective thermocline diffusion coefficient
found anywhere in the North Pacific is in this region (Hansen et al., 1984), where an oxygen
undersaturation as much as 20% probably occurs in winter (Chen, 1988). Strong winds coupled
with strong cooling increase the gas solubility (causing the high degree of undersaturation) and
induce intensive vertical mixing in the winter in the Sea of Okhotsk to enhance the penetration
of excess CO2 . Subsequently the winter-formed subsurface water contributes to the formation of
the North Pacific Intermediate Water which spreads out into a large portion of the North Pacific
(Reid, 1973; Watanabe et al., 1991). Winter data in the formation region of the Intermediate
Water is needed in order to provide the “preformed” concentrations which are needed for
obtaining complete information on the excess CO2 penetration in the North Pacific (Chen, 1985).
The 100% aragonite saturation horizon in the North Pacific Ocean corresponds to roughly
the σt =26.7 surface (Chen et al., 1986, 1988). Figures 2(a), 3, 5(a) and 6 indicate that excess CO2
has penetrated deeper than the aragonite saturation horizon everywhere in the North Pacific
(Chen et al., 1986). Except in the equatorial region, the excess CO2 has also penetrated deeper
than the calcite saturation horizon (Chen et al., 1986). Continued increase of excess CO2 in the
oceans will cause upward migration of the saturation horizons and induce calcite and aragonite
dissolution at a shallower depth (Feely and Chen, 1982; Chen, 1985), providing a sink for excess
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C. T. A. Chen
CO2 . In this connection data from the Yellow Sea, and the East and the South China Seas are also
important because the shallow bottoms of these seas are covered by CaCO3 sediments. Water
chemistry and sediment data on the shelf are needed in order to assess the potential of the shelf
carbonate as an excess CO2 sink.
The estimates of in situ calcium carbonate dissolution need to be verified by sediment trap
studies. Seasonal variations of calcium carbonate fluxes need to be determined for subarctic,
subtropical and equatorial regions.
Carbonate data in the Kuroshio formation region in the Philippine Sea are also needed in
order to understand the carbonate dynamics in the most important current system in the Pacific
Ocean. Effects of El Niño on the air/sea oxygen and CO2 exchanges in the western North Pacific
also need to be studied.
8. Conclusion
The penetration depth of fossil-fuel CO2 in the North Pacific Ocean was constructed. The
penetration depth is influenced not only by vertical diffusion and advection but also by horizontal
advection. Pycnoclines also affect the penetration depth in the region. The deepest penetration
depth is greater than 2000 m, which is found in the western North Pacific Ocean. The shallowest
penetration depth is less than 300 m, which is found in the eastern equatorial region.
The excess CO2 signal agrees qualitatively with the carbon-14, tritium and freons data. Some
preliminary investigations also indicate a quantitative agreement with the freons data for stations
in the Subarctic Gyre. Overall the North Pacific Ocean contains 14.7 ± 4 × 1015 g excess carbon
around 1980.
Acknowledgements
This paper was presented at the International Symposium on Global Change, Tokyo, 27–29
March, 1992. I thank the National Science Council for supporting this research and two
anonymous reviewers whose comments strengthened the manuscript.
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