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Journal of Oceanography, Vol. **, pp. ** to **, **04

Roles of Continental Shelves and Marginal Seas in the

Biogeochemical Cycles of the North Pacific Ocean

CHEN-TUNG ARTHUR CHEN1*, ANDREY ANDREEV2, KYUNG-RYUL KIM3 and MICHIYO YAMAMOTO4

Institute of Marine Geology and Chemistry, National Sun Yat-Sen University,

Kaohsiung 804, Taiwan, R.O.C.

Pacific Oceanological Institute Far Eastern Branch, Russian Academy of Sciences,

Vladivostok, Russia

OCEAN Laboratory/RIO, SEES, Seoul National University,

Shilim-Dong, Gwanak-Gu, Seoul 151-747, Korea

Frontier Observational Research System for Global Change, International Arctic Research Center,

University of Alaska Fairbanks, Fairbanks, AK 99775-7335, U.S.A.

(Received 25 August 2003; in revised form 29 October 2003; accepted 1 November 2003)

Keywords:

Most marginal seas in the North Pacific are fed by nutrients supported mainly by

Bering Sea,

upwelling and many are undersaturated with respect to atmospheric CO 2 in the sur-

Japan/East Sea,

face water mainly as a result of the biological pump and winter cooling. These seas

Okhotsk Sea,

absorb CO 2 at an average rate of 1.1 0.3 mol C m 2yr 1 but release N2/N2O at an

East China Sea,

average rate of 0.07 0.03 mol N m 2yr 1. Most of primary production, however, is

South China Sea,

regenerated on the shelves, and only less than 15% is transported to the open oceans

Kuroshio,

as dissolved and particulate organic carbon (POC) with a small amount of POC de- Sulu Sea,

posited in the sediments. It is estimated that seawater in the marginal seas in the Gulf of California,

North Pacific alone may have taken up 1.6 0.3 Gt (1015 g) of excess carbon, includ- nutrients,

ing 0.21 0.05 Gt for the Bering Sea, 0.18 0.08 Gt for the Okhotsk Sea; 0.31 0.05 denitrification,

Gt for the Japan/East Sea; 0.07 0.02 Gt for the East China and Yellow Seas; 0.80 carbon,

0.15 Gt for the South China Sea; and 0.015 0.005 Gt for the Gulf of California. anthropogenic

CO 2,

More importantly, high latitude marginal seas such as the Bering and Okhotsk Seas

budgets,

may act as conveyer belts in exporting 0.1 0.08 Gt C anthropogenic, excess CO2 into

North Pacific

the North Pacific Intermediate Water per year. The upward migration of calcite and

Intermediate

aragonite saturation horizons due to the penetration of excess CO2 may also make

Water.

the shelf deposits on the Bering and Okhotsk Seas more susceptible to dissolution,

which would then neutralize excess CO 2 in the near future. Further, because most

nutrients come from upwelling, increased water consumption on land and damming

of major rivers may reduce freshwater output and the buoyancy effect on the shelves.

As a result, upwelling, nutrient input and biological productivity may all be reduced

in the future. As a final note, the Japan/East Sea has started to show responses to

global warming. Warmer surface layer has reduced upwelling of nutrient-rich sub-

surface water, resulting in a decline of spring phytoplankton biomass. Less bottom

water formation because of less winter cooling may lead to the disappearance of the

bottom water as early as 2040. Or else, an anoxic condition may form as early as 2200

AD.

gins and in the coastal ocean although continental mar-

1. Introduction

Many global assessments of the oceanic carbon cy- gins influence global biogeochemical cycles much more

cle (e.g. Chen and Drake, 1986) have not fully accounted than their areal extent might imply (Wollast, 1998; Liu et

for the carbon fluxes and dynamics on continental mar- al., 2000a, b). Coastal regions are also especially suscep-

tible to anthropogenic influence at a time when humans

are strongly interfering in the global biogeochemical cy-

* Corresponding author. E-mail: abplj8@r.postjobfree.com

cle of carbon (C), nitrogen (N) and phosphorus (P). This

interference has led to substantially increased loadings

of the land and atmosphere with chemicals such as nutri-

ents from these activities, especially in the North Pacific.

These discharges might have significantly stimulated car-

bon fixation (up to 0.5 1 Gt C yr 1 ) (Smith and

Hollibaugh, 1993; Meybeck, 1993; Chen e t al ., 1994;

LOICZ, 1997; Mackenzie e t al ., 1998, 2001, 2002a,

2002b; Mackenzie and Ver, 2001). At present, however,

it is not known how much of this excess organic carbon

(OC) is simply re-oxidized, and how much is permanently

sequestered by export to the interior ocean, or in sediments

on the shelves and shallow seas. Jahnke et al. (1990) es-

timated that about half of the OC input into the seabed of Fig. 1. Marginal Seas in the North Pacific. The hatched area

the North Pacific occurs within 500 km of the margin. indicates where the dominant water mass transfer is from

Walsh et al. (1981, 1991) and Walsh (1991) state that large the interior ocean to the surface mixed layer (redrawn from

Blanke et al., 2002).

amounts of OC are recycled on the Bering Sea shelves.

Of the small fraction that is not remineralized, the major-

ity is not buried on the shelves, but transported over the

shelf edge and deposited on the continental slopes. Hedges

major sources of most forms of nutrients to the sea, to

and Keil (1995) state that continental shelf deposits alone

which they are usually carried by terrestrial drainage in-

are responsible for the storage of about 45% of the pre-

cluding rivers, freshwater runoff, groundwater discharge

served OC in the world. The number is doubled if deltas

and point source discharges (Mackenzie et al ., 1998). For

are included. Bauer and Druffel (1998) also pointed out

coastal oceans the human inputs, including aeolian, are

that the ocean margins can contribute an order of magni-

beginning to become signicificant compared to weather-

tude greater amount of dissolved and particulate organic

ing (Hong et al., 1995; Hu et al., 1998; V r smarty et

matter (DOM and POM, respectively) to the interior of

al., 1998; Rabouille et al ., 2001).

the Pacific than those derived from the surface open ocean.

The marine organisms act as a biological pump,

These aspects of the oceanic cycles of carbon and associ-

thus removing CO2 and nutrients from the surface ocean

ated elements, and their horizontal and vertical fluxes in

and transferring these elements into the deeper ocean and

the marginal seas of the North Pacific, are the subject of

ocean bottom. The ratios among carbon, nitrogen and

the JGOFS North Pacific Synthesis Group (NPSG), and

phosphorus in phytoplankton vary but the Redfield Ratio

the results are hereby reported.

is still a good representation (Chen et al., 1996b; Hedges

The North Pacific has some of the largest marginal

et al ., 2002). Assuming C:N:P ratios of 106:16:1,

seas in the world, including the Bering Sea (2.27 106

remineralization of these material can be represented by

km2), the Okhotsk Sea (1.53 106 km 2), the Japan/East

the equation (Redfield et al., 1963):

Sea* (JES 1.0 1 0 6 k m2 ), the East China Sea (ECS,

0.77 106 km2 plus the Bohai Gulf at 0.077 106 km2

(CH2O) 106(NH 3)16H 3PO4 + 138O2

and the Yellow Sea at 0.38 106 km2), and the South

106CO2 + 122H2O + 16HNO3 + H3PO4. (1)

China Sea (SCS, 3.5 106 km 2) (Fig. 1). These seas form

the linkage between the largest continent and the largest

Nitrate values in coastal waters usually decrease

ocean, ventilate the deep oceans, receive much land run-

going seaward in surface waters and increase along the

off (Milliman et al., 1995; Dai and Trenberth, 2002) and

same axis in deep waters. Upwelling regions have major

exchange much material with the open oceans. General

inputs of new nitrogen from the subsurface open ocean

descriptions of the area studied are summarized in Table

and the surface nitrate concentrations are higher, and of-

1. The role of these marginal seas in the context of car-

ten have strong oxygen depletion and periodically show

bon and nutrient cycles are briefly discussed with more

a high degree of denitrification, resulting in high subsur-

emphasis on the CO 2 sink. Several national and interna-

face ammonium, N2O and nitrite values. One of the ma-

tional projects have also been identified (Appendix I).

jor reactions is as follows:

2. Carbon and Nutrient Cycles

(CH2O) 106(NH 3)16H 3PO4 + 94.4HNO3

Rock weathering and the decay of organic material,

106CO2 + 177.2H2O + H 3PO4 + 55.2N 2. (2)

together with fertilizers and waste discharges, are the

The concentrations of phosphate and silicate in

coastal waters are also often higher than in the water fur-

*The Editor-in-Chief does not recommend the usage of

ther offshore due to land runoff, sewage outflow, and

the term East Sea in place of Japan Sea .

18 C.-T. A. Chen e t al .

Table 1. North Pacific Continental Margins: Site Descriptions.

Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean

1. Chen et al. (2003a); 2. JGOFS (1997); 3. Loder et al. (1998); 4. Hill et al . (1998); 5. Matsumura et al . (2002); 6. Church et al . (1998).

*Narrow, 300 km.

coastal upwelling. When denitrification occurs N2O or

N2 may escape from the water column making DIN/DIP

ratio lower (Eq. (2); Gordon e t al ., 1996; Seitzinger,

2000). Since Si exists in the extracellular structures of

diatoms, silicoflagellates and radiolarians, whereas N and

P exist in the soft tissue, Si frequently does not covary

with N or P.

There is clear evidence of changes in the concentra-

tions of fixed N in some coastal systems due to human

activities (Kemp, 1995). These nutrients are carried to

Fig. 2. Schematic diagram for the annual nutrient budgets (num-

the oceans mainly via rivers and groundwater. A recent

bers in 109 mol yr 1) in the East China Sea where Q is the

report (IGBP, 1994) estimated the total anthropogenic

flux, subscripts Ri, P, TSW, KSW, KTW, KIW and SSW

inorganic N and P inputs to the coastal zone as 1.5 1012

denote river input, precipitation, Taiwan Strait Water,

mol N and 0.52 1011 mol P per year. Galloway et al.

Kuroshio Surface Water, Kuroshio Tropical Water, Kuroshio

(1995), however, estimated that 4.2 1012 mol N yr 1 of Intermediate Water and Shelf Surface Water, respectively;

the nitrogen fixed by human activities are injected into P denotes DIP, N denotes DIN, Si denotes silicate, Re de-

oceans, of which 2.9 1012 mol N yr 1 are from river notes the release from sediments, AS denotes the air-sea

flow and the rest are from the atmospheric deposition. exchange, B denotes the nutrients buried, and SS denotes

Smith et al. (2003) reported that the total loads for the suspended sediments transported offshore (Chen e t al .,

2003a).

1990s are about three times Meybeck s (1993) estimates

mainly for the 1970s. No doubt, part of the increase must

have been due to human influences.

and KIW have net onshore transports. In addition, there

3. Case Studies on North Pacific Marginal Seas is an input through the Taiwan Strait (TSW).

The relevant known nutrient fluxes are provided in

3.1 East China Sea Fig. 2. It is clear that the rivers play only a very minor

Since the ECS has been studied the most thoroughly, role as they contribute only 7% of the dissolved inorganic

we will discuss it first. The ECS, including the Gulf of phosphorus (DIP) input. The major contributors of P, in

Bohai and the Yellow Sea has a total area of 1.24 106 the form of DIP, are the KTW, KIW and TSW. Most of

km 2 of which about 0.9 106 km2 is the continental shelf, the incoming DIP is converted to the organic form which

one of the largest in the world. It is also one of the most is either deposited on the shelf or is transported offshore

productive areas of the world oceans. Two of the largest as particulate. Note even if there is any man-made

rivers in the world, the Yangtze River (Changjiang) and eutrophication or increased biomass production due to the

the Yellow River (Huanghe), empty into the shelf with increased anthropogenic input of P, the increase is prob-

large, and ever increasing nutrient and carbon inputs. For ably very small. On the other hand, enhanced or damped

instance, the NO3 concentration near the Yangtze River upwelling due to changes in climatic forcing or damming

(Changjiang) estuary increased four-fold between 1963 of large rivers would make a large difference in the bio-

and 1983, because of the 2.25-fold increase in the use of logical pump. There are evidences showing that the

N-containing fertilizers in China. The DIN/DIP ratio in aeolian fluxes may be increasing during the past several

the estuarine water has also doubled in 20 years, but net hundred years (Lou et al., 1997; Chen and Wann, 1998).

primary productivity (PP) has not shown much variation Anthropogenic iron input or enhanced aeolian input of

(Gu, 1991; Zhang, 1991; Zhang and Gu, 1994; Note that loess which is rich in Fe and P may increase PP in the

net primary productivity is frequently reported but it is high nutrient-low chlorophyll regions or in waters where

not always clear in the original references). molecular nitrogen-fixing cyanobacteria may bloom.

The Kuroshio Surface Water (KSW), Kuroshio Tropi- As for N, the riverine input is 37% of the total in-

cal Water (KTW), Kuroshio Intermediate Water (KIW) flux, still smaller than the contribution from the incom-

and the Shelf Surface Water (SSW) make up the major ing water masses but the difference is not as dramatic as

that for P. It is worth noting that the 15N data (Minagawa

water masses near the ECS continental shelf break (Chen

et al., 1995b). Although the major currents are parallel to et al., 2001) supports the conclusion that the major source

the isobath, the SSW has a net transport offshore because of N for the ECS is from the NPIW (Chen, 1996). Tsunogai

of the buoyancy induced by the net precipitation (Q P QE, et al. (2003) also confirm that half of N in the ECS is

where Q denotes the flux in mass unit, subscripts P and E supplied from the lower layer of the Kuroshio. Not much

are precipitation and evaporation, respectively) and the N leaves ECS with the outflowing seawater. Instead, the

fresh water discharge from rivers (QRi), while KSW, KTW largest sinks are the net burial on shelf, the offshore trans-

20 C.-T. A. Chen e t al .

port in the form of sediments, and losses in the form of

degassing as N2 and N2O (Watanabe et al., 1997; Oguri

et al., 2003) with an net denitrification rate of 0.103

0.050 mol N m 2yr 1 (note the sign usually denotes

standard deviation although it is not always clear from

the original sources). This figure is small compared to

those estimated in the coastal areas worldwide (Seitzinger

et al., 2000). This may be because the ECS value is aver-

aged over the entire shelf area rather than only in the

nearshore coastal region. Further, because of the abun-

dant Trichodesmium in the ECS (Chen et al., 1996b), ni- Fig. 3. Schematic diagram for the annual carbon budget in the

East China Sea (numbers in 109 mol C yr 1) (taken from

trogen fixation may be a significant source of N. Includ-

Chen and Wang, 1999). Subscripts as in Fig. 2.

ing this in the budget would increase the denitrification

rate accordingly.

After the completion of the Shelf Edge Exchanges

Processes (SEEP) project in the Atlantic Bight the sources

port rate as 0.76 0 .38 mol C m 2yr 1. This value is

of N for the shelf to support the measured PP was consid-

equivalent to 5.7% of the average PP of 13.3 mol org C

ered as an unresolved question. The question was the dif-

m 2yr 1 (Zhang, 1991). By way of comparison, the off-

ficulty to reconcile the flux of nitrate onto the shelf with-

shore transport of POC of the global continental margins

out imposing an net export of water (Biscaye et al., 1994).

amounts to 6% PP on the shelf. Dividing the POC trans-

Upwelling of nutrient-rich subsurface water however,

port rate by 106 gives the particulate organic P (POP)

would balance the export of nutrient-depleted surface

transport rate as 6.6 3.3 109 mol yr 1, in good agree-

water. Chen and Wang (1999) noted that the riverine DIN/

ment with the estimate of 7.9 4 10 9 mol yr 1 obtained

DIP ratio is 111 which is much higher than the Redfield

earlier.

N/P ratio of 16 for phytoplankton. This makes P more

Much of the land-derived DOC and POC is highly

limiting to net organic production than N in the estuaries.

inert and mixes with seawater conservatively. Should this

The total seawater flux of N and P to the ECS, however,

carbon partially or totally respire on the shelf it would

has a ratio of 13 which is much closer to the Redfield

form an natural source of atmospheric CO 2 ( Kempe,

ratio. As a result, the P shortage in the ECS as a whole is

1995). The DOC outflow, however, is higher than the to-

not as dramatic thanks to the large influx due to the sub-

tal input. Thus 1.14 0.38 mol C m 2yr 1 new DOC is

surface Kuroshio waters. Changes in nutrient structure

produced on the shelf and is transported to the open oceans

of small riverine input are also not expected to affect the

each year. On the other hand, the ECS shelf waters pro-

stoichiometric nutrient balance of phytoplankton ecosys-

duce 0.79 0.28 mol C m 2yr 1 POC for export out of the

tem in the ECS. The amount of Si transported by the riv-

water column. Overall the ECS gets 6.24 2.1 mol C

ers is 31% of the total influx. Note in Fig. 2 only the dis-

m 2yr 1 OC from outside sources and exports 6.75 2.25

solved or the easily dissolvable Si is included. Since it is

mol C m 2 yr 1 O C excluding the 0.67 0 .28 mol C

not known how much of these convert into sediments,

m 2yr 1 buried. So the ECS currently is a net exporter of

the offshore transport of SS can not be calculated.

OC and is a net sink of atmospheric CO2. It should be

The carbon fluxes are more complicated. On the

noted that Jickells et al. (1991) reported that the major

Kuroshio side the surface water CO2 is more or less at

portion of POC in continental margin sediments is de-

equilibrium with the atmosphere because of the low nu-

rived from algal cells that have been produced in the over-

trient contents and PP. On the shelf the pCO2 is all under-

lying water. They reported that little of the POC in the

saturated yearround because of the high PP. The influx of

CO2 is about 2 0.8 mol m 2yr 1 (Chen and Wang, 1999; sediment is of terrestrial origin.

The study of Chen and Wang (1999), however, indi-

Tsunogai et al., 1999; Oh et al ., 2000). By way of com-

cates that about 20% of the ECS shelf deposit is relic.

parison, the global shelf average is about 1.2

The results of Honda et al. (2000), Bauer et al. (2001)

mol m 2yr 1 (Chen, 2003b).

and Lin et al. (2002a) also indicate that a large portion of

The major rivers bring in carbon in the form of dis-

the off-shelf transport of POC may be old terrestrial or

solved inorganic carbon (DIC), dissolved organic carbon

relic matter. Further, Ogawa et al. (2003) estimated a ter-

(DOC), particulate inorganic carbon (PIC) and particulate

restrial DOC input of 400 109 mol C yr 1 in the area

organic carbon (POC). A mass-balance calculation gives

close to the Changjiang River mouth, which is substan-

the downslope contemporary particulate carbon transport

tially larger than the total input of 170 10 9 mol C yr 1

rate as 2.8 1.4 mol C m 2yr 1 (Fig. 3). Chen and Wang

that Chen and Wang (1999) chose to use (Fig. 3). A higher

(1999) estimated the contemporary offshore POC trans-

Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean 21

riverine DOC input also implies that more shelf produced

OC is recycled on the shelf. The net conversion of IC to

OC is by the new production of 1.95 0.98 mol org C

m 2yr 1 which is 15% of the average PP with the rest re-

generated on the shelf, mostly aerobically. Note the new

production rate obtained from the P budget is 0.16 0.05

mol C m 2yr 1. Results from the SEEP II program on the

eastern US continental shelf indicate that only less than

5% of the PP is exported to the adjacent slope (Anderson

et al ., 1994; Biscaye et al., 1994). In the North Sea only

0.14% of the PP is accumulated on the shelf as POC, and

2 3% of the PP is exported over the margin (de Haas et

al ., 1997). Fig. 4. Penetration depth of excess CO2 in the Bering Sea (modi-

The continental shelf waters are generally high in fied from Chen, 1993a).

titration alkalinity (TA) because of river discharge and in

situ generation due to oxidation of organic material, which

reduces TA by 17 moles for the aerobic regeneration of

however, the Bering Sea transports 0.8 Sv (106 m3 sec 1)

106 moles of OC (Chen et al., 1982). When the dissolved

oxygen is exhausted in the sediments, the system turns to of seawater to the Arctic Ocean to account for 19% of the

the next most abundant source for the oxidation of or- DOC found there (Wheeler et al., 1997). Takahashi (1999)

ganic material, NO3, followed by the manganese, iron, went even further and claimed that ... Much of the bio-

sulfate and methane reductions (Noriki et al., 1997; Chen, logical production of organic matter and associated nu-

2002a). Chen and Wang (1999) estimated that on the or- trients flowing into the Arctic Ocean today is due to this

der of 1 1012 mol yr 1 of alkalinity is generated on the northerly current direction .

ECS shelf, mainly because of iron and sulfate reductions The Bering Sea is relatively nutrient rich compared

(Lin et al., 2002b). Fermentation, however, is probably to the open oceans and the PP, even in the deep basin, is

responsible for the high CH4 found in the ECS, about 35% almost twice of that found in the Gulf of Alaska immedi-

supersaturated (Tsurushima et al., 1996). The sea-to-air ately south of the Aleutian Islands. The diatom fluxes are

flux of CH4 is about 3 109 mol yr 1. The DMS concen- approximately one order of magnitude higher in the

tration in he surface water is about 80 and 17 ng S l 1, Bering Sea than in the Gulf of Alaska although the rela-

respectively, in summer and winter, with the correspond- tively minor CaCO3 fluxes are similar on either side of

ing fluxes of 2.2 and 0.6 mmol g S m 2yr 1 (Uzuka et the Aleutian Islands (Tsunogai et al., 1979; Takahashi,

al ., 1997). 1995; Takahashi et al., 2002). The shelf water has a pCO2

of about 100 atm below saturation in winter, mainly

The anthropogenic, excess CO2 p enetrates to ap-

proximately 600 m in the ECS. The entire ECS contains because of the cooling effect. Nevertheless the rate of air-

0.07 ( 0.02) Gt C excess carbon in 1992. Since the wa- sea exchange is probably low because of the sea ice cov-

ters on the ECS shelf are highly supersaturated with re- erage. On the other hand, near the shelf break the pCO2

spect to calcite and aragonite, sediments on the ECS shelf is supersaturated because of vertical mixing (Chen,

are not expected to neutralize excess CO2 in the coming 1993a). In summer, however, the shelf area is highly

understurated with pCO2 as low as 125 atm because of

century.

high productivity, especially those from siliceous shell-

bearing plankton (Codispoti e t al ., 1986; Takahashi,

3.2 Bering Sea

The Bering Sea is the third largest marginal sea in 1999). The PP on the shelf is on the order of 14 mol C

m 2yr 1, supported by nutrients from influxes of subsur-

the world and the second largest in the North Pacific. It

is divided about equally into a wide shelf and a deep ba- face waters from offshore (Fig. 1). The f ratio is 49% on

sin with a maximum depth of 5121 m. The average water the outer shelf and 17% at mid-shelf (Walsh et al., 1985;

depth of the 500 km wide shelf is less than 50 m with a Walsh and McRoy, 1986; Walsh and Dieterle, 1994;

shelf break located at 100 200 m water depth. The shelf Wollast, 1998). DMS and N2O have a concentration of

about 1.6 nmol S and 313 atm, respectively, in the sur-

area is covered by ice from November to May. The Bering

Strait is about 85 km wide and less than 50 m deep. This face water (Nojiri e t al ., 1997; Uzuka e t al ., 1997).

is the only northern gateway where water of the Pacific Aranami et al . (2002) reported a sea-to-air DMS flux of

4.1 mmol m 2yr 1.

origin flows into the Atlantic through the Arctic Sea, and

has one of the highest biological productivities in the Walsh and Dieterle (1994) calculated a mean inva-

sion rate of 4.3 mol C m 2yr 1 on shelves of the Bering

world (Walsh et al ., 1990). Despite of its shallow depth,

22 C.-T. A. Chen e t al .

Fig. 5. Excess of pCFCs in the Okhotsk Sea over the Pacific water at the same density levels (cited from Yamamoto et al ., 2003).

Sea. These are converted to DIC, DOC and POC, and are dissolution rates of carbonates.

An increase of approximately 200 atm in pCO2,

transported to the deep basin. They suggested that, as a

consequence of the rising levels of atmospheric CO2 since however, would make the shelf water undersaturated with

the Industrial Revolution, the biophysical CO2 status of respect to aragonite and high-magnesium calcite, which

the southeastern shelf may have switched over the last would then dissolve and neutralize the excess CO2. A

250 years, from a prior source to the present sink. Chen doubling of the current CO2 level in the atmosphere by

(1993a) reported that the excess CO2 in the Bering Sea the later part of the next century would cause the calcites

penetrates to approximately 1000 m. This is the depth on the shelves to dissolve, providing another large sink

where the excess CO2 is at about its detection limit of 5 for CO2 (Chen, 1993a).

mol kg 1. Some excess CO2 may be present below this

depth. The penetration is deeper in the eastern and south- 3.3 The Okhotsk Sea

ern regions, with slightly shallower penetration off the The Okhotsk Sea is the sixth largest marginal sea in

Kamchatka Peninsula (Fig. 4). Tritium and freons show the world and the third largest in the North Pacific and

similar penetration, but the 14C concentration does not has a maximum depth of 3475 m. It is enclosed on three

level off until a slightly deeper depth (Chen, 1993a; sides by land and the shelves occupy 40% of the surface

Warner and Roden, 1995). The Bering Sea contains about area. Although this sea is geographically located at a tem-

0.21 ( 0.05) Gt excess carbon around 1980. Although this perate latitude, it has many characteristics of a polar

value is small, export of the dichothermal water (Miura ocean, namely, large seasonal variations in water tempera-

et al., 2002) through the Kamchatka Strait could feed the tures and a subarctic water column structure. It also has a

NPIW with excess CO2. Further, the carbonate deposits seasonal ice cover from December through April with a

on the vast Bering and Okhotsk shelves could provide a thickness of about 1 m and an areal coverage comparable

large sink for excess CO2 in the near future. The shelf to that of the Bering Sea. It is connected to the open North

waters are currently about 210 and 150% saturated with Pacific through the Kuril Island chain. The two most im-

respect to calcite and aragonite, respectively. Because of portant passages are Bussol (2300 m sill depth) and

the shallow depth of the shelves (less than 200 m) rela- Kruzenstern (1900 m sill depth) Straits. The net exchange

tive to the saturation horizons of calcite (400 m) and between the open North Pacific and the Okhotsk Sea is

aragonite (350 m), the upward migration of these satura- on the order of 3 5 Sv (Talley, 1996). The Okhotsk Sea is

tion horizons due to the excess CO2 input (Feely and Chen, connected to the JES by two narrow, shallow straits: the

1982; Sharma et al., 1999) probably has not changed the Soya (La Perouse) Strait between Sakhalin and Hokkaido

Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean 23

with a sill depth of 40 m, and Tatar Strait between Sakhalin 60N

and the Eurasian continent with a sill depth of 10 m. At

northwestern shelf region, the Amur River, one of the larg-

est rivers in the world, supplies freshwater, heat (Ogi et 60

55N

al ., 2001), nutrients, and organic matters (Nakatsuka et 40

al ., 2003) into the Okhotsk Sea.

Within the Okhotsk Sea, the flow is basically cy- 50

clonic. Local circulation features include the northern

50N

shelf region where coastal polynyas are often found;

upwelling on the Kashevarov Bank; the Soya Current 45 40

which transports saline JES water into the Okhotsk Sea,

and; an anticyclonic eddy field in the Kuril Basin. The 50

Ocean Data View

45N

230

cold, fresh but dense Okhotsk Sea Intermediate Water 30

(OSIW) is believed to be an important source of the North

Pacific Intermediate Water (NPIW) (Wakatsuchi and 135E 140E 145E 150E 155E

Martin, 1991; Talley and Nagata, 1995; Watanabe and

Fig. 6. Distribution of pCFC-12 on the potential density sur-

Wakatsuchi, 1998). Because of the influence of the north-

face of 27.4 i n the southern Okhotsk Sea (cited from

west Pacific water there is a maximum in potential tem- Yamamoto et al ., 2003).

perature at about 900 m in the deep basin, which is

deeper and colder than the max found in the northwest

Pacific. There is a weak min immediately beneath the

surface layer, representing the remnant winter water. change is low because of the sea ice coverage. The sur-

There is an apparent oxygen utilization (AOU) and nor- face waters of the ice-free zone of the Kuril Basin have a

pCO2 of about 10 40 atm below saturation mainly be-

malized total CO2 (NTCO2 = TCO2 35/S) maximum

but pH minimum in both the Okhotsk Sea and the north- cause of the strong salinity stratification and the cooling

west Pacific. The AOU and NTCO2 values are lower be- effect. On the other hand, near the Bussol Strait the sur-

tween about 200 m and the max layer, but higher above face water are supersaturated with pCO2 (20 80 atm)

200 m or near bottom in the Okhotsk Sea. This is a clear because of vertical mixing in the strait and winter con-

indication that the Okhotsk Sea has younger waters be- vection (unpublished data, R/V Dmitry Peskov March

tween 200 m and the max layer but older water near bot- 2003 cruise).

The excess CO2 penetrates to at least 1000 m ( =

tom as compared to the open ocean outside. Above 200

m AOU and NTCO2 values are higher but pH is lower in 27.35 27.5; Chen and Tsunogai, 1998; Andreev et al.,

the Okhotsk Sea. These features remain unchanged at least 1999). Wong et al. (1998) recently reported that CFCs

are found in the OSIW (ranging from = 26.8 to 27.4)

since the 1950 s when data became available although

inter-decadal variability has been reported (Andreev and which feeds the NPIW where high values of dissolved

Kusakabe, 2001). oxygen (DO), tritium, CFCs (Fig. 5) and excess CO2 are

detected in the = 26.6 to 27.2 range (Tsunogai et al.,

The normalized TA (NTA = TA 35/S) data seem to

show a systematic offset between the typical Okhotsk Sea 1995). The OSIW outflow is about 6 Sv (Takahashi, 1999;

and the northwest Pacific stations. Both, however, show Yamamoto et al., 2003). Wong et al. (1998) gave 2.7

23.3 Sv with most of the transport occurring above =

a large increase below about 300 m, which is already

deeper than the saturation depth for aragonite and cal- 27.0. Multiplying these fluxes by the excess CO2 con-

cite. The surface water is generally below saturation for centration gives the excess CO2 export of 0.011 0.18 Gt

C yr 1 to the North Pacific. Recently Chen and Tsunogai

pCO 2 except near the Kashevarou Bank and the Kuril

(1998) reported an inventory of 0.18 0.08 Gt excess

straits where tidal mixing is strong (Rogachev e t al .,

1997). In summer, the undersaturation of surface waters carbon in the Okhotsk Sea. This inventory is by itself not

with pCO2 between 20 and 90 atm is commonly ob- large but the export is significant. The CFC12 data (Fig.

served, while the recorded values range between 170 6) indicates that the upper water is indeed mixed into the

atm understurated to 50 atm supersaturated. The low- deeper layer diapycnally. Since the CFCs of the North

est undersaturation of CO 2 is measured along the eastern Pacific water are very low below 300 m (Fig. 6), the

coast of Sakhalin and northern shelf area. These outflowing CFC-rich OSIW transports CFCs and excess

undersaturations are attributed to enhanced phytoplankton CO2 to the interior of the North Pacific.

productivity. From May to September a mean invasion The nutrient and DIC budgets for the Okhotsk Sea

rate is 1 2 mol C m 2yr 1 (Biebow and Hutten, 1999; are determined by the water exchange with the Pacific

Biebow et al., 2000). In winter, the rate of air-sea ex- Ocean, Amur river run off and the supply of low-nutrient

24 C.-T. A. Chen e t al .

Table 2. Salt, dissolved inorganic phosphorus (DIP), nitrogen (DIN), and carbon (DIC) budgets for the Okhotsk Sea (OS).

*Residual fluxes are calculated taking the water transport rate between the Okhotsk Sea and the subarctic Pacific to be 5 Sv

(Talley, 1996).

20 3 kg of salt, 0.04 0.01 mole, 1.0 0.1 mole and 4 0.7 mole (1012, yr 1) are the accepted fluxes of, respectively, salt,

DIP, DIN and DIC from the subarctic Pacific and to the Okhotsk Sea.

Soya Current water. The fresh water supply to the Okhotsk Derygin Basin. In the northern and western parts of the

Okhotsk Sea N* decreases from 2 mol 1kg 1 to 15

Sea is ~0.026 Sv (Aota and Ishikawa, 1991) as excess

mol 1kg 1 (Fig. 8). Data collected during the German-

precipitation and river run off over evaporation. Thirty

seven % of the fresh water transport to the Okhotsk Sea Russian project KOMEX (Kurilen-Ochotskishes Meer

is from the Amur River. The computed salt, nutrient and Experiment) (Biebow and Hutten, 1999; Biebow et al.,

carbon budgets for the Okhotsk Sea are provided in Ta- 2000) show that in the surface sediments the DIN/DIP

ble 2. In the budget calculation differences in salinity, ratio decreases from 10 15 to 3 due to denitrification.

DIN, DIP and DIC between the subarctic Pacific and the This is equivalent to a spatially averaged denitrification

rate of 0.26 0.13 mol N m 2yr 1.

Kuril Basin of the Okhotsk Sea (Fig. 7) averaged in the

water column between 26.7 27.5 was used (Andreev Strong winds coupled with intensive vertical mixing

et al., 2002). The spatially averaged concentration of the and inter-leaving in the winter enhance the oceanic pen-

organic P, N and C in the upper sediment are respectively etration of excess CO2 in the Bering and Okhotsk Seas.

0.06%, 0.12% and 1.2% (Bruevich, 1956; Bezrukov, Although these seas have a limited capacity to store the

1960). Taking a mean sedimentation rate to be 0.01 cm excess CO2, they act as a conveyer belt. That is, the dense

yr 1 (Biebow and Hutten, 1999; Biebow et al., 2000) an- shelf water formed in winter (0.5 0.9 Sv; Wong et al.,

nually ~8 10 10 mole C, 0.6 10 10 mole N and 0.15 1998; Yamamoto et al., 2003) flows out to the deep ba-

10 10 mole P are converted into sediments. It is clear that sins and enters the intermediate layer. The outflowing

the major contributor of P, N and C in the dissolved inor- water from the highly biologically-productive shelves

ganic form is the Pacific Ocean. Annually, the Okhotsk contains large amounts of DOC and POC (Nakatsuka et

Sea obtains ~0.04 1012 mol P, 1.0 1012 mol N and 4 al., 2002, 2003) and is laden with excess CO2. At the re-

10 12 mol of DIC from the subarctic Pacific. As for P and gion off the east coast of Sakhalin Island, the lateral ex-

N, the riverine input is only about 2 3% of the total in- port of DOC and POC by the dense shelf water are much

flux, much smaller than the contribution from the incom- larger than the sinking POC flux from in-situ surface water

ing water masses. (Nakatsuka et al., 2003). In the Okhotsk Sea, it seems

The residual flux of DIN (Table 2) is 0.4 0.2 that the DOC and POC accumulated in the shelf water

10 12 m ol yr 1, which is taken to be the sedimentary are transported out of the shelf without significant removal

denitrificaton rate. As the result of denitrification there at the shelf edge.

is a significant decrease in N* ([NO3] + [NO2] 16 In addition to dense shelf water outflow, the water

[PO4]) on the shelf (Gruber and Sarmiento, 1997) and from the JES also imports excess CO2 into the intermedi-

Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean 25

Fig. 8. Vertical profiles of ([NO3] + [NO 2] 16[PO4]) mol

Fig. 7. Difference in salinity ( 200), dissolved inorganic kg 1 and [NH4] mol kg 1 in the northwestern Okhotsk Sea

carbon ( mol kg 1), DIN ( 4, mol kg 1), DIP ( 100, and Pacific.

mol kg 1) and silicate ( SiO4, mol kg 1) between

the Okhotsk Sea (Kuril Basin) and the subarctic Pacific

versus . The errors bars shown are 84% confidence inter-

maximum depth is 4036 m. The shelves are generally

vals.

narrow except in the far north and far south. While the

subsurface waters are characterized by the unique homo-

geneity, called Japan/East Sea Proper Water (Uda, 1934),

studies in recent years confirm that subsurface waters

ate layer of the Okhotsk Sea. The water, called the Fore-

display small but distinctive oceanic characteristics with

runner of Soya Warm Current water from JES, has a den-

sity of more than 26.8 in spring due to its high salinity several water masses (Gamo and Horibe, 1983; Kim and

Kim, 1996; Kim et al., 2001). Numerous results of hy-

and winter cooling, and can sink into the intermediate

drological measurements show anomalously high concen-

layer. It contains excess CO2 absorbed during its north-

tration of dissolved oxygen, suggesting rapid turn over

ward flow. According to Otsuki et al. (2003), CO 2 flux

of the subsurface waters: tracer studies have shown that

transported to the intermediate layer by this water is about

the turnover time of JES deep waters is on the order of

one tenth of that by dense shelf water. Subsequently, the

100 years (Watanabe et al., 1991; Chen et al., 1995a; Kim

intermediate water transports excess CO2 to the North

et al., 1999). The mean inflow and source of N (1 1012

Pacific Ocean (Chen, 1993b).

mol N yr 1) is through the Tsushima/Korea Strait with an

Further, these areas may have shown signs of cli-

annual mean transport of 2 2.5 Sv (Yanagi, 2002; Lyu

matic change. The warm Kamchatka Current intermedi-

and Kim, 2003). The Tsushima current is a branch of

ate water has warmed during the last decade while the

Kuroshio and the transport is strong in summer and fall.

upper layer in the Oyashio and the Okhotsk Sea has cooled

About 80% of the inflow leaves the JES and enters the

and become fresher (Rogachev, 2000). Andreev and

North Pacific through the Tsugaru Strait with a sill depth

Kusakabe (2001) also demonstrated that the DO concen-

of 130 m. The rest flows into the Okhotsk Sea through

trations in the intermediate waters of the Okhotsk Sea

the Soya Strait which is only 40 m deep. There is negligi-

and the western Subarctic Gyre have decreased since the

ble flow into the Okhotsk Sea through the 10 m-deep Tar-

1950s. Since the residence time of the intermediate water

tar Strait.

of the Okhotsk Sea is only several years (Yamamoto et

The pCO2 studies of JES surface waters show super-

al ., 2001, 2003), the inter-annual variability of the for-

saturation in summer and undersaturation in winter with

mation rate or properties of the Okhotsk Sea Intermedi-

an annual air to sea invasion of 0.045 Gt C (Oh et al.,

ate Water may directly affect the outflowing water which

1999; Kang and Kim, 2003). This influx rate of 3.7 mol

enters to the NPIW.

m 2yr 1 is more than twice the global average. With its

rapid turn over time, the entire JES has been penetrated

3.4 Japan/East Sea

by excess CO2, totaling 0.31 0.05 Gt C in 1992 (Chen

The JES is the eighth largest marginal sea in the

et al., 1995a; Kang, 1999). The ratio of CaCO3 dissolu-

world and the fourth largest in the North Pacific. The

tion and the organic carbon decomposition vary from 0.05

average depth is 1667 m but the three basins, namely the

at 300 m to about 0.17 below 2000 m. These values are

Japan, Yamato and Ulleung Basins exceed 2500 m. The

26 C.-T. A. Chen e t al .

Table 3. Consumption or production rates of CaCO3, organic carbon, oxygen and nutrients in the deep North Pacific Marginal

Sea Basins ( mol kg 1yr 1).

*Taken from Chen et al ., 1996a; **Taken from Chen et al ., 2001.

lower than the ratios of 0.14 and 0.36 found in the South deep connections with the outside are the 400 m-deep

and North Pacific, respectively (Chen et al ., 1982; Chen, Mindoro Strait which leads to the Sulu Sea, and the 2200

1990), or 0.54 in the Bering Sea (Chen, 1993a). Diatom m-deep Bashi Channel which opens to the Philippine Sea.

and silicoflagellates dominate the phytoplankton (Hong The seasonally reversing monsoon winds play an impor-

and Chen, 2002). As a result, the vertical gradient of SiO2 tant role in determining the upper ocean circulation. The

is several times that of calcium or alkalinity (Chen et al., combination of such variable atmospheric forcing and

1995a). complex geometry contributes to the complicated dynam-

Because the straits connecting the JES to the outside ics of the flow in the SCS. There is no deep or intermedi-

are all narrow and shallow, the deep waters have no di- ate water formation in the SCS. In addition, there is strong

rect exchange with the Pacific, the ECS and Okhotsk Sea. upwelling. As a result, the excess CO2 does not penetrate

Since the deep waters of the JES are all formed inside the more than 1500 m and the entire SCS probably contains

0.43 ( 0.1) Gt C anthropogenic carbon in 1994.

closed basin, they provide an unique opportunity to con-

firm the Redfield Ratios based on the mass-balance Particle fluxes in the SCS appear to be basically con-

method (Chen et al., 1996a). The consumption and pro- trolled by monsoon-related processes (Liu et al., 2002).

duction rates of CaCO3, OC, DO and nutrients are given Upwelling phenomena and wind-induced nutrient entrain-

in Table 3. The JES provides, furthermore, another unique ment into the euphotic zone may account for the overall

environment to study the carbon cycle within the closed enhanced production of biogenic components in the cen-

basin (Fig. 9; Kang

interference has led to substantially increased loadings

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