IEG 환경지질연구정보센터

Vol. 7, No. 2, p. 179
197, June 2003

Recent developments in chemical oceanography of the East (Japan) Sea with an emphasis on CREAMS findings: A review

ABSTRACT: The understanding on the chemistry of the East (Japan) Sea, a typical mid-latitude marginal sea, has been dra- matically improved through the CREAMS expeditions, an interna- tional cooperative study, carried out during 1990s. The CREAMS studies confirmed that the East Sea has undergone dramatic changes during the last 50 − 60 years. One of the most prominent characteristics of these changes is a rapid decrease of dissolved oxygen in deep waters. There even has been a claim that the East Sea might become an anoxic sea by next 200 years. While the causes for these changes are still under investigation, it has been shown that these changes are mainly due to the modification in the mode of deep water ventilation system in the East Sea: a slow down and complete stop of bottom water formation accompanied by an enhancement of upper water formation instead. A simple moving-boundary box model (MBBM) was developed in order to quantify the processes involved in such changes for the last 50 − ⁶⁰ years. The model predicts that the East Sea may remain as a well- oxygenated sea despites recent rapid oxygen decreases in deep waters in association with structural changes such as a shrinking of oxygen-depleted deeper waters and an expansion of oxygen-rich upper in the East Sea in next few decades. The sedimentary record, however, shows that the East Sea has undergone oscillation between well-oxygenated environment and anoxic environment during last glacial period in association with the eustatic sea-level change. Sev- eral flooding processes such as intrusion of cold Oyashio Current and less saline, nutrient-rich seawaters from East China Sea and Yellow Sea has been proposed. Being a semi-closed basin, the car- bon cycle of the East Sea has been a subject of CREAMS inves- tigation. The East Sea serves as a strong sink of atmospheric CO

; penetration of anthropogenic CO

all the way to the bottom is clear with its very rapid conveyor-belt system.

Key words: East (Japan) Sea, climate change, ocean conveyor belt, moving-boundary box model, anoxic sea, alkenone, paleoceanography, eustatic sea-level change, carbon cycle

1. INTRODUCTION

The East (Japan) Sea is a typical mid-latitude marginal sea in the western Pacific surrounded by Korea, Japan, and Russia (Fig. 1). With an area covering 0.6% of the Pacific Ocean, the East Sea is the eighth largest marginal sea in the world and the fourth largest in the North Pacific, consisting of deep basins exceeding 2500 meters such as Japan Basin,

Yamato Basin and Ulleung Basin. Even with its average depth of 1667 m, however, the exchange of seawaters between the East Sea and the North Pacific Ocean and adjacent seas is rather limited due to its 4 shallow sills such as Korea Strait in the south and Tsugaru Strait, Soya Strait (La Per- ouse Strait) and Tatarsky Strait in the north with depths much less than 200 meters. While 2.5×10

m

s

(Sv) of warm, saline Tsushima Current, on the average, is flowing into the East Sea through Korea Strait (Lyu and Kim, 2003), about two third of this is right back into the Pacific Ocean through Tsugaru Strait and the remaining one third is moving further north and flowing out into the Okhotsk Sea through Soya Strait, making the East Sea to behave as a semi-closed basin. However, the exchange of seawaters between northward, saline Tsushima Current and northern East Sea is the important source of salts into the East Sea, keeping its conveyor-belt in action.

In the early 1930s, Uda explored the seas around Korea by mobilizing 53 vessels and found that the entire basin of the East Sea below several hundred meters was filled with waters with quite uniform physico-chemical characteristics (Uda, 1934). Uda named this water body the East (Japan) Sea Proper Water, ESPW. He also observed that ESPW has quite high dissolved oxygen concentration over 250 mM and concluded that ESPW must be ventilated rather fast.

This ventilation process was later quantified by several tracer studies (Tsunogai et al., 1993; Kim et al., 2001) showing that the turnover time of the East Sea deep waters is on the order of 100 years, which is at least one of magnitude smaller than that of oceanic conveyor-belt system (Bro- ecker and Peng, 1982).

Gamo and Horibe (1983) through CTD observations noted later that ESPW has temperature and salinity structures, which are very similar to those in the open ocean, strongly indicating that different water masses such as Deep Water and adiabatic Bottom Water exist within the basins. Further- more, Gamo et al. (1986) showed that the oxygen profiles in the East Sea have been changing since at least the 1960s.

CREAMS (Circulation Research of the East Asian Mar- Dong-Jin Kang

Kyung-Eun Lee

Kyung-Ryul Kim* } OCEAN Laboratory/RIO, SEES, Seoul National University, Seoul 151-747, Korea

*Corresponding author: [email protected]

†

Present address: FB5 Geowissenschaften, University of Bremen, Klagenfurter Strasse, D-28359 Bremen, Germany

ginal Seas) Expeditions, a Japan−Korea−Russia international cooperative research program, which started in 1993, have provided a rare opportunity to carry out precise measure- ments of salinity, temperature and chemical tracers in all major basins of the East Sea. The studies also included the long-term mooring of deep sea current meters in Japan Basin. This was the first detailed study carried out in the East Sea in more than 60 years since Udas investigation (Kim and Kim, 1996; Kim et al., 2001). In 1998, US sci- entists joined the East Sea studies as CREAMS II, and expanded the research areas extensively.

2. MATERIALS AND METHODS

Figure 2 shows cruise tracks during the CREAMS expe- ditions. The cruise track of the 1996 study was severely modified from those of previous years mainly due to the limitation imposed on study areas by Japanese and Russian governments within the framework of the new “Law of Sea” in 1996. Figure 3 shows the stations, which were occu- pied during the CREAMS II study in 1999.

Chemical observations consisted of Rosette water sam- pling and continuous pCO

measurements in the surface waters along the cruise tracks. Nutrients, dissolved oxygen, alkalinity and pH were measured on board. CFC measure- ments in the water samples were also carried out on board

during the 1996 cruise and CREAMS II cruise (Min et al., 1996). Aliquots of C-14, tritium and the stable isotopes of oxygen and hydrogen were also taken for later studies.

Nutrient concentrations were measured by spectrophoto- metric methods (Strickland and Parsons, 1972). Dissolved oxygen was measured spectrophotometrically after samples were fixed according to the procedure in Winkler method (Pai et al., 1993). In 1996, continuous dissolved oxygen profiles were also obtained with a oxygen sensor attached to CTD and were calibrated with bottle data. pH (total hydrogen ion scale) and total alkalinity were also measured spectrophotometrically (Clayton and Byrne, 1993) and by potentiometric titration (Millero et al., 1993), respectively.

The total inorganic CO

was calculated from thermodynamic relationships. pCO

measurements in seawaters were car- ried out by an equilibration method with a Weiss-type equi- librator and NDIR detector system.

3. RESULTS

3.1. The East Sea, a Miniature Ocean

CREAMS studies confirmed earlier observation by Gamo and Horibe (1983) that profiles of T, S, and dissolved oxygen in the East Sea are reminiscent of typical oceanic structures as shown in Figure 4. Therefore, names for these water masses such as East Sea Central Water, East Sea Deep Water, and East Sea Bottom Water were newly proposed to identify these different water masses based on the θ−S analysis as shown in Figure 5 (Kim et al., 1996; Kim et al., 2001). In the figure, the boundary depth between Central Water and Deep Water is 0.11−0.15

C and the Bottom Water corresponds to the adiabatic well-mixed bottom boundary layer.

However, as seen in Figure 4, it should be noted that the ranges of variability are extremely small in the East Sea:

one order of magnitude smaller in temperature, and even two orders of magnitude in salinity when compared with those observed in the open ocean (Östlund et al., 1987), implying a very weak vertical stability.

3.2. The East Sea in Changes

CREAMS studies also confirmed that the East Sea has undergone changes continuously since at least more than last 50 years. Dissolved oxygen is an important oceano- graphic parameter showing such dramatic changes besides warming of whole water columns (Gamo et al. 1986; Kim and Kim, 1996; and Kim et al., 1999; Gamo, 1999; Kim et al., 2001). Figure 6 shows potential temperature and dis- solved oxygen profiles at a station in central Japan Basin for the last 30 years. Dissolved oxygen profiles were further extended to the Udas investigation in early 1930s.

The dissolved oxygen profiles obtained by Russian sci- entists in 1950s are essentially same as those obtained by Fig. 1. A topographic map of the East Sea. The East Sea has three

deep basins, Japan Basin, Yamoto Basin, and Ulleung Basin. The

water exchange with the Pacific occurs through 4 shallow straits

(Korea, Tsugaru, Soya, and Tatarsky Straits).

Uda in 1930s (USSR Academy of Science, 1957). Since then, however, the depths of the oxygen minima have deep- ened from a few hundred meters in the late 1960s to below 1500 meters in the mid-1990s. This deepening of oxygen minimum depths is accompanied by decreases in oxygen concentrations in deep waters by more than 20 µM. It is important to note, however, that the oxygen concentrations

in intermediate depths above the oxygen minimum did show an increase instead.

3.3. Baseline Data during CREAMS II Studies

CREAMS II provided an opportunity to study the entire basins of the East Sea most extensively as clearly seen in the Fig. 2. The cruise tracks of CREAMS expedition from 1993 to 1996. The cruise track of 1996 was severely modified from those of pre- vious years mainly due to the limitation imposed on study areas by Japanese and Russian governments within the framework of the new

“ Law of Sea” in 1996.

cruise track (Fig. 3). The CREAMS II was in many senses very similar to the GEOCECS (Geochemical Ocean Sec- tions) studies, carried out over all major Oceans as a part of the IDOE (International Decade of Ocean Exploration) dur- ing 1970s (Craig and Turekian, 1980). During CREAMS II studies, major chemical tracers including CFCs, tritium besides basic nutrients and carbon chemistry were measured along with extensive physical studies. Considering the fact that the East Sea is in rapid changes, the study will provide a valuable baseline data to be compared for later studies.

In Figure 7, typical summer-time profiles of temperature, salinity, dissolved oxygen, nitrate, phosphate and silicate for three stations in Japan Basin, Ulleung Basin and Yamato Basin were shown. Even with general similarities of pro- files among basins with its fast conveyor-belt system, some differences may be noticeable, for example, in silicate profiles, possibly reflecting the direction of the conveyor-belt system, deserving further careful investigations in the future.

In Figure 8, vertical sections of the principal parameters along the N−S transect (Fig. 3) were shown. Sections for the upper 400 meters were vertically expanded, for the most of the variability can be observed in these depth ranges.

Temperature and salinity sections in shallow depths clearly show the polar front at ∼ 38

N in latitude near stations 44 and 179 (around 500 km distant from the north end). Prop- erties in deep waters below 400 meters are fairly homoge- neous except silicate, reflecting its fast conveyor-belt system.

Figure 9 shows several property-property plots for the

Fig. 4. Temperature and salinity pro- files in the northern East Sea. Profiles at a station in the South Pacific, obtained during the GEOSECS Expe- dition (Östlund et al., 1987), are also shown for comparison, demonstrating the oceanic nature of the East Sea.

While similarities in profiles are quite remarkable, it is worth noting that the range of variability is much smaller in the East Sea; one order of magnitude in temperature and even two orders of magnitude in salinity.

Fig. 3. The cruise track of CREAMS II study in 1999. The stations

cover the entire basins of the East Sea.

same N−S transect. Nitrate and phosphate show a good lin- ear relationship except near the surface with the Redfield N/

P ratio of 12.8, a slightly lower than global average value.

But, the AOU to phosphate ratio in deep waters show ∼136, which is very close to the global average.

4. DISCUSSIONS

4.1. Changes in the East Sea Conveyor Belt System Many studies have shown that changes in temperature, dissolved oxygen are mainly due to a transition in the mode

of its conveyor-belt system from bottom-water formation in the past to intermediate-water formation at the present time (Kim and Kim, 1996; Kim et al., 1999; Gamo et al., 2001;

Kang et al., 2003a). A clue for this transition was first dis- covered by analyzing the oxygen profiles shown in Figure 6 (Kim and Kim, 1996).

Assuming that the system is in a quasi-steady-state mode, an equation for a steady-state one-dimensional advection- diffusion model (Craig, 1969; Gamo and Horibe, 1983) is written as follows,

where K is the vertical eddy diffusion coefficient, w is the upwelling velocity, J is the term representing the non-con- servative nature of dissolved oxygen, which is positive for production and negative for the consumption process, q rep- resents the parameters such as potential temperature, θ and dissolved oxygen, and z is the upward distance from the lower boundary.

From the θ−S plot (Fig. 5), deep waters are divided into two layers showing a linear θ−S relationship. The upper layer corresponds to East Sea Central Water (CW) and the lower layer to East Sea Deep Water (DW). At first, poten- tial temperature was fitted with the mixing parameter K/w ( = Z*). Then, the oxygen profile was fitted with J/w. The details of the analysis are given elsewhere (Kim and Kim, 1996). The result of the model calculation is shown in Fig- ure 10.

The curvature of the temperature profile shows that there is an upward vertical velocity (w>0) with mixing parame- ters, Z*, 0.55 km and 0.71 km for CW and DW layer, respectively. These values are similar to those obtained for Bottom and Deep Waters in the Pacific Ocean, 0.4−1.1 km

0 = K d (

q dz ⁄

) – w dq dz ( ⁄ ) + J

Fig. 6. The changes in profiles of tem- perature and dissolved oxygen in the Japan Basin during the last several decades. The average profiles of dis- solved oxygen (O

) in 1930s and early 1950s are also shown. The property changes such as warming of deep waters and a drastic change of O

structures in the East Sea since some- time during 1950s are clear. Some data are from earlier studies in the area (Uda, 1934; USSR AOS, 1957; Gamo and Horibe, 1983; Sudo, 1986).

Fig. 5. A typical θ − S plot for deep waters in the Japan Basin with

recently proposed names of water masses (Kim et al., 1996).

(Chung, 1975). The J/w for DW is −21.2 µmol/kg/km, showing the consumption of oxygen in deep waters. However, J/w for CW is + 35.2 mmol/kg/km, that is, “positive”. This should imply a “net” production of dissolved oxygen in the CW layer, which can only be accomplished by the injection of oxygen-rich surface waters into this layer (Craig, 1994).

The transition in the mode of deep water ventilation sys- tem in the East Sea; from bottom water formation in the past to upper water formation at the present time could also be traced from historical temperature measurements as shown in Figure 11 (Kim and Kim, 1996; Kim, 1996). As described earlier, boundary depths between CW and DW were assumed to be the depths of 0.13 + /−0.02

C while those between DW and BW (Bottom Water) were the top of the bottom adiabatic layer. The historical data clearly show the expanding Central Water and the diminishing Bottom Water. The volume of Deep Water in between Central Water and Bottom Water has also been shrinking.

There has been some claims that conveyor-belt in the

East Sea has slowed down in recent time (for example, Gamo, 1999). However, this is only one-side of the coin. It is more appropriate to say that the over-all ventilation sys- tem in the East Sea is still very active, and that the decrease of bottom water formation is counterbalanced by the enhancement of intermediate water formation. The imme- diate consequence of this shift in conveyor-belt system, thus, is a fast expansion of oxygen-rich Central Water in mid- depths in the East Sea in recent years (Kim et al., 1999).

4.2. A Moving-Boundary Box Model (MBBM)

A simple moving-boundary box model was developed to quantify these changes, which is based on a static box model by Watanabe et al. (1991) and Tsunogai et al. (1993).

The MBBM is a non-steady-state box model, which consists

of four vertical layers, representing surface waters, CW,

DW, and BW from the top to the bottom as shown sche-

matically in Figure 12. The top layer is further divided into

Fig. 7. Typical profiles of temperature,

salinity, dissolved oxygen, and nutri-

ents at each basin in the East Sea

obtained from CREAMS II expedition

in 1999.

two boxes, which represent Warm Surface Water (SW) in the southern sub-tropic region and Cold Surface Water (SC) in the northern sub-polar sea.

A steady-state box model (Tsunogai et al., 1993) is mod- ified to describe the East Sea in such a way that the volume

of boxes, V

(for CW), V

(for DW), and V

(for BW)

can alter with time according to historical observations as

shown in Figure 11, by moving the boundaries between

these boxes without changing the total volume of all the

boxes (Kim and Kim, 1996; Kang et al., 2003a). In this

Fig. 8. Vertical sections of the temperature, salinity, dissolved oxygen, and nutrients along the N − S transect. The transect crossing the

western Japan Basin and Ulleung Basin was shown in Fig. 3.

MBBM, the formations of DW and BW directly from CS were further introduced which were essential in order to describe the chemical data such as CFC-11 and tritium.

The basic assumptions of the model are as follow:

1) Formation of deep waters (CW, DW, and BW) occurs only in the area of cold surface waters (SC).

2) The total rate of deep water formation, i.e. the sum of D1, D2, and D3 (=U1) remains constant, while individual D1, D2 and/or D3 can change through time,

3) U1 is equally divided and moved upward to SW and SC, being the same area of SW and SC,

4) The rate of BW formation (D3) started to decrease lin- early from year 1952 (i.e., t

=1952), following the observed data (Fig. 11),

5) The decrease in D3 is compensated by the increase in CW formation (D1), keeping D2 being constant.

The mass balance equations for t>t

are as follows:

1) Surface Waters (SW and SC)

= constant (1)

2) Central Water (CW)

(2) 3) Deep Water (DW)

= -5.14×10

(3)

4) Bottom Water (BW)

(4) Coefficients for the volume change in the above Equations (2), (3), and (4) are obtained from Figure 11, the observed U1 = D1 + D2 + D3

dV

--- m dt (

yr

) = D1 + U2 U1 –

7.61 × 10

( t t –

) + 1.18 × 10

–

=

dV

--- m dt (

yr

) = U3 + D2 U2 –

dV

--- m dt (

yr

) = D3 U3 –

7.61 × 10

( t t –

) – 6.66 × 10

=

Fig. 9. Property-property plots for the

same transect of Fig. 8. (a) J-S dia-

gram, (b) Apparent Oxygen Utiliza-

tion (AOU) versus phosphate, (c)

Nitrate versus phosphate, and (d)

Silicate versus phosphate.

change of boundary depths between water masses and the depth-area relationship between area and depth for the East Sea (Kim et al., 2002).

Using a commercially available system software (STELLA

), a time-dependent box model was constructed with the vol- ume changes of each box in time being prescribed as Equa- tions (2), (3) and (4).

Then, the concentrations of two tracers, CFC-11 and tri-

tium for CW, DW, and BW were estimated by injecting these tracers into the surface waters, SW and SC. The CFC- 11 concentration of SC, C

(CFC-11), was assumed to be saturated with the atmospheric mean concentration of CFC- 11 in the mid-latitude Northern Hemisphere at 5ºC. The tri- tium concentration in SC, C

(T), was assumed to be same as that in the North Pacific waters (Broecker et al., 1986).

The profiles of CFC-11 collected in 1996 (Fig. 13) and of tritium in 1987 (Watanabe et al., 1991) in the East Sea were used to calibrate the model (Fig. 3). In Figure 13, the profile of CFC-12 and the CFC-11/CFC-12 age was also given.

The details of model calibration were given elsewhere (Kang et al., 2003a).

The results are shown in the Figure 14. One of the most Fig. 12. A schematic diagram of the moving-boundary box model for the East Sea (SW: Surface Warm Water, SC: Surface Cold Water, CW: Central Water, DW: Deep Water, BW: Bottom Water).

D1, D2, D3, U1, U2, and U3 represent fluxes between boxes.

Boundaries of CW/DW and DW/BW have been moving with time according to the changes in D1 and D3.

Fig. 11. (a) Changes in the depths of the boundaries between CW and DW, and DW and BW over time, estimated from the historical data (Kim, 1996).

(b) Volume changes of water masses over time, calculated from the linear relationship between depth and area of the East Sea (Kim et al., 2002) and (a).

Fig. 10. The results of potential temperature and dissolved oxygen profiles by an one-dimensional advection and diffusion model.

The curvature of the temperature profile shows that there is an

upward vertical velocity (w>0) with mixing parameters, Z*, 0.55

km and 0.71 km for CW and DW layer. The J/w for DW is − 21.2

µmol/kg/km, showing the consumption of oxygen. However, J/w

for CW is + 35.2 mmol/kg/km, implying a “net” production of dis-

solved oxygen in CW layer.

important conclusion is that the formation of Bottom Water (D3) of 0.02 Sv in magnitude in the past completely stopped in between mid 1980s and late 1990s (t

=1993 ± 8).

Further studies on other man-made tracers such as SF

and CFC-113 will reduce the present uncertainties in the model.

Figure 15 shows the model results of the structural changes of the East Sea in time.

4.3. The Future of the East Sea

Assuming the current changes to continue, the expected structure of the East Sea in 2040 is also given in the Figure 15, clearly showing the disappearance of Bottom Water in the East Sea. It is still not clear at the present time what the real causes of these changes are. Such change could be a part of the natural cycle that the East Sea has experienced in the past; similar behavior has already been observed over much larger areas and over much longer time scales (for example, Broecker and Denton, 1989; Appenzeller et al., 1998; Stoker, 1998; Broecker et al., 1999). Alternatively, they could be a result of recent global warming manifested on a regional scale. Regardless of the reasons thereof, the MBBM strongly imply that not much can be said about the future of

the East Sea beyond 2040 until we fully understand the sys- tem working behind such changes.

In Figure 16, the model prediction of dissolved oxygen contents in deeper waters in the near future is also given.

The results show that the East Sea may remain as a well- oxygenated sea despites recent rapid oxygen decreases in deep waters. This is mainly due to the associated structural changes such as a shrinking of oxygen-depleted deeper waters and an expansion of oxygen-rich upper in the East Sea in next few decades (Kang et al., 2003b). This is quite contrary to some claims of anoxemia in the East Sea in next few hundred years and might allay recent concerns that the bottom waters in the East Sea may become anoxic creating an ecological disaster in 200 years (Chen at al., 1999; Gamo 1999).

4.4. The Paleoceanography of the East Sea

However, several studies of sediment cores in the East Sea have shown that the East has undergone through anoxic environments in the past, especially during the last glacial period (for example, Oba et al., 1991; Keigwin and Gor- barenko, 1992; Tada et al., 1999). The presence of dark lay- ers in the sediment columns unequivocally proves the existence of such changes.

The history of the East Sea has been reconstructed pri- marily with cores from KH-79-3, which were recovered from the top of Oki Ridge in the southeastern part of the East Sea at the water depth of 935 m (Oba et al., 1991; Oba et al., 1995; Crusius et al., 1999; Ishiwatari et al., 2001) and cores from ODP site 797 at the south central part of the East Sea with the water depth of 2874 m (Tada et al., 1999).

Recently cores recovered from the southern Ulleung Basin were also investigated (Lee and Kim, 2002). The positions of these stations are shown in Figure 17. These investiga- Fig. 14. The variation of fluxes between boxes with time calcu- lated from MBBM.

Fig. 13. Profiles of CFC-11 and CFC-12 obtained in 1996 at a sta-

tion in the central Japan Basin. The profile near the bottom are

shown in an expanded scale. The CFC ratio age is also shown in

the figure. The CFC ratio age of the waters are no younger than 20

years for deep waters.

tions applied various methods such as AMS C-14 for monospecific planktonic foraminifera, alkenone and diatom and foraminiferal assemblage as well as oxygen isotopes in order to decipher paleotemperature, and paleosalinity of the East Sea during the past glacial-interglacial period.

The eustatic sea-level depletion of 120 m below the

present level during the last glacial maximum made the northern Tatarsky Strait (shallower than 25 m) and Soya Fig. 16. Variation of oxygen inventories of each water mass in the

East Sea simulated by O

-MBBM. The sources of data: 1952;

Chen et al. (1999), 1954; Kim and Kim (1996), 1969; Gamo and Horibe (1983), 1977, 1979, 1984; Gamo et al. (1986), 1992; Chen et al. (1999), 1995, 1996; Kim et al. (1999).

Fig. 17. Bathymetric map of the East Sea with the locations of cores recovered for studies.

Fig. 15. The structural changes with time estimated from the MBBM. The model predicts that the bottom water disappears completely in 2040.

Strait (sill depth of ∼55 m) being sub-aerial and the Korea Strait and Tsugaru Strait being almost closed with minimal opening with the Pacific Ocean and the Okhotsk Sea with the sill depth of 20 + /−m (Tada et al., 1999).

The major consequence of the isolation of East Sea from the Pacific Ocean was the depletion of salts needed for driv- ing deep convection within basins (Oba et al., 1991; Tada et al., 1999). The warm, saline water entering into the East Sea through the Korea Strait as a branch of Kuroshio Cur- rent at the present time is the most important source of salts in order for northern surface waters to become dense enough to sink down and initiate the conveyor belt system.

How the East Sea was flooded as the sea-level rose is still a very interesting scientific question to be resolved. Oba claimed that the flooding of the East Sea was initially done by the intrusion of cold Oyashio current from the north through the Tsugaru Strait prior to the intrusion of warm, salty waters from the south through the Korea Strait as in the present time (Oba et al., 1991; Oba et al., 1995). This claim originated from the observation of the initial drop in SST prior to the rise of SST, reaching present day warm temperature. This SST changes can also be observed in the cores from the Ulleung Basin as shown in Figure 18 (Lee and Kim, 2002). Between 15 and 11 kyr BP, the tempera- tures were about 3

C lower than today. However, how the

Oyashio current intruded into the East Sea is still an impor- tant subject to be further investigated (for example, Ikeda et al., 1999). It could also be simply due to the inflow of cold ice-melt water.

Another interesting question associated with paleo-SST of the East Sea is that the alkenone proxy results show an apparent warming of surface water during the Last Glacial Maximum period contrary to conventional belief (Ishiwa- tari et al., 2001; Lee and Kim, 2002). Figure 18 clearly shows the warming of SST by about 3°C than today during 15−19 ka. Considering various factors, which might influ- ence the SST reconstruction from the alkenone, U

values, it was concluded that the alkenone temperature estimates are reliable (Lee and Kim, 2002). However, the reason for this warming is still in debate.

Warming at the early Holocene could be due to inflow of warm water into the East Sea through the Korea Strait. But, there is also a possibility that it could be caused by shift in the season of maximum alkenone production. Core top alk- enone SST calibration with modern surface temperatures and sediment trap data indicate that the SST estimated from alkenones most likely represents the temperatures of late fall as shown in Figure 19. Therefore, there is a possibility that the maximum production of alkenone was done in summer time during the last glaciation whereas it is in late fall during the Holocene.

Tada et al. (1999) proposed a different mode of flooding process during the initial stage of flooding in order to explain the banded structure of alternating dark and light layers. They proposed two types of flooding mode prior to the final stabilization to the current situation: one by the intrusion of warm, saline Tsushima Current, which is sim- ilar to the current situation and the other by the intrusion of less saline, high nutrient waters from the East China Sea and the Yellow Sea. The oscillation between these two

Fig. 19. Annual variation of modern sea surface temperature at a location close to TY99PC18. Alkenone SST for coretop sediment (18

C) is indicated as a horizontal line.

Fig. 18. (a) Total C

alkenone concentration versus

C ages in core TY99PC18. Shading indicates the interval of alternating lay- ers of crudely laminated mud and homogenous mud. (b) U

-

based temperature estimates versus

C ages in core TY99PC18.

modes could produce the observed layered structures in the sediment column. Furthermore, the intrusion of seawaters from the East China Sea and the Yellow Sea into the East Sea at the present time became a subject of scientific debate in recent years (for example, Suk et al., 1996). The connec- tivity between the East Sea and Yellow Sea and East China Sea in the past and also at the present time will be a very exciting subject to be further explored in future studies.

4.5. Carbon Cycle Studies in the East Sea

The role of the ocean is very crucial in the overall bio- geochemical cycle of CO

with its special pumping mech- anisms (Volk and Hoffer, 1985) such as solubility pump at the air-sea interface, biological pump in the surface waters, the carbonate pump associated with carbonate deposition at the sea floor and dynamic pump associated with ocean cir- culation. Therefore, the quantification of processes involved in the biogeochemical cycle of CO

is one of the most

important research areas for understanding the effect of human perturbation on global climate in recent years.

The CREAMS studies demonstrated that the East Sea can serve as a miniature ocean for studying the oceanic pro- cesses associated with recent global changes such as global warming (Kim et al., 2001) and thus strongly suggested that the East Sea is one of a few ideal places to study oceanic carbon cycle on a small scale. Furthermore, the role of mar- ginal seas, such as the East Sea, in global carbon cycle has not been fully assessed yet (Fasham et al., 2001). Some studies in continental margins suggest that the regions are net sinks for CO

(Monaco et al., 1999; Liu et al., 2000). If similar results can be applied to other regions, the total uptake of CO

in the continental margins could be as much as 0.2−1 Gt-C in a year, a significant component in the glo- bal carbon cycle. Therefore, a new term, continental shelf pump, as a mechanism for the absorption of atmospheric CO

is also proposed (Tsunogai et al., 1999).

The carbon cycle studies in the East Sea have been car-

Fig. 20. The distribution of ∆fCO

(fCO

− fCO

) in the East Sea.

The surface seawaters in the East Sea

are, in general, supersaturated in sum-

mer and undersaturated in winter. In

some areas in the northern East Sea in

winter, however, high supersaturation

of CO

was also observed, which

could be due to intensive vertical mix-

ing of upper waters.

ried out through CREAMS studies. Initial emphasis was on the solubility pump in the East Sea (Oh et al., 1999). The fCO

of surface waters varied in a range from 350 to 400 µatm in summer and from 300 to 350 µatm in winter as shown in Figure 20. Comparisons of these concentrations with those in the atmosphere revealed that surface seawa- ters in the East Sea are, in general, supersaturated with respect to atmospheric CO

in summer and are undersatu- rated in winter. In winter, some areas in the northern East Sea show high supersaturation of CO

, which could be due to intensive vertical mixing of upper waters. This abnormal feature suggests that the winter fCO

of surface water in subpolar ocean could serve as a real-time in situ tracer for deep convection (Kang et al., 2003c).

A simple model was devised for describing the variation of fCO

in surface waters in time and for estimating the annual flux of CO

at the air-sea interface (Oh et al., 1999).

The model considers the annual variation of parameters such as the atmospheric CO

concentration, SST and salin- ity, mixed layer depth (MLD), transfer velocity (k

) and biological activity. Due to differences in SST variations, the East Sea was divided into two regions (cold and warm regions) along the 40

N in latitude: the northern East Sea represents the subpolar ocean and the southern part the sub- tropical one. The net flux of CO

is from the sea into the atmosphere from June through September, while the net flux is reversed (from the atmosphere into the sea) from October to May. The net annual CO

flux at the air-sea interface was estimated to be 0.045 Gt-C per year (Kang, 1999). This amount corresponds to about 2% of the annual oceanic uptake of CO

, 2.1 Gt-C (Tans et al., 1993), and about 6 times the global average CO

uptake rate per unit area of the ocean. The fluxes of various components involved in the solubility pump in the East Sea were shown in Figure 21.

Studies on the carbonate system in the East Sea were also carried out in order to construct a model for a total geochemical cycle of CO

in the East Sea (Park, 1996;

Kang and Kim, 2003). The analysis of data for total alka- linity, pH and the total inorganic CO

shows that penetra- tion of anthropogenic CO

all the way to the bottom within entire basins as shown in Figure 22 due to a rapid turnover time in the area (Kim and Kim, 1996; Kim et al., 1999). The East Sea appears to have 0.3 Gt-C of excess CO

already in deep waters (Kang, 1999). Chen and his colleagues also reported the excess CO

penetration all to the bottom (Chen et al., 1995)

The degree of saturation of calcium carbonate in the East Sea was also estimated from the CREAMS data (Lee et al., 2001). Data suggest that the crossover from the supersatu- ration to undersaturation for calcium carbonate occurs at the depth of approximately 200−400 meters for calcite, and 100−300 meters for aragonite as shown in Figure 23. Chen et al. reported different depths; 1300 meters for calcite, and 300 meters for aragonite (Chen et al., 1995), requiring fur- ther studies. Compared to the open ocean, however, these levels in the East Sea are much shallower possibly due to lower temperature of seawater and lower alkalinity in this region.

Dynamic pump represents the carbon exchange between Fig. 21. The overall exchange of CO

in the East Sea by solubility

pump. Anthropogenic emissions of CO

from nearby nations (Marland et al., 1994) are also shown in the figure for comparison.

Fig. 22. Vertical profile of excess CO

in the East Sea. The data

show that penetration of CO

all the way to the bottom within

entire basins, supporting a rapid turnover time. The East Sea

appears to have 0.3 Gt-C of excess CO

already in deep waters.

the surface and deep water by the movement of sweater.

Hence, the carbon flux by dynamic pump can be expressed by the multiplication of amount of water movement and inorganic carbon content. The dynamic pump in the East Sea was estimated using the moving-boundary box model (MBBM) to be 0.031 Gt-C of carbon from the surface to the deep water in a year (Kang and Kim, 2003).

The marine biota act as a biological pump for carbon by producing particulate and dissolve organic carbon, which is then exported to deeper layers in the ocean where it is decomposed. In order to understand the biological pump, studies on primary production, respiration, and exported particle fluxes from the surface to deep waters, and dis- solved organic carbon are essential.

During CREAMS studies, sediment traps were deployed for more than a year along with one of the current meter moorings from July, 1994 to August, 1995 in the south- western Japan Basin at depths of 2800 m and 300 m above the bottom. The initial results show that the annual mass

flux was 96 g/m

/yr, of which 66% consists of biogenic par- ticles and about 32% consists of lithogenic particles (Kim et al., 1996).

New production is also one of the important factors affecting the biological pump. During CREAMS studies, Helium isotope method was applied (Hahm and Kim, 2001) and the results thus obtained were compared with the clas- sical estimates based on bottle incubation using

N isotope (Yang, 1997, 1998; Moon et al., 1998). The annual new production of 64 g-C m

yr

with monthly variation of the nitrate flux (Fig. 24) was obtained, which is very compa- rable to estimates in the northwest Pacific (Hahm and Kim, 2001).

A preliminary carbon cycle model based on the solubility, dynamic, biological pumps was constructed and shown in Figure 25 (Kang and Kim, 2003). The East Sea absorbs 0.045 Gt-C of CO

from the atmosphere each year. The dynamic pump transports 0.031 Gt-C of carbon annually from the surface to the deep water. Biota absorb 0.038 Gt- C of carbon per year from the surface water and transport to deep water. The carbon flux removed from the water to the sediment is less than 0.001 Gt-C each year. While the carbon inventory of surface water reservoir decreases by 0.023 Gt-C annually, the reservoir for deep waters increases by 0.068 Gt-C yr

, thus producing 0.045 Gt-C of annual

“net” increase in carbon inventory for the whole East Sea.

This means that more carbon is transported from the surface to deep water than the amount of carbon the East Sea absorbs from the atmosphere. This could be due to an active intermediate water formation in the East Sea (Kang and Kim, 2003).

While the in situ observation at sea is necessary, it has Fig. 23. Vertical profiles of the degree of saturation for (a) calcite

and (b) aragonite along the stations. Contours are 0.5 interval. Dash lines indicate the depth where Ω=0.9 for calcite and aragonite.

Fig. 24. The monthly mean nitrate flux into the mixed layer, which

estimated from the correlation between

He and nitrate. The sea-

sonal variation of the flux is similar to the new production rate of

Yang (1997, 1998) and Moon et al. (1998).

time and spatial limitation. In order to overcome this lim- itation, application of satellite data is essential. Figure 26 shows a schematic diagram describing how to apply the sat- ellite information to the carbon flux study. The carbon flux at the air-sea interface can expressed by multiplication of gas transfer velocity (k), solubility (S), and ∆fCO

. The gas transfer velocity, k, is determined by sea surface tempera- ture and wind velocity. The solubility, S, is a function of sea surface temperature and salinity. ∆fCO

is controlled not only by physical parameter such as temperature and salinity but also by biological factor in the surface seawa- ter. Satellites can provide the sea surface temperature, wind velocity, and chlorophyll concentration, which can be translated into the biological activity of sea surface. Figure 27 shows the gas transfer velocity in the East Sea esti- mated by using satellite data (Hahm et al., 2002). Param- eterizations of ∆fCO

as a function of salinity, temperature and chlorophyll from satellite data could enable us to esti- mate of the CO

flux at the air-sea interface, thus tremen- dously enhancing our understanding on the solubility pump of the ocean in the future.

5. CONCLUSION

The understanding on the chemistry of the East (Japan) Sea has been dramatically improved through the CREAMS expeditions, an international cooperative study, carried out during 1990s. The CREAMS studies confirmed that the East Sea has undergone dramatic changes during the last 50−60 years. One of the most prominent characteristics of these changes is a rapid decrease of dissolved oxygen in deep waters.

While the causes for these changes are still under inves- tigation, it has been shown that these changes are mainly due to the modification in the mode of deep water venti- lation system in the East Sea: a slow down and complete stop of bottom water formation accompanied by an enhance- ment of upper water formation instead.

It is extremely interesting to note that shift in the con- veyor-belt system in the East Sea has remarkable resem- blance to possible changes in the ocean conveyor-belt system in this century associated with recent global warm- ing: a weakening of Atlantic thermohaline circulation (Manabe and Stouffer, 1993; Broecker et al., 1999; Latif et al., 2000).

A simple moving-boundary box model (MBBM) was developed in order to quantify the processes involved in such changes for the last 50−60 years. The model predicts that the East Sea may remain as a well-oxygenated sea despites recent rapid oxygen decreases in deep waters in association with structural changes such as a shrinking of oxygen-depleted deeper waters and an expansion of oxy- gen-rich upper in the East Sea in next few decades.

The sedimentary record, however, shows that the East Sea has undergone oscillation between well-oxygenated environment and anoxic environment during last glacial period in association with the eustatic sea-level change.

Several flooding processes such as intrusion of cold Oyashio Fig. 26. A schematic diagram of the carbon flux study using sat- ellite data.

Fig. 25. Preliminary result of the overall carbon budget in the East

Sea. Arrows represent the fluxes by solubility, dynamic, biologi-

cal, and carbonate pumps. Units of reservoir size and fluxes are in

Gt-C and Gt-C yr

, respectively.

Current and less saline, nutrient-rich seawaters from East China Sea and Yellow Sea has been proposed, deserving further exploration.

Being a semi-closed basin, the carbon cycle of the East Sea has been a subject of CREAMS investigation. The East Sea serves as a strong sink of atmospheric CO

; penetration of anthropogenic CO

all the way to the bottom is clear with its very rapid conveyor-belt system.

ACKNOWLEDGMENTS: The authors would like to express their sincere appreciation to Prof. Takematsu of Kyushu University for his extraordinary leadership carrying out CREAMS expedition success- fully. Never-ending cooperation with Prof. Kuh Kim of Seoul National University was the most important ingredient for making our CREAMS studies productive and alive. The enthusiasm of Prof. Jong Hwan Yoon of Kyushu University and Dr. Volkov at Hydrometeoro- logical Institute, Vladivostok was also essential for making CREAMS in action. The authors would also like to express their thanks to the Captain, officers, all crew members, in particular Messrs. Scherbinin

Fig. 27. Monthly variation of CO

transfer velocity (cm/h) in 2000. Wind velocities and SST are obtained from QuikSCAT and NOAA/AVHRR, respectively.

and Yarosh of R/V Professor Khromov for their professionalism in sup- porting science at sea. Mr. Ossi of Otronix Systems, Inc. deserves our special appreciation for his unselfish support for our research in most needed time. Full supports from all members of OCEAN Laboratory at sea and in the laboratory are highly appreciated. This work was sup- ported by MOST, Korea, through the NRL program. This is OCEAN Laboratory contribution No. 16.

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Manuscript received May 2, 2003

Manuscript accepted June 5, 2003