- Email: [email protected]
Japan Sea become an anoxic sea in the next century?
Marine Chemistry 91 (2004) 77 – 84 www.elsevier.com/locate/marchem
Will the East/Japan Sea become an anoxic sea in the next century? Dong-Jin Kang a,*, Jae-Yeon Kim b, Tongsup Lee b, Kyung-Ryul Kim a a
OCEAN Laboratory, Research Institute of Oceanography, School of Earth and Environment Sciences, Seoul National University, Seoul 151-742, South Korea b Department of Marine Science, Pusan National University, Busan 609-735, South Korea Received 13 January 2003; received in revised form 12 March 2004; accepted 17 March 2004 Available online 1 July 2004
Abstract The deep interior of the East/Japan Sea is governed by its unique thermohaline circulation which is far smaller but much faster than the Great Ocean Conveyer Belt. Analysis of oxygen profiles confirms that, at present, less dense waters no longer penetrate to the bottom. Instead they start to generate intermediate waters. Consequently, the volume of each water mass is changing with time. Recently, a Moving-Boundary Box Model (MBBM) of the region, which can accommodate this feature, has been proposed. The model predicts that the current bottom water will be replaced by deep water by the year 2040. The physical supply of dissolved oxygen into the deep waters and the biological oxygen utilization rates in the East Sea are estimated using the MBBM under the structural change of ventilation scenario. The oxygen utilization rates of the deep water masses [Central Water (CW), Deep Water (DW), and Bottom Water (BW)] in the East Sea are estimated as 2.0, 1.1, and 0.8 Amol kg 1 year 1, respectively. This study also confirms that the dissolved oxygen concentration of the deep waters in the East Sea has decreased over the years, as recent studies have claimed. However, this trend can only be extended until the year 2040 since the Bottom Water will disappear by then and the deep layer structure will be changed into a two-layer system. Our model suggests that the previous predictions of the imminent anoxic interior of the East Sea will not be realized because the current structural change will replace the bottom water before it becomes hypoxic. D 2004 Elsevier B.V. All rights reserved. Keywords: Dissolved oxygen; Oxygen utilization rate; Anoxic; Moving-Boundary Box Model; East/Japan Sea
1. Introduction The East/Japan Sea is a small mid-latitude marginal sea in the western Pacific surrounded by Korea, Japan, and Russia. Nevertheless, the East Sea, with deeper basins exceeding 3000 m (Fig. 1), exhibits vertical profiles similar to open oceans in temperature, salinity, * Corresponding author. Tel.: +82-2-877-6741; fax: +82-2-8857164. E-mail address: [email protected] (D.-J. Kang). 0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2004.03.020
and dissolved oxygen (Fig. 2), which are ultimately driven by a conveyer belt system. The time scale of the East Sea conveyer belt is on the order of 100 years (Watanabe et al., 1991; Tsunogai et al., 1993; Kim et al., 2001), much faster than the Great Ocean Conveyer Belt. Currently, several water masses are identified from the h –S diagram (Fig. 2) such as Central, Deep, and Bottom Waters (Kim et al., 1996). Central Water (CW) represents a water mass at about 1 jC, typically occupying from 200 m depth to the deep salinity minimum layer. Deep Water (DW) resides in between
78
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
Fig. 1. Topographic map of the East/Japan Sea. Area at depth z m (Az) is described very accurately as a function of depth and the surface area (A0) of 1.013 1012 m2 by the equation (Az/A0) = 2.82 10 4z + 0.9647 (r2>0.99) (Kim et al., 2002).
the deep salinity minimum and the top of the bottom adiabatic mixed layer. The homogeneous bottom mixed layer is designated as Bottom Water (BW). Recent studies (Kim and Kim, 1996; Kim et al., 1999, 2001; Chen et al., 1999; Gamo, 1999) demonstrated clearly that the East Sea has undergone a dramatic structural change at least during the last 50 years, confirming an earlier observation of Gamo et al. (1986). The most striking change is the decrease in the dissolved oxygen concentration of the deep waters (Kim and Kim, 1996; Kim et al., 1999; Gamo, 1999, Chen et al., 1999). The profiles of dissolved oxygen in the Japan Basin since the 1930s (Kim et al., 2001) clearly show that dissolved oxygen went through a drastic change in such a way that the depth of the oxygen minimum, which is located in DW, has gradually deepened from a few hundred meters in the late 1960s to deeper than 1500 m at the present (Fig. 3). The deepening of oxygen minimum depth is accom-
panied by decreases in O2 concentrations of up to 20 AM in deep waters. Judging from the similarity of O2 profiles in the 1930s and early 1950s, it seems probable that the change started sometime in the 1950s. It has been shown (Kim and Kim, 1996; Kim et al., 1999, 2001) and further confirmed by Gamo et al. (2001) that these changes are due to a shift in the mode of the ventilation system in the East Sea. That is, while the bottom water formation of the past had been reduced, the enhanced formation of Central Water compensates for the deficit. Considering only the phenomenon of oxygen decrease in the deep layer, some researchers predict that the deep water of the East Sea will become anoxic within a few hundred years (Chen et al., 1999; Gamo, 1999). Since not only the concentration of dissolved oxygen but also the water mass structure has changed, however, the dissolved oxygen content in the deep layer of the East Sea should be predicted considering
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
79
Fig. 2. Typical potential temperature (h), salinity (S), and dissolved oxygen (O2) profiles of the Japan Basin in 1996 and h – S plot for deep waters showing three water masses in deep waters: Central Water (CW), Deep Water (DW), and Bottom Water (BW).
the accompanying changes in water mass structures. Will the interior of the East Sea really become anoxic within a few hundred years regardless of the change in its ventilation system? Using a simple box model describing the observed changes of the dissolved oxygen as well as the water mass structure in the East Sea (Kang et al., 2003), the simulation results regarding the upcoming anoxia will be presented in this paper.
2. Historical trends of dissolved oxygen in the East/ Japan Sea Kim et al. (1999) confirmed the earlier observation of Gamo et al. (1986) that the East Sea is currently experiencing an ongoing structural change, at least since the 1960s, when abrupt changes in dissolved oxygen profiles were first noticed, as shown in Fig. 3.
The profiles obtained by Uda (1934) in the 1930s are essentially indistinguishable from those obtained by Russian scientists in the 1950s (USSR Academy of Science, 1957). However, profiles obtained in later years show remarkable differences from earlier ones (Fig. 3). The depths of the oxygen minima have been steadily deepening from a few hundred meters in the late 1960s to below 1500 m in the mid-1990s, and accompanied by decrease in oxygen concentration of up to 20 AM. The latter is interpreted as a clear indication of a slow down of bottom water formation as long as the biological pump acts as usual, which it more than likely does. However, it does not necessarily endorse a gross weakening of the conveyer belt as previously claimed by Gamo (1999). The deepening of the oxygen minimum depth sheds light on the rerouting of the conveyer belt, i.e., to a shallower depth than before possibly due to insufficient winter cooling, caused in turn by the
80
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
Fig. 3. Profiles of dissolved oxygen obtained in the Japan Basin from the 1930s until 1996, clearly reveal property changes in the East Sea. In 1996, a continuous oxygen profile was obtained with a dissolved oxygen sensor and then calibrated with bottle data; both profiles are shown in the figure.
winter warming of the last century. As one might expect, a slow down and rerouting may exert totally different effects on the deep oxygen content. If only the warmer winter temperature affected the deepwater formation and the other conditions were the same, then it would be plausible to guess that only the density of the surface water decreased. Then the result would be a shallower winter convection, rather than a slow-down or cessation. A plausible explanation for this would be an increment of Central Water formation in response to the weakening of the bottom water formation, which would result in a decrease of the Bottom Water volume and a concurrent expansion of Central Water as is indeed observed. It is important to note that the dissolved oxygen profiles in the 1930s and early 1950s are quite similar, which means that the changes started sometime after the mid-1950s.
3. Oxygen utilization rate (OUR) The dissolved oxygen concentrations of deep waters are controlled by the convective supply of
air-saturated surface seawater and biological consumption. This source-minus-sink term ( JO2) has been analyzed by Chen et al. (1999) using the classical one-dimensional advection – diffusion model of Craig (1969). They obtained a value of JO2/ w = 24.1 F 2.7 (Amol kg 1 km 1) between depths of 600 and 2000 m. Their value is quite close to the reported value of 27.0 Amol kg 1 km 1 obtained from the 800– 1400 m interval in the East Sea by Kim and Kim (1996). Unlike to conventional oxygen profiles, the East Sea shows ‘‘positive’’ JO2/w, which indicates that production dominates over consumption at depth. Since the sporadic lateral advection is not significant, Chen et al. (1999) interpreted the production of dissolved oxygen in the deep layer as that transported from the surface. This explanation goes very well with the claim that the ventilation system of the East Sea is changing. Kim and Kim (1996) and Kim et al. (1999) have already discussed the positive JO2 of the deep levels in the East Sea and interpreted it as a result of a fresh input of surface waters in recent years. Chen et al. (1999) report that the oxygen utilization rates (OUR) from the nitrate and phosphate produc-
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
tion at a depth of 600 m in the East Sea are 2.85 and 3.86 Amol kg 1 year 1, respectively. The oxygen production rate ( JO2) obtained, 2.63 Amol kg 1 year 1, is a remnant of the convective supply less biological consumption. Therefore, the convective supply rate of dissolved oxygen at a depth of 600 m would be either 2.63-( 2.85) = 5.48 Amol kg 1 year 1 or 2.63-( 3.86) = 6.49 Amol kg 1 year 1, giving a mean of about 6.0 Amol kg 1 year 1. Applying the method, the convective supply rate of oxygen at a depth of 2000 m is calculated as 0.9 Amol kg 1 year 1. It is expected that the convective supply of dissolved oxygen from the surface to the intermediate or deep waters occurs in winter in the northern cold regions of the East Sea. If we assume that the dissolved oxygen of the surface water at the cold region is saturated, with its salinity and temperature 34 and 5jC, respectively, the dissolved oxygen concentration of the cold surface water should be around 320 Amol kg 1. The convective addition of dissolved oxygen in the deep water can be expressed as follows; DC ¼ C0
Q V
where, DC is the annual oxygen supplied from the surface waters (6.0 Amol kg 1 year 1 at 600 m and 0.9 Amol kg 1 year 1 at 2000 m), C0 is the dissolved
81
oxygen concentration in the surface water (320 Amol kg 1), Q is the seawater flux from the surface to the deep layer in m3 year 1, and V is the volume of seawater between 600 and 2000 m (5.76 1014 m3). From the equation, Q is estimated to be 1.1 1013 3 m year 1 ( = 0.35 Sv) at 600 m, and 1.6 1012 m3 year 1 ( = 0.05 Sv) at 2000 m depth. Recently, a Moving-Boundary Box Model (MBBM), a model which describes the ventilation flux of the East Sea in change, was suggested (Kang et al., 2003). The MBBM estimates the annual vertical fluxes from the cold surface to the deep waters (CW, DW, and BW) considering the change in the East Sea (Fig. 4). The flux to CW which was located between 200 and 1000 m in the 1990s is 0.45 Sv (1.42 1013 m3 year 1), and the flux from the surface to DW (1000 – 2500 m depth) is 0.02 Sv (0.6 1012 m3 year 1). The Q values obtained above are in good agreement with the fluxes estimated in the MBBM (Kang et al., 2003). Since the flux of water to the deep layers is estimated by the MBBM (Fig. 4), we can calculate the convective flux of oxygen from the surface into the deep layers by multiplying the concentration of dissolved oxygen in the surface. The convective flux can occur only in the cold region of the East Sea. We assumed that the dissolved oxygen in the surface was saturated at a temperature and salinity of 5 and 34 jC, respectively (320 Amol kg 1). The biological con-
Fig. 4. (a) A schematic diagram of the Moving-Boundary Box Model for the East Sea and (b) the variation of the fluxes between boxes with time. The formation of Bottom Water (D3) of 0.02 Sv in magnitude in the past completely stopped in between mid 1980s and late 1990s (tf = 1993 F 8).
82
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
sumption rate, that is the oxygen utilization rate (OUR), can be estimated by comparison of the model results with the measured oxygen values. We assumed that the OURs in each water mass remain constant with time, but that the convective fluxes changed with time as the water fluxes changed (Fig. 5a). The OURs are estimated by least-square fitting to the historic oxygen data of each water mass with the model results. The OURs obtained for CW, DW, and BW are 2.0, 1.1, and 0.8 Amol kg 1 year 1, respectively (Fig. 5b). The values are an order of magnitude higher than the reported world ocean average of about 0.1 Amol kg 1 year 1 (Fiadeiro and Craig, 1978). Indeed, Gamo (1999) pointed out that such higher OUR values render the East Sea susceptible to anoxia in the case where the deep conveyor belt slow downs. There are previous studies on the OUR in the East Sea. A number of different methods were applied to obtain the numbers: the static mass balance model (Chen et al., 1996, 1999), the deep convection stagnation assumption (Chen et al., 1999), the one-dimensional advection –diffusion model (Chen et al., 1999),
Fig. 6. Graphical compilation of the available data of deep-water oxygen utilization rate in the East Sea.
Fig. 5. (a) A schematic diagram of the Oxygen-MBBM. [Surface Warm Water (SW), Surface Cold Water (SC), Central Water (CW), Deep Water (DW), Bottom Water (BW)]. D1O, D2O, D3O, U1O, U2O, and U3O represent fluxes of oxygen between boxes. OURCW, OURDW, and OURBW are the oxygen utilization rates in each box, which are constant with time. (b) Variations of dissolved oxygen concentration in deep water masses and best-fit lines for the observed dissolved oxygen of each deep-water mass obtained from Oxygen-MBBM with time. Error bars and symbols represent the maximum/minimum value and the median value of measured historical data, respectively. Dashed lines represent simulation results of the secular change of dissolved oxygen in the deep-water masses of the East Sea. Note that BW is expected to be completely depleted by the year 2040 much earlier than anoxia develops. 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).
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
and the use of CFC ages (Min, 1999). The results are summarized in Fig. 6 for successive depth intervals. Most of them are fairly consistent with each other. The results of this study also agree well with previous data. The simulation results of oxygen by the MBBM for deep water masses are juxtaposed with the historical data shown in Fig. 5b. Although some deviations are noticeable, the model reproduces the decreasing oxygen in the deep quite satisfactorily. The model indicates that the oxygen levels will drop continuously in the near future. When only the concentration decrease in deep waters is considered, it seems unavoidable that we face the anoxic East Sea foreseen by Chen et al. (1999) and Gamo (1999). There exists one more result worthy of mentioning regarding the anoxic bottom. The MBBM also reproduces the deepening of the oxygen minimum, which lies inside DW. This indicates that for precise prediction one should also consider the structural change of the deep interior of the East Sea.
4. The future of dissolved oxygen in the East/Japan Sea Because the MBBM describes the structural change of the East Sea in a quantitative manner, the dissolved oxygen of the future can be simulated under the usual scenario. The results drawn in Fig. 5b can be extrapolated to show that the O2 concentration of BW will be depleted by the year 2270. However, this carries no realistic meaning since the MBBM dictates that the BW will be gone by the year 2040 much earlier than the anoxia development. The position of the oxygen minimum layer is deepening. It is currently located in DW (Fig. 2). Judging from the current rate of deepening, the oxygen minimum may soon appear in BW; however, to estimate the dissolved oxygen concentration of the deep, not only the convective supply but also the biological consumption should be known precisely. Many uncertainties related to the biology hamper reliable prediction. In the mean time, the MBBM suggests that because of the change in structure of the deep interior, anoxia will not occur. The model predicts the two-layer system of the future, in which CW expands to occupy 90% of the deep. If this should happen, then anoxia cannot develop at all.
83
Fig. 7. Variation of oxygen inventories of each water mass in the East Sea simulated by Oxygen-MBBM. Symbols represent the inventories calculated from the same observed data as in Fig. 5b.
The total oxygen inventory of the East Sea also supports this. It has decreased slightly, but still converges to around 700 mol m 2 (Fig. 7), although the inventory of BW diminishes to zero.
5. Conclusion Considering the oxygen supply from the surface as well as the biological consumption in the deeper layer, the temporal change of the dissolved oxygen concentration in the deeper layers of the East Sea was estimated. These two terms of the East Sea are estimated using the Moving-Boundary Box Model which can handle the secular change of ventilation system. Our best estimates of oxygen utilization rate for the deep-water masses (Central Water, Deep Water, and Bottom Water) in the East Sea are 2.0, 1.1, and 0.8 Amol kg 1 year 1, respectively. As other recent studies insist (Kim and Kim, 1996; Kim et al., 1999; Chen et al., 1999; Gamo, 1999), this study also confirms that the dissolved oxygen concentration of the deep waters in the East Sea has decreased over the years. However, this trend can only be extended until the year 2040 since Bottom Water will disappear by then and the interior structure will be shifted into a two-layer system. Due to this structural shift, the behavior of the oxygen concentration under the two-layer system is difficult to predict
84
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
after the year 2040. However, our model predicts that the previous predictions of an imminent anoxic interior of the East Sea will not be realized because the current structural change will replace the bottom water before it becomes hypoxic.
Acknowledgements This work was partially supported by the National Research Laboratory Program, ‘‘OCEAN Laboratory: Real Time Monitoring of Ocean Environmental Change’’ (2000-N-NL-01-C-012) of the Ministry of Science and Technology, Korea, by the Eco-technopia 21 program of the Ministry of the Environment, Korea, and by the BK21 project. OCEAN Laboratory contribution No. 23. Associate editor: Dr. Andrew Watson.
References Chen, C.T.A., Gong, G.C., Wang, S.L., Bychkov, A.S., 1996. Redfield ratios and regeneration rates of particulate matter in the Sea of Japan as a model of closed system. Geophys. Res. Lett. 23, 1785 – 1788. Chen, C.T.A., Bychkov, A.S., Wang, S.L., Pavlova, G.Yu., 1999. An anoxic Sea of Japan by the year 2200? Mar. Chem. 67, 249 – 265. Craig, H., 1969. Abyssal carbon and radiocarbon in the Pacific. J. Geophys. Res. 74, 5491 – 5506. Fiadeiro, M.E., Craig, H., 1978. Three-dimensional modeling of tracers in the deep Pacific Ocean: I. Salinity and oxygen. J. Mar. Res. 36, 323 – 355. Gamo, T., 1999. Global warming may have showed down the deep conveyor belt of a marginal sea of the northwestern Pacific: Japan Sea. Geophys. Res. Lett. 26, 3137 – 3140. Gamo, T., Horibe, Y., 1983. Abyssal circulation in the Japan Sea. J. Oceanogr. Soc. Jpn. 39, 220 – 230. Gamo, T., Nozaki, Y., Sakai, H., Nakai, T., Tsubota, H., 1986. Spacial and temporal variations of water characteristics in the Japan Sea bottom layer. J. Mar. Res. 44, 781 – 793.
Gamo, T., Momoshima, N., Tolmachyov, S., 2001. Recent upward shift of the deep convection system in the Japan Sea, as inferred from the geochemical tracers tritium, oxygen, and nutrients. Geophys. Res. Lett. 28, 4143 – 4146. Kang, D.-J., Park, S., Kim, Y.-G., Kim, K., Kim, K.-R., 2003. A moving-boundary box model (MBBM) for oceans in change: an application to the East/Japan Sea. Geophys. Res. Lett. 30, 1299 (doi 10.1029/2002GL016486). Kim, K.-R., Kim, K., 1996. What is happening in the East Sea (Japan Sea)?: recent chemical observations during CREAMS 93 – 96. J. Korean Soc. Oceanogr. 31, 164 – 172. Kim, K., Kim, K.-R., Chung, J.-Y., Choi, B.-H., Byun, S.-K., Hong, G.H., Takematsu, M., Yoon, J.-H., Volkov, Y., Danchenkov, M., 1996. New findings from CREAMS observations: water masses and eddies in the East Sea. J. Korean Soc. Oceanogr. 31, 155 – 163. Kim, K.-R., Kim, K., Kang, D.-J., Park, S.Y., Park, M.-K., Kim, Y.-G., Min, H.S., Min, D., 1999. The East Sea (Japan Sea) in change: a story of dissolved oxygen. MTS J. 33, 15 – 22. Kim, K., Kim, K.-R., Min, D., Volkov, Y., Yoon, J.-H., Takematsu, M., 2001. Warming and structural changes in the East Sea (Japan Sea): a clue to the future changes in global oceans? Geophys. Res. Lett. 28, 3293 – 3296. Kim, K.-R., Kim, K., Kang, D.-J., Volkov, Y.N., Yoon, J.-H., Takematsu, M., 2002. The changes in the East/Japan Sea found by CREAMS. Oceanogr. Jpn. 11 (3), 419 – 429 in Japanese with English Abstr. Min, D.-H. 1999. Studies of large-scale intermediate and deep water circulation and ventilation in the North Atlantic, South Indian and Northeast Pacific Oceans, and in the East Sea (Sea of Japan), using chlorofluorocarbons as tracers. PhD. thesis, Univ. of California, San Diego, USA, unpublished. Sudo, H., 1986. A note on the Japan Sea proper water. Prog. Oceanogr. 17, 313 – 336. Tsunogai, S., Watanabe, Y.W., Harada, K., Watanabe, S., Saito, S., Nakajima, M., 1993. Dynamics of the Japan Sea deep water studied with chemical and radiochemical tracers. In: Teramoto, T. (Ed.), Deep Ocean Circulation, Physical and Chemical Aspects. Elsevier, Amsterdam, pp. 105 – 119. Uda, M., 1934. The results of simultaneous oceanographic investigations in the Japan Sea and its adjacent waters in May and June 1932. J. Imp. Fish. Exp. Stn. 5, 57 – 190. USSR Academy of Science 1957. Reports of multi-disciplinary oceanography expeditions on R/V Vityaz. v.6, unpublished. Watanabe, Y.W., Watanabe, S., Tsunogai, S., 1991. Tritium in the Japan Sea and the renewal time of the Japan Sea deep water. Mar. Chem. 34, 97 – 108.
Will the East/Japan Sea become an anoxic sea in the next century? Dong-Jin Kang a,*, Jae-Yeon Kim b, Tongsup Lee b, Kyung-Ryul Kim a a
OCEAN Laboratory, Research Institute of Oceanography, School of Earth and Environment Sciences, Seoul National University, Seoul 151-742, South Korea b Department of Marine Science, Pusan National University, Busan 609-735, South Korea Received 13 January 2003; received in revised form 12 March 2004; accepted 17 March 2004 Available online 1 July 2004
Abstract The deep interior of the East/Japan Sea is governed by its unique thermohaline circulation which is far smaller but much faster than the Great Ocean Conveyer Belt. Analysis of oxygen profiles confirms that, at present, less dense waters no longer penetrate to the bottom. Instead they start to generate intermediate waters. Consequently, the volume of each water mass is changing with time. Recently, a Moving-Boundary Box Model (MBBM) of the region, which can accommodate this feature, has been proposed. The model predicts that the current bottom water will be replaced by deep water by the year 2040. The physical supply of dissolved oxygen into the deep waters and the biological oxygen utilization rates in the East Sea are estimated using the MBBM under the structural change of ventilation scenario. The oxygen utilization rates of the deep water masses [Central Water (CW), Deep Water (DW), and Bottom Water (BW)] in the East Sea are estimated as 2.0, 1.1, and 0.8 Amol kg 1 year 1, respectively. This study also confirms that the dissolved oxygen concentration of the deep waters in the East Sea has decreased over the years, as recent studies have claimed. However, this trend can only be extended until the year 2040 since the Bottom Water will disappear by then and the deep layer structure will be changed into a two-layer system. Our model suggests that the previous predictions of the imminent anoxic interior of the East Sea will not be realized because the current structural change will replace the bottom water before it becomes hypoxic. D 2004 Elsevier B.V. All rights reserved. Keywords: Dissolved oxygen; Oxygen utilization rate; Anoxic; Moving-Boundary Box Model; East/Japan Sea
1. Introduction The East/Japan Sea is a small mid-latitude marginal sea in the western Pacific surrounded by Korea, Japan, and Russia. Nevertheless, the East Sea, with deeper basins exceeding 3000 m (Fig. 1), exhibits vertical profiles similar to open oceans in temperature, salinity, * Corresponding author. Tel.: +82-2-877-6741; fax: +82-2-8857164. E-mail address: [email protected] (D.-J. Kang). 0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2004.03.020
and dissolved oxygen (Fig. 2), which are ultimately driven by a conveyer belt system. The time scale of the East Sea conveyer belt is on the order of 100 years (Watanabe et al., 1991; Tsunogai et al., 1993; Kim et al., 2001), much faster than the Great Ocean Conveyer Belt. Currently, several water masses are identified from the h –S diagram (Fig. 2) such as Central, Deep, and Bottom Waters (Kim et al., 1996). Central Water (CW) represents a water mass at about 1 jC, typically occupying from 200 m depth to the deep salinity minimum layer. Deep Water (DW) resides in between
78
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
Fig. 1. Topographic map of the East/Japan Sea. Area at depth z m (Az) is described very accurately as a function of depth and the surface area (A0) of 1.013 1012 m2 by the equation (Az/A0) = 2.82 10 4z + 0.9647 (r2>0.99) (Kim et al., 2002).
the deep salinity minimum and the top of the bottom adiabatic mixed layer. The homogeneous bottom mixed layer is designated as Bottom Water (BW). Recent studies (Kim and Kim, 1996; Kim et al., 1999, 2001; Chen et al., 1999; Gamo, 1999) demonstrated clearly that the East Sea has undergone a dramatic structural change at least during the last 50 years, confirming an earlier observation of Gamo et al. (1986). The most striking change is the decrease in the dissolved oxygen concentration of the deep waters (Kim and Kim, 1996; Kim et al., 1999; Gamo, 1999, Chen et al., 1999). The profiles of dissolved oxygen in the Japan Basin since the 1930s (Kim et al., 2001) clearly show that dissolved oxygen went through a drastic change in such a way that the depth of the oxygen minimum, which is located in DW, has gradually deepened from a few hundred meters in the late 1960s to deeper than 1500 m at the present (Fig. 3). The deepening of oxygen minimum depth is accom-
panied by decreases in O2 concentrations of up to 20 AM in deep waters. Judging from the similarity of O2 profiles in the 1930s and early 1950s, it seems probable that the change started sometime in the 1950s. It has been shown (Kim and Kim, 1996; Kim et al., 1999, 2001) and further confirmed by Gamo et al. (2001) that these changes are due to a shift in the mode of the ventilation system in the East Sea. That is, while the bottom water formation of the past had been reduced, the enhanced formation of Central Water compensates for the deficit. Considering only the phenomenon of oxygen decrease in the deep layer, some researchers predict that the deep water of the East Sea will become anoxic within a few hundred years (Chen et al., 1999; Gamo, 1999). Since not only the concentration of dissolved oxygen but also the water mass structure has changed, however, the dissolved oxygen content in the deep layer of the East Sea should be predicted considering
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
79
Fig. 2. Typical potential temperature (h), salinity (S), and dissolved oxygen (O2) profiles of the Japan Basin in 1996 and h – S plot for deep waters showing three water masses in deep waters: Central Water (CW), Deep Water (DW), and Bottom Water (BW).
the accompanying changes in water mass structures. Will the interior of the East Sea really become anoxic within a few hundred years regardless of the change in its ventilation system? Using a simple box model describing the observed changes of the dissolved oxygen as well as the water mass structure in the East Sea (Kang et al., 2003), the simulation results regarding the upcoming anoxia will be presented in this paper.
2. Historical trends of dissolved oxygen in the East/ Japan Sea Kim et al. (1999) confirmed the earlier observation of Gamo et al. (1986) that the East Sea is currently experiencing an ongoing structural change, at least since the 1960s, when abrupt changes in dissolved oxygen profiles were first noticed, as shown in Fig. 3.
The profiles obtained by Uda (1934) in the 1930s are essentially indistinguishable from those obtained by Russian scientists in the 1950s (USSR Academy of Science, 1957). However, profiles obtained in later years show remarkable differences from earlier ones (Fig. 3). The depths of the oxygen minima have been steadily deepening from a few hundred meters in the late 1960s to below 1500 m in the mid-1990s, and accompanied by decrease in oxygen concentration of up to 20 AM. The latter is interpreted as a clear indication of a slow down of bottom water formation as long as the biological pump acts as usual, which it more than likely does. However, it does not necessarily endorse a gross weakening of the conveyer belt as previously claimed by Gamo (1999). The deepening of the oxygen minimum depth sheds light on the rerouting of the conveyer belt, i.e., to a shallower depth than before possibly due to insufficient winter cooling, caused in turn by the
80
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
Fig. 3. Profiles of dissolved oxygen obtained in the Japan Basin from the 1930s until 1996, clearly reveal property changes in the East Sea. In 1996, a continuous oxygen profile was obtained with a dissolved oxygen sensor and then calibrated with bottle data; both profiles are shown in the figure.
winter warming of the last century. As one might expect, a slow down and rerouting may exert totally different effects on the deep oxygen content. If only the warmer winter temperature affected the deepwater formation and the other conditions were the same, then it would be plausible to guess that only the density of the surface water decreased. Then the result would be a shallower winter convection, rather than a slow-down or cessation. A plausible explanation for this would be an increment of Central Water formation in response to the weakening of the bottom water formation, which would result in a decrease of the Bottom Water volume and a concurrent expansion of Central Water as is indeed observed. It is important to note that the dissolved oxygen profiles in the 1930s and early 1950s are quite similar, which means that the changes started sometime after the mid-1950s.
3. Oxygen utilization rate (OUR) The dissolved oxygen concentrations of deep waters are controlled by the convective supply of
air-saturated surface seawater and biological consumption. This source-minus-sink term ( JO2) has been analyzed by Chen et al. (1999) using the classical one-dimensional advection – diffusion model of Craig (1969). They obtained a value of JO2/ w = 24.1 F 2.7 (Amol kg 1 km 1) between depths of 600 and 2000 m. Their value is quite close to the reported value of 27.0 Amol kg 1 km 1 obtained from the 800– 1400 m interval in the East Sea by Kim and Kim (1996). Unlike to conventional oxygen profiles, the East Sea shows ‘‘positive’’ JO2/w, which indicates that production dominates over consumption at depth. Since the sporadic lateral advection is not significant, Chen et al. (1999) interpreted the production of dissolved oxygen in the deep layer as that transported from the surface. This explanation goes very well with the claim that the ventilation system of the East Sea is changing. Kim and Kim (1996) and Kim et al. (1999) have already discussed the positive JO2 of the deep levels in the East Sea and interpreted it as a result of a fresh input of surface waters in recent years. Chen et al. (1999) report that the oxygen utilization rates (OUR) from the nitrate and phosphate produc-
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
tion at a depth of 600 m in the East Sea are 2.85 and 3.86 Amol kg 1 year 1, respectively. The oxygen production rate ( JO2) obtained, 2.63 Amol kg 1 year 1, is a remnant of the convective supply less biological consumption. Therefore, the convective supply rate of dissolved oxygen at a depth of 600 m would be either 2.63-( 2.85) = 5.48 Amol kg 1 year 1 or 2.63-( 3.86) = 6.49 Amol kg 1 year 1, giving a mean of about 6.0 Amol kg 1 year 1. Applying the method, the convective supply rate of oxygen at a depth of 2000 m is calculated as 0.9 Amol kg 1 year 1. It is expected that the convective supply of dissolved oxygen from the surface to the intermediate or deep waters occurs in winter in the northern cold regions of the East Sea. If we assume that the dissolved oxygen of the surface water at the cold region is saturated, with its salinity and temperature 34 and 5jC, respectively, the dissolved oxygen concentration of the cold surface water should be around 320 Amol kg 1. The convective addition of dissolved oxygen in the deep water can be expressed as follows; DC ¼ C0
Q V
where, DC is the annual oxygen supplied from the surface waters (6.0 Amol kg 1 year 1 at 600 m and 0.9 Amol kg 1 year 1 at 2000 m), C0 is the dissolved
81
oxygen concentration in the surface water (320 Amol kg 1), Q is the seawater flux from the surface to the deep layer in m3 year 1, and V is the volume of seawater between 600 and 2000 m (5.76 1014 m3). From the equation, Q is estimated to be 1.1 1013 3 m year 1 ( = 0.35 Sv) at 600 m, and 1.6 1012 m3 year 1 ( = 0.05 Sv) at 2000 m depth. Recently, a Moving-Boundary Box Model (MBBM), a model which describes the ventilation flux of the East Sea in change, was suggested (Kang et al., 2003). The MBBM estimates the annual vertical fluxes from the cold surface to the deep waters (CW, DW, and BW) considering the change in the East Sea (Fig. 4). The flux to CW which was located between 200 and 1000 m in the 1990s is 0.45 Sv (1.42 1013 m3 year 1), and the flux from the surface to DW (1000 – 2500 m depth) is 0.02 Sv (0.6 1012 m3 year 1). The Q values obtained above are in good agreement with the fluxes estimated in the MBBM (Kang et al., 2003). Since the flux of water to the deep layers is estimated by the MBBM (Fig. 4), we can calculate the convective flux of oxygen from the surface into the deep layers by multiplying the concentration of dissolved oxygen in the surface. The convective flux can occur only in the cold region of the East Sea. We assumed that the dissolved oxygen in the surface was saturated at a temperature and salinity of 5 and 34 jC, respectively (320 Amol kg 1). The biological con-
Fig. 4. (a) A schematic diagram of the Moving-Boundary Box Model for the East Sea and (b) the variation of the fluxes between boxes with time. The formation of Bottom Water (D3) of 0.02 Sv in magnitude in the past completely stopped in between mid 1980s and late 1990s (tf = 1993 F 8).
82
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
sumption rate, that is the oxygen utilization rate (OUR), can be estimated by comparison of the model results with the measured oxygen values. We assumed that the OURs in each water mass remain constant with time, but that the convective fluxes changed with time as the water fluxes changed (Fig. 5a). The OURs are estimated by least-square fitting to the historic oxygen data of each water mass with the model results. The OURs obtained for CW, DW, and BW are 2.0, 1.1, and 0.8 Amol kg 1 year 1, respectively (Fig. 5b). The values are an order of magnitude higher than the reported world ocean average of about 0.1 Amol kg 1 year 1 (Fiadeiro and Craig, 1978). Indeed, Gamo (1999) pointed out that such higher OUR values render the East Sea susceptible to anoxia in the case where the deep conveyor belt slow downs. There are previous studies on the OUR in the East Sea. A number of different methods were applied to obtain the numbers: the static mass balance model (Chen et al., 1996, 1999), the deep convection stagnation assumption (Chen et al., 1999), the one-dimensional advection –diffusion model (Chen et al., 1999),
Fig. 6. Graphical compilation of the available data of deep-water oxygen utilization rate in the East Sea.
Fig. 5. (a) A schematic diagram of the Oxygen-MBBM. [Surface Warm Water (SW), Surface Cold Water (SC), Central Water (CW), Deep Water (DW), Bottom Water (BW)]. D1O, D2O, D3O, U1O, U2O, and U3O represent fluxes of oxygen between boxes. OURCW, OURDW, and OURBW are the oxygen utilization rates in each box, which are constant with time. (b) Variations of dissolved oxygen concentration in deep water masses and best-fit lines for the observed dissolved oxygen of each deep-water mass obtained from Oxygen-MBBM with time. Error bars and symbols represent the maximum/minimum value and the median value of measured historical data, respectively. Dashed lines represent simulation results of the secular change of dissolved oxygen in the deep-water masses of the East Sea. Note that BW is expected to be completely depleted by the year 2040 much earlier than anoxia develops. 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).
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
and the use of CFC ages (Min, 1999). The results are summarized in Fig. 6 for successive depth intervals. Most of them are fairly consistent with each other. The results of this study also agree well with previous data. The simulation results of oxygen by the MBBM for deep water masses are juxtaposed with the historical data shown in Fig. 5b. Although some deviations are noticeable, the model reproduces the decreasing oxygen in the deep quite satisfactorily. The model indicates that the oxygen levels will drop continuously in the near future. When only the concentration decrease in deep waters is considered, it seems unavoidable that we face the anoxic East Sea foreseen by Chen et al. (1999) and Gamo (1999). There exists one more result worthy of mentioning regarding the anoxic bottom. The MBBM also reproduces the deepening of the oxygen minimum, which lies inside DW. This indicates that for precise prediction one should also consider the structural change of the deep interior of the East Sea.
4. The future of dissolved oxygen in the East/Japan Sea Because the MBBM describes the structural change of the East Sea in a quantitative manner, the dissolved oxygen of the future can be simulated under the usual scenario. The results drawn in Fig. 5b can be extrapolated to show that the O2 concentration of BW will be depleted by the year 2270. However, this carries no realistic meaning since the MBBM dictates that the BW will be gone by the year 2040 much earlier than the anoxia development. The position of the oxygen minimum layer is deepening. It is currently located in DW (Fig. 2). Judging from the current rate of deepening, the oxygen minimum may soon appear in BW; however, to estimate the dissolved oxygen concentration of the deep, not only the convective supply but also the biological consumption should be known precisely. Many uncertainties related to the biology hamper reliable prediction. In the mean time, the MBBM suggests that because of the change in structure of the deep interior, anoxia will not occur. The model predicts the two-layer system of the future, in which CW expands to occupy 90% of the deep. If this should happen, then anoxia cannot develop at all.
83
Fig. 7. Variation of oxygen inventories of each water mass in the East Sea simulated by Oxygen-MBBM. Symbols represent the inventories calculated from the same observed data as in Fig. 5b.
The total oxygen inventory of the East Sea also supports this. It has decreased slightly, but still converges to around 700 mol m 2 (Fig. 7), although the inventory of BW diminishes to zero.
5. Conclusion Considering the oxygen supply from the surface as well as the biological consumption in the deeper layer, the temporal change of the dissolved oxygen concentration in the deeper layers of the East Sea was estimated. These two terms of the East Sea are estimated using the Moving-Boundary Box Model which can handle the secular change of ventilation system. Our best estimates of oxygen utilization rate for the deep-water masses (Central Water, Deep Water, and Bottom Water) in the East Sea are 2.0, 1.1, and 0.8 Amol kg 1 year 1, respectively. As other recent studies insist (Kim and Kim, 1996; Kim et al., 1999; Chen et al., 1999; Gamo, 1999), this study also confirms that the dissolved oxygen concentration of the deep waters in the East Sea has decreased over the years. However, this trend can only be extended until the year 2040 since Bottom Water will disappear by then and the interior structure will be shifted into a two-layer system. Due to this structural shift, the behavior of the oxygen concentration under the two-layer system is difficult to predict
84
D.-J. Kang et al. / Marine Chemistry 91 (2004) 77–84
after the year 2040. However, our model predicts that the previous predictions of an imminent anoxic interior of the East Sea will not be realized because the current structural change will replace the bottom water before it becomes hypoxic.
Acknowledgements This work was partially supported by the National Research Laboratory Program, ‘‘OCEAN Laboratory: Real Time Monitoring of Ocean Environmental Change’’ (2000-N-NL-01-C-012) of the Ministry of Science and Technology, Korea, by the Eco-technopia 21 program of the Ministry of the Environment, Korea, and by the BK21 project. OCEAN Laboratory contribution No. 23. Associate editor: Dr. Andrew Watson.
References Chen, C.T.A., Gong, G.C., Wang, S.L., Bychkov, A.S., 1996. Redfield ratios and regeneration rates of particulate matter in the Sea of Japan as a model of closed system. Geophys. Res. Lett. 23, 1785 – 1788. Chen, C.T.A., Bychkov, A.S., Wang, S.L., Pavlova, G.Yu., 1999. An anoxic Sea of Japan by the year 2200? Mar. Chem. 67, 249 – 265. Craig, H., 1969. Abyssal carbon and radiocarbon in the Pacific. J. Geophys. Res. 74, 5491 – 5506. Fiadeiro, M.E., Craig, H., 1978. Three-dimensional modeling of tracers in the deep Pacific Ocean: I. Salinity and oxygen. J. Mar. Res. 36, 323 – 355. Gamo, T., 1999. Global warming may have showed down the deep conveyor belt of a marginal sea of the northwestern Pacific: Japan Sea. Geophys. Res. Lett. 26, 3137 – 3140. Gamo, T., Horibe, Y., 1983. Abyssal circulation in the Japan Sea. J. Oceanogr. Soc. Jpn. 39, 220 – 230. Gamo, T., Nozaki, Y., Sakai, H., Nakai, T., Tsubota, H., 1986. Spacial and temporal variations of water characteristics in the Japan Sea bottom layer. J. Mar. Res. 44, 781 – 793.
Gamo, T., Momoshima, N., Tolmachyov, S., 2001. Recent upward shift of the deep convection system in the Japan Sea, as inferred from the geochemical tracers tritium, oxygen, and nutrients. Geophys. Res. Lett. 28, 4143 – 4146. Kang, D.-J., Park, S., Kim, Y.-G., Kim, K., Kim, K.-R., 2003. A moving-boundary box model (MBBM) for oceans in change: an application to the East/Japan Sea. Geophys. Res. Lett. 30, 1299 (doi 10.1029/2002GL016486). Kim, K.-R., Kim, K., 1996. What is happening in the East Sea (Japan Sea)?: recent chemical observations during CREAMS 93 – 96. J. Korean Soc. Oceanogr. 31, 164 – 172. Kim, K., Kim, K.-R., Chung, J.-Y., Choi, B.-H., Byun, S.-K., Hong, G.H., Takematsu, M., Yoon, J.-H., Volkov, Y., Danchenkov, M., 1996. New findings from CREAMS observations: water masses and eddies in the East Sea. J. Korean Soc. Oceanogr. 31, 155 – 163. Kim, K.-R., Kim, K., Kang, D.-J., Park, S.Y., Park, M.-K., Kim, Y.-G., Min, H.S., Min, D., 1999. The East Sea (Japan Sea) in change: a story of dissolved oxygen. MTS J. 33, 15 – 22. Kim, K., Kim, K.-R., Min, D., Volkov, Y., Yoon, J.-H., Takematsu, M., 2001. Warming and structural changes in the East Sea (Japan Sea): a clue to the future changes in global oceans? Geophys. Res. Lett. 28, 3293 – 3296. Kim, K.-R., Kim, K., Kang, D.-J., Volkov, Y.N., Yoon, J.-H., Takematsu, M., 2002. The changes in the East/Japan Sea found by CREAMS. Oceanogr. Jpn. 11 (3), 419 – 429 in Japanese with English Abstr. Min, D.-H. 1999. Studies of large-scale intermediate and deep water circulation and ventilation in the North Atlantic, South Indian and Northeast Pacific Oceans, and in the East Sea (Sea of Japan), using chlorofluorocarbons as tracers. PhD. thesis, Univ. of California, San Diego, USA, unpublished. Sudo, H., 1986. A note on the Japan Sea proper water. Prog. Oceanogr. 17, 313 – 336. Tsunogai, S., Watanabe, Y.W., Harada, K., Watanabe, S., Saito, S., Nakajima, M., 1993. Dynamics of the Japan Sea deep water studied with chemical and radiochemical tracers. In: Teramoto, T. (Ed.), Deep Ocean Circulation, Physical and Chemical Aspects. Elsevier, Amsterdam, pp. 105 – 119. Uda, M., 1934. The results of simultaneous oceanographic investigations in the Japan Sea and its adjacent waters in May and June 1932. J. Imp. Fish. Exp. Stn. 5, 57 – 190. USSR Academy of Science 1957. Reports of multi-disciplinary oceanography expeditions on R/V Vityaz. v.6, unpublished. Watanabe, Y.W., Watanabe, S., Tsunogai, S., 1991. Tritium in the Japan Sea and the renewal time of the Japan Sea deep water. Mar. Chem. 34, 97 – 108.