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Observation of Bottom Water Renewal and Export Production in the Japan Basin, East Sea Using Tritium and Helium Isotopes
Doshik Hahm*† and Kyung-Ryul Kim
School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea Received 12 September 2007; Revised 1 February 2008; Accepted 10 March 2008
Abstract − Tritium (3H or T) has been produced mostly by atmospheric nuclear weapon tests, and entered the ocean in the form of water (HTO). As tritium exists as water itself, it has been regarded as an ideal tool to study the transport of water masses. In April 2001 we collected water samples in the western Japan Basin (WJB) for tritium and helium measurement. The timely sampling provided direct evidence of the bottom water formation, resulting in the drastic increase in tritium concentration from 0.3 TU in 2000 to 0.67 TU in 2001.
Considering that the new bottom waters were found mostly in the WJB, it implies that maximum 1% of the whole bottom layer below 2600 m should be replaced with the surface water during the severely cold winter 2000–2001. 3H-3He age, showing the elapsed time since the water left from the surface, can be used to calculate oxygen utilization rate by dividing AOU by the age. Under the condition of 90% oxygen saturation in the surface water, the integration of OUR in the water column below 200 m yields net oxygen consumption of 12 mol (O2) m-2 yr-1, which corresponds to the export production of 99 g C m-2 yr-1. This estimate is comparable to a previous estimate based on satellite data and implies that the ratio of export to primary production (f -ratio) is as high as 0.5 in the WJB.
Key words − East Sea, water renewal, export production, helium isotope, tritium
1. Introduction
Tritium (3H or T), the heaviest isotope of hydrogen, has been produced mostly by atmospheric nuclear weapon tests peaking early in the 1960s, and entered the ocean in the
form of water (HTO). As tritium exists as water itself, it has been regarded as an ideal tool to study the transport of water masses. A classic example of the utility of tritium is the work of Östlund and Fine (1979), clearly demonstrating the formation of North Atlantic deep waters.
The utility of tritium was increased further by the advent of mass spectrometric method for tritium measurement by the 3He ingrowth method (Clarke et al. 1976). The mass spectrometric method improved the precision of tritium data and provided another useful tracer, 3He, the daughter nuclide of 3H. The combination of 3H and 3He provides
3H-3He age (τ), which is defined by
(1) where T1/2 is the half-life of tritium (12.33 years, Lucas and Unterweger, 2000), [3He]trit is the concentration of 3He supported by tritium decay, and [3H] is the tritium concentration. τ shows the elapsed time since the water left the surface. Because of the relatively short half-life of tritium, τ has been used to study upper ocean circulation for timescales from a few months to decades (Jenkins 1987, 1998). Jenkins (1998), for instance, calculated isopycnal diffusivities and absolute velocities in the eastern subtropical North Atlantic, an area characterized by direct subduction and ventilation. Considering that the relatively short overturning time scale of deep waters of several decades (Tsunogai et al. 1993; Min 1999), 3H and 3He can be useful tools to study the ventilation in the East Sea.
The East Sea, consisting of three deep basins, namely the Ulleung, Yamato and Japan Basins, has drawn much
τ T1 2⁄
2
---ln 1 [3He]trit 3H ---[ ]
⎝ + ⎠
⎛ ⎞
ln
=
*Corresponding author. E-mail: dhahm@ucsd.edu
†Present address: Geosciences Research Division, Scripps Institution of Oceanography, University of Califonia, San Diego, La Jolla, CA 92093- 0244, USA
Article
attention of researchers because of its similarity in water column structures to that of open ocean (Kim and Kim 1996) and the existence of its own deep water circulation like a ‘ocean conveyor belt’. Among the basins the Japan Basin has been considered as the place where deep water formation occurs through deep convection and/or brine rejection (Sudo 1986; Seung and Yoon 1995; Ponomarev and Zuenko 1995).
The East Sea, like many other seas over the world, is also undergoing change in its water mass structure and circulation mechanisms. For example, a simple box model with boundaries moving with the volume change of deep waters (namely, Central, Deep and Bottom Waters) showed that the bottom water formation of 0.02 Sv in the 1950s slowed down and came to a complete halt in the mid-1980s (Kang et al. 2003). Surprisingly, a series of observations in 2001 (Kim et al. 2002; Senjyu et al. 2002; Tsunogai et al.
2003) found decrease in temperature and nutrients, and increase in salinity, oxygen and CFC-11 at depths below 3000 m, which strongly suggest the direct input of surface water into the bottom layer. Kim et al. (2002) suggested that the bottom water formation should be caused by a severely cold winter, and estimated the renewal of 8% of bottom water using anomalous signals of nutrients and oxygen. On the other hand, Talley et al. (2003) found that although deep convection occurs every winter, it does not reach past mid- depths; they concluded that the main ventilation mechanisms for the bottommost waters is brine rejection. 3H and 3He data will give another set of independent information on the magnitude and rate of renewal in the bottom layer.
There are several studies of carbon cycle in the East Sea:
for example, in terms of air-sea gas exchange (Oh et al.
1999; Hahm et al. 2003) and anthropogenic CO2 (Park et al.
2006). However, there are very few studies on annual export production. The export production is regarded as a measure of biological pump, by which CO2 fixed during photosynthesis is transferred to the interior ocean, resulting in a sequestration of carbon. The work of Hahm and Kim (2001), based on the mass balance of tritium-supported 3He in the mixed layer, provided annual new production of 64 g C m-2 yr-1 in the Ulleung Basin. This estimate may not be applicable to the northern East Sea because the subpolar front running along 40°N divides the East Sea into two regions with different physical and biological properties.
Another interesting approach to estimating export production using 3H and 3He can be found in Jenkins (1977). The author
observed 3H and 3He and derived oxygen utilization rate (OUR) in the Sargasso Sea. Since the first simple estimate by dividing apparent oxygen utilization (AOU) by τ, more delicate estimates have been made by carrying out many measurements on the same density surface to obtain more concrete correlation between AOU and τ (Jenkins 1982, 1998; Zheng et al. 1997). Given that oxygen utilization occurs as a result of oxidation of sinking organic particles, OUR could be converted, using the stoichiometry of carbon- oxygen, to export production.
In this paper we present the results of tritium and helium isotopes in the western Japan Basin (WJB; 132°20’E, 40°30’N), where the renewal of bottom water was observed in 2001. We observed significant increase in tritium concentration in the bottom water, and tried to calculate the renewal rate of the bottom water. In addition, we estimated oxygen utilization rate (OUR) by dividing apparent oxygen utilization (AOU) by 3H-3He age. The OUR was further used to estimate export production.
2. Materials and Methods
In April 2001 water samples for helium, neon and tritium measurement were collected at Station 016, where bottom water formation was observed in the winter of 2000-2001 (Kim et al. 2002). Because only one sample (3340 m) was available below 2500 m in 2001, water samples collected at the same location (station 019) in the next year were analyzed to make up for the sparse sampling and to check any change in deep layer (Fig. 1).
Water samples for helium and neon analysis were collected from Niskin Bottles and stored in cold welded copper tubes. The dissolved gases were vacuum extracted from the water samples into glass ampoules with low helium permeability and analyzed by mass spectrometric peak height manometry using automated techniques described by Lott and Jenkins (1984). Measurements were made to a precision of 0.8% for He, 3% for tritium in a magnetic sector mass spectrometer, and to 1.1% for Ne in a quadrupole mass spectrometer. The results were corrected for processing blanks and extraction efficiencies by an amount generally less than measurement precision. Helium isotope ratio measurements were made relative to atmospheric helium standards, to an accuracy of 0.25%, and were corrected for instrumental linearity, which was determined by measurement of differing sized air standards.
3. Results
Hydrography
The vertical distribution of potential temperature and salinity in the WJB were shown in the Fig. 2. While the deep water below 2500 m in the year 2000 (March) and 2002 (April) carried rather uniform temperature of around 0.07oC, the water in 2001 showed abrupt decrease in temperature at 3100 m and was as cold as 0.01oC at the bottom. Similar abrupt change was also found in salinity.
While salinity of the deep water in 2000 and 2002 showed a quite similar trend, salinity in 2001 showed clear increase between 3100 and 3300 m. The deviation started at the exactly same water depth of 3100 m as in potential temperature.
The anomaly was also confirmed independently by high oxygen (Fig. 2), low nutrients (Kim et al. 2002; Senjyu et al. 2002), and high CFCs (Tsunogai et al. 2003) in the winter 2000-2001. Kim et al. (2002) and subsequent studies suggested that the anomaly indicates a massive renewal of bottom waters through brine rejection or deep convection.
The distribution of helium, neon, and tritium
δ3He, relative abundance 3He to 4He, is defined by δ3He = (Rsample/ Ratm -1) × 100% (2)
where R stands for the ratio of 3He to 4He in the sample or atmosphere (Ratm=1.384×10-6, Clarke et al. 1976). The values of δ3He (Fig. 3a) were close to the static equilibrium of -1.8% at surface (Benson and Krause 1980), and gradually Fig. 1. A map showing the location where helium isotope (dot)
and/or tritium (circle) data are available in the Japan Basin. In this study the samples from station 016 and 019 were collected in 2001 and 2002 respectively. The others were collected in 1999 and presented in Postlethwaite et al. (2005). The triangle shows the location where CFCs samples were collected in 1996 (Min 1999).
Fig. 2. Potential temperature (a), salinity (b) and dissolved oxygen (c) in the Western Japan Basin, 2001 (red). The values in 2000 (black) and 2002 (brown) obtained at the same place was also shown for comparison. Insets show the properties below 2000 m.
increase to around 6% at 1000 m depth. The maximum is lower than the values of over 7% found in 1999 (Postlethwaite et al. 2005). Tsunogai et al. (2003) found a 20% increase in the inventory of CFC-11 between 500 and 1500 m depths in the 2000-2001 winter, and argued that the severely cold winter should be important for the renewal of the entire deep water rather than that of bottom water only. Although the recent addition of surface water carrying values close to equilibrium may decrease δ3He, it is not certain whether the decrease is due to the surface water or just reflects geographical difference. The lower maximum might be attributed to the lack of samples between 1000 m and 1500 m depth. Below the maximum δ3He decrease to 2.5%, and the value is consistent with those in 1999 in general. A remarkable feature is the extraordinary low value at the bottom in 2001. δ3He of - 1.6%, together with temperature, salinity and oxygen anomalies (Fig. 2), strongly implies a significant input of surface water.
Tritium ranged from 1.3 to 1.6 TU (Tritium Unit: equal to 1 3H atom per 1018 H atoms) at surface, and gradually decreased to 0.3 TU at 2000 m (Fig. 3b). In this depth range, the tritium concentrations at St. 016 (WJB in 2001) were either similar to or slightly higher than those at St. 96 and 156 in the Eastern Japan Basin (EJB) in 1999. Considering that radioactive decay between 1999 and 2001 results in decrease of tritium by 10%, if measured at the same time, the tritium concentration in the WJB is likely to be higher than that in the EJB, especially in the range between 1000 and 2000 m. The deep layer between 2000 m and 3100 m carried uniform concentration of 0.3 TU, and at the bottom layer the concentration slightly increased to 0.4 TU in 2002.
Overall, the tritium concentrations below 2000 m were significantly higher than those at St. 96 and 156. This implies that direct input of tritium-enriched surface water is larger in the west than east of the Japan Basin. The concentration in 2001 was even higher than in 2002, again indicating bottom water formation in the winter 2000-2001.
A quantitative discussion of the renewal will be presented in the section 4.1.
According to Hahm et al. (2004), the distributions of helium and neon are influenced by the brine rejection, responsible for the bottom water formation in the Japan Basin (Talley et al. 2003). That is, due to higher solubility of helium in sea-ice than in seawater, helium may be depleted in brine-enriched water (BEW); On the other hand, neon, more
soluble in seawater than in sea-ice, can be enriched in BEW.
Unfortunately, both helium and neon did not show enough difference between 2001 and 2002 to resolve any mechanisms affecting the distributions (Fig. 3), probably because the magnitude of brine rejection is not enough to generate significant saturation anomalies of helium and neon. The BEW in Peter the Great Bay (34.6 psu, Ponomarev and Zuenko, 1995) can be produced by freezing of only 5% of seawater of 33.1 psu (the lowest winter value in the bay from the World Ocean Database
’01). This small fraction of ice to water results in less than 2% saturation anomalies of helium and neon in BEW, which can be easily eroded by mixing with ambient water.
Considering that the relatively small magnitude of brine rejection in Peter the Great Bay, the adoption of argon, virtually insoluble in ice and hence more sensitive to brine rejection than helium or neon, will increase the possibility to resolve deep water formation mechanisms in the East Sea.
Fig. 3. δ3He (a), tritium (b), 4He (c), and Ne (d) in the Western Japan Basin, 2001 (open square) and 2002 (filled square).
The values of δ3He and tritium from Postlethwaite et al.
(2005) were also shown for comparison. The same color scheme as in Fig. 1 was applied to the symbols.
4. Discussion
Renewal of bottom water
By assuming that the increase in tritium below 3100 m in 2001 resulted from the addition of surface water in the winter, we calculated how much bottom water was replaced with the tritium-enriched surface water. First, the concentration below 2600 m (boundary depth between deep and bottom water, Kang et al. 2004) in the winter 2000 (just before the bottom water formation) was assumed constant with the concentration of 0.32 TU, decay-corrected value for the average (0.3 TU) between 2500 and 3100 m in 2002. Next, the thickness of the bottom water equivalent to the surface water (DSEt ) was calculated by
(3) where is the mean concentration of tritium in the bottom water at time t. DB is the thickness of the bottom water, and is the mean concentration in the surface water. It was assumed that DB was 800 m (2600-3400 m) and was 1.6 TU and their variations were insignificant during 2000–2001 (Fig. 3b). When and were set to 0.32 TU and 0.42 TU, and were calculated as 160 m and 210 m respectively. For the calculation of , it was assumed that the concentration of tritium increases linearly from 3100 (0.32 TU) to 3240 m (0.67 TU), and the concentration below 3240 m is constant with the value of 0.67 TU. The difference in the surface-equivalent thickness of 50 m between 2000 and 2001 indicates that it takes about 16 years to renew the bottom water of 800 m thick completely; in other words, around 6% of the bottom water was replenished by the sudden-bottom water formation.
This value is comparable to the estimate of 8% by Kim et al.
(2002), based on anomalous signal of nutrients and dissolved oxygen. The rate of 50 m/yr is also similar to the CFC-derived values of Tsunogai et al. (2003) ranging 32-45 m/yr.
Lobanov et al. (2005) found that the remnants of the new bottom water still existed in 2002. In order to determine how fast the new bottom water mixed with the ambient water, of 178 m was also calculated as following the above calculations. When it is compared to that in 2000, the difference in thickness decreased from 50 m in 2001 to 18 m in 2002; That is, 36% of the new bottom waters survived over 1 year. If the same survival rate is assumed, it is expected that only 13% of the new bottom water will exist
in the next year. Considering that the magnitudes of anomalies in 2001, the amount of 13% is hardly detectable by most physical and chemical tracers such as temperature, salinity, oxygen and tritium.
The renewal of bottom water did not seem to happen all over the Japan Basin. In fact, all the new bottom waters found in April (Kim et al. 2002) and July (Senjyu et al.
2002) were located in the WJB, north of 40°N and west of 133°30’E. If it is assumed that the surface water filled evenly from bottom to 3350 m (bottom depth at St. 016 minus 50 m) in the WJB, the amount of surface water injected to the bottom layer is estimated to be 1.8×1012 m3, corresponding to 1% of the total volume of bottom layer below 2600 m. However, the actual amount of new bottom water could be much less than this estimate because the new bottom waters did not seem to fill up the bottom layer but rather exist as ‘patches’ in the WJB (Lobanov et al. 2005).
In addition to tritium, 3H-3He age (τ), showing the elapsed time since the water left the surface (see eq. 1), can be used to explore water renewal. Fig. 4 shows the vertical distribution of τ in 2001 and 2002. The water shallower than 200 m, corresponding to the East Sea Intermediate Water (ESIW), was around 1 year old, supporting the idea that ESIW is formed locally every winter in the WJB (Kim et al. 2004). Below ESIW τ gradually increased and reached over 20 years at 1700 m. In this layer τ is significantly younger than CFC-11/CFC-12 ratio age (τCFC, Min 1999) at each corresponding depth. It is likely because the severely cold winter 2000-2001 supplied young surface water to this layer through deep convection (Talley et al. 2003). The increase in inventory of CFC-11 by 20% was also observed in this layer between 2000 and 2001 (Tsunogai et al. 2003). In the water column deeper than 1700 m τ and τCFC were similar to each other except for the decrease in τ below 3100 m. The change of τ from 1 in 2001 to 21 years in 2002 shows well the formation and reduction of new bottom water over 1 year.
3H-3He age (and τCFC), however, should not be regarded as
‘mean’ age of the water masses as τ is not conservative with respect to mixing and is typically biased towards younger values. For example, a mixture of recently ventilated ‘young’
water and ‘old’ tracer-free water will have an age equal to that of the younger fraction (Warner et al. 1996; Doney et al. 1997). CFC-12 partial pressure age (Fig. 4) can also be biased younger or older than the mean age because of the non-linear increases in atmospheric CFCs (Sonnerup DSEt CBt
DB
CS
---
= CBt
CS
CS
CB2000
CB2001
DSE2000
DSE2001
CB2001
DSE2002
2001). The effect of mixing on τ can be examined by comparing with the distribution of idealized “age tracer”
(Thiele and Sarmiento 1990), conservative with respect to mixing and hence providing a reference point to tracer- based age. However, 2- or 3-dimensional tracer data and a decent numerical model simulating tracer fields are prerequisites for the comparison. Instead of the modeling approach, we considered a Peclect number, a measure of the relative importance of advection to diffusion (Jenkins and Wallace 1992). The Peclet number (Pe) is defined by
(4) It was assumed that water velocity (u) and isopycnal eddy diffusivity (κ) in the East Sea are 4 × 10-2 m s-1 (Park 2002) and 2 × 103 m2 s-1 (Oh et al. 2000) respectively. The horizontal length scale (L) was assumed to be around 106 m, which was inferred from the size of gyre circulating in the Japan Basin at 800 m depth (Park 2002). The resultant Pe of 20 implies that an error of about 5% is introduced by ignoring mixing. In the following discussion on oxygen utilization rate it is assumed that the bias of τ due to mixing is not significant.
Oxygen utilization rate and export production
Although both 3H and 3He themselves are not related any biological processes, τ can be used to obtain valuable information on biological processes. In the following, we calculated oxygen utilization rate (OUR) and export production, a measure of photosynthesized-CO2 transferred to the interior ocean resulting in a sequestration of carbon. First, OUR was calculated by dividing apparent oxygen utilization (AOU) by τ (Jenkins 1977). AOU has been defined as the difference between the saturation oxygen concentration and the observed oxygen concentration:
AOU=[O2]0- [O2]. (5)
However, Ito et al. (2004) observed significant disequilibrium of oxygen in high latitude surface oceans. They expected a large under-saturation when the rate of solubility increases due to heat loss or the rate of entrainment flux is relatively rapid compared to that of air-sea gas exchange. A similar phenomenon seems to happen in the East Sea. Figure 5 shows the saturation percent of oxygen at surface in February. The saturation percents decrease to the north, and
Pe uL
---κ
=
Fig. 4. pCFC-11/pCFC-12 ratio (Min, 1999) and 3H-3He ages in the Japan Basin. The samples for CFCs analysis were collected at about 1° south of St. 016 (Fig. 1).
Fig. 5. Oxygen saturation (%) at surface in February. The contour map was produced with objectively analyzed climatological mean of ‘World Ocean Atlas 2001’ (NODC/NOAA).
become below 90% near Primorye coast. The saturation percents in the region higher than 46°N are even lower than 90%, possibly due to sea-ice prohibiting air-sea gas exchange. Indeed, Talley et al. (2003) found many locations with oxygen saturation lower than 94% in the WJB (>40°N) during their winter expeditions in 2001. As the OUR estimate by the present approach is highly dependent on the appropriate estimate of AOU, in addition to AOU assuming 100% oxygen saturation at surface water (AOU100), AOUs when the surface water is saturated with oxygen by 80 and 90% (AOU80 and AOU90) were also considered throughout the following calculations.
The OUR results were shown in Fig. 6. The OUR estimate by assuming that the surface water was saturated with oxygen by 80% of the solubility equilibrium values ranged from 3 to 8 µmol kg-1 yr-1 at depths between 300 and 1600 m, where most of the oxygen consumption occurs.
The present estimates are significantly higher than the previous estimates. For example, the results of Min (1999) for the entire East Sea ranged from 1 to 4 µmol kg-1 yr-1 at corresponding water depths. Most of the difference could be attributed to the difference in ages applied to the calculation. Although Min (1999) made the estimates by dividing AOU by age as in the present study, he used ‘mean mixing age’ deduced from a CFC-calibrated steady state box model, intrinsically averaging all CFC data in the East Sea. Given that the WJB is the place where deep convection occurs most strongly, resulting in younger tracer age, it is not surprising that the present estimate in the WJB is higher than those of Min (1999), representing OUR in the entire East Sea. Therefore, the present estimate should be regarded as a representative of the WJB rather than that of the whole Japan Basin or the East Sea.
To calculate the net oxygen consumption rates in the water column, the OURs were fitted with exponential functions for each oxygen saturation percentage. The values at the depths shallower than 200 m were not included in the fitting because they had large errors due to the low 3H-
3He ages. In addition, the values below 1600 m were not
used because the gradients of oxygen and τ along water depths were not significant in these water columns. The integration of OUR was performed in the water column between 200 and 1600 m, showing relatively strong correlation between AOU and τ. The carbon fluxes equivalent to oxygen consumption were also calculated with the Redfield oxygen:carbon ratio of 1.45 (Anderson and Sarmiento 1994). The results are summarized in Table 1.
The present estimates of export production are highly variable with the initial degree of oxygen saturation.
Although it is not certain where the waters between 200 and 1600 m come from, 90% are likely an appropriate assumption of initial saturation percent (Fig. 5) considering that many of deep convection areas were located around 42°N near Vladivostok (Talley et al. 2003). The export production of 99 g C m-2 yr-1, based on the integration of water column from 200 to 1600 m, should be regarded as Fig. 6. Oxygen utilization rates with AOU100 (circle), AOU90 (triangle) and AOU80 (square) respectively. The dashed lines show the fittings based on log linear consumption rate-depth dependence.
Table 1. Net oxygen consumptions and export productions in the Western Japan Basin for AOU80, AOU90 and AOU100. The integrations were performed between 200 and 1600 m.
Initial oxygen (% saturation) Fitting equation O2 consumption (O2 mol m-2 yr-1) Export production (g C m-2 yr-1)
80 OUR=6.6 exp (-0.00039z) 6.6 54
90 OUR=14 exp(0.00061z) 12 99
100 OUR=29 exp (-0.00095z) 19 160
the minimum of the export production. If the integration interval is extended to 3500 m depth, although the deep layer is not well constrained, the estimate would increase to 150 g C m-2 yr-1.
As there is no tracer-based estimate of annual export production in the Japan Basin, the present estimate was compared with a global map based on satellite chlorophyll data (Laws et al. 2000). To estimate export from primary production Laws et al. (2000) introduced a food web model, which assumes that primary production is partitioned through both large and small phytoplankton. Although details of the model may be problematic (Dunne et al. 2005;
Richardson and Jackson 2007), it is one of few estimates of global export production which have been referred to (Buesseler et al. 2007; Najjar et al. 2007). According to the model, the export production in the WJB would be around 80 g C m-2 yr-1 , similar to the estimate in this study. Similar estimates (70 ± 20 g C m-2 yr-1) were also made by the model of Goes et al. (2004), predicting export production with empirical relationships between nitrate and its predictor variables, temperature and chlorophyll-a.
The present estimate of 99 g C m-2 yr-1 in the WJB is higher than 64 g C m-2 yr-1 in the Ulleung Basin (UB), based on the mass balance of tritium-supported 3He in the mixed layer (Hahm and Kim 2001). However, this does not necessarily mean that the productivity in the WJB is significantly higher than that in the UB. Laws et al. (2000) argued that f-ratio (export production/primary production) should be high in the region of low temperature of 0-10oC because most organic matter is being routed through the large phytoplankton-based food chain, resulting in higher particulate flux. Assuming that the primary productions of 200 g C m-2 yr-1 in the WJB and 220 g C m-2 yr-1 in the UB (Yamada 2004), the f-ratios in the WJB and UB should be 0.5 and 0.3 respectively.
5. Conclusion
Water samples for helium isotopes and tritium were collected in the WJB. The timely sampling in April 2001 provided a direct observation of the bottom water formation, resulting in the drastic increase in tritium concentration from 0.3 TU in 2000 to 0.67 TU in 2001.
Considering that the new bottom waters were found mostly in the WJB, it implies that maximum 1% of the whole bottom water should be replaced with the surface water
during the severely cold winter 2000-2001. 3H-3He age, showing the elapsed time since the water left from the surface, can be used to calculate oxygen utilization rate by dividing AOU by the age. As the OUR estimate using the method is sensitive to the degree of oxygen saturation in the surface water, each OUR estimate was made for the surface water with 80, 90 and 100% oxygen saturations respectively (Table 1). Under the condition of 90% oxygen saturation in the surface water, the integration of OUR in the water column below 200 m yields net oxygen consumption of 12 mol (O2) m-2 yr-1, which corresponds to the export production of 99 g C m-2 yr-1. This estimate is comparable to the estimate based on satellite data (Laws et al. 2000; Goes et al. 2004) and higher than that in the Ulleung Basin, located in the south of the subpolar front running along 40°N (Hahm and Kim 2001). Assuming that primary production in the WJB and UB are not significantly different from each other (200 g C m-2 yr-1; Yamada 2004), higher export production implies higher f-ratio in the WJB of 0.5 than that in the UB of 0.3. The f-ratios in the East Sea are similar to or higher than the global averages in the range of 0.2-0.35 (Falkowski et al. 1998; Laws et al. 2000; Najjar et al. 2007).
The present estimate of OUR carries a large degree of error due to our ignorance of initial oxygen concentration when the water stays at the sea surface. This drawback could be overcome by measuring AOU and 3H-3He age along the isopycnals having significant gradients for those values. The correlation between AOU and τ can be used to predict initial oxygen on the isopycnal (Jenkins 1982;
Zheng et al. 1997).
Acknowledgements
The authors appreciate the reviewers for their constructive comments for improving the manuscript. We also thank the scientists and crews of the expeditions in 2001 and 2001 for their help on board. C. F. Postlethwaite provided great help in the sample analysis and data interpretation. D. Hahm was financially supported by the BK21 Project of the Korean Government.
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