End Members of Water-Masses in the Western Japan Sea

At least thirteen distinct end members of water-masses are identified in the western Japan Sea. Their origins and conditions of formation are discussed, including the effects of continental water inputs, photosynthesis in remnant winter waters, microbial oxidat. ion of organic matter in the water column or at water-sediments interface, and the cooling of surface waters along the Siberian coast and the northern Japan Sea.


INTRODUCTION
The Japan Sea is bounded by the Japanese islands, the Siberia and the Ko1•ean Peninsula, and has a total area of 1.013 x 10 6 km 2 and a \1olume of 1.69 x I 0 6 km 3 .Its mean water depth is 1669 m and its deepest de.pth 4036 m.It is connected to the Pacific Ocean by four shallo\v straits: Tsushima, Tsugaru, Soya, and Tatar Straits (Figure 1 ), each with a sill depth of less than 130 m (Tsunogai el: rLl . .I 993 ).The Tsushima Wann Current branches oft' from the Kuroshio Current and flows into the Japan Sea through the Tsushima Strait with a magnitude of about 3 to 4 x 10 6 m 3 /se.c(Miita and Ogawa, 1984 ).Then, it flows northward mainl)' along the west coast of Japan.A portion of water flows out of the Japan Sea through the Tsugaru and Soya Straits.The other portion turns anti-clockwise, mixes with fresh water inputs from Siberia and Korea and becomes the North Korean Cold Current, which fto\VS southward a1ong the Siberian and the north Korean coasts.Thus, the Tsushima Warm Current and the North Korean Cold Current form a cyclonic gyre and associated subsurface upwelling dome ( Figures 41 to 44 of Stepanov, 1961 ).These two currents form a front 'h'ith an E-W dire.ction around 39-40°N and a NE-SW or N-S direction above 40°N (Figure 28 of Stepanov, 1961 ).
Major water masses in the Japan Sea were identified by Yasui et al. ( 1967), using the distribution of estimated wate.r \,olume as functions of potential temperature (8) and salinity.
However, their approach has a few shortcomings in that the identified water masses have wide ranges in both potential temperature and salinity, thus, making it difficult to pinpoint the properties of the water-mass end members.Additionally, the approach smooths out small but sometimes important changes within their water masses, and it does not take advantage of additional geochemical parameters, such as dissolved oxygen, nutrients, and ''PO" (=138 P04 + 02 ) or 1'NO'' ( = 8.6 N03 + 02 ; Broecker, 1974) to distinguish other possible water masses.
The objectives of this paper are to (I) identify all possible end members of water-masses in the Japan Sea using the high quality data obtained during the Kuroshio Edge Exchange Processes-Marginal Seas Study (KEEP-MASS) expedition (Chen and Bychkov, 1992) and (2) discuss their origins.

SAMPLING AND ANALYTICAL METHODS
The KEEP-MASS expedition was conducted by scientists from Taiwan aboard the RN Vinogradov from July 10 to August 5, 1992.The locations of the major sampling stations in the western Japan Sea are shown in Figure 1.Yuan-Hui Li & Gwo-Ching Gong

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• A Guildline CTD model-875 was deployed to obtain the vertical.in""" situ temperature and salinity profiles.The temperature was calibrated against reversing thermometers, and the salinity against bottled samples.Water samples were collected by 2.5 liter Niskin or Go-Flo samplers.All chemical parameters were measured on s•hipboard soon after sample collection.
umber within an open cube identifies the \\"ater sample that is an end member (EM) of a water-mass.An end member is so chosen that at least one parameter among 8, salinity and 02 cannot be reproduced by the mixing of two neighboring end members.Chemical properties of all end members are summarized in Table 1.Figures 5a to 5e show the cross-section of salinity, (} , 02 , P04 and ''PO" along the track between Stations 2 and 14 (dotted line in Figure 1).To avoid an overcrowding of the data, Station 9 data are excluded in Figure 5.This does not change the main features in Figure 5 because the water column properties of Station 9 are similar to those of Station 13 nearby (Figures 2 to 4).The end members identified from Figures 2 to 4 are also indicated in Figures 5a to 5e.
End members 8, 9 and 10 are all surface waters.EMlO is characterized by high temperature (24 ° C), low salinity (33 per mil) and a high NTA (Figures 2a and 2c ), indicating inputs of continental freshwater with high NTA.One may call EMlO the northward-flowing surface Tsushima Warm Current.EM9 is characterized by high salinity (Figure 2a) and was probably fon-ned by the upwelled EM7 mixing with EM 10.The geographical extent of EM9-type water is limited (Figure 5a).EMS is slightly low in B (20°C in Figure 2a) and represents the southward-flowing Surface North Korea Cold Current (Kim et a l ., 1991).In principle, EM8 can be produced by mixing EMlO and EM7, except that EM8 contains more 02 or less P04 than predicted values from mixing.
End members 4 to 7 are subsurface waters at depths between 30 m and 250 m.EM7 is characterized by the highest salinity and lowest NTA (Figures 2a and 2c), representing the core of the Tsushima Warm Current (Figure 5d).EM7' is closely related to EM7 in terms of conservative tracers but has relatively low 02 (Figure 5d) and high P04 and Si02 (Figures 2e and 2f), indicating the effect of n1icrobial oxidation of organic matter in the water column or somewhere at the water-sediment interface on the continental shelf.•EM6 is characterized by low P04 (Figure 2e) and very high 02, which is much higher than the 02 saturation value at the given temperature (dotted curve in Figure 2d).EM6 probably represents the remnant of the winter surface mixed-layer water that was originally high in P04 but became low in P04  [I] _.. .'tPO'' CuMl   TAO, vT ol. 7, No.l, March 1996 0 .5 r-r-.---r-,-,:--r-... ..-.-.-r --;:�-,---,-7"T" 1"'1'"i1 As in Figur � 2, but poten tial tempe rature is on an expan ded scale of 0 to 0.5 °C.and high in 02 through photosynthesis in summer.The extent of EM6-type water is limited (Figure 5d).EMS is distinguished by a subsurface salinity minimum (Figures 2a, 3a ----------• l
• . - -150 -I � �  Siberian coast.As shown in Figure 2b (as well as in Figure 2d), the dotted line represents the saturated 02 conc .entrations at given q and salinity (Weiss, 1970).Therefore, for the surface water more than 90% of ''PO'' is contributed by 02 and less than 10% by P04.In contrast, if EM3 was forrr1ed at surface and fully equilibrated with 02 in the air, it should contain 1.1 µM P04 in order to have the observed ''PO'' value (Figure 2b ).Interestingly, EM4 and EMS also have the same amount of P04 (Figure 2e).It is quite feasible that the cooling of EM4 and EM5 waters and the uptake of 02 at surface with a suitable adjustment of salinity in winter can produce EM3.It is well known that the surface mixed-layer can be as deep as 150 to 200 m along the Siberian and north Korean coasts in winter ( H idaka, 1966).Therefore, EM4-and EMS-type waters can easily mix up to the ocean surface.EM2 is characterized by salinity, ''P01' and 02 minima (Figures 4a,4b,4d,5a,5e,and 5d), and P04 and silicate maxima ( Figures 4e and 4f).One notices that the 02 minimum in Figure 5d is around 2000 m, which is much deeper than the 1400 m obtained for the eastern Japan Sea in 1984 (Garno et al., 1986).This difference is likely real because the depth of 02 minimum has deepened only about 200 m in one decade in the eastern Japan Sea (Garno et al., 1986).Between EM2 and EM3, there are additional end members 2' and 2'', which are identified by breaks in the plots of 8 versus 02 (Figure 4d), P04 (Figure 4e) and Si02 (Figure 4f).The 02 concentrations of EM2' and EM2'' cannot be reproduced by mixing between EM2 and EM3 (Figure 4d) without acquiring additional 02 sources.The formative modes of EM2, 2', and 2-" are probably similar to that of EM3, except that the former must have been formed at the surface under more severe winter conditions than the latter (Nitani, 1972).Vasilev and Makashin ( 1992) suggested that the frequency of a severe cold winter is about once in a decade.EMl is the bottom water with 8 less than 0.05°C at a depth below 2000 to 2500 m (Figure Sb) and is relatively homogenous in chemistry except for NTA (Figures 4a to 4f).EMl' has an unusually high ''PO'' value (Figure 4b); otherwise, is indistinguishable from EMl (Figure 4).EMl and EM l' must have been for111ed under even colder conditions than EM2 and EM3.The end members 1 to 2 were most likely formed in the surface.areas of northern and northeastern Japan Sea (Garno et al., 1986).
The re-plotting of the tritium data given by Watanabe et al. (199 1) as a function of depth in the Japan Sea is shown in Figure 6.The tritium concentration is high between the surface and 800 m, indicating good \,entilation betVv'een the sutfac .e ocean and subsurface end members 3 to 7. The moderate.tritium concentrations at depths between 800 m and 2000 m indicate moderate ventilation of EM2' and 2''.A model calculation by Watanabe et al. (1991) gives a turn-0\1er ti1ne of 100 years for the Japan Sea water be.tween 200 m and 1500 m.EM2 around 2000 m has the lowest 0 2 (Figure 5d), and below 2000 m the lowest tritium ( Figure 6) conce.ntrations.Therefore, EM2 is likely the least v•entilated \Vater in the Japan Sea.EM 1 has onl)1 one tritium datum with a finite value (Figure 6), probably indicating a finite \1entilation.Certainly, additional tritium as well as freon data are badly needed to constrain the tum-over time of the bottom \\1ater.Also, the future deployment of the oxygen probe.along with CTD probe will provide important information on the formative processes of the deep waters in the Japan Sea.The deployment should be focused in the northern part of the Japan Sea as suggested by Garno et al. ( 1986) .(2) The effect of river water inputs to surface water of the Japan Sea is to decrease salinity and incre.ase normalized total alkalinity (NTA).
(3) The localized intense oxidation of organic matter at the sediment-water interlace or in the water column or both resulted in low oxygen and high nutrient concentrations for some Tsushima Warm Current water (EM7').(4) Intense photosynthesis in remanent winter mixed-layer during warmer seasons can pro duce a subsurface water with unusually high 02 and low nutrient contents (EM6).
(5) The East Sea Intermediate Water consists of at least two end members EM4 and EM5.
The former was probably fonne.d along the Siberian coast and the latter in the bay around the Tumen River mouth.(6) At least five end members are identified at a depth below 500 m to the bottom.They were probably formed intermittently in the surface areas of the northern or northeastern Japan Sea under extremely cold and variable winter conditions.

Fig. 1 .
Fig. 1.Locations of major sampling stations in the western Japan Sea during the KEEP-MASS expedition.The depth contours are in meters.

.Fig. 3 .
Fig. 3.As in Figure 2, except that the potential temperature is on an expanded scale of 0 to 3 ° C. The portions belo�1 the dotted lines are expanded into Figure 4.

Fig. 5 .
Fig. 5. Cross-sectional profiles along stations between 12 and 114 for (a) salinity (b) potential temperature (c) P04 (d) 02 and (e) ''PO''.All symbols are the same as .in Figure 2. The capital letters H and L represent the high and low values respectively .

Table 1 .
Properties of water-mass end members in the v.1estem Japan Sea.