Interfrontal Zone. Many low-salinity water patches were found down to depths of 640 m. .... constants of carbonic acid given by Dickson and Millero. (1987) and ...
Journal of Oceanography, Vol. 54, pp. 681 to 694. 1998
Chemical Alternation of Waters in the Kuroshio/Oyashio Interfrontal Zone T. ONO1*, I. YASUDA2, H. NARITA3 and S. TSUNOGAI3 1Graduate
School of Fisheries Sciences, Hokkaido University, Hakodate 041-0821, Japan of Earth and Planetary Physics, Graduate School of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan 3Graduate School of Environmental Earth Sciences, Hokkaido University, Sapporo 060-0810, Japan 2Department
(Received 10 February 1998; in revised form 27 July 1998; accepted 8 August 1998)
An intensive survey has been conducted of the distributions of some chemical properties (dissolved oxygen, nutrients and carbonate properties) in the Kuroshio/Oyashio Interfrontal Zone. Many low-salinity water patches were found down to depths of 640 m. Each chemical property also showed anomalies in these patches, but the degree of variation showed a low correlation with salinity. This may be due to the high variability of biological processes in the surface waters where these patches are formed. Vertical profiles of the chemical properties were also observed along the Kuroshio extension axis from 140.50°E to 146.75°E. The concentrations of nutrients and total carbonate (TC) in the water having densities greater than σθ = 26.60 can be regarded as being formed by the isopycnal mixing of the Kuroshio component water and Oyashio component water and biological degradation within the density surfaces. This implies that the transport of chemical properties by the diapycnal mixing is negligible in these density layers in the K/O zone.
1. Introduction The Kuroshio/Oyashio Interfrontal Zone (K/O zone: Yasuda et al., 1996) is a key area when considering the transport of heat, freshwater and chemical substances across the subpolar front in the North Pacific. Complex mixing of the subtropical-gyre-originated Kuroshio water with the subarctic-gyre-originated Oyashio water occurs in the K/O zone and achieves effective cross-gyre transport (Talley, 1993; Talley et al., 1995; Yasuda et al., 1996; Maksimenko et al., 1997; Yasuda, 1997). In the subtropical gyre, North Pacific Intermediate Water (NPIW) formed in the K/O zone efficiently spreads the Oyashio-oriented substances farther into the gyre (Tsunogai et al., 1995). The transport of anthropogenic carbon from the subarctic surface to the subtropic subsurface carried by the newly formed NPIW is of particular importance (e.g., Tsunogai et al., 1993; Tsunogai, 1997). Processes relating to the transport of chemical substances from the Oyashio to NPIW through the K/O zone are highly complex. The density surfaces of NPIW (σθ = 26.7– 27.0; Yasuda, 1997) and those of lighter densities lie in
shallow layers in the K/O zone, and the concentration of each chemical property in the newly formed NPIW may be determined not only by the isopycnal mixing but also by non-hydrological processes within this area, such as biological activities and gas exchange with the atmosphere. The contribution of each process in determining the chemical characteristics of the new NPIW formed at the K/O zone has not yet been quantitatively evaluated. For the carbonate properties such as total carbonate, even observed data are limited for this region (Rogachev et al., 1996) despite its obvious significance. In spring 1996 we made intensive observations of the chemical properties including carbonate properties in the K/ O region by several cruises. In this paper we describe the distribution of chemical properties in the K/O region, longitudinal changes in the chemical characteristics of the NPIW and other water masses along the Kuroshio extension axis and evaluate the contribution of each process affecting these changes. 2. Observation The data were obtained during two cruises carried out in May 1996, the WK9605 Cruise of the R/V WakatakaMaru of Tohoku National Fisheries Research Institute (TNFRI) and the HK9605 Cruise of the R/V Hokko-Maru of Hokkaido National Fisheries Research Institute (HNFRI).
*Present address: Marine Productivity Division, National Research Institute of Fisheries Sciences (NRIFS), Hukuura, Kanazawa-ku, Yokohama 236-8648, Japan.
681 Copyright The Oceanographic Society of Japan.
Keywords: ⋅ K/O Zone, ⋅ chemical alternation/transport.
Table 1. List of observation lines used in this study. Code
Cruise
A B 147E 149E 152E
HK9605 WK9605 WK9605 WK9605 WK9605
Sampling location
(42°50N, 144°50E)–(39°00N, 146°45E) (34°51N, 140°50E)–(32°30N, 146°45E) (39°00N, 146°45E)–(32°30N, 146°45E) (35°01N, 149°31E) (37°39N, 152°00E)–(36°20N, 152°00E)
Fig. 1. Map of the observation stations. The 100 m water temperatures during the observation period (TNFRI, 1996) are also shown.
Observation stations were established along five observation lines listed in Table 1 at approximately a 30 mile intervals. Locations of stations together with isotherm of 100 m depth (TNFRI, 1996) during the observation period are given in Fig. 1. 2.1 Measurements during the WK9605 Cruise Water samples were taken by CTD-RMS using 10L Niskin Bottles. Samples from 24 depths were taken from the seasurface to 3000 m depth at each station. At several stations where regional salinity minima were observed, the sampling depth was adapted so that water samples were taken at each salinity minimum.
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Frequency of chemical sampling
Depth range
Number of bottles measured
each 0.5°N 7 stns./line each 0.5°N 1 stn./line each 0.33°N
0–1000 m 0–1500 m 0–1500 m 0–1500 m 0–1500 m
13/stn. 24/stn. 24/stn. 24/stn. 24/stn.
Salinity (S), dissolved oxygen (DO), phosphate (P), nitrate (NO3), nitrite (NO2), silicate (Si), pH and total alkalinity (TA) were measured at each station. Salinity samples were measured by an Autosal located in the temperaturecontrolled room on board. DO subsamples were placed in 100 ml Fram Bottles and measured with a HIRAMA ART3DO-1 autotitrator within 6 hours of sampling. The average difference between measurement results of duplicate samples was 1 µmol/kg. Nutrient samples were placed in 10 ml polypropylene tubes and stored at 4°C. Measurements were carried out within 3 days of sampling using a TRACCS-800 autoanalyser. Special grade reagents of Na2 HPO4, KNO3, NaNO2, SiF6 (Kanto-Kagaku Co.) were used for primary standards of P, NO3, NO2 and Si, respectively. Concentrated standard solutions prepared in the laboratory were mixed and diluted with 3.5% NaCl solution on board and used as working standards. Samples for pH and TA were placed in 120 ml polyethylene bottles and stored at room temperature. Measurement of pH was carried out within 12 hours of sampling using a glass electrode calibrated with total hydrogen ion concentration scale buffers (Dickson, 1993; DOE, 1994) in a 25°C water bath. TA measurements were carried out within 48 hours of pH measurements by the modified onepoint method (Culberson et al., 1970). The average difference between duplicate samples was 0.004 pHu for pH and 3 µeq./kg for TA. The total carbonate contents of water samples (TC) were calculated from pH and TA using the dissociation constants of carbonic acid given by Dickson and Millero (1987) and that of boric acid given by Hansson (1973). Onboard TC measurements were also carried out at 8 stations by coulometry following the procedures of Ono et al. (1998). The ratio of measurement results of TC to the calculated values at these 8 stations were 0.9963 ± 0.0036 on average. The precision of on board measurement of TC is below 0.15% (Ono et al., 1998). Thus the variation in the ratio of 0.36% (or approximately ±7 µmol/kg) is thought to reveal the overall uncertainty of the calculated TC values. In the following discussions we use the calculated TC values only.
2.2 Measurement during the HK9605 Cruise Water samples were taken by CTD-RMS with 1.7L Niskin Bottles. Samples of 13 depths were taken from seasurface to 1000 m depth at each station. S, P, NO3, NO2 , Si, TA and total carbonate (TC) were measured at each station. Salinity samples were placed in 500 ml glass bottles, stored at room temperature and measured by an Autosal on land. Nutrient samples were placed in 10 ml polypropylene tubes, stored frozen and measured on land using a TRACCS800 autoanalyser. Samples of TA and TC were placed in 120 ml glass vial bottles (Maruemu Co., Ltd.). Approximately 0.02 g of solid HgCl2 was added to each sample and stored at room temperature. TA samples were measured on land within 1 month of sampling following the procedures as for WK9605. TC samples for HK9605 were measured by coulometry on land within 40 days of sampling following the procedure of Ono et al. (1998). To evaluate the stability of the TC samples during storage, additional TC samples were collected at several stations during the WK9605 Cruise besides those collected for on-board TC measurement. The results of on-and TC measurement 30 days after sampling agreed closely with the on-board measurements, with a difference of 3.1 ± 4.6 µmol/ kg (on-board minus on-land) on average. Thus we conclude that the precision of TC measurements during the HK9605 Cruise are about ±4.6 µmol/kg, and that the concentrate changes in TC samples during storage (3.1 µmol/kg on an average) was negligible within the measurement precision. The preservation procedure of the TC samples described above has also been used elsewhere: Shitashima et al. (1996) preserved TC samples collected in the Philippine Sea for 4 months using a similar preservation procedure. The water samples collected during winter in the K/O zone have been preserved without significant change for over 100 days by the same method (Ono, unpublished data). 3. Results and Discussion 3.1 Distribution of chemical properties along the 147°E section Figures 2a–2d shows the distribution of S, P, TC and σθ, respectively, along the 147°E section (section codes listed in Table 1). In the salinity section (Fig. 2a), a distinct, large low-salinity patch was found between latitudes 35.5°N– 37.8°N with a core depth between 200–300 m. Under the Kuroshio Extension (represented by the S = 34.80 isobar just south of 33.5°N) and just north of it, many small low salinity patches were found, with major low-salinity peaks located at 34°N (150 m), 34°N (250 m), 33.66°N (400 m), 33.33°N (520 m) and 33°N (640 m). Some minor low salinity peaks also occurred at 35°N (200 m) and 35°N (300 m). These patch-like structures also occurred in both cross
sections of P and TC (Figs. 2b and 2c, respectively). The detailed location of each patch, however, seems to differ somewhat from the salinity profile. In the P section (Fig. 2b) the isobars show no clear patch between the latitudes 35.5°N– 37.8°N and above 300 m depth. The signal also seems weak at 37.5°N in the TC section (Fig. 2c). Instead another patchlike structure is found at 39°N (500 m) in both P and TC cross sections. Signals of patches at the circumference of the Kuroshio Extension were also weak in Figs. 2b and 2c. No patches was found just under the Kuroshio Extension except for small signals located at 33°N (500 m) and 33.33°N (520 m). At the north side of the stream there seems to be some patch-like structures similar to the structures shown in Fig. 2a. However, the center of these patches shown in Figs. 2b and 2c are found at around 35°N rather than 34°N, the center of the salinity minima. This disagreement in the distribution pattern between the salinity and the chemical properties is also clear in the vertical profiles at each station. At 34.67°N, 146.75°E (Fig. 3a), the AOU peak with a density of σθ = 26.80 seems very small, despite the corresponding salinity peak being the largest of all the peaks observed at this station. The other peak, with a density of σθ = 25.79, seems remarkable in the AOU profile despite the fact that the salinity is rather faint. The AOU peak with a density of σθ = 27.08 is even positive, in contrast to other regional AOU peaks. At 33.53°N, 146.75°E (Fig. 3b), a large low salinity peak is observed with its center located at σθ = 26.57 in density. A corresponding peak is also observed in the AOU profile. However its center is located at σθ = 26.42, which is significantly lighter in density than that of salinity. 3.2 Chemical characteristics of the low salinity patches in the K/O zone To evaluate the chemical characteristics of the low salinity patches quantitatively, the Oyashio content of each patch was calculated using both the salinity and phosphate concentration as in the following equation; Oy(x) = (Xm – Xk )/(Xo – Xk)
(1)
where Oy(x) is the Oyashio content in each water patch calculated from the property x, Xm the observed value of x at the core of the patch, Xk and Xo the values of x in each end member of Kuroshio and Oyashio water at the density surface of the patch core, which ranged from σθ = 26.41 to σθ = 27.06, respectively. The x we show here is either salinity (s) or phosphate (p). (If we use nitrate, TC and TA instead of phosphate, the resulting Oy(x) shows similar distributions as Oy(p).) Xk was calculated in each density from the vertical profile observed at WK9605 Station 1 (34.86°N, 140.83°E), the closest station to the Kuroshio axis off Japan. Xo was calculated by averaging the profiles of the Chemical Alternation in K/O Zone
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Fig. 2. Distributions of a) salinity (psu), b) phosphate (µmol/kg), c) total carbonate (µmol/kg) and d) potential density (σθ) along the 147°E section.
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Fig. 2. (continued).
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Fig. 3. Examples of vertical profiles of AOU (circles) and salinity (solid line) at a) WK9605 Stn. 17 (34.67°N, 146.75°E) and b) WK9605 Stn. 19 (35.33°N, 146.75°E), respectively. The x-axis is the potential density (σθ), and the y-axis psu (left side) and in µmol/kg, respectively.
HK9605 stations in which the 100 m water temperature was below 5°C. In calculating the Oy(p) we assumed that the water mixing occurred mainly isopycnally in the K/O zone, and that the life times of the low salinity water patches in the K/O zone were short enough that the change in phosphate concentration by biological processes in the water mass was negligible. We may be able to extend the above assumptions (and thus Eq. (1)) further for the wide range of watermasses 686
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in the K/O zone, but in this section we would like to focus our interest on the chemical transport processes by the low salinity patches, the characteristic transport process of Oyashio component water through K/O zone. The overall transport processes of chemical properties in the K/O zone will be discussed in the following sections. Figure 4 shows the correlation of Oy(p) and Oy(s) at 28 patches with the density of 26.41 ≤ σθ ≤ 27.06 observed along
Fig. 4. Scatter plot of Oy(p) against Oy(s) (for definition of the terms see text). Solid line indicates the regression line. The line Oy(p) = Oy(s) is also shown as a dashed line.
the 147°E section and B section. The standard deviation of Oy(p) around the regression line with Oy(s) is 0.47. This means that if we assume a low salinity water patch with its Oy(s) of 0.5, this water mass can have any value of Oy(p) from 0 to 1. This implies that salinity gives no indication about the relative contribution of each patch in regard to the transport of phosphate (or other chemical properties) from subpolar (Oyashio) to subtropical gyre water (Kuroshio) carried by it. The regression line itself, on the other hand, is Oy(p) = 0.99 (±0.24)·Oy(s) + 0.38 (±0.12) (r2 = 0.41, n = 27). The bulk amount of the phosphate transport, integrated across many water patches, thus seems to be proportional to that of salinity, although the uncertainty in the regression slope is rather large. As to the cause of this low correlation between Oy(p) and Oy(s) as shown in Fig. 4, we can consider following two possibilities: (1) Phosphate concentration changed after the formation of patches by biodegradation within the water mass. (2) There are some end members of the Oyashio water (with variable phosphate concentrations but the same salinity) in a density layer where the water patches are formed. For possibility (1) we must conclude that this effect is not the main source of the variation because almost half of the data in Fig. 4 are plotted above the Oy(p) = Oy(s) line. Even if the phosphate concentration changes significantly by biological processes during the transport of water patches, such underwater biological activity (i.e., degradation of organic matter) cannot produce water with Oy(p) smaller than Oy(s). For possibility (2), it seems possible that the phosphate concentration varies significantly in hydrologically the same
water mass because of the strong biological activity in the spring K/O zone. The sea surface fCO2 distribution in the spring K/O zone, for example, clearly shows a large variability with a finer horizontal scale than that of the salinity variation (Nojiri et al., 1997). On the other hand Yasuda et al. (1997) showed that the low-salinity water patches in this area have two origins; i.e., patches having relatively low potential vorticity (PV) originating from the Okhotsk Sea water and those with relatively high PV originating from open Western Subarctic Gyre (WSAG) water. In the 147°E section the patches observed at around 34°N, which are clearly observable in the salinity section (Fig. 2a) but less clear in the P/TC sections (Figs. 2b and 2c), are found to have a high density gradient (Fig. 2d) suggesting that they are WSAG-originated high-PV patches. On the other hand those observed at around 35°N, which are clear in the P/TC sections than in the salinity section, seem to have a low density gradient indicating that they are Okhotsk-originated ones. Similarly the patch observed at 37.5°N (Fig. 2a) seems to have high-PV and that observed at 38.5°N (Figs. 2b and 2c) seems to have low-PV. This implies that the Okhotskoriented low-PV patches are relatively more effective than WSAG-originated ones when considering the transport of bio-reactive (or atmosphere-oriented) chemical properties through the K/O zone carried by low-salinity water patches. In this section we assume that the diapycnal mixing is negligible compared to the isopycnal mixing in the K/O zone. If the diapycnal mixing between a patch water and its circumferences is significant, however, this may give an additional source of variation of the data shown in Fig. 4. To estimate the extent of this process more information is Chemical Alternation in K/O Zone
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Fig. 5. Vertical profiles of a) density, b) salinity, c) phosphate and d) NTC in four Kuroshio extension axes from off the Boso Peninsula, Japan to 152°E.
needed about the hydrography of the upper layers in the K/ O zone. 3.3 Change in the vertical profiles of chemical properties along Kuroshio Extension Bulk transport of chemical substances appears primarily in the changes in vertical profiles of chemical properties 688
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at the axis of Kuroshio Extension. To investigate this the averaged vertical profiles of density, S, P and TC at the axis of the Kuroshio Extension observed in each section (147°E, 149°E, 152°E, and the Japan side axis of the B line, the axis in B line referred to as the “off Boso” axis) were calculated. Stations in which the water temperature was within 15– 10°C at 200 m depth were selected for the calculation in each
Fig. 5. (continued).
section. In this calculation the TC values were normalized to 35 psu with salinity (NTC). In the 149°E section chemical sampling was carried out only at one station, so we use this as the profile of Kuroshio Extension axis in this section. The 200 m temperature at this station was 12.5°C and thus within the above criteria.
The results are shown in Figs. 5a–5d. The y-axis in each profile is shown as the potential density (σθ), with the exception of the averaged density profile (Fig. 5a). The typical profile of each chemical characteristic in the Oyashio water is also shown in Figs. 5b–5d. These Oyashio profiles are derived from the average values of the stations of the A line Chemical Alternation in K/O Zone
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(Table 1) with 100 m water temperature less than 5°C. In the axis east of 149°E each density surface with σθ smaller than 26.60 seems to lie at depths shallower than the off Boso axis (Fig. 5a). This indicates the cooling of surface water within the Kuroshio Extension and/or K/O zone on the way of the water flow. The overall succession pattern of the salinity profiles from off Boso to 152°E is similar to that described in Yasuda et al. (1996). The profile with the salinity minimum of 34.2 psu in the σθ = 26.90–27.00 density range (Type 1 water in Yasuda et al., 1996) in the off Boso axis changes to a profile with a minimum of 34.0 psu at around σθ = 26.70 density (Type 3 water in Yasuda et al., 1996) in the axis east of 147°E. The salinity of the water with a σθ = 26.70–26.80 density range in the axis east of 147°E approximates the value when the off-Boso axis water and typical Oyashio water mix isopycnally with a 1:1 ratio. Note that each profile in Fig. 5b is of averaged data of several profiles for each section. Some stations have salinity profiles containing several regional salinity minima (Type 4 water in Yasuda et al., 1996) and consequently averaged profiles in Fig. 5b occasionally show small regional fluctuations. In the density surfaces of 26.80 < σθ < 27.20 phosphate concentration is higher in the Oyashio profile (Fig. 5c) than in the Kuroshio profile because waters are older in the subpolar region than in the subtropical region in these density surfaces (e.g., Tsunogai et al., 1995). In Levitus and Boyer’s (1994) data set the phosphate concentration in the Oyashio region is lower than that in the Kuroshio region even in the density surfaces of 26.80 < σθ < 27.20, but this may be due to the difference in the sampling season in their data sources. The profiles east of 147°E lie roughly midway between the Oyashio and the Kuroshio profiles, which is the same feature as the salinity profiles. The phosphate concentration on the density surfaces of σθ < 26.40, on the other hand, tend to have smaller values than the off-Boso line in the sections east of 147°E. (Fig. 5c). NTC profiles also show a similar pattern to that of phosphate (Fig. 5d). The smallest concentrations of phosphate and/or TC in the Oyashio profile exceed those of the waters with the density of σθ < 26.40 in the Boso line profile (Figs. 5c and 5d). The decline of phosphate and TC concentration in each σθ < 26.40 density surface east of the Off-Boso line thus cannot be explained by the mixing of Oyashio component water. Some mixed layer processes such as biological consumption of phosphate/TC are likely the course of these non-conservative concentration changes. 3.4 Non-conservative concentration changes in the chemical properties of the Kuroshio extension water through the K/O region Yasuda et al. (1996) suggested that the water at around σθ = 26.80 density in the Kuroshio Extension axis east of Japan is formed by isopycnal mixing between the Oyashio
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water and the off-Boso Kuroshio axis water. If we assume that such isopycnal mixing processes occurs throughout the density surfaces, and that there are no non-conservative processes in the K/O zone, the concentration of the chemical property x in the Kuroshio extension axis east of 147°E CC(x) can be calculated by the following equation: CC(x) = (1 – m)CK(x) +mCO(x) (m = (SK – Sm )/(SK – SO))
(2)
where CK(x), CO(x) and SK, SO the concentration of property x and salinity, respectively, in the corresponding density surface at the off Boso and the typical Oyashio axes, respectively. Sm the salinity of the corresponding water mass, and m the content of the newly added Oyashio water. CK(x) and SK were calculated in each density from the vertical profile observed at WK9605 Station 1. CO(x) and SO were calculated by averaging the profiles of the HK9605 stations in which the 100 m water temperature was lower than 5°C. In the density surfaces of σθ < 26.40 we can not use Eq. (2) because typical Oyashio profiles occur only in the 26.40 ≤ σθ density range (Figs. 5b–5d). We thus assume that there is no water mixing process between Kuroshio and Oyashio components in these density surfaces; i.e., CC(x) = CK(x).
(3)
Equation (3) well explains the salinity profiles of the Kuroshio extension waters in the density range of σθ ≤ 26.00 but not so well in the σθ = 26.20 density surface (Fig. 5b). We thus have not calculated CC(x) for this density. Finally, the difference of actual concentration of x, Cm (x), from the CC(x) are then calculated; i.e., dCX = Cm(x) – CC(x).
(4)
Stations in which the water temperature was within 15– 10°C at 200 m depth were selected for the calculation of Cm (x) in each section. If our assumption about the water mixing is consistent, this reveals the amount of non-conservative concentration change of properties at each density surface within the K/O zone. Tables 2a and 2b shows the calculated dCX of various chemical properties for both 149°E and 152°E axes, respectively. In the 149°E axis, dCP and dCTC becomes nearly zero within the observation error in the density surfaces of σθ ≥ 27.00 and σθ ≥ 26.90, respectively. In the 152°E axis again dCP and dCTC are nearly zero in these density ranges. This suggests that our assumption about water mixing is appropriate and that these properties are nearly conservative in the K/O zone in these deep layers. This feature is also consistent with the fact that the integrated transport of phosphate by low salinity patches is proportional to that of salinity (Fig. 4).
Table 2. Non-conservative concentration changes of chemical properties along the Kuroshio extension. a) off Boso—149°E Density
Sal (psu)
AOU
P
N
Si
TA
NTA
TC
NTC
NTC-6.6N
24.80 25.00 25.20 25.40 25.60 25.80 26.00 26.20 26.40 26.60 26.70 26.80 26.90 27.00 27.10 27.20 27.30 27.40
–0.051 –0.057 –0.013 0.011 –0.013 –0.021 –0.025
–34 –38 –33 –35 –38 –37 –34
–0.15 –0.16 –0.14 –0.16 –0.16 –0.15 –0.17
–3.01 –4.10 –3.63 –3.68 –3.54 –3.05 –2.76
1.0 0.8 0.7 –0.2 –1.0 –1.2 –4.7
3 3 0 0 4 7 –1
8 10 5 2 7 9 –1
–25 –26 –20 –22 –32 –31 –30
–20 –20 –15 –20 –30 –29 –31
0 7 9 4 –6 –9 –12
–0.21 0.03 –0.03 –0.07 –0.09 –0.02 0.01 0.04 0.01 0.04
–4.05 –2.05 –3.54 –1.95 1.43 0.19 1.11 1.11 0.48 0.77
–7.9 –1.3 –3.9 –5.2 4.9 –0.8 0.6 2.2 –2.5 –2.2
–9 4 –4 2 1 –2 –1 2 4 1
–3 2 7 3 3 1 0 4 4 1
–39 –3 –4 –7 3 1 3 7 0 2
–39 –10 –16 8 4 3 3 7 0 2
–12 3 8 21 –6 1 –5 0 –3 –3
b) off Boso—152°E Density 24.80 25.00 25.20 25.40 25.60 25.80 26.00 26.20 26.40 26.60 26.70 26.80 26.90 27.00 27.10 27.20 27.30 27.40
Sal (psu)
AOU
P
NO3 + NO 2
Si
TA
NTA
TC
NTC
NTC-6.6N
–0.004 0.049 0.054 0.010
–46 –59 –63 –79
–0.11 –0.18 –0.21 –0.30
–3.21 –4.19 –4.55 –5.43
2.7 0.8 –0.7 –6.1
11 14 13 7
11 11 10 6
–29 –37 –38 –43
–29 –40 –42 –44
–8 –12 –11 8
–0.48 0.08 0.02 –0.04 –0.08 0.00 0.04 0.09 0.10 0.06
–8.27 –1.89 –2.41 –1.74 0.73 0.18 1.02 1.10 1.29 –0.09
–15.1 1.4 –1.8 –2.6 2.9 –1.2 0.8 2.1 0.6 –2.8
–11 –4 0 3 1 –1 2 1 2 1
–10 –2 1 1 –2 –4 –2 –1 0 2
–61 –16 –7 –5 1 1 7 10 7 5
–62 –17 –8 –6 0 1 6 10 7 5
–7 –4 8 5 –4 0 0 2 –1 6
>27.00 av. ±
In the shallow layers, phosphate shows significant negative dC values in the density surfaces of σθ ≤ 26.40 in both 149°E and 152°E sections. Such significant negative dC values are also found in other properties in the shallow density surfaces (Tables 2a and 2b). The average values of
1 2.9
dC in the density ranges of σθ = 24.80–26.00 in the 149°E section are –36 µmol/kg for AOU, –0.16 µmol/kg for P, –3.4 µmol/kg for N and –26 µmol/kg for TC, respectively. There are some potential uncertainty in the dC values due to the uncertainties in the determination of end memChemical Alternation in K/O Zone
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bers, CK(x) and CO(x), and this must be considered in addition to the simple observation errors. For CO(x) there is a possibility that the Okhotsk-oriented low PV waters (Yasuda, 1997) is the significant source of Oyashio component water in the K/O zone in addition to the WSAG-oriented typical Oyashio water. In our dataset low PV waters are observed at HK9605 Stn. 2, the northernmost station of the A line. If we use the data of this station when calculating CO(x) instead of typical Oyashio profiles, the resulting dCP decreases by 0.1 µmol/kg in each density surface of 26.40 ≤ σθ < 26.90 in the both sections. In the remaining density surfaces, dCp vary by ±0.04 µmol/kg due to the change of CO(x). The variation of dCTC due to the same operation is within the range of ±3 µmol/kg for all sections and densities. It is difficult to evaluate the potential uncertainty due to the CK(x) calculation, because WK9605 Stn. 1 is the only station observed within the off Boso Kuroshio axis (Fig. 1). If we use WK9605 Stn. 2 (34.50°N, 141.49°E) instead Stn. 1 for CK(x) calculation dCp and dCTC increase by 0.1 µmol/ kg and 10 µmol/kg, respectively, in each density surface of σθ < 25.90 in the both sections. In the remaining density surfaces dCp and dCTC vary by ±0.04 µmol/kg and 5 µmol/ kg, respectively. Finally, to evaluate the uncertainty due to the Cm(x) calculation in Eq. (4), one more station in the 152°E line peripheral to the original ones are added to the Cm (x) data source. The resulting dCp and dCTC varied by ±0.04 µmol/ kg and ±5 µmol/kg, respectively, in each density surface. To sum up the above arguments, the potential uncertainty of dCp due to the selection of end members are ±0.04 µmol/kg in the density surfaces of 26.40 ≤ σθ and ±0.1 µmol/ kg in the density surfaces of σθ < 26.40, respectively. Similarly the potential uncertainty of dCTC due to the selection of end members are ±5 µmol/kg in the density surfaces of 25.90 ≤ σθ and ±10 µmol/kg in the density surfaces of σθ < 25.90, respectively. The uncertainty in the density surfaces of σθ < 25.90 might be overestimated, however, because the uncertainties in these densities originate mainly from the uncertainty of CK(x) and the extent of this is not properly estimated (see previous discussion). Isopycnal differences in the phoshate concentrations within the Kuroshio axis should be smaller than the difference between inside the Kuroshio axis and outside it. The decline of phosphate and TC in the density surfaces of σθ < 26.00 in the 149°E line (Table 2a) is thus concluded to have some significance. The potential uncertainty of other properties were also evaluated and significance of concentration changes in these light density surfaces determined. If a body of water flows through the Kuroshio extension directly, the residence time of this body in the K/ O zone will be only a few weeks. In the case that the water body is incorporated in warm core rings, however, this body remains in the K/O region over a few years until the ring declines (e.g., Yasuda et al., 1992). The observed large dC
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values in the shallow layers will thus be mainly sustained by the mixed layer processes occurring within such long-lived water bodies. Silicate in each axis also shows significant negative dC values in the σθ = 26.00–26.40 density layers. However in shallower density surfaces dCSi becomes almost zero, regardless of the properties of the other nutrients. This can be explained by the fact that the density surfaces of σθ ≤ 26.00 outcrops mainly in the area south of the subpolar front in the K/O region (e.g., Talley, 1988), where diatom bloom is not so strong as that in the subpolar gyre. In the density surfaces with the density of 26.40 ≤ σθ < 26.90, it is difficult to conclude whether each dC value is zero or not when considering the potential uncertainties discussed previously. The overall trend shows that the dCs of each property have extensive minus values in the density surfaces of around σθ = 26.40, and approach zero with increasing density. If these negative dC values are real, this means either that there are some non-conservative concentration changes (i.e., biological degradation) in these density surfaces or that the diapycnal mixing occurs among these density surfaces. If the negative dC values are mainly caused by the biological degradation, the corresponding dCN and dCTC shows a Redfield relationship; i.e., if we calculate following equation, dCred must be around zero. dCred = dCTC – 6.6·dCN.
(5)
The coefficient of dCN in Eq. 5 was derived from the C:N ratio given by Redfield et al. (1963). We did not calculate dCred based on the phosphate variation, because dCred varies too widely with a small fluctuation of dCP. The rightmost column in Tables 2a and 2b shows the calculated dCred for each axis. In the density surfaces of σθ ≥ 26.60 dCred seems to be constantly zero with a scatter of few µmol/kg, despite large potential uncertainty in both dCN and dCTC in these density surfaces. This implies that the main source of negative dC values is the biological degradation in the density surfaces, and diapycnal mixing is still not significant in these density surfaces. This also implies that our calculations of CO(x) are appropriate and actual errors of dC values due to the CO(x) error are rather small. If we calculate CO(x) based on KH9605 Stn. 2 data, the resulting dCred varies by ±16 µmol/kg in the density surfaces of 26.60 ≤ σθ < 26.90. In both axes a pulse of significant positive dCred values are observed in the density surfaces around σθ = 26.80. This seems to be because of the erroneously high dCN values compared to dCP and dCTC induced from the small difference in P, N and TC concentration between Kuroshio and Oyashio in these density surfaces, rather than the influence of an actual influx of TC. If the dCred is due to the diapycnal mixing the surrounding density surface must also have significant dCred value.
In the density surfaces of σθ < 26.60 the dCred shows some scatter with a trend of negative dCred values. This implies that the diapycnal mixing is also the source of dC values in these upper density surfaces. There is some uncertainty about the C:N ratio in the biological processes in Eq. (5). Many global-scale estimations (Takahashi et al., 1985; Anderson and Sarmiento, 1994) have found a C:N ratio similar to that of the original Redfield ratio of 6.6, but recent studies have shown that there may be large local variations in the surface ocean with a general trend towards a high C:N ratio (e.g., Sambrotto et al., 1993; Karl et al., 1996; Chen et al., 1997). This may also be the cause of negative dCred values in the near-surface layers, although the extent is difficult to estimate. 4. Conclusive Summary Throughout this study it has been exhibited that the concentration of chemical properties in the K/O region show more complex variations than that of salinity due to the processes in the source region such as biological processes and gas exchange. The low-PV low-salinity eddies seems to serve particular characters in the distributions of chemical properties in the K/O zone. The integrated transport of chemical properties into the Kuroshio Extension axis, on the other hand, is shown to be almost equal to that of salinity in the density surfaces with σθ greater than 26.90. The change in chemical properties in each density surfaces of 26.60 ≤ σθ < 26.90 along the Kuroshio Extension is explained only by isopycnal mixing of Kuroshio water with the Oyashio water and biological processes within the K/O zone. The influence of diapycnal mixing is estimated to be small in the density surfaces greater than σθ = 26.60. This implies that the amount of anthropogenic carbon injected within the K/O region is also negligible in these density layers, because the density surfaces of σθ ≥ 26.60 are not outcropped within the K/O zone and thus injection of anthropogenic carbon in these density surfaces must require diapycnal mixing. Many recent studies show that a significant amount of anthropogenic carbon is accumulated even in the density surface with σθ greater than 26.60 in the NPIW (e.g., Chen, 1993; Tsunogai et al., 1993, 1995; Tokieda et al., 1996; Ono et al., 1998). Because the K/O zone is thought to be the area of NPIW formation, observed small dCred values indicate that anthropogenic carbon in NPIW observed in previous studies is injected into the NPIW source water before the water is introduced into the K/O region. The Okhotsk Sea and/or the area around the Kuril Islands may be candidates for such gas injection areas (Yasuda, 1997). Strictly, there are some time lags between the source water’s inflow into the K/O region and its outflow through the Kuroshio extension. Since our results are based on synoptic surveys there may be some bias due to the lack of information of the time lag. Further surveys with a smaller time interval are thus essential to study this area, which will
be carried out in future studies such as the Subarctic Gyre Experiment (SAGE). Acknowledgements The observational data used in this study were taken during the collaborative study of Hokkaido University, Tohoku National Fisheries Research Institute and Hokkaido National Fisheries Research Institute. We wish to express our gratitude to all participants of WK9605 and HK9605 cruises, the officers and crew of R/V Wakataka-Maru and Hokko-Maru for their kind cooperation with the field work. We also thank all the co-workers of the above institutes for their valuable discussions and suggestions. References Anderson, L. A. and J. L. Sarmiento (1994): Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochem. Cycles, 8, 65–80. Chen, C.-T. A. (1993): Anthropogenic CO2 distribution in the North Pacific Ocean. J. Oceanogr., 49, 257–270. Chen, C.-T. A., C.-M. Lin, B.-T. Huang and L.-F. Chang (1997): Stoichiometry of carbon, hydrogen, nitrogen, sulfur and oxygen in the particulate matter of the western North Pacific marginal seas. Mar. Chem., 54, 179–190. Culberson, C., R. M. Pytkowitcz and J. E. Hawley (1970): Seawater alkalinity determination by the pH method. J. Mar. Res., 28, 15–21. Dickson, A. G. (1993): pH buffers for sea water media based on the total hydrogen ion concentration scale. Deep-Sea Res., 40, 107–118. Dickson, A. G. and F. J. Millero (1987): A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res., 34, 1733–1743. DOE (1994): Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; ver. 2, edited by A. G. Dickson and C. Goyet, ORNL/CDIAC-74. Hansson, I. (1973): A new set of constants for carbonic acid and boric acid in seawater. Deep-Sea Res., 20, 461–478. Karl, D. M., J. R. Christian, J. E. Dore, D. V. Hebel, R. M. Letelier, L. M. Tupas and C. D. Winn (1996): Seasonal and interannual variability in primary production and particle flux at Station ALOHA. Deep-Sea Res. 2, 43, 539–568. Levitus, S. and T. P. Boyer (1994): World Ocean Atlas 1994. NODC. NOAA Atlas NESDIS 4. Maksimenko, N. A., T. Yamagata and K. Okuda (1997): Frontal convection in Kuroshio and at the subarctic front. Oceanology, 37, 295–300. Nojiri, Y., J. Zeng, C. S. Wong and T. Kimoto (1997): Ship-ofOpportunity measurement of pCO2 in the Northern Pacific with complete seasonal coverage. p. 87–90. In Biogeochemical Processes in the North Pacific: Proceedings of the International Marine Science Symposium, ed. by S. Tsunogai, Japan Marine Science Foundation, Tokyo. Ono, T., S. Watanabe, K. Okuda and M. Fukasawa (1998): Distribution of total carbonate and related properties in the North Pacific along 30°N. J. Geophys. Res. (in press). Redfield, A. C., B. H. Ketchum and F. A. Richards (1963): The
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