Absorption of Atmospheric CO 2 and Its Transport to the Intermediate Layer in the Okhotsk Sea

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1 Journal of Oceanography, Vol. 59, pp. 709 to 717, 2003 Absorption of Atmospheric CO 2 and Its Transport to the Intermediate Layer in the Okhotsk Sea AKIHISA S. OTSUKI*, SHUICHI WATANABE and SHIZUO TSUNOGAI Graduate School of Environmental Earth Science, Hokkaido University, Sapporo , Japan (Received 12 July 2002; in revised form 18 February 2003; accepted 24 February 2003) In the southwestern Okhotsk Sea off Hokkaido we observed chemical components related to the carbonate system for 1 year from August 1997 to June Using the conservative components salinity and water temperature, we confirmed the existence of two water masses flowing into the intermediate layer of the Okhotsk Sea, the East Sakhalin Current Water (ESCW) which becomes denser by mixing of brine water, and the Forerunner of Soya Warm Current Water (FSWW) which becomes denser due to cooling of the saline Kuroshio water. The NTC x values were calculated by comparing the ESCW and the FSWW with the Pacific Deep Water (PDW). The NTC x values obtained are µmol/kg and µmol/kg for the ESCW and the FSWW off Hokkaido, respectively, which are considerably larger than that of the Kuroshio water. These large NTC x values may be due to both low DIC concentration in the surface water and intense gas exchange under the cold and stormy winter conditions for the ESCW and the cooling of the FSWW as it flows northward. Since the flow rates of dense waters concerned with the ESCW and the FSWW have previously been estimated as 0.9 Sv and 0.2 Sv, respectively, the amount of atmospheric CO 2 absorbed and transported to the intermediate layer turns out to be gc/yr. This flux is small on a global scale, but the flux divided by the surface layer of the Okhotsk Sea is 30 gc/m 2 /yr, which is 5 times greater than the mean absorption flux of anthropogenic CO 2 in the world s oceans. It is thus considered that atmospheric CO 2 is efficiently absorbed in the Okhotsk Sea. Keywords: Anthropogenic CO 2, Okhotsk Sea, North Pacific Intermediate Water, dense water formation, East Sakhalin Current Water, Forerunner of Soya Warm Current Water. 1. Introduction The ocean is the most important sink and reservoir of the anthropogenic CO 2, which has increased after the Industrial Revolution, in both the oceans and the atmosphere. In the Pacific Ocean, the increase in dissolved inorganic carbon (DIC) is large in the subtropical surface and intermediate waters (Chen, 1993) including the North Pacific Intermediate Water (Tsunogai et al., 1993) and the Antarctic Intermediate Water (McNeil et al., 2001). It is thus important for the global carbon cycle to investigate the processes by which these intermediate water masses and their carbonate systems are formed. The North Pacific Intermediate Water (NPIW), which is characterized by a salinity minimum layer in the subtropical North * Corresponding author. otsuki@ees.hokudai.ac.jp Now at Japan Marine Science and Technology Center, Yokosuka , Japan. Copyright The Oceanographic Society of Japan. Pacific (Talley, 1991), is formed by the mixing of various water masses, but the isopycnal layer of its density (26.8σ θ ) outcrops to the sea surface in the western Okhotsk Sea in winter. One of the processes by which water of density greater than 26.8σ θ is formed due to the mixing of brine water rejected by sea ice formation in the northwestern region (around the Shantarskiye Islands) and the east coast of Sakhalin Island (Kitani, 1973; Martin et al., 1998; Gladyshev et al., 2000). In these regions in winter, the atmospheric CO 2 may be actively absorbed, because the cooling lowers the fco 2 of surface water and the rough sea surface enhances the gas exchange at the air-sea interface. Furthermore, the saline subtropical surface water (called the Soya Warm Current Water: SWCW) flows from the Japan Sea through the Soya (La Perouse) Strait into the southwestern area of the Okhotsk Sea (Watanabe and Wakatsuchi, 1998). The SWCW becomes denser than 26.8σ θ in early spring due to wintertime cooling. The cooled SWCW is called the Forerunner of Soya Warm 709

2 Table 1. Cruise period and vessel names. Period Aug. 1 4, 1997 Sept. 29 Oct. 2, 1997 Dec. 9 11, 1997 Feb. 4 10, 1998 Apr , 1998 June 8 11, 1998 Vessel Soya Water (FSWW). It sinks gradually from the coastal region to the intermediate layer ( m depth) in the Kuril Basin (Takizawa, 1982). These two intermediate water formation processes are enhanced by the cooling due to the cold wind from Siberia. Although they occur independently and at a distance from each other, the surface water in the northwestern Okhotsk Sea flows into the southwestern part together with the sea ice due to a strong northerly wind. This advected northwestern water is called the East Sakhalin Current Water (ESCW) (Watanabe, 1963). It is considered that the ESCW gives information on the water properties in the sea ice formation region. The ESCW, characterized by low salinity (<32.0), is observed in the southwestern Okhotsk Sea during the period from November to February (Itoh and Ohshima, 2000). According to Mochizuki et al. (1995), the flow rate of the sea ice is about 35 cm/s in the western Okhotsk Sea. On the other hand, the subsurface ( m depths) current speed of the ESCW was m/s according to an estimate from a drifter experiment conducted in autumn (Ohshima et al., 2002). If the surface water also moves at the same speed as the sea ice, it appears that the ESCW observed in the southwestern part in February existed one or two months previously in the sea ice formation region. We have measured the properties of the FSWW and the ESCW, such as dissolved inorganic carbon and nutrient concentration, and have investigated the carbonate chemistry in the area of the Okhotsk Sea off Hokkaido. Based on the distributions found, the amount of CO 2 absorbed from the atmosphere and transported to the pelagic intermediate layer has been estimated, taking account of the previously reported physical oceanographic information. Fig. 1. Maps of the entire Okhotsk Sea (a) and its southwestern region (b). Sampling stations, the rough bathymetry and major currents are also shown in these figures. ESC, SWC and TWC represent respectively the East Sakhalin Current, the Soya Warm Current and the Tsushima Warm Current in (a). 2. Methods of Observation and Chemical Analysis Seawater samples and hydrographic data were obtained during periodic hydrographic survey cruises conducted by the vessels of the Hokkaido-Wakkanai Fisheries Experimental Station (R/V ) and the Hydrographic and Oceanographic Department of the Japan Coast Guard (icebreaker-type patrol vessel Soya) in the southwestern part of the Okhotsk Sea (Table 1). Observations were carried out every 2 months from August 1997 to June 1998 (Fig. 1). Seawater samples were collected at various depths from the surface to 500 m deep at several stations, shown in Fig. 1. However, several sampling stations on line O2 moved slightly in Feb owing to heavy sea ice conditions and time restrictions. 710 A. S. Otsuki et al.

3 Fig. 2. Comparison of vertical sections of a) temperature, b) salinity, c) density, d) n-pa, e) n-dic and f) n-po 4 at O2-line between February and April Absorption of Atmospheric CO 2 and Its Transport to the Intermediate Layer in the Okhotsk Sea 711

4 Fig. 3. Vertical profile changes of a) temperature, b) salinity, c) density, d) n-pa, e) n-dic and f) n-po 4 at the off shore point (St. O4-6) from April to June We have measured the dissolved inorganic carbon concentration (DIC), total alkalinity (TA) and nutrient (NO 3, NO 2, PO 4 and SiO 2 ) concentrations. TA was measured on board. DIC and nutrients samples were carried back to our laboratory and analyzed there. The DIC and TA values were measured by coulometry and the one-point titration method, respectively (Tsunogai et al., 1993). The DIC and TA values were calibrated against reference materials (these running standards were referred to the Certified Reference Material kindly provided by Dr. A. G. Dickson), and their precisions were ±2 µmol/kg and ±2 µeq/kg, respectively. The nutrients were determined by an auto-analyzing system (Bran+Luebbe AACS-II) according to the JGOFS method (JGOFS, 1994). The precision of PO 4 determination was ±0.010 µmol/kg. 3. Results Figure 2 shows vertical sections of temperature, salinity, density (σ θ ), normalized potential alkalinity (n-pa), normalized dissolved inorganic carbon concentration (n- DIC) and normalized phosphate concentration (n-po 4 ) at the O2-line observation in February and April The normalization to a salinity value eliminates the effect of dilution due to precipitation and river water as well as that of evaporation (e.g. n-dic = [DIC]/Sal. 35.0). The potential alkalinity is the sum of total alkalinity and [NO 3 ] + [NO 2 ] + [PO 4 ], which removes the effect of the alkalinity change due to the formation and decomposition of organic matter containing nitrogen and phosphate. In February 1998, the sea ice covered the surface of a region between Sta. O2-3 and Sta. O2-5, and it disappeared after two months, in April On the continental shelf region along the O2-line the temperature and salinity increased remarkably with time from February to April (Figs. 2(a) and (b)). In winter, the low temperature and less saline ESCW predominated due to the prevailing northerly wind in the southwestern Okhotsk Sea (Itoh and Ohshima, 2000; Ohshima et al., 2001). In spring, the northerly wind was weakened and the warmer, saline SWCW began to inflow to occupy the coastal area of Hokkaido. This alternation of the water masses is also confirmed in the climatological data set edited by Itoh and Ohshima (2000). The isopycnal surface shallowed and the density (σ θ ) reached 27.0σ θ (Fig. 2(c)) at the bottom of the shelf in April. This slightly warmer, more saline, denser water compared to the subarctic water masses is the FSWW. The n-pa, n-dic and n-po 4 values of the FSWW were lower than those of the ESCW (Figs. 2(d) (f)). The FSWW retained the characteristics of the subtropical water of low alkalinity, DIC and nutrients. The difference in the n-pa values between the ESCW and the FSWW was µmol/kg. In the Okhotsk Sea, because of the low activity of CaCO 3 shell production by corals 712 A. S. Otsuki et al.

5 Fig. 4. Seasonal variation of vertical profile of a) temperature, b) salinity, c) density, d) n-pa, e) n-dic and f) n-po 4 at St. O2-4 (150 m depth) from August 1997 to June and coccolithophorids, the n-pa may not changed seriously. Consequently, the n-pa value is available as a tracer to distinguish the ESCW from the FSWW in the Okhotsk Sea. At 500 m depth corresponding to the σ θ of Sta. O4-6 (the depth of which is 2750 m) in the southwestern Kuril Basin, the temperature and salinity increased and the n-pa decreased slightly during the period from April to June 1998 (Fig. 3). These changes indicate that the denser FSWW advected and penetrated to the intermediate layer of the southwestern Kuril Basin from the surface layer of the continental shelf zone of Hokkaido. The n-pa value (2390 µeq/kg) of 500 m depth at Sta. O4-6 in June 1998 was a mean value between those of the FSWW and the sub-arctic intermediate water (n- PA = 2415 µeq/kg). The n-pa value indicates that the FSWW was mixed with nearly the same amount of subarctic water by isopycnal mixing when it descended to the intermediate layer of the Kuril Basin. This result agrees with Takizawa (1982), who analyzed the relationship between salinity and dissolved oxygen concentration used as a tracer of the FSWW. The water properties at Sta. O2-4 changed vertically and seasonally during from August 1997 to June 1998, as shown in Fig. 4. The water mass exchanges that occurred were quite drastic, as seen in all these properties. The less saline, high n-dic and high nutrient ESCW, namely, dominated in the surface in winter, and the saline, dense, low n-dic and low nutrient FSWW was predominant in the spring-summer bottom layer. On the other hand, n- DIC and n-po 4 in the surface water ranged µmol/kg and µmol/kg, respectively. These large variations in the properties of the surface water were caused by the supply of DIC and nutrients due to the vertical convection in winter and by their consumption, underlying the large primary production in summer. An interesting question is how much so-called anthropogenic CO 2 is contained in these water masses? It is not easy to estimate this, since we have to derive the distribution of CO 2 in the pre-industrial era using many assumptions, even if the water mass has not changed. In this study we have refused to make such assumptions, simply calculating the NTC x values. The NTC x is a relative amount of carbonate change due to gas exchange at the surface. We have chosen the Pacific Deep Water (PDW) as the reference water mass to estimate NTC x values. Because the PDW is the oldest water in the global deep water circulation, it is still not contaminated by anthropogenic CO 2 (Tsunogai, 2002). Furthermore, the PDW is upwelled and mixed gradually with the surface water in the northwestern North Pacific due to the cyclonic circulation of the Subarctic Gyre; it then spreads Absorption of Atmospheric CO 2 and Its Transport to the Intermediate Layer in the Okhotsk Sea 713

6 Fig. 5. Seasonal variation of vertical profile of estimated NTC x at St. O2-4 (150 m depth) from August 1997 to June Light and dark shaded areas represent the part of the ESCW and FSWW, respectively. widely to marginal seas and the subtropical region of the North Pacific. For these reasons, we considered the PDW to be a suitable reference water mass. The NTC x values were estimated by the following equations. NTC x = [(n-dic obs R C/P n-po 4-obs ) A] + [B (n-ta obs + R N/P n-po 4-obs )]/2 A = 2,050 µmol/kg = n-dic PDW R C/P n-po 4-PDW B = 2,490 µeq/kg = n-ta PDW + R N/P n-po 4-PDW. These n-dic obs, n-ta obs and n-po 4-obs are observed values of each water sample. The R C/P (=117) and R N/P (=15) represent Redfield ratios reported by Anderson and Sarmiento (1994). Furthermore, A and B are potential n- DIC and n-ta values of the PDW, respectively. The annual variation in the NTC x at Sta. O2-4 is shown in Fig. 5. The water column always contained a large amount of NTC x, especially in the ESCW and FSWW, containing about µmol/kg NTC x. Although these NTC x values do not accurately represent the amount of anthropogenic CO 2, we believe that the ESCW and FSWW do contain excess n-dic compared to the PDW. These waters will be transported to the intermediate layer of the Okhotsk Sea thanks to the two processes discussed in the earlier part of this paper. 4. Discussion As it has been shown, water masses sinking and forming the intermediate water in the western Okhotsk Sea transport a considerable amount of atmospheric CO 2. First, we consider the reason why the ESCW and the FSWW can absorb such a large amount of atmospheric CO 2. Secondly, we estimate the amounts of atmospheric CO 2 absorbed and transported to the intermediate layer of the Okhotsk Sea. Fig. 6. Schematic water mass distributions in summer (a) and winter (b), and winter dense water formation due to brine rejection from the sea ice at the eastern or northern coastal area of Sakhalin. 4.1 The ESCW system absorbing atmospheric CO 2 As shown in Fig. 4, the advected ESCW in winter drastically lowers the salinity and density in the southwestern region of the Okhotsk Sea. It is considered that the accumulation of a less saline water mass occurs due to the Ekman convergence of the summer surface water driven by the winter northerly wind (Itoh and Ohshima, 2000; Ohshima et al., 2001). In summer, the surface layer of the Okhotsk Sea becomes less saline due to mixing of sea ice melt water, rain and river waters. As indicated by the profiles in June in Fig. 3, less saline summer surface water also contains relatively little DIC and nutrients. According to Sapozhnikov et al. (1998), the decline in salinity and nutrients in the surface water in summer are remarkable in the region off the Sakhalin Island where the ESCW has its origin. Nitrate especially is almost exhausted (<0.5 µm) due to active primary production. Consequently, the surface water will also be depleted in DIC (Fig. 6(a)). Generally the DIC and nutrients will be supplied from the subsurface layer to the surface due to the vertical mixing in winter. In the region off the eastern 714 A. S. Otsuki et al.

7 coast of Sakhalin, the supply of the DIC and nutrients from deeper layer will be hindered, because less saline summer surface water are accumulated and covers the surface due to the Ekman convergence, although CO 2 is still supplied from the atmosphere (Fig. 6(b)). The accumulated, less saline surface water is suitable for the absorption of atmospheric CO 2, because the surface water has a low fco 2 value due to the low DIC concentration and the low temperature; the strong wind in winter enhances the air-sea CO 2 exchange. According to the meteorological observational data, the air temperature drops to 30 C and the wind speed frequently exceeds 20 m/s in the eastern coast of Sakhalin Island (at Nogliki: 52 N, 143 E) in winter. Moreover, the surface fco 2 values of the ESCW at the O2-line in February 1998, calculated from observed TA and DIC values, were µatm. The winter ESCW thus has the capacity to absorb atmospheric CO 2. Because the ESCW surface fco 2 value was 70 µatm lower than the atmospheric fco 2, the flux of CO 2 from the atmosphere to the winter ESCW surface water was estimated as 20 mmol/m 2 /day using the gas exchange coefficient given by Wanninkhof (1992) and assuming that SST is 0 C and wind speed is 10 m/s. This CO 2 inversion process could raise the n-dic of the ESCW of 50 m thickness by 50 µmol/kg for 2 months. Moreover, the existence of coastal polynyas or thin ice areas is always observed in these regions on satellite images (Martin et al., 1998), and this will permit an airsea gas exchange. Thus, it is considered that the surface water off the eastern Sakhalin in winter, the ESCW, is enriched in atmospheric CO 2, having a large amount of NTC x. The eastern coast area of Sakhalin Island is one of sea ice formed region. The sinking of the surface water enriched in atmospheric CO 2 occurs due to the mixing of brine rejected from the sea ice (Fig. 6(b)). It is considered that the ESCW surface water will be able to absorb atmospheric CO 2 only in the coastal polynya region, because the existence of sea ice prevents the air-sea gas exchange. The NTC x of the ESCW (=100 µmol/kg: the light-gray part in Fig. 5) observed in the area off Hokkaido was almost all absorbed in the polynya region. Consequently, brine water rejected from the sea ice will also contain about 100 µmol/kg NTC x. Since the annual mean flux of dense water formed by brine mixing has been estimated at about 0.9 Sv (= m 3 /yr) (Wong et al., 1998), the amount of CO 2 increased relative to the PDW and transported to the intermediate layer by sea ice production in the region off the eastern coast of Sakhalin turns out to be gc/yr. Fig. 7. Schematic flow patterns of the Kuroshio and its branch currents. Broken lines represent wintertime (in February) mean sea surface temperature ( C). 4.2 The FSWW system absorbing atmospheric CO 2 The FSWW has its origin in the Tsushima Warm Current Water (TWCW), a branch of the Kuroshio, flowing northward in the East China Sea and then to the northeast of the Japan Sea (Fig. 7). Thus, the TWCW constitutes a part of the North Pacific Subtropical Gyre. We thus consider the difference in water properties between the Kuroshio surface water and the FSWW (Table 2) may give us a key showing the reason why the FSWW contains such large NTC x. The Tsushima Warm Current separates from a main stream of the Kuroshio at the northwestern end of the Okinawa Islands. As the TWCW flows into the higher latitudes, passing near the Asian continent, it is gradually cooled (Fig. 7). If the surface water near Okinawa Island in summer moves to the Soya Strait in winter, the water temperature will be lowered by 25 C at most. On the other hand, the n-dic increased by µmol/kg (Table 2). This increase in the n-dic is considered to be due to increase in the solubility of CO 2 in cooled seawater and the enhanced gas exchange rate during wintertime due to strong northerly winds. However, this increase in the n- DIC is not fully derived from the atmospheric CO 2 invasion, since the mixing of river water (e.g. Chang Jiang) and the Japan Sea deep water may contribute to the increase. We have also calculated the NTC x value for the Kuroshio surface water based on the data (Table 2) obtained by Tsunogai et al. (1997) and Tsunogai et al. (1999). The NTC x value we obtained is almost 0 µmol/kg in summer, and 40 µmol/kg in winter for the Kuroshio surface water near Okinawa Island. Since the FSWW (>26.8σ θ ) contains µmol/kg of NTC x in the southwestern Okhotsk Sea off Hokkaido (the darkgray part in Fig. 5), the increase in the n-dic of the Absorption of Atmospheric CO 2 and Its Transport to the Intermediate Layer in the Okhotsk Sea 715

8 Table 2. Comparison of water properties between Kuroshio surface water (northwestern region of Okinawa Island) and FSWW (at the bottom of Sta. O2-2 in Apr. 98). Season Temp. Sal. n-ta n-dic n-po 4 ( C) (µeq/kg) (µmol/kg) (µmol/kg) Okinawa winter summer FSWW early spring Difference max (FSWW Okinawa) min TWCW due to the absorption of atmospheric CO 2 is estimated to be µmol/kg from the subtropical area (near the Okinawa) to the Okhotsk Sea (O2-line). The annual mean flux of dense FSWW (>26.8σ θ ) has been estimated to be about 0.2 Sv (= m 3 /yr) (Watanabe and Wakatsuchi, 1998). Thus, the amount of excess CO 2 transported to the intermediate layer due to the sinking of the FSWW in the southwestern Okhotsk Sea off Hokkaido turns out to be gc/yr. Although this flux is lower than the former process, induced by the sea ice formation, the latter process is also worthwhile investigating, because it is a unique system in the North Pacific that the subtropical surface water is transported directly to the subarctic region, sinking to the intermediate layer of the Okhotsk Sea. 4.3 Estimation of the amount of atmospheric CO 2 absorbed and transported to the intermediate layer of the Okhotsk Sea Based on these observations of the carbonate system in the southwestern Okhotsk Sea for 1 year, fluxes of CO 2 absorbed from the atmosphere and transported to the intermediate layer are estimated to be gc/yr for the water sinking due to the mixing of brine rejected from sea ice formation, and about gc/yr for the water sinking due to cooling of the subtropical water. When we sum these two processes, the fluxes of CO 2 are gc/yr, which will not return to the atmosphere for several decades, or even one hundred years. Although this flux is less than 1% of the annual anthropogenic emission of CO 2 (= gc/yr), the flux divided by the area of the Okhotsk Sea is 30 gc/m 2 /yr (= gc/yr/ km 2 ), which is 5 times greater than the mean oceanic absorption rate of anthropogenic CO 2 (=6 gc/m 2 /yr = gc/yr/ km 2 ). This is an efficient process and it is thus important to study the transportation of CO 2 to the intermediate layers. We should pay attention to the uncertainty of the Redfield ratio and estimated water fluxes. In particular, it is known that the Redfield ratio fluctuates during the growth of specific phytoplankton assemblies. According to Arrigo et al. (1999), a wide drift of R C/P values was observed (from 93 to 147) in the summer Ross Sea. The drift of the R C/P in the Okhotsk Sea and the surrounding area is not clear, but the influence of the R C/P change on NTC x values of the FSWW and ESCW is small, because the nutrients concentrations in these waters are relatively lower than in other Subarctic water masses (Fig. 4(f)). Consequently, it is considered that estimated NTC x values in this study will not decrease drastically. These processes in the western Okhotsk Sea are local, but their efficiency in absorbing CO 2 seems to be large enough not to ignore. 5. Conclusion The FSWW (Forerunner of the Soya Warm Water) and the ESCW (East Sakhalin Current Water) contribute to intermediate water mass formation, containing about µmol/kg of excess n-dic referred to the PDW. This excess n-dic ( NTC x ) is believed to be atmospheric CO 2 absorbed by the solubility pump under cool, stormy conditions. The ESCW exists in the sea ice formation area, before it advects to the coastal area of Hokkaido. Because the gas exchange in the ESCW is prevented by the cover of sea ice during the advection from Sakhalin to Hokkaido, the brine water, rejected from the new sea ice, is considered to contain 100 µmol/kg of NTC x. Estimated annual fluxes of the dense FSWW and the brine water rejected from sea ice are m 3 /yr and m 3 /yr, respectively. Total fluxes of CO 2 absorbed from the atmosphere and transported to the intermediate layer in the Okhotsk Sea are estimated to be gc/yr. These processes of atmospheric CO 2 absorption and dense water formation in the western Okhotsk Sea and the northeastern Japan Sea are promoted by the winter cold weather, similar to condition in the Arctic or Antarctic. Global warming will cause the air temperature to rise. This influence will appear clearly in these polar seas located in the mid-latitudes. The rise of air tem- 716 A. S. Otsuki et al.

9 perature will decrease the production of dense water and increase surface water fco 2. Based on the relationship between the mean air temperature and the amount of the sea ice observed at Abashiri, Hokkaido, Aota (1999) predicted that the production rate of the sea ice in the Okhotsk Sea will decrease greatly due to global warming. This will affect the decrease in intermediate water formation. We shall need to pay continuous attention to the production rate of the intermediate water and the associated carbonate system in the western Okhotsk Sea. Acknowledgements We thank the captains and crews of R/V Hokuyo- Maru and P/V Soya for the observation and seawater sampling. We are grateful to T. Suzuuchi, T. Ohtsuki and J. Nakata (on the staff of Hokkaido-Wakkanai Fisheries Experimental Station) and Y. Nabae and K. Suehiro (staff of Hydrographic and Oceanographic Department of Japan Coast Guard) for their help with the field observations. Moreover we also thank M. Wakatsuchi, K. I. 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