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Artificial Upwelling of Deep Seawater Using the Perpetual Salt Fountain for Cultivation of Ocean Desert

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Artificial Upwelling of Deep Seawater Using the Perpetual Salt Fountain for Cultivation of Ocean Desert Journal of Oceanography, Vol. 60, pp. 563 to 568, 2004 Artificial Upwelling of Deep Seawater Using the Perpetual Salt Fountain for Cultivation of Ocean Desert 1 1 2 3 SHIGENAO MARUYAMA *, KOUTARO TSUBAKI , KEISUKE TAIRA and SEIGO SAKAI 1Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan 2Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan 3Graduate School of Engineering, Yokohama National University, 79-1 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan (Received 20 January 2003; in revised form 16 July 2003; accepted 17 July 2003) Deep seawater in the ocean contains a great deal of nutrients. Stommel et al. have Keywords: proposed the notion of a “perpetual salt fountain” (Stommel et al., 1956). They noted ⋅ Deep seawater, the possibility of a permanent upwelling of deep seawater with no additional external ⋅ upwelling, ⋅ energy source. If we can cause deep seawater to upwell extensively, we can achieve an perpetual salt ocean farm. We have succeeded in measuring the upwelling velocity by an experi- fountain, ⋅ natural convection, ment in the Mariana Trench area using a special measurement system. A 0.3 m diam- ⋅ heat transfer, eter, 280 m long soft pipe made of PVC sheet was used in the experiment. The mea- ⋅ enhancement of sured data, a verification experiment, and numerical simulation results, gave an esti- bio-production, mate of upwelling velocity of 212 m/day. ⋅ flow measurement. 1. Introduction salt fountain” (Stommel et al., 1956). In ocean deserts, The explosion of the earth’s population necessitates the salinity of surface seawater is higher than that of deep a greater food productivity. Primary food productivity seawater. As a result, seawater is stably stratified, and from the ocean is gradually increasing (Berger and Wefer, little convective motion occurs in the vertical direction. 1991), due to aquaculture, but number of captured fish When a pipe is inserted to connect deep seawater and such as salmon is declining because the number of salmon surface seawater, and the pipe is filled with the low-sa- flies released is reaching the limit of the ocean’s produc- linity deep seawater, the salinity of the water inside the tivity. Large areas in the middle ocean have very low pro- pipe is lower than that outside, and hence a buoyant force ductivity of the phytoplankton that forms the beginning occurs in the pipe. The upwelling continues as long as of the food chain (Nozaki, 1998). These areas have suffi- the differences of the temperature and salinity exist. Af- cient sunshine on the surface but little nutrient. The mid- ter Stommel’s proposal, Howard and Stommel tried to dle southern part of the North Pacific is classified as an measure the upwelling velocity using a 1000 m pipe in ocean desert. the ocean, but could not detect any upwelling due to low So-called deep seawater, meaning generally seawater velocity and strong wave motion (Huppert and Turner, deeper than 200 m below the surface (Takahashi and Iseki, 1981). 2000), where sunlight does not penetrate, is cooler and We have proposed “the Laputa project” (Maruyama contains much nutrient compared to surface seawater. To et al., 2001a) which has the potential to solve the global draw up deep seawater, several artificial methods such as food problem without destroying land forests. Figure 1 OTEC (Ocean Thermal Energy Conversion) (Otsuka et shows a diagram of the Laputa project. A number of float- al., 2000) have been proposed. But these methods are ing pipes are deployed with buoys in an ocean desert. expensive to deploy in large areas, and storms might de- The nutrient-rich deep seawater is upwelled by the per- stroy the mechanical structure on the sea during long pe- petual salt fountain to the region where the sunlight riods of operation. Furthermore, pumped-up, cold, deep reaches to cultivate phytoplankton. The drawn up deep seawater may sink down due to its higher density. seawater is heated during its flow up through the pipe Stommel et al. have proposed the idea of a “perpetual and then remains on the surface. The increase in phytoplankton will enhance the food chain, and eventu- ally fisheries and sea plant fields will be formed in the area. Without any mechanical structures except buoys on * Corresponding author. E-mail: [email protected] the ocean surface, the system can be operated for a long Copyright © The Oceanographic Society of Japan. time without maintenance. Since the pressure inside and 563 Sun Plankton Seaweed Buoy High Temp. Fish & Salinity Upwelling Heat 200~1000m Low Temp. Pipe & Salinity Nutrient Rich Deep Sea Water Fig. 1. Diagram of the Laputa project: draw up the nutrient- rich deep sea water to the photic region and cultivate phytoplankton using a large number of vertical floating pipes. An independent ecosystem of ocean farm is thus con- structed in an ocean desert. Fig. 3. Diagram of experimental apparatus. Pipe is 280 m long and 0.3 m diameter. Top of the pipe is maintained at 55 m depth, and the velocity measurement system is placed at the 15 m below top of the pipe. laminar natural convection method in a tube (Aihara et al., 1976, 1986) and obtained an average velocity of 10 to 30 m/day (Takahashi et al., 2002). We also conducted a laboratory experiment in a 0.4 m deep pool, inserting a plastic tube of 5 mm diameter. The velocity in the tube was approximately 2.2–3.5 mm/s, which is in good agree- ment with the numerical simulation (Maruyama et al., 2001b). Fig. 2. Temperature and salinity distribution measured outside 2. Measurement Method the pipe. Salinity around the bottom of the pipe is lower In order to evaluate the feasibility of the Laputa than most of the upper part. This density difference achieves Project, it is essential to measure the actual velocity of the upwelling. upwelling by the perpetual salt fountain in the ocean. The field experiment was carried out in the Mariana Trench area of the Pacific Ocean from August 2–5, 2002, using a outside of the pipe is balanced at each depth, lightweight, research vessel, Hakuho-maru of the University of To- flexible pipes made of plastic can be used. kyo (KH-02-2). The experimental point was located at The above mentioned massive upwelling of deep longitude 142°24′96″ E and latitude 11°25′91″ N in the seawater using floating pipes is called as the Laputa Mariana Trench. The temperature and salinity distribu- Project. Laputa was described by Jonathan Swift (1726) tions measured at the experiment point at noon of 4th in his “Gulliver’s Travels” as a floating island, and a re- August 2002 are shown in Fig. 2. At the same point, the lated animated movie was later released (1986) by H. temperature and salinity distributions were taken on 22nd Miyazaki. If the deployed pipes can upwell deep seawater June 2001, and both records shows similar distributions, in a large area, an ocean farm and consequently a green which shows that these distributions should be almost the floating island can be realized in an ocean desert. same every summer. Fluctuation of the distributions is We have estimated the average velocity by the expected to be small within a period of few days. 564 S. Maruyama et al. The distribution of the salinity near the surface in Fig. 2 is no higher than that at depth. It would seem diffi- cult to induce “the perpetual salt fountain” in this area. A salt fountain occurs due to the difference of integrated density value between the inside and outside of the pipe. At the pipe position shown in Fig. 2, the salinity outside the pipe is mostly higher than the salinity inside it, i.e. salinity at the bottom of the pipe, and the integrated den- sity value outside it is higher than that inside the pipe. Figure 3 shows a diagram of the experimental appa- ratus. A vertical pipe of 280 m length and 0.3 m diameter was deployed. The pipe was made of nylon-reinforced PVC sheet. The length was chosen to obtain a reasonably high velocity using our numerical simulations (Aihara et Fig. 4. Time variation of temperature and depth at the top and al., 1986) of the experimental conditions. Steel coil rings bottom of the pipe. Measurement was started after deploy- were attached at every 0.25 m to maintain a circular cross ment of the pipe. section. Spherical buoys were attached to decrease the tension of the ropes due to the pipe weight. The buoys and ropes were adjusted so that the pipe exit was approxi- a battery to operate the injection system are installed in- mately 55 m below sea level. side the buoy on the surface. Time variation of the tem- In order to measure the very low velocity, much lower perature and depth at top and bottom of the pipe are de- than that of the undulating motion of the pipe, a special picted in Fig. 4. According to the difference of the depth measurement system was constructed, as shown in Fig. between upper and bottom end, the maximum inclination 3. A tracer injector was sequentially operated, and the two of the pipe is approximately 30 degrees, due to the cur- sensors were placed above and below the injector, which rent in the ocean. This inclination reduces the buoyancy was located 15 m below the pipe exit. The measurement force by up to 15% compared with a vertical pipe. This system was placed at the center of the pipe. The tracer value is not negligible but is not essential to the upwelling.
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