Journal of Oceanography, Vol. 60, pp. 625 to 630, 2004
3He/4He ratios, mantle helium, mid-depth, Pacific water, Izu-OgasawaraRidge.
* Corresponding author. E-mail: email@example.com Present address: Centre de Recherches Petrographiques et
Geochimiques, Centre National de la Recherche Scientifique UPR 1167,Rue Notre-Dame des Pauvres, B.P. 20, 54501 Vandoeuvre les NancyCedex, France.
Copyright The Oceanographic Society of Japan.
Helium Isotopes in Pacific Waters from Adjacent Regionof Honshu, JapanYUJI SANO1*, TAKUMA KOSUGI2, NAOTO TAKAHATA1 and REIKA YOKOCHI2
1Center for Environmental Research, Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan2Department of Earth and Planetary Science, Hiroshima University, Kagamiyama, Higashi Hiroshima 739-8526, Japan
(Received 7 May 2002; in revised form 29 May 2003; accepted 5 June 2003)
We have measured helium isotopic ratios of thirty-seven Pacific water samples fromvarious depths collected in adjacent regions of Honshu, Japan. The 3He/4He ratiosvary significantly from 0.989 Ratm to 1.208 Ratm where Ratm is the atmospheric ratio of1.39 106. The mid-depth (7501500 m) profile of 3He/4He ratios at ST-1 locatedNorthwestern Pacific Ocean east of Japan (Off Joban; 3700 N, 14240 E) is signifi-cantly different from that at ST-2 of the Northern Philippine Sea south of Japan(Nankai Trough; 3307 N, 13959 E), suggesting that these waters were separated bya topographic barrier, the Izu-Ogasawara Ridge. Taking 3He/4He data of the Geosecsexpeditions in the western North Pacific, an extensive plume of 15% excess 3He rela-tive to the air may be traced at ST-1 over 12,000 kilometers to the northwest of theEast Pacific Rise where the mantle helium may originate. The 20% excess found atST-2 may be attributable to the additional source of the subduction-type mantle he-lium in the Okinawa Trough. A 15% excess of 3He has also been discovered at a depthof about 1000~1500 m at ST-3 adjacent to Miyakejima Island (3357 N, 13922 E)and ST-4 of Sagami Bay (3500 N, 13922 E). It is confirmed that mid-depth all overthe western North Pacific water is affected by the mantle helium with a high 3He/4Heratio.
and Tolstikhin, 1984; Sano and Wakita, 1985). The 3He/4He ratio is thus one of the most sensitive and conserva-tive tracers in chemical oceanography (e.g. Jenkins et al.,1972; Craig et al., 1975) because of the primordial sig-nature, rapid mobility and chemical inertness of the iso-topes.
Lupton and Craig (1981) discovered a striking in-tensity and lateral extent of excess 3He relative to air-saturated water in the deep Pacific Ocean at latitude 15Son the East Pacific Rise. The plume-shaped 3He anomalyoriginated from the volcanic activity of the ridge andspread westwards at the depth of the crest by abyssal cur-rents. This interpretation is still controversial, since thewestward advection implied by the 3He distribution iscontrary to the classic view of deep-water geostrophiccirculation which predicts a southeasterly flow through-out the eastern Pacific at 15S latitude (Stommel andArons, 1960). Sano et al. (1995) reported excess 3He inSouth Pacific water in the Solomon Sea and the central
1. IntroductionHelium has two stable isotopes of mass number 3
(3He) and 4 (4He). Helium-3 is often said to be derivedfrom the upper mantle where it was trapped at the time ofthe Earths accretion. On the other hand, most 4He is pro-duced by the radioactive decay of uranium and thoriumin the crustal rocks. The 3He/4He ratios of mantle-derivedsamples, such as mid-ocean ridge basalts and volcanicgases in island arcs, have a high value of about 1 105,while those of granitic rocks and natural gases are low,with a ratio of about 1 107 (Lupton, 1983; Mamyrin
626 Y. Sano et al.
Coral Sea Basin. In consideration of 3He/4He data of theGeosecs (stlund et al., 1987) and the South Tow (Luptonand Craig, 1981) expeditions, Sano et al. (1995) suggestedthat 25% excess 3He plume can be traced over 5000kilometers to the west of the East Pacific Rise.
Since the South Tow expedition (Lupton and Craig,1981), more than 1700 3He/4He measurements have beencarried out in the three main oceans. However, heliumisotope data in the waters neighboring the Japanese Is-lands are very sparse in the literature (stlund et al., 1987;Igarashi et al., 1987; Ishibashi et al., 1988; Sano et al.,1989) and there are no data from the coastal region. Inthis work, we study the 3He/4He ratios of Pacific waterfrom a region adjacent to Honshu, Japan in order to verifythat the northwestern end of 3He anomaly originated fromthe East Pacific Rise.
2. Experimental MethodSeawater sampling was conducted on the KT-01-12
cruise of the Research Vessel, Tansei Maru of the OceanResearch Institute, the University of Tokyo (August 39,2001) in the Pacific adjacent region of Honshu, Japan,viz., regions ST-1 (Northwestern Pacific Ocean east ofJapan; Off Joban), ST-2 (Northern Philippine Sea southof Japan; Nankai Trough), ST-3 (Adjacent MiyakejimaIsland), and ST-4 (Sagami Bay). The sampling points areshown in Fig. 1. Samples for helium isotope measure-ments were collected from 12 depths at ST-1 and ST-2and from 810 depths at ST-3 and ST-4 using a CTD car-ousel system equipped with 10-liter Niskin bottles. De-tails are listed in Table 1. Seawater was transferred with-out exposure to the atmosphere from the Niskin bottlesinto about 30 cm3 copper-tubing containers for storage(Sano et al., 1989).
At the laboratory the copper container was connectedto a stainless steel high vacuum line and dissolved gaseswere extracted from the sea water samples in vacuo. He-lium in the exsolved gases was purified using hot tita-nium-zirconium getters and charcoal traps held at liquidnitrogen temperature. The 3He/4He ratios were measuredwith a conventional noble-gas mass spectrometer (6-60-SGA, Nuclide Co.) which was moved from HiroshimaUniversity and re-installed at the Ocean Research Insti-
tute, The University of Tokyo (Sano et al., 1998). Ionbeams of 3He and 4He were measured by a double collec-tor system. A resolving power of ~600 at 1% of the peakheight was used for the complete separation of the 3He+beam from those of H3+ and HD+. After thorough tuningof the ion source and electromagnet, the sensitivity to 4Hewas enhanced about five times the value when it was in-stalled in Hiroshima. The observed 3He/4He ratios of sam-ples were calibrated against atmospheric helium collectedin August 2000 in Higashi Hiroshima. Helium was notseparated from neon in this work, which may introduce asmall error in the 3He/4He ratio of the terrestrial samples
Table 1. Location and bottom depth of sampling stations together with sampling depth of Pacific water.
Fig. 1. Sampling sites (ST-1, ST-2, ST-3 and ST-4) of Pacificwaters from adjacent region of Honshu, Japan. Approxi-mate surface currents (Oyashio and Kuroshio) in summerare shown.
Station Location Bottom depth (m) Sampling depth (m)
ST-1 3700 N, 14240 E 4087 5, 50, 100, 200, 300, 500, 750, 1000, 1500, 2000, 2500, 3000
ST-2 3307 N, 13759 E 4078 5, 50, 100, 200, 300, 500, 750, 1000, 1500, 2000, 2500, 3000
ST-3 3357 N, 13922 E 1025 5, 50, 100, 200, 300, 500, 800, 1000
ST-4 3500 N, 13920 E 1508 5, 50, 100, 200, 300, 500, 800, 1000, 1200, 1400
Helium Isotopes in Pacific Waters from Adjacent Region of Honshu, Japan 627
ratio is about 2.5%, estimated by repeated measurementsof standard air.
3. Results and Discussion
3.1 Helium isotopes in western North PacificObserved 3He/4He ratios are listed in Table 2 using
Ratm notation where Ratm is the atmospheric ratio of1.39 106 (Lupton, 1983). Temperature and salinity arealso listed in Table 2. Figure 2 shows salinity profiles ofall stations (ST-1, ST-2, ST-3 and ST-4). The salinity databetween 300 m and 750 m depth at ST-1 is apparentlylower than those of the other stations such as ST-2, eventhough there is no significant difference in the tempera-ture. The discrepancy at ST-1 may be attributable to theeffect of the Oyashio component with a low salinity, sinceit is located in the boundary region where the Oyashio isopposed by the Kuroshio (Mizuno and White, 1983). Theother stations are situated in the area of the Kuroshioproper and their salinities are well explained by theKuroshio.
The 3He/4He ratios of ST-1 and ST-2 range from1.000 Ratm to 1.208 Ratm. Generally, surface water is satu-rated with atmospheric noble gases since their exchangerate is rapid. It is well documented that 3He is slightlyless soluble (1.2 0.5%) in water than 4He (Weiss, 1971;Benson and Krause, 1980). Then surface water showsabout 1% lower 3He/4He ratio than the atmosphere(stlund et al., 1987; Sano et al., 1995). However, thepresent data are not distinguishable from the air value
Station Depth Temperature Salinity 3He/4He(m) (C) (Ratm)
ST-1 4.8 26.378 34.156 1.00249.4 17.985 34.462 1.00099.7 13.541 34.425 1.010
200 11.033 34.407 1.051299 7.090 34.003 1.061501 4.932 34.104 1.079749 3.684 34.271 1.068
1001 2.905 34.395 1.0861500 2.063 34.528 1.1092002 1.635 34.604 1.1622502 1.391 34.650 1.1692999 1.255 34.675 1.159
ST-2 4.5 27.101 33.557 1.00049.4 16.483 34.470 1.01099.1 11.759 34.461 1.020
201 8.285 34.340 299 6.621 34.280 1.059502 4.793 34.309 1.079750 3.600 34.387 1.129
1000 2.902 34.458 1.1671499 2.182 34.546 1.1982001 1.810 34.599 2501 1.544 34.655 1.2082998 1.335 34.671 1.178
ST-3 4.9 24.978 34.200 1.00549.0 17.947 34.498 0.99099.7 14.588 34.526
199 11.121 34.414 301 9.013 34.325 1.117500 5.498 34.254 1.119800 3.453 34.371 1.148
1002 2.793 34.446 1.158
ST-4 5.5 24.939 34.112 0.98949.9 16.107 34.534 0.999
100 13.529 34.540 1.019200 10.754 34.392 1.038301 9.008 34.321 1.067500 5.750 34.257 1.116799 3.589 34.356
1001 2.958 34.414 1.1601201 2.574 34.598 1.1561401 2.416 34.481 1.174
Table 2. Temperature, salinity and helium isotope ratios ofPacific waters.
(Rison and Craig, 1983; Sano and Wakita, 1988). How-ever, the possible interference may be negligibly small,since the 4He/20Ne ratio of sea water is rather similar tothat of air and the effect may be compensated (Sano etal., 1995). Experimental error in the detected 3He/4He
Fig. 2. Salinity profiles of the sampling sites. Data at Off Joban(ST-1) is apparently lower than those of the other sites (ST-2, ST-3 and ST-4), probably due to the influence of Oyashiocomponent.
628 Y. Sano et al.
because of the relatively large experimental error of 2.5%.Figure 3 shows the helium isotope profiles of Pa-
cific water at ST-1 and ST-2. The ST-1 profile is identicalto ST-2 from the surface to 500 m depth. In contrast, thereis a significant difference of data from 750 m to 2500 mdepth. This suggests that Pacific water at ST-1 is wellmixed with that of ST-2 above 500 m and they are prob-ably separated or independent beneath 500 m. This maybe explained by the existence of an obstacle, that is, thetopographic high of the Izu-Ogasawara ridge. TheKuroshio in the depth range from the surface to 500 mmay penetrate into Off Joban through the ridge. On theother hand, the abyssal current deeper than 500 m cannotpass through the ridge. It should be noted that the heliumisotope ratios at 300 m and 500 m are very similar, eventhough there is a significant difference of salinity. Thushelium isotopes and salinity may give discrepant infor-mation on the water mass.
It is difficult to explain the excess 3He of 1.208 Ratmin terms of radioactive decay of natural tritium (Craigand Clarke, 1970). The excess is possibly due to the man-tle-derived helium with a high 3He/4He ratio (Lupton,1983; Mamyrin and Tolstikhin, 1984). Sano et al. (1995)suggested that the 25% excess 3He plume in South Pa-cific water can be traced over 5000 kilometers to the westof the East Pacific Rise. The observed excess of 3He may
also be attributable to the helium plume originating inthe East Pacific Rise. In order to check the hypothesis, acontour map of helium isotopes at mid-depth of the NorthPacific is needed. Only four helium isotopic profiles ofthe western North Pacific have been reported in the lit-erature (stlund et al., 1987). Figure 4 shows the loca-tions (222, 223, 226 and 227) of the Geosecs data togetherwith the topography of the region. It is apparent that ST-2 is separated from ST-1 by Izu-Ogasawara ridge whileST-1 is continuous with Geosecs stations in terms of 2000m depth. Figure 5 indicates the depth profiles of heliumisotopes in the Geosecs locations (stlund et al., 1987).Their profiles are similar to those observed in South Pa-cific water, while the maximum 3He/4He ratio is relativelylower. The 20% excess can be traced in the South Pacificbut not in the western North Pacific (Lupton, 1995). Onthe other hand, the 15% excess is found in ST-1 and allGeosecs stations (222, 223, 226 and 227). The extensiveplume of 15% excess 3He may be traced at ST-1 over12,000 kilometers to the northwest of the East PacificRise. This is consistent with a contour map of helium iso-topes along latitude 24N and 47N (Pacific Marine En-vironment Laboratory, 2003). Thus helium isotopes are asignificantly sensitive and conservative tracer of the man-tle signature.
Fig. 3. Helium isotopic profiles of Off Joban (ST-1) and NankaiTrough (ST-2). There is a significant discrepancy at mid-depth from 750 m to 2000 m.
Fig. 4. Bathymetry of western North Pacific together with sam-pling sites of this work and Geosecs expedition. The topo-graphic barrier of Izu-Ogasawara Ridge and the OkinawaTrough are shown.
Helium Isotopes in Pacific Waters from Adjacent Region of Honshu, Japan 629
3.2 Excess helium-3 in Nankai Trough, AdjacentMiyakejima Island and Sagami BayThere is a difference of 3He/4He ratio at mid-depth
(from 750 m to 1500 m) of Pacific water between ST-1and ST-2 (Fig. 3). All of the 20% excess observed at ST-2 cannot be explained by the helium plume originatingfrom the East Pacific Rise, since the excess is constrainedto about 15% in the western North Pacific. Another sourceof mantle helium (additional 5% excess) should be con-sidered in ST-2. Hydrothermal activity in the OkinawaTrough may be a candidate to explain this additional 5%excess (see Fig. 4). Two high-temperature hydrothermalsites, the JADE and CLAM sites, were discovered in theOkinawa Trough where significant mantle helium with a3He/4He ratio of 3~6 Ratm is currently being released(Ishibashi et al., 1995). The summit of the two hydro-thermal sites is about 1400 m deep. It is well known thatthe maximum excess 3He was discovered at about 200above the summit of a volcano in the East Pacific Rise(Lupton and Craig, 1981). Maximum excess 3He mayappear at the depth of 1200 m if it originates from theJADE and CLAM. This is the case with the present study.
Fig. 6. Helium isotopic profiles of Adjacent Miyakejima Is-land (ST-3) and Sagami Bay (ST-4). They are almost iden-tical except for 300 m deep of the Miyakejima sample.
The difference of 3He/4He ratios between ST-1 and ST-2reaches a maximum at around 1200 m (Fig. 3). There-fore, an...