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Helium Isotopes of Seawater in the Philippine Sea and the Western North Pacific

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Helium Isotopes of Seawater in the Philippine Sea and the Western North Pacific Geochemical Journal, Vol. 44, pp. 451 to 460, 2010 Helium isotopes of seawater in the Philippine Sea and the western North Pacific NAOTO TAKAHATA,*# TAICHI TOKUKAKE,** KOTARO SHIRAI,† SHINZOU FUJIO,# KIYOSHI TANAKA# and YUJI SANO# Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan (Received June 4, 2009; Accepted June 4, 2010) We measured helium isotopic ratios of 84 seawater samples from various depths collected in the western North Pacific Ocean and the western Philippine Sea. The 3He/4He ratios varied significantly from δ3He of 0.1% to 22.9%, where δ3He is defined as the percent deviation of the helium isotopic ratio relative to the atmospheric standard. Maximum δ3He > 20% was observed at mid-depth (2000–2500 m) in the western Philippine Sea and in the southern part (~10°N) of the western North Pacific, though not in the northern part (~30°N) at the same depth. Contour maps of the lateral δ3He distribution at mid-depth suggest that the helium-3 plume derived from the East Pacific Rise does not flow northward along the Izu– Ogasawara–Mariana Ridge but westward through the Caroline Basin or the Yap–Mariana Junction into the Philippine Sea. It then flows northward in the western Philippine Sea to a region adjacent to the Japanese Islands. Although these flows inferred from the δ3He distribution are roughly similar to those estimated from water properties such as isopycnal distri- butions, the δ3He distribution could reveal that deep-water circulation seems to be different at each depth (2000, 2500, 3000 m). Keywords: 3He/4He ratios, abyssal current, Philippine Sea, North Pacific, seawater a dissolved oxygen sensor (CTDO ). Most of it then flows INTRODUCTION 2 into the West Caroline Basin. The remaining western The Philippine Sea, at the western end of the North boundary current flows over the middle and lower Solo- Pacific Ocean, consists mainly of three basins, the mon Rise, and then proceeds westward, where it is di- Shikoku, the West Mariana, and the Philippine basins. It vided by the Caroline Seamounts into southern and north- is mostly isolated from the main North Pacific below 2500 ern branches. The northern branch current enters the West m depth by the Izu–Ogasawara, the Mariana and the Yap Mariana Basin through the Yap–Mariana Junction. To ridges (Fig. 1). This topographic barrier restricts the hori- understand deep circulation more precisely at mid-depth zontal exchange of abyssal water, and the inflow of deep (2000–3000 m) in the western North Pacific Ocean, how- water is possible only through some narrow gaps. ever, it is important to investigate water mass structure Isopycnal maps constructed by Reid (1997) suggest that both chemically and physically. Especially at mid-depth deep seawater from the South Pacific enters the Philip- in the North Pacific Ocean, it is difficult to observe deep- pine Sea along its western boundary. Kawabe et al. (2003) sea current due to weak flow rate compared with subsur- reported that a deep western boundary current at 2000– face waters or bottom waters. It is also difficult to distin- 3000 m depth may flow from the Melanesian Basin, guish water masses due to small difference of water prop- change direction and flow around the upper Solomon Rise erties such as temperature and salinity. As we discuss to the southwestern, and proceed into the East Caroline below, the isotopic ratio of helium dissolved in seawater Basin, as suggested by hydrographic data observed with is at the maximum at mid-depth rather than in bottom a conductivity-temperature-depth profiler equipped with waters in the Pacific, which is derived from the crest of the East Pacific Rise. So excess 3He can be used for trac- ing movement and mixing of different water masses in such regions. *Corresponding author (e-mail: [email protected]) The 3He/4He ratio of the atmosphere is 1.386 × 10–6, #Present address: Atmosphere and Ocean Research Institute, The Uni- versity of Tokyo, Kashiwa, Chiba 277-8564, Japan. and it is considered to be constant on a global scale within 3 4 **Present address: Tokyo Research Laboratory, Mitsubishi Gas Chemi- an experimental error of 5%. The He/ He ratios of cal Co. Inc., Katsushika-ku, Tokyo 125-0061, Japan. mantle-derived samples, such as those from mid-ocean †Present address: Department of Earth and Planetary Science, School ridge basalts and volcanic gases in island arcs, are rela- of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, tively high at about 1 × 10–5, whereas those of granitic Japan. rocks and continental natural gases are low, with ratios Copyright © 2010 by The Geochemical Society of Japan. of around 1 × 10–7 (Lupton, 1983; Mamyrin and 451 (a) IOR NPB SB North Pacific Ocean Philippine Sea MR PB WMB EMB YR CS WCB ECB MB SR (b) 9 10 1 2 12 11 8 7 6 5 3 4 Fig. 1. (a) Map of a portion of the Philippine Sea and the western North Pacific Ocean with place names. Depth of the ocean is contoured with the 2500 m, 4000 m isobaths, and depths shallower than 4000 m are shaded. IOR, Izu–Ogasawara Ridge; MR, Mariana Ridge; YR, Yap Ridge; SR, Solomon Rise; CS, Caroline Seamounts; SB, Shikoku Basin; PB, Philippine Basin; WMB, West Mariana Basin; EMB, East Mariana Basin; WCB, West Caroline Basin; ECB, East Caroline Basin; MB, Melanesian Basin; NPB, Northwest Pacific Basin. (b) Sampling sites of seawaters in the western North Pacific (ST-1 to -5 and -9 to -12) and the western Philippine Sea (ST-6 to -8). 452 N. Takahata et al. Table 1. Location and bottom depth of sampling stations together with sampling depth of western North Pacific water and western Philippine Sea water Cruise Station Location Bottom depth (m) Sampling depth (m) KH-04-4 ST-1 32°30′ N, 143°09′ E 5612 397, 596, 991, 1485, 1977, 2469, 4423, 5393 ST-2 32°30′ N, 150°00′ E 5575 2468 ST-3 12°52′ N, 147°13′ E 5508 2471 ST-4 12°40′ N, 155°00′ E 5869 2470 ST-5 14°34′ N, 170°54′ E 5655 596, 792, 992, 1486, 1979, 2471, 2962, 4427, 5647 KH-06-2 ST-6 15°00′ N, 128°00′ E 5807 199, 496, 744, 991, 1485, 1978, 2469, 2959, 3449, 3939 ST-7 20°00′ N, 128°00′ E 5660 199, 496, 745, 992, 1485, 1976, 2470, 2961, 3451, 3937 ST-8 25°00′ N, 128°00′ E 7020 198, 496, 744, 992, 1486, 1979, 2471, 2961, 3451, 3941 KH-07-1 ST-9 37°59′ N, 150°00′ E 5921 544, 989, 1483, 1975, 2466, 2954, 3443, 3930, 4417, 4902, 5387 ST-10 38°00′ N, 157°00′ E 5717 542, 990, 1236, 1728, 1975, 2467, 2955, 3444, 3932, 4418, 4904, 5390 ST-11 32°29′ N, 166°01′ E 6224 594, 988, 1484, 1975, 2463, 2959, 3446, 4422, 4908, 5876 ST-12 32°30′ N, 160°01′ E 4631 542, 989, 1479, 1978, 2465, 3447, 3936, 4423 Tolstikhin, 1984; Sano and Wakita, 1985). The 3He/4He 2004; KH-07-1, May 2007) and in the Philippine Sea (KH- ratio is one of the most sensitive and conservative tracers 06-2, June 2006). Stations visited were ST-6 to -8 (west- in chemical oceanography (e.g., Jenkins et al., 1972; Craig ern Philippine Sea, along 128°E longitude), ST-1 and -2 et al., 1975; Sano et al., 1995) owing to the primordial and ST-9 to -12 (western North Pacific near the Japanese signature, rapid mobility, and chemical inertness of the Islands, ~30°N latitude), and ST-3 to -5 (western North isotopes. During the South Tow expedition in 1972, Pacific near the Mariana Arc, ~10°N). Figure 1 shows Lupton and Craig (1981) discovered a striking intensity the sampling points used in this study. Samples for he- and lateral extent of excess 3He relative to air-saturated lium isotope measurements were collected from 8–12 seawater in the deep Pacific Ocean at latitude 15°S on depths at each station with a CTD carousel system the East Pacific Rise. This plume-shaped 3He anomaly, equipped with 10-L Niskin bottles. Details are shown in which originated from volcanic activity on the ridge, Table 1. Seawater was transferred without exposure to spread westward by abyssal currents at the depth of the the atmosphere from the Niskin bottles into containers of crest. Since this expedition, more than 5000 3He/4He about 30 cm3 made of copper tubing for storage (Sano et measurements have been carried out in three oceans. A al., 1989). thorough understanding of deep-sea circulation in the In the laboratory, each 30-cm3 copper container was Philippine Sea and adjacent North Pacific is hindered by connected to a stainless steel high vacuum line and dis- sparse helium isotopic data in seawater samples from these solved gases were extracted from the seawater samples regions (Östlund et al., 1987; Igarashi et al., 1987; Sano in vacuo. Helium in the exsolved gases was purified with et al., 2004; Takahata et al., 2004). Sano et al. (2004) hot titanium-zirconium getters and charcoal traps held at and Takahata et al. (2004) have reported excess 3He of liquid nitrogen temperature. The 4He/20Ne ratios were more than 20% at mid-depth (2000–3000 m) in the north- measured by an inline quadrupole mass spectrometer. ern Philippine Sea, in contrast to profiles of 3He/4He ra- Helium was then separated from neon by a cryogenic tios in the western North Pacific.
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