Nd isotopic composition and concentration distributions were determined in surface waters of the Northwest Pacific Ocean and its adjacent seas, i.e., the Okhotsk and Japan Seas. The Okhotsk and Japan Seas samples showed higher Nd concentrations (19∼31 pmol/kg) than the Northwest Pacific Ocean samples, suggesting a large amount of Nd input from the land areas surrounding the Okhotsk and Japan Seas. Although the Nd isotopic composition data were limited for those regions, each oceanic area showed a distinctive value. The lone sample from the Okhotsk Sea had radiogenic Nd (ε_Nd = −3.6) that was comparable to samples from the Pacific Ocean, indicating a large amount of radiogenic Nd supply from the Kuril Islands. On the other hand, the Japan Sea samples had the most highly unradiogenic Nd (ε_Nd = −8.9∼−7.2). The southernmost sample had the most negative value, due to the influence of the Tsushima Warm Current (TWC) having a low ε_Nd value. The latitudinal distributions of Nd isotopic composition and concentration in the surface waters of the Northwest Pacific Ocean showed marked variations. Nd concentration was low at low latitudes (3∼8 pmol/kg) and was increased with increasing latitude (16∼20 pmol/kg). On the other hand, Nd isotopic composition showed a mid-latitude minimum (ε_Nd = ∼−6) and high ε_Nd values at low and high latitudes (ε_Nd = −2∼−1). The distribution of the ε_Nd values seemed to be controlled by three currents: Kuroshio Extension, Oyashio Current and North Equatorial Current (NEC). The Kuroshio Extension, which showed a low ε_Nd value, seemed to play an important role in transporting continentally derived Nd to the central Pacific. The Oyashio Current and NEC are supplied with large amounts of radiogenic Nd from the Kuril and Aleutian Islands, and the Hawaiian Islands, respectively. Based on the Nd isotopic composition and concentration distributions, we calculated the radiogenic Nd flux to the Pacific Ocean surface waters and estimated the Nd residence time. Our calculation suggested that the radiogenic Nd flux required to account for the Nd isotopic composition is higher than 70% of the unradiogenic Nd flux including Nd remobilization from coastal and shelf sediments and atmospheric input. It was also revealed that a large Nd remobilization flux is necessary to attain a global Nd residence time of ∼400 y as estimated in previous studies.

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This observation was carried out on the surface seawater of Jinhae Bay, southeastern Korea during a red tide in July, 1991 in order to understand the removal processes of reactive trace elements using natural radionuclides of ²³⁴Th and ²¹⁰Po as co-tracers. Comparing the results of ²³⁴Th and ²¹⁰Po activities against chlorophyll a and TSM (total suspended matter), the two nuclides have different scavenging process in the surface coastal seawater during algal blooming. Particulate and dissolved ²³⁴Th and particulate ²¹⁰Po are subject to dilution by chlorophyll a and TSM, showing the negative correlation with ²³⁴Th and particulate ²¹⁰Po, while they are positively correlated with dissolved ²¹⁰Po. This suggests that scavenged ²¹⁰Po may be preferentially re-dissolved at the bottom waters and/or surface sediments and reappear at the surface by tidal current. The calculated distribution coefficients (K_d) of ²¹⁰Po to particle (TSM) relative to that of ²³⁴Th was relatively large, demonstrating that the affinity of ²¹⁰Po to TSM is greater than that of ²³⁴Th during algal blooming. These results suggest that particulate matter, especially biogenic matter, plays a significant role in controlling the scavenging of reactive trace metals in coastal seawater.

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In order to understand the origins and transport processes of lithogenic aluminosilicate in the western North Pacific, time-series sediment traps were deployed at Stn. KNOT at 44°N 155°E (water depth: 5320 m). Annual mean content of aluminum (Al) which is a proxy of lithogenic material in settling particles was 0.38% at 1 km depth, 0.78% at 3 km depth and 1.14% at 5 km depth, respectively. Mass fluxes were corrected by ²³⁰Th and ²³¹Pa activities in particles and seawater. Annual mean corrected Al fluxes at 1, 3 and 5 km depths were calculated to 0.45, 0.46 and 0.33 g/m²/year, respectively. Lanthanum/ytterbium (La/Yb) and thorium/scandium (Th/Sc) ratios were used as proxies for estimating the origin of lithogenic aluminosilicate in settling particles. Both La/Yb and Th/Sc ratios in settling particles increased with depth, and the seasonal variation was the smallest at 5 km depth. It was considered that variations of La/Yb and Th/Sc ratios in settling particles reflected the change of source material of lithogenic aluminosilicates. Origins of lithogenic materials in the settling particles were summarized using two end-members, loess from central Asian continent (AL) and basalts from the Kurile-Kamchatka Islands (KK). Contributions of end-members in settling particles were estimated from the La/Yb and Th/Sc ratios. The contribution of KK particles in lithogenic materials was calculated to be 84% at 1 km depth, and 61% at 3 and 5 km depths, respectively. Although the KK material was the dominant constituent of the terrigenous component in settling particles at Stn. KNOT, the KK flux decreased with depth. On the other hand, the AL flux increased with depth and the significant increase of the AL flux was presumably due to the lateral import of AL particles to the deep layer at Stn. KNOT. From these results we conclude that the distribution of lithogenic materials recorded in sediments in the western North Pacific are influenced by the northward transport of AL within the deep layer.

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This paper reports a series of studies leading to the discovery of a submarine hydrothermal field (called Nakayama Field) at an arc seamount (12°43′N, 143°32′E) in the southernmost part of the Mariana Trough, western Pacific Ocean. We first detected hydrothermal plumes characterized by water column anomalies of temperature, light transmission, Mn, Fe, Al, O₂, CH₄, and δ¹³C of CH₄ above the summit caldera of the seamount. Then deep-tow camera surveys confirmed the existence of hydrothermal activity inside the caldera, and an ROV dive finally discovered white smoker-type fluid venting associated with vent fauna. A high concentration of aluminum in the plume and white smoker-type emissions imply acidic hydrothermal activity similar to that observed at the DESMOS Caldera in the eastern Manus Basin, Papua New Guinea. Anomalously low δ¹³C (CH₄) of −38‰ of a vent fluid sample compared to other arc hydrothermal systems along the Izu-Bonin and Mariana Arcs suggests an incorporation of biogenic methane based on a subsurface microbial ecosystem.

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Dissolved and acid-dissolvable Fe, Co, Ni, Cu, Zn, Cd, and Pb were measured in the North Pacific Ocean (40–17°N, 151°E–160°W) in June–July 2000. Immediately after the collection, a portion of seawater for dissolved species was filtered with a 0.2-μm filter and acidified to pH 2.2 with HCl. A portion of seawater for acid dissolvable species was acidified to pH 2.2 with HCl without filtration and stored for 3 years at an ambient temperature. In our laboratory, the dissolved and acid dissolvable trace metals were concentrated using a chelating column technique and determined by ICP-MS. Profiles of dissolved and acid dissolvable Fe show a large intra-basin variation. The concentrations are 0.3–1.4 nM for dissolved Fe (D Fe) and 0.7–4.7 nM for acid dissolvable Fe (AD Fe) in deep water. The concentrations are high at stations in the northwestern area and southwest of the Hawaiian Islands. AD Fe increases below 4000 m suggesting resuspension of sedimentary material. For Co, Ni, Cu, Zn, and Cd, the concentrations of dissolved and acid dissolved species mostly agree within an experimental error. Deep water distributions of Ni, Cu, Zn, and Cd are fairly uniform in the basin. AD Pb is significantly higher than D Pb especially in surface water. Comparison to our previous data on the dissolved trace metals in the subarctic North Pacific in July–September 1997 (Fujishima et al., 2001) indicates non-uniform distributions of the bioactive trace metals in the surface mixed layer in the North Pacific. In the subtropical region, chlorophyll biomass is very low owing to depletion of major nutrients, resulting in surface maxima of D Fe and D Zn. In the subarctic region, D Fe and D Zn are depleted in the surface mixed layer. In contrast, D Co and D Ni are high in the northwestern subarctic area and low in the subtropical gyre.

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We report here the first measurements of dissolved rare earth elements (REE) in the headwaters of the Yamuna river draining through the southern slopes of Himalaya. Due to intense weathering of the surface rocks of different lithologies and influence of tributaries, Yamuna river waters have variable dissolved REE contents (87 < ΣREE < 1374 ng L^-1, mean = 288.6 ng L^-1) and exhibit negative Eu anomaly (0.49 < Eu/Eu* < 0.73, mean = 0.63). While most of the samples do not show discernable Cc anomalies; a negative Cc anomaly, however, found in a few of them, which can be explained by the colloidal pool preferentially enriched in Ce. A comparison among the river waters and bed sediments suggests that dissolved composition of REE is strongly fractionated and is enriched in MREE (Nd-Gd) with respect to sediments; presumably due to preferential dissolution of phosphate minerals such as apatite during weathering processes. Along with the Yamuna river, bed sediments from the Chambal river (a Peninsular river) have also been analyzed for REE composition. Bed sediments in the Yamuna and the Chambal river basins are characterized by ΣREE concentrations in the range of 78 to 291 μg g^-1 (mean = 165 μg g^-1) and 96 to 157 μg g^-1 (mean = 134 μg g^-1), respectively. A characteristic feature observed in the REE-normalized patterns of bed sediments is a strong HREE enrichment and a relatively positive Eu anomaly with respect to the granites in the Yamuna river catchment. In contrast, the bed sediment samples of the Chambal river show significant LREE enrichment and Eu enrichment with respect to the Deccan basalts in its catchment. The feldspars and their secondary products, which are enriched in Eu, might be the cause of the Eu anomaly. In river sediments of both these basins, the enrichment factors (EF), with respect to PAAS are ≤2 suggesting that REE composition is mainly derived from weathering processes.

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We have measured the vertical profiles of selenite, selenate and organic selenide in the Southeast Asian Basins (the Celebes Sea, the Sulu Sea and the South China Sea) for the two years, 1996 and 2002. Total selenium, selenite and selenate showed nutrient-like profiles. In particular, the vertical profiles of selenate and organic selenide in the Sulu Sea differed greatly between 1996 and 2002. The salinity and selenite/selenate ratio in the surface layer showed that the Sulu Sea surface water in these two years was derived from different sources. The vertical profiles of selenite, selenate and organic selenide in the South China Sea in 1996 and 2002 were almost the same. Both selenite and selenate were slightly elevated at the surface in the South China Sea. These inorganic selenium species seemed to be derived from atmospheric input, admixture of the East China Sea surface water, West Philippine Sea surface water, and fluvial, coastal and shelf waters of the South China Sea. The relationship between the concentration of organic selenide and the relative fluorescent intensity of fluorescent organic matter in the South China Sea suggested the organic selenide exists in the refractory DOC fraction possibility as humic-like substances. The total dissolved selenium concentration and the selenite/selenate ratio in deep water indicated that the water mass age of the Celebes Sea deep water was similar to that of the western North Pacific deep water. On the other hand, the water mass age of the Sulu Sea is younger than that of the western North Pacific. The selenite/selenate ratio in the South China Sea deep water was two times higher than that of the western North Pacific. This result suggested that a large amount of organic detritus was supplied to the South China Sea, and then the organic selenide in the organic detritus rapidly oxidized to selenite.

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Activities of particulate and dissolved ²³⁴Th and concentrations of particulate organic carbon (POC) were measured in the northwestern North Pacific in winters and springs. The export fluxes of POC from surface waters were estimated by using ²³⁴Th as a tracer. The POC fluxes in winter showed a wide variation (20–190 mg-C m^-2d^-1) and were higher in the western region than in the eastern region. The influence of the Oyashio Current and the supply of substances from the continent to the study area also appeared to be higher in the western region. Therefore, the horizontal distribution of POC fluxes in winter was influenced by continental materials. In the spring bloom, the fluxes of POC were much higher (up to 520 mg-C m^-2d^-1), as were the e-ratios (up to 70%). In the northwestern North Pacific, the efficiencies of export of POC from the euphotic zone were higher than in other areas of the world ocean. The increase in POC fluxes in spring was considered to depend not only on the increased abundance of phytoplankton, but also on the transition of phytoplankton species, as evidenced by the relationship between ²³⁴Th adsorption rates and the concentrations of POC.

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We have measured helium isotopic ratios of 43 water samples with various depths collected in Eastern East China Sea and Northwestern Philippine Sea, adjacent region of Nansei Islands, Southwestern Japan. The ³He/⁴He ratios vary significantly from 0.994 R_atm to 1.242 R_atm where R_atm is the atmospheric ratio of 1.39 × 10^-6. Mid-depth (750–1500 m) samples in this study would be affected by the source of subduction-type mantle helium in the Okinawa Trough. Noble gas abundances (neon, argon, krypton and xenon) were measured in 24 water samples with various depths collected in Northwestern Pacific Ocean and Northwestern Philippine Sea (Nansei Trench). Neon abundances show slight excess relative to air saturated seawater at ambient temperature and salinity. This may be due to either air bubble effect or contamination during the sampling. When these effects are corrected using the neon anomaly, heavier noble gas abundances (argon, krypton and xenon) of samples with the temperature higher than 5°C (shallower than 500 m) agree well with those of calculated air saturated seawater, while the lower temperature samples (deeper than 500 m) indicate anomaly of −7% to +10%.

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Size-fractionated particulate ²³⁴Th and particulate organic carbon (POC) fluxes were measured in the Gulf of Mexico during 2000 and 2001 in order to obtain a better estimation of upper ocean organic carbon export out of the euphotic zone within cold core and warm core rings, and to assess the relative merit of sediment trap and POC/²³⁴Th methods. In 2000, the flux of POC measured by sediment traps at 120 m ranged from 60 to 148 mg C m^-2d^-1, while ²³⁴Th-derived POC fluxes in large particles (>53 μm) varied from 18 to 61 mg C m^-2d^-1 using the ratio of POC/²³⁴Th at 120 m, and from 51 to 163 mg C m^-2d^-1 using an average ratio of POC/²³⁴Th for the upper 120 m water column. In 2001, the fluxes of POC measured by traps deployed at 120 m water depth ranged from 39 to 48 mg C m^-2d^-1, while the ²³⁴Th-derived POC fluxes in large particles (>53 μm) varied from 7 to 37 mg C m^-2d^-1 using a ratio of POC/²³⁴Th at 120 m, and from 37 to 45 mg C m^-2d^-1 using an average ratio of POC/²³⁴Th within the 0–120 m interval. The results show that POC fluxes estimated by the ²³⁴Th method using the average ratio of POC/²³⁴Th within the euphotic zone are similar to those measured by sediment traps. Furthermore, the results demonstrate that the variability in POC export fluxes estimated by the ²³⁴Th/²³⁸U disequilibrium approach is strongly related to the ratio of POC/²³⁴Th that is taken, and for which we have independent evidence that it may be controlled by the chemical composition of the suspended particles. The results also reveal that using POC/²³⁴Th ratios in small particles may result in an estimate of the POC export flux that is considerably higher than when using POC/²³⁴Th ratios in large particles (>53 μm). The POC flux calculated from ratios in large particles is, however, more comparable to the POC flux determined directly by sediment traps, but both of these estimates are much lower than that determined by using the POC/²³⁴Th ratios in sinking particles. Therefore, without reliable flux values to compare with, ²³⁴Th-based and sediment trap approaches are complementary methods for estimating upper ocean POC export, with comparable uncertainties for both of these approaches.

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Using living corals collected from Okinawan coral reefs, laboratory experiments were performed to investigate the relationship between coral calcification and aragonite saturation state (Ω) of seawater at 25°C. Calcification rate of a massive coral Porites lutea cultured in a beaker showed a linear increase with increasing Ωaragonite values (1.08–7.77) of seawater. The increasing trend of calcification rate (c) for Ω is expressed as an equation, c = aΩ + b (a, b: constants). When Ω was larger than ∼4, the coral samples calcified during nighttime, indicating an evidence of dark calcification. This study strongly suggests that calcification of Porites lutea depends on Ω of ambient seawater. A decrease in saturation state of seawater due to increased pCO₂ may decrease reef-building capacity of corals through reducing calcification rate of corals.

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Detailed vertical profiles of dissolved Ag in the Bering Sea, the central North and South Pacific and the Southern Ocean have been present here. These profiles are comparable to our previous report in the western North Pacific, the Okhotsk Sea and the Japan Sea (Zhang et al., 2001). There is a systematic enrichment of Ag concentrations in the deep waters (North Atlantic < South Atlantic < South Pacific < North Pacific), which is parallel with the distributions of dissolved Si and consistent with the route of the global ocean circulation. In the surface waters, the latitudinal distributions of both Ag and Si showed higher concentrations at higher latitude, suggesting that Ag in the oceanic surface waters are supplied from the deep water by upwelling and vertical mixing similar to that of Si. However, the Ag/Si ratios in surface waters are significantly higher than those in deep waters. Neither anthropogenic nor natural input from the atmosphere can play an important role for the elevated Ag/Si ratios in the surface waters. It is assumed that the elevated Ag/Si ratios in the surface waters are due to the difference in the biological uptake between the two elements.

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A submersible analyzer (Geochemical Anomaly MOnitoring System, GAMOS-IV) powered by micro-diaphragm pumps has been developed for in situ monitoring of time-series changes in manganese concentrations. The analyzer determines dissolved manganese in a continuous manner using a H₂O₂-luminol chemiluminescence method. The analyzer operates with a sample and reagent flow rate of approximately 60 μl/min, with a detection range up to 10 μM manganese, and has a 98% response time of approximately 6 min. The GAMOS-IV was tested during test deployment at 1, 379 m in diffused hydrothermal fields at the Suiyo seamount. The ratio of manganese concentration to temperature (Mn/T) varied with different admixtures of source fluids.

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Analysis of chlorofluorocarbons (CFCs) data in seawater in the Kuroshio-Oyashio Transition Area obtained during the WOCE Campaign P-13 along 165°E in August to October 1992 and P-14 along 179°E in July to September 1993 revealed that CFC-12 and CFC-11 in the North Pacific Central Mode Water (NPCMW) were significantly under-saturated with respect to the atmospheric CFC-12 (75.5 ± 12.3% for P-13 and 85.9 ± 6.3% for P-14) and CFC-11 (72.9 ± 10.6% for P-13 and 82.6 ± 5.3% for P-14). Since the mode water is derived from the vertical convection in the surface layer in winter and is considered not greatly influenced by mixing with ambient water during the subsequent advection, the observed under-saturation of CFC-12 and CFC-11 in the mode water suggests that CFCs in the mixed layer in winter has already been under-saturated with respect to the atmospheric CFC-12 and CFC-11. A result from a simple one-dimensional model for the mixed layer also supports the entrainment of the water with lower degree of CFC-12 and CFC-11 saturation and the under-saturation in the wintertime mixed layer.

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Two soil profiles from the United States with radically different emplacement and climatic histories were analyzed for U, Th and members of the ²³⁸U decay series (²³⁴U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb), ¹³⁷Cs and osmium isotopes. The arid New Mexico profile is developed on an approximately 250, 000 years old colluvium while the temperate New Hampshire profile is formed on till after the last glaciation at about 10, 000 years ago. Both the profiles show significant ²³⁴U/²³⁸U ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁰Th disequilibria, however, in the New Hampshire profile, the disequilibria are far more pronounced in mid-depths (20–50 cm). High Os concentration with highly radiogenic ¹⁸⁷Os/¹⁸⁸Os is another characteristic of the mid-depths of the New Hampshire profile. This layer, particularly at about 30–40 cm depth has the characteristics of a soil developed on black shale, as evidenced from both the high U and Os concentrations and the large excess of ²³⁰Th over ²³⁸U. This profile clearly shows that the regolith on which the contemporary soil is developing was not homogeneous. The presence of measurable excess ²²⁶Ra activity over ²³⁰Th activity in both profiles suggests the need for a source of ²²⁶Ra external to the regolith in both cases. Atmospheric deposition of ²²⁶Ra is a possible source for this ²²⁶Ra excess and brings to light the important role of atmospheric deposition of nuclides and their transport in the soil profile in pedogenic processes. It also shows that regolith developed by glacial processes need not be homogeneous, thereby confounding the understanding of vertically modified soil profiles.

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