Hiroshi Amakawa

JNdi-1 : a neodymium isotopic reference in consistency with LaJolla neodymium

Abstract A neodymium oxide with relative 143Nd/144Nd ratio 1.000503±1(1 σm) to LaJolla Nd was prepared as a new isotopic reference. The neodymium reagent was selected from two points of view as follows. The first is low abundance of neighboring elements Ce and Sm, which affects isobaric interference. The second is high 143Nd/144Nd ratio, which is closer to those of chondritic and mantle-derived materials. The 143Nd/144Nd ratio of the reagent was measured alternately with LaJolla Nd to get a coherency with LaJolla Nd using 12 mass spectrometers in 11 laboratories in Japan. Aliquots of this neodymium oxide reagent named JNdi-1 are available upon request from the Geological Survey of Japan and may be useful for precise interlaboratory calibration of Nd isotopes.

The fractionation between Y and Ho in the marine environment

Abstract New measurements of Y and Ho in seawater, rivers and rain are presented. Based on the data and a two-box model calculation, we suggest that fractionation between Y and Ho takes place during their removal by particulate matter from the surface ocean to the deep sea. The fractionation factor, K D is calculated to be 0.50, implying that Ho is scavenged two times faster than Y. This presumably occurs due to differences between Y and Ho complexation behavior with respect to seawater inorganic ligands (mainly carbonate ions) and soft organic ligands (though unspecified) of the surface of particulate matter. Fractionation of Y and Ho during weathering and fluvial transport to the ocean appears to have minor influence on the relative abundance of Y and Ho in seawater. We also estimated the mean oceanic residence time to be 5100 years for Y and 2700 years for Ho. Y is less effectively scavenged from seawater than any of the trivalent rare earth elements and theY/Ho ratio in seawater is higher than those in rain, rivers and estuarine waters.

Nd isotopic composition and REE pattern in the surface waters of the eastern Indian Ocean and its adjacent seas

The Nd isotopic composition and dissolved rare earth elements (REEs) have been measured in the surface waters along the 1996/97 R.V. Hakuho-Maru Expedition route from Tokyo to the Southern Ocean, southwest of Australia, through the Philippine and Indonesian Archipelago, the eastern Indian Ocean, the Bay of Bengal and the South China Sea. The radiogenic eNd values of −1.3 and −1.4 were found in the Sulu Sea and near the Lombok Strait, indicating the strong influence of surrounding volcanic islands, whereas non-radiogenic eNd values of less than −10 were found in the Southern Ocean and the Bay of Bengal suggesting Nd of continental origin. The dissolved Nd concentrations also showed a wide range of variation from 2.8 to 19.6 pmol/kg and the trivalent REE patterns exhibited characteristic features that can be grouped into each different oceanic province. The geographical distribution of dissolved Nd is different from that of atmospherically derived 210Pb, but generally resembles that of coastally derived 228Ra. This strongly suggests that fluvial and coastal input predominates over eolian input for dissolved Nd in the surface ocean. However, the riverine dissolved Nd flux appears to be relatively minor, and remobilization of Nd from coastal and shelf sediments may play an important role in the total Nd input to the ocean. By modeling the distributions of the isotopic composition and concentration of Nd together with the activity ratio of 228Ra/226Ra in the southeastern Indian Ocean, we estimate a mean residence time of Nd in the surface mixed layer to be 1.5–2.6 years. The short mean residence time is comparable with, or slightly longer than that of 210Pb suggesting similar chemical reactivity.

The comparative behaviors of yttrium and lanthanides in the seawater of the North Pacific

Yttrium has long been recognized as an ekalanthanide, because of its chemical contiguity relative to the chemistries of rare earth elements which are, in recent years, intensively utilized for elucidation of metal scavenging processes in the ocean. Here, we present the first detailed depth-profile of Y(III) in seawater together with the lanthanides in the North Pacific Ocean. The concentrations of Y(III) range 70–290 pmol/kg and show the “nutrient-like” profile best resembling that of Ho(III) amongst the other rare earth elements. The results agree well with an expectation based on the similarity in their ionic radii and hence stability constants of complexation with carbonate ions. Yet the Ho(III)/Y(III;) ratios in seawater systematically increase with depth, suggesting that Y and Ho are fractionated during scavenging by natural marine particulates. This is likely to result from the different complexation behavior in that Y(III) is more weakly complexed than Ho(III) with soft organic ligands on the surface of particulate matter during scavenging in the surface water but, once released into seawater in the deep sea, Y(III) is complexed with carbonate ions equally or stronger than Ho. The pattern of deep water enrichment in the lanthanide series appears to be consistent with the recent observation of partitioning between suspended particles and seawater. Our precise measurements also indicated that Pr and Tb best resemble Nd and Dy, respectively in their oceanic behavior, whereas Ho and Tm are intermediate between their neighboring rare earth elements.

Dissolved rare earth elements and hydrography in the Sulu Sea

Because most Southeast Asian basins are flushed rapidly by waters from the western Pacific, the effects of respiration and silica dissolution within those basins are hardly discernible based on general hydrographic and geochemical observations (Broecker et al., 1987). However, a different situation is expected for the Sulu Sea because its deep water is isolated from other deep sea basins by shallow sills of ,400 m depth. Hence, any heterogeneity of elemental distributions within the basin must be ascribed to vertical physical and biogeochemical processes. We have obtained detailed vertical profiles of dissolved rare earth elements (REEs) together with conductivity-temperature-depth (CTD) and hydrographic measurements in the Sulu Sea during the 1996 -97 Hakuho-Maru cruise. Vertical profiles clearly indicate that the REE(III)s are enriched in the deep water due to regeneration and are involved in vertical biogeochemical cycling. The light REE(III)s (La, Pr, and Nd) showed a subsurface maximum at 500 m and a minimum;1500 m in depth. None of the hydrographic properties or nutrients shows such features; therefore, the REE(III)s and their elemental ratios may be useful as tracers of water masses. The relationship of REE(III)s versus dissolved Si suggests that the REE(III)s are regenerated in a delayed fashion relative to Si, and it is likely that their dissolution largely occurs at the bottom interface. The mean residence time of the deep water is estimated to be 300 6 150 yr. Consequently, the calculated benthic flux of REE(III)s becomes roughly comparable with the river and atmospheric fluxes. Because of oxidation to Ce(IV), the Ce profile shows a decrease from ;6 pmol/kg at the surface to a constant value at 3.9 pmol/kg below 1500 m, unlike the other REEs. The REEs in the Sulu Sea are enriched in the middle REEs and Ce relative to those of the North Pacific Deep Water and must derive from local sources around the Sulu Sea. Copyright

Neodymium isotopic variations in Northwest Pacific waters

Abstract Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (ϵ Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ϵ Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ϵ Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters. In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ϵ Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ϵ Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ϵ Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.

Oceanic profiles of dissolved silver: precise measurements in the basins of western North Pacific, Sea of Okhotsk, and the Japan Sea

We present here the detailed vertical profiles of dissolved (<0.04 μm) Ag in the three different oceanic basins in the western margin of the North Pacific. Those profiles confirmed that Ag is strongly involved in the biogeochemical cycling of biological uptake in the surface water, particulate sinking, and regeneration in the deep sea. The vertical profiles of Ag best resemble those of dissolved Si, but in detail there are differences between the two. In the western North Pacific, dissolved Ag reaches a maximum at 2500–3000 m, which is deeper than that of dissolved Si (∼2000 m). This implies that Ag is regenerated more slowly than Si. The greater inter-oceanic variation of Ag over Si in the deep waters between the North Atlantic and North Pacific is consistent with this interpretation. The other two profiles from the Sea of Okhotsk and the Japan Sea fit well in this biogeochemical and oceanographic trend. Our dissolved Ag concentrations (4.2–8.0 pmol/kg) in the surface waters are significantly higher than those reported previously for other locations, and it appears that the variability of Ag in the surface waters is greater than an order of magnitude. Although many factors are involved in controlling the Ag concentration in the surface waters, removal by biological uptake appears to be particularly important. Although there may be some indication of anthropogenic sources in the surface waters, it seems to be local, and the large-scale contamination of Ag in the ocean is not seen in our data.

Isotopic compositions of Ce, Nd and Sr in ferromanganese nodules from the Pacific and Atlantic Oceans, the Baltic and Barents Seas, and the Gulf of Bothnia

Abstract Ferromanganese nodules from the Pacific and Atlantic Oceans, the Barents and Baltic Seas, and the Gulf of Bothnia were analyzed for the isotopic compositions of Ce, Nd and Sr and the abundances of REE, Ba and Sr. REE patterns of Barents, Baltic and Bothnian samples show no Ce anomaly, or even a negative one, in contrast to the positive anomaly observed for the Pacific and Atlantic samples. Moreover, the Baltic and Bothnian samples have distinctly low e Nd values; average e Nd values of the four regions are as follows: Pacific −5, Barents −10, Atlantic −11 and Baltic inclusive of Gulf of Bothnia −19. The characteristic low e Nd values of the Baltic samples are indicative of the influence of Precambrian rocks from the Baltic shield. Of particular interest is the feature of the Ce isotopic composition that e Ce values of the samples from the Pacific are negative and those from the other three regions positive. This novel finding might suggest a difference in sources of Ce between the Pacific and other regions. These results demonstrate that Ce isotopic ratios can be a useful tracer in marine geochemistry, in combination with isotopic compositions of Nd and Sr.

Boron isotope variations in the atmosphere

We report here the first measurements of boron isotope ratios in the maritime atmosphere together with those of precipitation. The δ11B values of atmospheric condensates in the western North Pacific and Japanese coast and snow in Tokyo range from -12.8 to +5.1‰ and from -0.4 to +0.4‰, respectively, which are significantly lower than those of rainwater (+18.9 to +34.7‰) collected mostly over the North Pacific. Since the 11B/10B ratios of the atmosphere are lower than those of volcanic emissions (δ11B=+2.3 to +21.4‰), we must seek sources for atmospheric boron other than volcanism. We postulate that the sea may be an important supplier for atmospheric boron under some dynamic conditions and that boron isotope fractionation during evaporation from seawater and removal from the atmosphere may account for the large variations of 11B/10B ratios observed in the atmosphere and precipitation.

Sr–Nd isotopic and chemical characteristics of the silicic magma reservoir of the Aira pyroclastic eruption, southern Kyushu, Japan

Abstract Sr and Nd isotope and geochemical investigations were performed on a remarkably homogeneous, high-silica rhyolite magma reservoir of the Aira pyroclastic eruption (22,000 years ago), southern Kyushu, Japan. The Aira caldera was formed by this eruption with four flow units (Osumi pumice fall, Tsumaya pryoclastic flow, Kamewarizaka breccia and Ito pyroclastic flow). Quite narrow chemical compositions (e.g., 74.0–76.5 wt% of SiO2) and Sr and Nd isotopic values ( 87 Sr / 86 Sr =0.70584–0.70599 and eNd=−5.62 to −4.10) were detected for silicic pumices from the four units, with the exception of minor amounts of dark pumices in the units. The high Sr isotope ratios (0.7065–0.7076) for the dark pumices clearly suggest a different origin from the silicic pumices. Andesite to basalt lavas in pre-caldera (0.37–0.93 Ma) and post-caldera (historical) eruptions show lower 87 Sr / 86 Sr (0.70465–0.70540) and higher eNd (−1.03 to +0.96) values than those of the Aira silicic and dark pumices. Both andesites of pre- and post-caldera stages are very similar in major- and trace-element characteristics and isotope ratios, suggesting that the both andesites had a same source and experienced the same process of magma generation (magma mixing between basaltic and dacitic magmas). Elemental and isotopic signatures deny direct genetic relationships between the Aira pumices and pre- and post-caldera lavas. Relatively upper levels of crust (middle–upper crust) are assumed to have been involved for magma generation for the Aira silicic and dark pumices. The Aira silicic magma was derived by partial melting of a separate crust which had homogeneous chemistry and limited isotope compositions, while the magma for the Aira dark pumice was generated by AFC mixing process between the basement sedimentary rocks and basaltic parental magma, or by partial melting of crustal materials which underlay the basement sediments. The silicic magma did not occupy an upper part of a large magma body with strong compositional zonation, but formed an independent magma body within the crust. The input and mixing of the magma for dark pumices to the base of the Aira silicic magma reservoir might trigger the eruptions in the upper part of the magma body and could produce a slight Sr isotope gradient in the reservoir. An extremely high thermal structure within the crust, which was caused by the uprise and accumulation of the basaltic magma, is presumed to have formed the large volume of silicic magma of the Aira stage.