深海 雄介

A key driver to develop stable tungsten (W) isotope geochemistry is its unique relationship with molybdenum (Mo). Here, we establish a combined double-spike (DS) method for W (180W-184W spike) and Mo (97Mo-100Mo spike) to perform simple, efficient, and robust isotope measurements of these two chemically analogous elements in single sample aliquots. Based on previous column chemistry, we optimized two-stage anion-exchange procedures to remove matrix elements, particularly the critical interference of Ta and Hf on 180W, and to collect sharply separated W and Mo fractions. The obtained recoveries are quantitative for both elements, and their purities are sufficiently high to achieve high-precision measurements comparable to previous DS measurements of individual elements. The reproducibility of our isotope measurements for in-house standard solutions (2SD) was ±0.02‰ for δ186W and ±0.03‰ for δ98Mo. We applied our method to 27 geological reference materials including 10 igneous rocks (AGV-2, JA-3, JR-1, JB-1, JB-1a, JB-2, JB-3, W-2a, TDB-1, and WGB-1), 9 sediments (Nod-A-1, Nod-P-1, JMn-1, JMS-1, JMS-2, CRM7302-a, HISS-1, MESS-4, and PAC-3), and 8 sedimentary and metasedimentary rocks (SDC-1, SDO-1, SBC-1, SCO-1, SCO-2, JSL-1, JSL-2, and IOC-1) to produce a comprehensive data set. The data set confirmed the accuracy of our measurements and expanded the reference materials available for interlaboratory comparisons of δ186W and δ98Mo. The data set also indicates potential pitfalls in sample preparations for particular sample types and shows several variations of W and Mo isotopes possibly related to low-/high-temperature geochemical processes. Our new method, plus the reference data set, will facilitate the development of stable isotope geochemistry for W and Mo.

We examine the historical changes of 236U/238U and 235U/238U in a sediment core collected in Tokyo Bay and elucidate the anthropogenic sources of uranium in the 1960s-2000s. Uranium-236 was detected in samples deposited in the 1960s-2000s, and the 236U/238U ratio of the sediment core shows peak values in the 1970s. The 235U/238U isotopic ratios in samples deposited in the early 1960s are almost identical to that of natural uranium, implying that the 236U might have originated from global fallout. A decrease in 235U/238U was observed in the late 1960s-2000s, suggesting that depleted uranium from nuclear fuel reprocessing increased the 236U/238U ratios in the sediment. The 236U/238U values in sediments from the 1980s-2000s were lower than those in the 1970s but considerably higher than those in the 1960s, suggesting that the main source of depleted uranium still remains around Tokyo Bay. Our results demonstrated that the depleted uranium released in the 1970s should be considered as an important end-member when using uranium isotopic ratios as environmental tracers in closed aquatic environments around industrial cities.

We present the first report of the concentrations and stable isotope compositions of Te in the surface layer of ferromanganese crusts collected from the slope of two seamounts in the Northwest Pacific Ocean, Takuyo-Daigo and Takuyo-Daisan, which cover water depths from 1000 to 5500 m. The Te concentration in the surface layer of the ferromanganese crusts sharply decreases with increasing depth at shallow regions, but this decrease becomes more gradual at deeper regions. The Te isotope composition exhibits two trends that become (1) lighter with increasing water depth at shallow depths and (2) heavier with increasing water depth at greater depths. These profiles are the same for the two seamounts and indicate two types of correlations between the concentration and isotope composition with water depth. The turning points are located at approximately 2000 m for Takuyo-Daigo and 3200 m for Takuyo-Daisan, which likely correspond to different oxygen minimum zones. Co-precipitation processes with Fe and oxidation involving Mn may be responsible for the variation of Te concentration and isotope composition with water depth. These processes are also related to a change of the dissolved oxygen concentration in ambient seawater. The Te isotope compositions in ferromanganese crusts may therefore serve as a potential proxy for changes of the oxygen minimum zone of paleoceans.

We examine the temporal changes of 236U/238U and 235U/238U in atmospheric deposition from samples collected in Tokyo and Akita from 1963 to 1979 and elucidate the spatial distribution and historical changes of the anthropogenic sources of uranium in Japan. The 236U/238U ratio of atmospheric deposition in Tokyo peaked in 1963 and again during the 1970s, while the corresponding 235U/238U ratios of atmospheric deposition during the second peak period were lower than that of natural uranium. The 236U/238U ratios of atmospheric deposition in Akita samples peaked in 1963. The 235U/238U ratios in Akita samples were almost identical to that of the natural uranium ratios. These results suggest that the peak of 236U/238U in 1963 corresponds to what is recognized as representative for global fallout. The increase of 236U/238U and the decrease of 235U/238U observed simultaneously in the 1970s indicate that depleted uranium has subsequently been released into the environment around Tokyo. The cumulative deposition density of 236U for atmospheric fallout samples collected in Tokyo from 1968 to 1979 is an order of magnitude larger than that of the global fallout, suggesting that the depleted uranium in the 1970s is a major component of 236U in Tokyo and should be considered as an end-member when using 236U as an environmental tracer in the industrial city. This knowledge can facilitate future research using 236U as an effective environmental tracer.

Nickel (Ni), copper (Cu), and zinc (Zn) are commonly used in human activities and pollute aquatic environments including rivers and oceans. Recently, Ni, Cu, and Zn isotope ratios have been measured to identify their sources and cycles in environments. We precisely determined the Ni, Cu, and Zn isotope ratios in rain, snow, and rime collected from Uji City and Mt. Kajigamori in Japan, and investigated the potential of isotopic ratios as tracers of anthropogenic materials. The isotope and elemental ratios suggested that road dust is the main source of Cu in most rain, snow, and rime samples and that some of the Cu may originate from fossil fuel combustion. Zinc in the rain, snow, and rime samples may be partially attributed to Zn in road dust. Zinc isotope ratios in the Uji rain samples are lower than those in the road dust, which would be emitted via high temperature processes. Nickel isotope ratios are correlated with V/Ni ratios in the rain, snow, and rime samples, suggesting that their main source is heavy oil combustion. Furthermore, we analyzed water samples from the Uji and Tawara Rivers and the Kakita River spring in Japan. Nickel and Cu isotope ratios in the river water samples were significantly heavier than those in rain, snow, and rime samples, while Zn isotope ratios were similar. This is attributed to isotopic fractionation of Cu and Ni between particulate-dissolved phases in river water or soil.

© 2020 Elsevier B.V. We determined redox-sensitive element concentrations along with δ15NTN values from the Triassic-Jurassic shales interbedded within deep-sea cherts at the Katsuyama section, SW Japan, to clarify the redox and related nitrogen cycle conditions in mid-Panthalassa during the TJB biotic crisis, focusing particularly on the linkages with CAMP volcanism. Slight enrichments of Mn and Mo across the TJB suggest a vertically expanded mid-Panthalassic OMZ above a well‑oxygenated deep ocean. Moreover, high-resolution δ15NTN records during the Triassic-Jurassic transition exhibit a negative nitrogen isotope excursion (NNIE) just above the TJB. Relative to the previously reported negative carbon isotope excursion (NCIE) across the TJB, the NNIE can be divided into two intervals: negative interval 1 (NI-1) during the NCIE and NI-2 after the NCIE. The NNIE can be explained as an interval of nitrate-rich conditions, reflecting an oxic Panthalassic Ocean during the Triassic-Jurassic transition. Our geochemical dataset provides new insights into environmental perturbations in mid-Panthalassia during the TJB biotic crisis: (1) a vertically expanded OMZ and decreased primary productivity owing to global warming caused by CAMP volcanism across the TJB resulted in an increased nitrate concentration during the NI-1, and (2) eutrophic, nitrate-rich conditions developed in mid-Panthalassa during the NI-2 due to the enhanced continental weathering and deep-water upwelling. Significantly, our results are the first to demonstrate that the OMZs were expanded not only in the shallow-marine regions, but also in pelagic sites across the TJB, suggesting that the globally expanded OMZs can be regarded as a crucial driver for the TJB biotic crisis.

<p>The extinct, relatively short-lived nuclide ¹⁸²Hf produced ¹⁸²W as a decay product. Fractionation of Hf-W in the very early Earth led to variations in the ¹⁸²W/¹⁸⁴W ratios of terrestrial rocks; however, because these variations are very small, quantifying ¹⁸²W/¹⁸⁴W ratios requires an extremely precise method. Here, we propose an improved method for highly precise and accurate method for measuring the ¹⁸²W/¹⁸⁴W ratios of terrestrial rocks. Samples were extracted with 4-methyl-2-pentanone and purified by cation and anion exchange chromatography prior to determination of the W isotope ratio by multiple collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) system coupled with a desolvating nebulizer. Sample preparation removed matrix elements (e.g., Hf, Ta, Os, and dimers of Nb and Mo) with masses similar to those of W isotopes, resulting in these elements having a negligible influence on the measured ¹⁸²W/¹⁸⁴W ratios. A W standard solution processed by ion exchange chromatography and/or solvent extraction showed a ¹⁸³W deficiency, even after mass fractionation correction of the measured isotope data. As reported previously, mass-independent fractionation increases the ¹⁸²W/¹⁸⁴W ratio if the ¹⁸³W/¹⁸⁴W ratio is used to correct for mass fractionation to for better precision in natural samples. However, accurate ¹⁸²W/¹⁸⁴W ratios for a basalt reference material (JB-2) were obtained, even if ¹⁸³W was used for mass fractionation correction. Our results show that it is also possible to correct for the effects of mass-independent fractionation on the ¹⁸³W/¹⁸⁴W ratio by sample-standard bracketing using a W standard solution subjected to the same preparation procedure used for the samples. A major advantage of the newly developed method is that it requires a smaller amount of sample (0.2–0.3 g; 50–80 ng W for JB-2) compared with that needed for other reported methods (typically 0.7–15 g; 500–1000 ng W). This decrease in sample amount was possible by removing matrix elements from the sample solutions, and cleaning the membrane of the desolvating nebulizer between analyses also contribute to enhancing the W ion beam intensity and to high precision. Analysis of different basalts from the Loihi, Kilauea islands and Ontong Java Plateau with various W isotopic compositions consistent with the previous studies demonstrated the reliability of the method.</p>

Copyright © 2018 by the International Society of Offshore and Polar Engineers (ISOPE) Vertical profiles of seawater neodymium (Nd) concentration and isotopic composition were determined for a location near the Takuyo-Daigo Seamount in the northwest Pacific Ocean. The Nd isotopic composition profile is similar to the surface layer Nd isotopic composition of ferromanganese crusts collected from the Takuyo-Daigo Seamount. This confirms that Nd in the surface layer of ferromanganese crusts is supplied directly by ambient seawater.

Stable isotope ratios of nickel, copper, and zinc are powerful tools for elucidating the biogeochemical cycling of trace metals in the ocean. However, analytical difficulties have impeded isotopic studies of these metals. We present a simple and rapid method for simultaneous analysis of Ni, Cu, and Zn isotope ratios in seawater using NOBIAS Chelate-PA1 resin and anion exchange resin. A NOBIAS Chelate-PA1 resin column was used to quantitatively collect Ni, Cu, and Zn from seawater and thoroughly remove the seawater matrix. Subsequent anion exchange purified and separated the Ni, Cu, and Zn from each other. The blanks used in this method (0.22 ng for Ni, 0.29 ng for Cu, and 0.53 ng for Zn) were sufficiently low to determine the isotope ratios of Ni, Cu, and Zn in surface seawater. Using this method, we analyzed GEOTRACES reference seawater samples (i.e., SAFe D1 and SAFe D2), National Research Council Canada certified materials (i.e., CASS-5 and NASS-6), and seawater samples collected from different depths in the subarctic South Pacific. The results were consistent with previously reported values. This method is expected to accelerate isotopic research and contribute to our understanding of biogeochemical cycling in the ocean. (C) 2017 Elsevier B. V. All rights reserved.

<p>We present the tellurium (Te) isotope compositions of the acid leachates and residues from three carbonaceous chondrites, namely, Allende, Murchison, and Tagish Lake. Most of the Te isotope compositions in the acid leachates and residues were indistinguishable from that of the terrestrial standard within analytical uncertainties, indicating a homogeneous distribution of Te isotopes in the solar nebula. Previous studies have reported nucleosynthetic isotope anomalies for Sr, Mo, W, and Os in the leachates and residues from the same meteorites. This suggests that the anomalous Te isotope signatures within the carbonaceous chondrites, including presolar phases, have presumably been nearly completely erased via a temperature-controlled nebular processes that acted on the relatively volatile elements before the onset of parent body formation. In contrast, the final residue of the Allende chrondrite displays a small but resolvable Te isotope anomaly. We performed mixing calculations to reproduce the observed Te isotopic pattern for the Allende final residue, which can be explained by the depletion of a theoretical r-process component. This result suggests that our Allende final residue was depleted in presolar nanodiamonds, which were enriched in the theoretical r-process component, because nanodiamonds are strongly acid resistant and can survive the leaching steps used in this study. The presolar SiC is not responsible for the observed r-process depletion. The discrepancy might instead be attributed to the existence of another presolar phase, including glassy carbon, in the final Allende residue.</p>

Precise Sr isotopic compositions in samples from sequential acid leaching experiments have been determined for three carbonaceous chondrites, Allende, Murchison, and Tagish Lake, together with those in the bulk aliquots of these meteorites. The chondritic acid leachates and residues were characterized by Sr isotope anomalies with variable mu Sr-84 values (10(6) relative deviation from a standard material) ranging from +120 to -4700 ppm, documenting multiple nucleosynthetic sources within a single meteorite. In addition, the mu Sr-84 patterns across leaching samples for individual chondrites differed from one another. The highest mu Sr-84 values were observed for leaching Step 3 (HCl+H2O, 75 degrees C) for Allende and Murchison likely because of the incorporation of calcium and aluminum-rich inclusions (CAIs). In contrast, extremely low mu Sr-84 values were observed in the later fractions (Steps 6 and 7) for Murchison and Tagish Lake, suggesting the existence of s-process-enriched presolar SiC grains derived from AGB stars. A mu Sr-84-epsilon Cr-54 diagram was prepared with the CAIs and bulk aliquots of carbonaceous chondrites and other meteorites (noncarbonaceous) that were plotted separately; however, they still formed a global positive correlation. CAIs presented the highest mu Sr-84 and epsilon Cr-54 values, whereas carbonaceous chondrites and noncarbonaceous meteorites had intermediate and the lowest mu Sr-84 and epsilon Cr-54 values, respectively. The positive trend was interpreted as resulting from global thermal processing in which sublimation of high mu Sr-84 and epsilon Cr-54 carriers generated the excess mu Sr-84 and epsilon Cr-54 signatures in CAls, while noncarbonaceous planetesimals accreted from materials that underwent significant thermal processing and thus had relatively low mu Sr-84 and epsilon Cr-54 values. Apart from the global trend, the carbonaceous chondrites and noncarbonaceous meteorites both exhibited intrinsic variations that highlight an isotopic dichotomy similar to that observed in other isotope combinations (e.g., epsilon Cr-54-epsilon Ti-50, epsilon Cr-54-Delta O-17). A plausible scenario for creation of the intrinsic variations involves local thermal processing (e.g., flash heating for chondrule formation) caused by additional selective destruction of presolar grains differently than that caused by global thermal processing. The existence of such a global positive trend and local variations for two meteorite groups suggests a complicated dynamic history for the dust grains with respect to thermal processing, material transportation, and mixing in the protoplanetary disk prior to planetesimal formation. (C) 2015 Elsevier B.V. All rights reserved.

We present a new method for precise isotope analysis of Te by thermal ionization mass spectrometry in negative ion mode (N-TIMS) along with an improved technique for chemical separation of Te from geochemical samples. In the chemical separation, Te was purified by a three-step column chemistry method employing an anion exchange resin and an extraction chromatographic resin, yielding 98% recovery. To achieve strong Te- beam intensity in N-TIMS, we evaluated the optimum analytical conditions with respect to the filament material, ionization activator, and filament heating protocol. The reproducibilities of Te isotope ratios obtained by static multicollection using Faraday cups were 0.9%, 0.04%, 0.06%, 0.01%, 0.005%, and 0.008% for Te-120/Te-128, Te-122/Te-128, Te-123/Te-128, Te-125/Te-128 Te-126/Te-128 and Te-130/Te-128, respectively. This result is comparable to those of previous MC-ICP-MS studies. The reproducibilities of Te isotopes obtained by SEM jumping measurements were approximately ten times worse than those obtained by Faraday cup runs, excluding the Te-120/Te-128 ratio for which the reproducibility was comparable to that obtained by Faraday cup runs due to the extremely low abundance of Te-120. To further improve the analytical precision, the Te-126/Te-128 ratio was determined by multidynamic collection for the study of Sn-126-Te-126 chronology. The 2s of the Te-126/Te-128 ratio of the multidynamic collection was 1.6 times better than that obtained by static measurements. Finally, the accuracy of our method was confirmed by the analysis of two carbonaceous chondrites and one terrestrial standard rock, which was consistent with previously reported values.