Both molybdenum (Mo) and tungsten (W) form soluble oxyanions in oxic seawater, whereas Mo forms insoluble thiomolybdate and W forms soluble thiotungstate in sulfidic seawater. Thus, concentrations and stable isotope ratios of Mo and W in sediments may fluctuate due to changes in redox conditions and can be used to estimate paleoenvironmental changes. The modern Japan Sea is oxic from the surface to the bottom, whereas deep water became anoxic several times from the late Pleistocene to the Holocene. Detailed information on redox conditions is still lacking. In this study, we analyzed a sediment core that was collected from offshore Iwanai, Hokkaido (43°22ʹ36ʺ N, 140°04ʹ10ʺ E, water depth 900 m). To the best of our knowledge, our study is the first to report stable isotope data of Mo and W in sediments of the Japan Sea. We observed maxima in the Mo concentration of up to 29 ppm in the sediment layers of 11–10 ka, 17–14 ka (the last glacial maximum), 31 ka, and 45 ka in accordance with the maxima of total sulfur, thereby indicating the deposition of thiomolybdate MoOxS4−x2− (0 ≤ x ≤ 3). δ98Mo, however, was between –0.19 and 0.69‰ at these ages, suggesting that the H2S concentration in bottom water never exceeded 11 μmol kg−1. The concentration and isotopic ratio of W were relatively constant throughout the core; W = 1.2 ± 0.2 ppm and δ186W = 0.03 ± 0.03‰ (ave ± sd). The authigenic Mo and W ratio, Moauth/Wauth (mol/mol), was 10.5 ± 7.3 except for the above four ages, supporting the control of Mn and Fe (oxyhydr)oxides on Moauth and Wauth under oxic conditions.
To explore the shallow marine redox state from the Ediacaran to the Cambrian (ca. 635–488.3 Ma) Yangtze Platform in southern China, this study presents traditional rare-earth element (e.g., δCe, δEu, REE distribution pattern and Y/Ho), enrichment factor, and paleoenvironment proxy data from the Kaiyang shelf from Nantuo to Loushanguan Formations. Our data from a drill core from Kaiyang suggest the occurrence of a fluctuating redox state from the Ediacaran to the Cambrian ocean, in which the fluctuating redox state of shallow water coexisted with eustatic change. We observe an obvious oxygenation signal of seawater during Ediacaran-Cambrian transition and a distinctive redox state in Ediacaran and Cambrian shallow seawater. We posit that the large-scale oxygenation of the atmospheric-oceanic system may have played a critical role in the main phase of Cambrian Explosion, and a fluctuating redox state of shallow seawater existed at the Cambrian stage.
An experimental method was developed for mutual separation of a series of rare earth elements (REE) from 57La to 71Lu. This method was applied in isotopic analyses of geological materials. Resin chemistry was used in a two-step chemical separation procedure. The first step is the separation of a fraction including a group of REE from the major component elements in common silicate rocks using a cation-exchange resin. The second successive step is the mutual separation of the individual REE using a lanthanoid element-specific resin (Ln resin). To confirm the validity of our chemical separation techniques for isotopic studies of geochemical applications, isotopic analyses of Dy, Er, and Yb separated from two geological materials, JB-2 and JG-2, were performed by thermal ionization mass spectrometry (TIMS). Our analytical techniques provide high-quality isotopic data within 0.004% of analytical reproducibility (95% confidence level) for major isotopic ratios of Dy, Er and Yb in geological samples.
Isotopic analyses of Sr and Ba and quantitative analyses of Rb, Sr, Cs, Ba, and rare earth elements from chemical separates of four chondrites, i.e., Cold Bokkeveld (CM2.2), Murray (CM2.4/2.5), Nogoya (CM2.2), and NWA 4428 (H4), were performed to investigate the origin of Sr and Ba isotopic variations. Most Ba isotopic deviations of the chemical separates showed positive isotopic anomalies of 135Ba that correlated with 137Ba and 138Ba, suggesting a heterogeneous distribution of s-process isotopes in the early solar system. The Sr and Ba isotopic data for the acid residues showed significant isotopic deficits of 84Sr, 130Ba, 132Ba, 135Ba, 137Ba, and 138Ba derived from the enrichments of s-process isotopes caused by the presence of presolar SiC grains. Furthermore, the correlation between the isotopic data for 135Ba, 137Ba, and 138Ba with the isotopic data for the acid residues suggested the contribution of n-process nucleosynthetic components. Assuming that the Ba isotopic compositions of the acid residues were a mixture of s- and n-process nucleosynthetic components and radiogenic components of 135Ba, the upper limits of the initial 135Cs isotopic abundance expressed as the 135Cs/133Cs ratio in the early solar system could be estimated as (2.4 ± 0.4) × 10–4 for Cold Bokkeveld, (1.6 ± 0.9) × 10–4 for Murray, and (4.6 ± 0.7) × 10–4 for Nogoya, respectively.
In order to better understand the tectono-magmatic history of the Miocene Ashizuri igneous complex in Shikoku, SW Japan, we conducted U-Pb isotopes and trace element analyses of zircons from an alkali gabbro, a dolerite and A-type granites in this complex by laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). The weighted mean U-Pb ages of zircons in all samples are in the range of ca. 14–13 Ma. The chondrite-normalized REE patterns show positive Ce anomalies, negative Eu anomalies, and enriched HREE relative to LREE and MREE. On the other hand, a sandstone in the Shimizu Formation intruded by the Ashizuri complex contains detrital zircons older than 35 Ma. These results suggest that the ca. 14–13 Ma Ashizuri felsic magma was less affected by crustal contamination and/or assimilation during its emplacement and solidification. Moreover, the gabbros, in spite of their mafic composition, contain igneous zircons most likely derived from a felsic magma. This indicates that magma mixing/mingling took place between the mafic and felsic magmas. Integration of our results with previously published data suggests that the Ashizuri granitic magma underwent fractionation from an OIB-type magma.
Elemental and isotopic analyses of geological samples require several sample-preparation steps that can introduce contamination and result in analytical inaccuracies. In this study, the influence of different materials used as rock and mineral powdering equipment (agate, stainless steel and tungsten carbide) on Re-Os contents and Os isotope ratios was evaluated by crushing quartz sand with variable pre-powdering grain sizes (2–4, 6–10, and 8–16 mesh). Rhenium-Os contents were least affected by crushing in agate, and increased by up to one and two orders of magnitude after crushing in stainless steel and tungsten carbide, respectively. Osmium contamination is affected by the duration of powdering, whereas this is not the case for Re. 187Os/188Os ratios decrease due to contamination. Agate powdering equipment is optimal for sample preparation for Re-Os concentration and isotopic analysis. Moreover, a small pre-powdering grain size (8–16 mesh) can reduce contamination.