TY - JOUR
T1 - Stable isotope fractionation of tungsten during adsorption on Fe and Mn (oxyhydr)oxides
AU - Kashiwabara, Teruhiko
AU - Kubo, Sayuri
AU - Tanaka, Masato
AU - Senda, Ryoko
AU - Iizuka, Tsuyoshi
AU - Tanimizu, Masaharu
AU - Takahashi, Yoshio
N1 - Funding Information:
We thank Dr. Katsuhiko Suzuki, Dr. Yusuke Fukami, Mrs. Yukari Otsuki, Mrs. Junko Saka (JAMSTEC), and Dr. Keisuke Fukushi (Kanazawa Univ.) for their contributions to this work. This study was supported by Grant-in-Aid for Research Activity Start-up from Japan Society for the Promotion of Science (JSPS KAKENHI Grant Number: 23840058), and the Sasakawa Scientific Research Grant from The Japan Science Society. This work was conducted based on the experiments with approvals from JASRI (Proposal Nos. 2010A1612, 2010B1664, 2011B1400, 2014B1307, and 2016A1350) and KEK (Proposal Nos. 2011G635, and 2013G562). Finally, we thank associate editor Dr. Jan Wiederhold and three anonymous referees for their constructive comments and suggestion.
Publisher Copyright:
© 2017 Elsevier Ltd
PY - 2017/5/1
Y1 - 2017/5/1
N2 - The similar, but not identical chemical properties of W compared with Mo suggest that the stable isotope system of W could be a novel proxy to explore the modern and ancient ocean as is the case in the well-established utility of Mo isotopes. We experimentally investigated the isotopic fractionation of W during adsorption on Fe and Mn (oxyhydr)oxides (ferrihydrite and δ-MnO2), a key process in the global ocean budget of this element. Our adsorption experiments confirmed that W isotopes fractionate substantially on both ferrihydrite and δ-MnO2: lighter W isotopes are preferentially adsorbed on both oxides as a result of equilibrium isotopic exchange between dissolved and adsorbed species, and the obtained values of Δ186/183Wliquid–solid ( = δ186Wdissolved − δ186Wadsorbed) are 0.76 ± 0.09‰ for ferrihydrite and 0.88 ± 0.21‰ for δ-MnO2 (2σ, n = 6). Compared with the case of Mo isotopes, fractionation of W isotopes is (i) of comparable magnitude between ferrihydrite and δ-MnO2, and (ii) much smaller than that of Mo on δ-MnO2. Our previous XAFS observations and newly-performed DFT calculations both indicate that the observed W isotopic fractionations are caused by the symmetry change from Td (tetrahedral) WO42− to distorted Oh (octahedral) monomeric W species via formation of inner-sphere complexes on both ferrihydrite and δ-MnO2. The similar isotopic fractionations between the two oxides relate to the strong tendency for W to form inner-sphere complexes, which causes the symmetry change, in contrast to the outer-sphere complex of Mo on ferrihydrite. The smaller isotopic fractionation of W compared with Mo on δ-MnO2 despite their similar molecular symmetry seems to be due to their different degrees of distortion of Oh species. Our findings imply that the isotopic composition of W in modern oxic seawater is likely to become heavier relative to the input by removal of lighter W isotopes via adsorption on ferromanganese oxides in analogy with the Mo isotope budget. In contrast, the isotopic composition of W in ancient seawater should have evolved in response to the extent of deposition of both Fe and Mn oxides; this is likely to be different compared with that of the Mo isotopes, which is strongly associated with the occurrence of Mn oxides relative to Fe oxides.
AB - The similar, but not identical chemical properties of W compared with Mo suggest that the stable isotope system of W could be a novel proxy to explore the modern and ancient ocean as is the case in the well-established utility of Mo isotopes. We experimentally investigated the isotopic fractionation of W during adsorption on Fe and Mn (oxyhydr)oxides (ferrihydrite and δ-MnO2), a key process in the global ocean budget of this element. Our adsorption experiments confirmed that W isotopes fractionate substantially on both ferrihydrite and δ-MnO2: lighter W isotopes are preferentially adsorbed on both oxides as a result of equilibrium isotopic exchange between dissolved and adsorbed species, and the obtained values of Δ186/183Wliquid–solid ( = δ186Wdissolved − δ186Wadsorbed) are 0.76 ± 0.09‰ for ferrihydrite and 0.88 ± 0.21‰ for δ-MnO2 (2σ, n = 6). Compared with the case of Mo isotopes, fractionation of W isotopes is (i) of comparable magnitude between ferrihydrite and δ-MnO2, and (ii) much smaller than that of Mo on δ-MnO2. Our previous XAFS observations and newly-performed DFT calculations both indicate that the observed W isotopic fractionations are caused by the symmetry change from Td (tetrahedral) WO42− to distorted Oh (octahedral) monomeric W species via formation of inner-sphere complexes on both ferrihydrite and δ-MnO2. The similar isotopic fractionations between the two oxides relate to the strong tendency for W to form inner-sphere complexes, which causes the symmetry change, in contrast to the outer-sphere complex of Mo on ferrihydrite. The smaller isotopic fractionation of W compared with Mo on δ-MnO2 despite their similar molecular symmetry seems to be due to their different degrees of distortion of Oh species. Our findings imply that the isotopic composition of W in modern oxic seawater is likely to become heavier relative to the input by removal of lighter W isotopes via adsorption on ferromanganese oxides in analogy with the Mo isotope budget. In contrast, the isotopic composition of W in ancient seawater should have evolved in response to the extent of deposition of both Fe and Mn oxides; this is likely to be different compared with that of the Mo isotopes, which is strongly associated with the occurrence of Mn oxides relative to Fe oxides.
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U2 - 10.1016/j.gca.2017.01.031
DO - 10.1016/j.gca.2017.01.031
M3 - Article
AN - SCOPUS:85013176287
SN - 0016-7037
VL - 204
SP - 52
EP - 67
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -