The molybdenum isotopic composition of the modern ocean

作者：Yusuke Nakagawa, Shotaro Takano, Mochamad Lutfi Firdaus, Kazuhiro Norisuye, Takafumi Hirata, Derek Vance, Yoshiki Sohrin · 发表于：GEOCHEMICAL JOURNAL · 年份：2012 · DOI：10.2343/geochemj.1.0158 · 被引用次数：144 · 研究领域：Paleontology and Stratigraphy of Fossils、Geochemistry and Elemental Analysis、Radioactive element chemistry and processing

Natural variations in the isotopic composition of molybdenum (Mo) are showing increasing potential as a tool in geochemistry. Although the ocean is an important reservoir of Mo, data on the isotopic composition of Mo in seawater are scarce. We have recently developed a new method for the precise determination of Mo isotope ratios on the basis of preconcentration using a chelating resin and measurement by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), which allows us to measure every stable Mo isotope (Nakagawa et al., 2008). In this study, 172 seawater samples obtained from 9 stations in the Pacific, Atlantic, and Southern Oceans were analyzed, giving global coverage and the first full depth-profiles. The average isotope composition in δA/95Mo (relative to a Johnson Matthey Mo standard solution) was as follows: δ92/95Mo = -2.54 ± 0.16‰ (2SD), δ94/95Mo = -0.73 ± 0.19‰, δ96/95Mo = 0.85 ± 0.07‰, δ97/95Mo = 1.68 ± 0.08‰, δ98/95Mo = 2.48 ± 0.10‰, and δ100/95Mo = 4.07 ± 0.18‰. The δ values showed an excellent linear correlation with atomic mass of AMo (R2 = 0.999). Three-isotope plots for the Mo isotopes were fitted with straight lines whose slopes agreed with theoretical values for mass-dependent isotope fractionation. These results demonstrate that Mo isotopes are both uniformly distributed and follow a mass-dependent fractionation law in the modern oxic ocean. In addition, Mo isotopic analysis revealed that δ98/95Mo of the standard used in this stud...