The molybdenum isotopic composition of the modern ocean

Abstract

Wang et al., 2011 (1) and is converted to δ 97/95 Mo by multiplying 0.67 when the system follows mass-dependent fractionation. Though no commonly-used Mo isotopic standard currently exists (see later), the standards used by different laboratories do appear to be isotopically similar at the level of 0.1-0.2‰. Continental rock samples, such as granite, basalt, and clastic sediments, have δ 98/95 Mo of ~0.1‰ Siebert et al., 2003). Ferromanganese oxides, which scavenge Mo from seawater in oxic conditions, have a light isotopic signature (δ 98/95 Mo = -1.0 to -0.5‰; Siebert et al., 2003), whereas sediments formed in euxinic conditions (aqueous H 2 S concentrations higher than ~11 µM, the action point pro-The molybdenum isotopic composition of the modern ocean (Received September 13, 2011; Accepted January 5, 2012) 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 . 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 δ 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 δ 98/95 Mo of the standard used in this study was 0.117 ± 0.009‰ lighter than the Mo standard that was used by Archer and Vance (2008). A common Mo standard is urgently required for the precise comparison of Mo isotopic compositions measured in different laboratories. On the other hand, our results strongly support the possibility of seawater as an international reference material for Mo isotopic composition.

Molybdenum isotopes in hydrothermal manganese crust from the Ryukyu arc system: Implications for the source of molybdenum

“…The alternative is that Mo was incorporated from seawater, in which dissolved Mo is predominantly in the tetrahedrally coordinated oxyanion MoO 4 2 − (Byrne, 2002) and has a nominally homogeneous δ 98/95 Mo value of~2.1‰ (Barling et al, 2001;Siebert et al, 2003;Nakagawa et al, 2012;Goldberg et al, 2013). If most of the Mo in the sample was adsorbed from seawater, we would expect a δ 98/95 Mo value of approximately −0.6‰, which is in good agreement with our results (Fig.…”

“…The striped area shows the range of Os isotopic compositions in hydrothermal fluids ( 187 Os/ 188 Os = 0.11-0.40) (Sharma et al, 2000). (Barling et al, 2001;Siebert et al, 2003;Nakagawa et al, 2012;Goldberg et al, 2013). The dashed line show the Mo isotopic compositions in hydrothermal fluids from the low-temperature ridge-flank hydrothermal system in the Cascadia Basin (δ 98/95 Mo =~0.5) (McManus et al, 2002).…”

“…In the modern ocean, dissolved Mo is dominated by the tetrahedrally coordinated oxyanion MoO 4 2 − (Byrne, 2002) and is generally conservative in aqueous solutions (oceanic residence time of Mo =~400-800 kyr) (Emerson and Huested, 1991;Miller et al, 2011). As a result, Mo is the most abundant transition metal in the ocean (~102 nmol/kg; Collier, 1985), and its isotopic composition in seawater is nominally homogeneous (δ 98/95 Mo =~2.1‰; Barling et al, 2001;Siebert et al, 2003;Nakagawa et al, 2012;Goldberg et al, 2013). The remarkably uniform δ 98/95 Mo values in two hydrogenous Fe-Mn crusts suggest that seawater δ 98/95 Mo may/might have not changed significantly during Cenozoic time (Siebert et al, 2003).…”

What the ~1.4 Ga Xiamaling Formation can and cannot tell us about the mid‐Proterozoic ocean

“…In such cases, authigenic Mo is typically ~1‰ lighter than the associated seawater (Arnold et al., ; Barling et al., ; Neubert, Nägler, & Böttcher, ; Poulson Brucker et al., ; Poulson, Siebert, McManus, & Berelson, ; Siebert, McManus, Bice, Poulson, & Berelson, ). In the modern ocean, the isotopic composition of Mo is about +2.3‰ (Barling et al., ; Nägler et al., ; Nakagawa et al., ; Siebert et al., ), which is significantly heavier than the riverine input of +0.7‰ (Archer & Vance, ). This disparity is maintained through dominance by the Mn oxide sink in today's well‐ventilated oceans, exporting large amounts of the lighter 95 Mo.…”

Molybdenum Mass Fractions and Isotopic Compositions of International Geological Reference Materials

“…The accuracy and longterm reproducibility were further evaluated by analysing seawater samples from the Atlantic Ocean over a period of 2 years (Table 1, Figure 2). Previous studies showed that the d 98/95 Mo of modern open-ocean water accounts for some of the variation, and reflect a small inherent heterogeneity within ± 0.10‰ (2s) (Siebert et al 2003, Greber et al 2012, Nakagawa et al 2012, Goldberg et al 2013. Greber et al (2012) renormalised their Mo isotope data for seawater to NIST SRM 3134 = 0‰ and obtained a value of 2.09 ± 0.07‰ (2s, n = 5).…”