Natural mass-dependent variations in the isotopic composition of molybdenum

Earth and Planetary Science Letters 193 (2001) 447^457 www.elsevier.com/locate/epsl

Natural mass-dependent variations in the isotopic composition of molybdenum J. Barling a; *, G.L. Arnold a , A.D. Anbar a;b a

Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA b Department of Chemistry, University of Rochester, Rochester, NY 14627, USA Received 17 May 2001; received in revised form 7 September 2001; accepted 13 September 2001

Abstract We present the first observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Mo), using multi-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are reported as N97=95 Mo ( = ((97 Mo/95 Mo)sample /(97 Mo/95 Mo)standard 31)U1000x). External analytical precision of N97=95 Mo is 6 þ 0.25x (2c) on natural samples. Our data demonstrate a clear offset of s 1x between sediments deposited under anoxic conditions (N97=95 Mo = +1.02 to +1.52x relative to our in-house standard) and ferromanganese nodules (N97=95 Mo = 30.63 to 30.42x). N97=95 Mo of Pacific Ocean seawater (N97=95 Mo = +1.48x) lies within the range of values for anoxic sediments, closest to modern Black Sea anoxic sediments. Molybdenites from continental ore deposits have intermediate N97=95 Mo ranging from 30.26 to +0.09x. Variations in the abundances of 92 Mo, 95 Mo, 96 Mo, 97 Mo and 98 Mo are consistent with mass-dependent fractionation. A sporadic unidentified interference occurs at mass 94 and 100 Mo is not measured. We hypothesize that the N97=95 Mo offset between anoxic sediments and ferromanganese nodules results from Mo isotope fractionation during inefficient scavenging of Mo from seawater by Mn oxides under oxic conditions. The similarity in N97=95 Mo of anoxic sediments and seawater is consistent with the very efficient removal of Mo from seawater under anoxic conditions in the presence of H2 S. The data can be interpreted in terms of a steady-state mass balance between the Mo flux into the oceans from the continents and the Mo flux out of the oceans into oxic and anoxic sediments. Such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of Mo to anoxic and oxic sediments. These findings suggest that N97=95 Mo in seawater may co-vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans, and that this variation may be recorded in N97=95 Mo of anoxic sediments. Hence, the Mo isotope system may be useful in paleoredox investigations. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: molybdenum; stable isotopes; isotope fractionation; Eh; anaerobic environment; paleoenvironment

1. Introduction High-precision isotopic analyses using multi-

* Corresponding author. Fax: +1-716-244-5689. E-mail address: [email protected] (J. Barling).

collector, magnetic-sector inductively coupled plasma mass spectrometry (MC-ICP-MS) have recently revealed that the isotopes of Mo can be chemically fractionated in the laboratory, and that the isotopic composition of this element in puri¢ed industrial Mo is di¡erent from molybdenite (MoS2 ) ore [1]. However, mass-dependent

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 1 4 - 3

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Mo isotopic variations in natural materials have not yet been reported. With the advent of MCICP-MS, such variations have been reported for isotopes of Fe, Cu, Zn and Tl [2^10]. Laboratory studies suggest that these variations result from both equilibrium and kinetic chemical isotope fractionation, in some cases biologically mediated [2,9,11,12]. Mo isotopes have a percent mass spread of V10%, larger than for any of the above elements, and Mo environmental chemistry is characterized by pronounced changes in redox state and coordination. Therefore, it is reasonable to expect signi¢cant variations in the isotopic composition of Mo in nature. Here, we present the ¢ndings of an initial survey of mass-dependent variations of the isotopic composition of Mo in some geological materials of importance in the environmental geochemical cycle of this element. The aqueous geochemistry of Mo makes it a particularly interesting element for isotopic investigation. As a transition metal of Group VIB of the periodic table, Mo has a wide range of oxidation states. Under oxidizing conditions Mo(VI) is the stable form. In natural waters under oxic conditions it occurs as the highly stable molybdate ion (MoO23 4 [13]); in the oceans its concentration is uniform at 10.3 þ 0.48 (2c) Wg/l [14] and recent estimates of its residence time are of the order of 800 000 years (e.g. [15]). However, in the presence of reduced sulfur Mo forms oxothiomolybdate ions (e.g. MoO43x S23 x ). These ions are highly particle-reactive and hence are rapidly removed from solution [16^18]. As a result, in comparison to oxic seawater, Mo is generally de¢cient in waters with free H2 S [19]. pH also plays an important role in Mo solubility, with insoluble forms favored at low pH (e.g. [13]). Due to this sensitivity of Mo to environmental redox conditions, we have focussed on sediments deposited under di¡erent redox conditions and on the possibility that Mo stable isotope geochemistry may prove useful in paleoredox applications. Materials studied include anoxic sediments, ferromanganese nodules, seawater and a small number of continentally derived samples.

2. Analytical methods 2.1. Sample preparation In order for Mo isotopes to be measured by MC-ICP-MS, Mo in natural samples must be extracted and puri¢ed. The reasons for this are twofold. Firstly, for our method of analysis (see [1], and below) a Zr `spike' is used to correct both samples and standards for instrumental mass fractionation. Therefore all naturally occurring Zr must be stripped from the samples. Secondly, all other elements must be eliminated as far as possible in order to minimize both isobaric interferences and matrix e¡ects. Matrix e¡ects include variations in the relative ionization e¤ciency of elements caused by competition for ionization in the plasma when sample solutions contain high dissolved solids. Such variations can a¡ect the relative fractionation of Mo and Zr and thus reduce the reliability of the correction for instrumental fractionation. Sample puri¢cation was carried out in a metalfree clean room ¢tted with a HEPA-¢ltered air supply and laminar £ow benches. All labware, including Te£on and disposable plastics, was cleaned in concentrated reagent-grade acids and 18 M6 de-ionized H2 O prior to use. Acids used during separation chemistry were ultra-pure ; either Seastar1 sub-boiling, quartz-distilled acids, or acids puri¢ed in-house by sub-boiling distillation in a Te£on still. All dilutions were performed with 18 M6 de-ionized H2 O. Sample preparation varied according to sample type. Seawater Mo was provided by C. Tuit (WHOI) as a puri¢ed Mo concentrate that had been processed through a chelating resin ion exchange chemistry modi¢ed from Riley and Taylor [20]. As a test of this separation chemistry, another seawater sample was heavily doped with a Mo standard before puri¢cation. The isotopic composition of the puri¢ed Mo was found to be identical within error to that of the Mo standard, indicating that no isotopic fractionation had occurred as a result of the puri¢cation process and/or that there was quantitative recovery of Mo. Molybdenite (MoS2 ) has high Mo concentra-

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tions and, apart from sulfur, no other major constituents. Molybdenite samples were therefore dissolved in aqua regia and reconstituted in 0.05 M HNO3 without need for pre-concentration or puri¢cation of Mo. All other samples were dissolved by a standard HF/HNO3 attack in Savillex0 screw top PFA Teflon bombs, followed by solution in 6 M HCl. Prior to this, organic carbon-rich samples were ashed at 500³C. Molybdenum was then puri¢ed by a two-column ion exchange separation. An anion exchange column (Bio-Rad1 AG1-X8) was used to separate Zr from Mo. Samples were loaded and rinsed in 6 M HCl to remove Zr and substantial amounts of other elements. Mo was then eluted in 1 M HCl. The anion column does not separate Fe (a major element in most samples) from Mo. In order to do this a cation exchange column (Bio-Rad1 AG50W-X8) was employed. Samples were loaded and Mo eluted in 1.5 M HCl. Mo, which forms anionic complexes in this solution, passed straight through the column, while cations and cationic complexes such as those formed by Fe were retained by the resin. Mo recovery from this column is quantitative, presumably due to the minimal interaction of Mo with the resin. A visual test of the purity of the ¢nal dry Mo sample is its deep inky blue color, believed to result from the formation of reduced mixed valence Mo oxide/hydroxides (`molybdenum blue'). If necessary, the cation column separation was repeated until a pure Mo sample was achieved. Samples were reconstituted in 0.05 M HNO3 for introduction into the instrument. Sample stability was ensured by the addition of a few microliters of concentrated HF. Fractionation of Mo isotopes occurs during separation on the anion exchange column [1]. It is therefore essential to have greater than 90% recovery of Mo in order to ensure that fractionation artifacts produced by sample preparation are smaller than the external precision of the analysis ( þ 0.25x; see below). In order to monitor this, precisely weighed aliquots of every sample are taken from the sample solution before chemistry and from the ¢nal puri¢ed Mo solution. The Mo concentrations of these solutions are determined

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by isotope dilution/ICP-MS (VG Elemental PQ II+). Recovery is calculated from the di¡erence between the total amount of Mo in the sample solution before chemistry and the total amount of Mo in the ¢nal puri¢ed solution. 2.2. Mass spectrometry All measurements were made on the Plasma54 MC^ICP^MS (VG Elemental) at the University of Rochester. Procedures are discussed in detail in Anbar et al. [1] and are summarized brie£y here, with attention given to modi¢cations for the analysis of natural samples. Immediately prior to analysis, samples are diluted to give a 98 Mo beam intensity of at least 1 V. The concentration of Mo in such a run solution varied from day to day but was typically around 1.5 ppm. The Zr elemental spike used to correct for mass bias [5,21] is added from a concentrated Zr stock standard (Johnson Matthey Specpure0 Zirconium Plasma Standard, Lot No. 700193E) in su¤cient quantity to give a Zr concentration half that of the concentration of Mo. Samples are introduced to the instrument via a microconcentric nebulizer and desolvating system (Transgenomic/CETAC Aridus I). Each isotopic analysis represents the average of 30^50 ratio measurements, each of which was integrated over 5 s. Molybdenum intensities are corrected for Zr interference on masses 92, 94 and 96, and Mo ratios determined as 9x Mo/97 Mo for masses 92, 94, 95, 96 and 98. A sporadic, unidenti¢ed interference observed at mass 94 precludes the use of 94 Mo data [1]. However, the correction for Zr interferences results in measurements of 92 Mo/ 97 Mo and 96 Mo/97 Mo ratios that are su¤ciently precise to permit us to verify that fractionation is mass-dependent across all other measured Mo isotopes [1]. Although we use four measured 9x Mo/97 Mo ratios to assess data reliability (see below), samples are compared based on 95 Mo/97 Mo in order to avoid possible uncertainties arising from the correction for isobaric interferences at other Mo isotopes. To conform with standard stable isotope conventions, these data are reported in Table 1

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Table 1 Mo isotope data for anoxic sediments, ferromanganese nodules, continentally derived samples and seawater Sample

N97=95 Moa þ S.D.

N9x=97 Mo/amu N92=97 Mo/amu

N95=97 Mo/amu

N96=97 Mo/amu

N98=97 Mo/amu

Mean

1.60 1.44 1.52 þ 0.11

30.812 30.806

30.797 30.721

30.791 30.755

30.768 30.811

Unit 2 (BC21-3) Unit 2 (BC25-2)

1.11 1.17

30.569 30.574

30.552 30.582

30.553 30.549

30.533 30.560

0.87 1.14 1.05 0.82 0.98 0.96 0.99 0.90 1.12 1.12 1.09 1.16 1.02 þ 0.11

30.408 30.494 30.449 30.416 30.506 30.494 30.542 30.509 30.603 30.608 30.594 30.631

30.436 30.568 30.522 30.410 30.492 30.479 30.496 30.451 30.561 30.557 30.545 30.581

30.414 30.499 30.465 30.409 30.451 30.489 30.542 30.484 30.576 30.589 30.563 30.594

30.405 30.513 30.458 30.419 30.532 30.454 30.487 30.504 30.591 30.543 30.572 30.595

30.42 30.46 30.38 30.40 30.41 30.26 30.60 30.42 þ 0.10

0.186 0.207 0.173 0.179 0.183 0.127 0.256

0.211 0.229 0.188 0.198 0.205 0.129 0.301

0.182 0.188 0.153 0.163 0.153 0.082 0.205

0.179 0.214 0.167 0.165 0.165 0.111 0.299

30.62 30.48 30.75 30.69 30.62 30.63 þ 0.10

0.231 0.356 0.318 0.358 0.330

0.240 0.373 0.311 0.344 0.312

0.271 0.324 0.317 0.362 0.336

0.193 0.388 0.325 0.338 0.323

0.00 30.02 30.10 30.04 þ 0.05

30.020 30.007 30.006

0.001 0.010 0.050

30.059 30.065 30.036

0.017 30.004 0.031

Anoxic sediments: Black Seab Unit 1 (BC21-2)

Devonian Ohio Shale USGS SDO-1

Mean Oxic sediments: Paci¢c Ocean USGS Nod-P-1

Mean Atlantic Ocean USGS Nod-A-1

Mean Continental material: Molybdenites 34844 Mean

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Table 1 (Continued) Sample 34843

Mean 34840

N97=95 Moa þ S.D. 0.18 30.07 0.16 0.11 0.03 0.11 0.09 þ 0.09

Mean

30.19 30.24 30.29 30.31 30.27 30.26 30.26 þ 0.04

Seawater: Paci¢c Oceanc

1.48

N9x=97 Mo/amu N92=97 Mo/amu

N95=97 Mo/amu

N96=97 Mo/amu

N98=97 Mo/amu

30.085 0.015 30.071 30.089 30.042 30.083

30.090 0.033 30.082 30.054 30.015 30.053

30.079 30.015 30.049 30.100 30.052 30.084

30.089 0.000 30.114 30.080 30.030 30.096

0.092 0.118 0.134 0.119 0.094 0.094

0.093 0.120 0.145 0.157 0.134 0.131

0.085 0.093 0.101 0.104 0.096 0.079

0.059 0.117 0.144 0.147 0.117 0.094

30.737

30.683

30.656

30.704 97=95

For consistency with standard practice, ¢nal data are reported as N Mo where N97=95 Mo = [((97 Mo/95 Mo)sample /(97 Mo/95 Mo)standard 31)U1000x. To assess data quality we use N9x=97 Mo/amu values ( = N9x=97 Mo/(M97 3M9x ) where N9x=97 Mo is calculated from the data, as collected, with 97 Mo as the denominator i.e. N9x=97 Mo = ((9x Mo/97 Mo)sample /(9x Mo/97 Mo)standard 31)U1000x. a Samples with more than three repeats were measured on multiple days. b Samples supplied by G. Ravizza (see also [22]). c Latitude 9³46.00N; longitude 104³22.00W; depth = 104.5 m.

in N notation with the heavy isotope as the numerator (i.e. N97=95 Mo = [((97 Mo/95 Mo)sample /(97 Mo/ 95 Mo)standard 31)U1000x) relative to our inhouse Mo reference standard (Johnson Matthey Specpure0 Molybdenum Plasma Standard, Lot No. 7024991). Prior to running any samples, the instrument is tuned and a sequence of standards analyzed in order to establish reliable running conditions. Since the data are collected as 9x Mo/97 Mo ratios, we assess data reliability in terms of N9x=97 Mo calculated from 9x Mo/97 Mo ratios, i.e. N9x=97 Mo = [((9x Mo/97 Mo)sample /(9x Mo/ 97 Mo)standard 31)U1000x and not in terms of 9x Mo/95 Mo ratios. To assess whether the isotopic variations we see are mass-dependent we compare N9x=97 Mo/amu ( = N9x=97 Mo/(M97 3M9x ); where M is the isotopic mass) for all isotopes measured except 94 Mo. The standard sequence includes our in-house Mo reference standard interleaved with a gravimetrically prepared 97 Mo-enriched standard of known isotopic shift (N95=97 Mo =

31.01x, N96=97 Mo and N98=97 Mo = 31.00x) and an in-house rock standard that has been puri¢ed through chemistry (USGS SDO-1; N9x=97 Mo/amu = 30.51x/amu). These standards are used to determine that instrumental conditions are stable and that data acquired are accurate, precise, internally consistent and reproducible. To satisfy these requirements, the N95=97 Mo, N96=97 Mo and N98=97 Mo for the gravimetric standard, must be within þ 0.20x of the expected value, and are typically within þ 0.10x. For the in-house rock standard N95=97 Mo = 31.02 þ 0.20x and for any single analysis, the total range of values for the four N9x=97 Mo/amu (i.e. excluding N94=97 Mo) must be 6 0.10x/amu. In practice the vast majority ( s 85%) of samples that pass this test have values for N95=97 Mo/amu and N98=97 Mo/amu (the masses without Zr interferences) that agree to better than 0.05x/amu. This test (also applied to all samples) is critical in order to establish that observed shifts in Mo isotopic composition result from mass-dependent

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isotopic fractionation rather than from uncorrected isobaric interferences or matrix e¡ects. Once these conditions are satis¢ed, samples are run in triplicate (sample size permitting) interleaved with our in-house reference standard and occasional repeats of the gravimetric standard. The internal precision of each 95 Mo/97 Mo ratio measurement was typically 9 þ 0.0040% (2cmean ). The reported N97=95 Mo and N9x=97 Mo used in data assessment are calculated using the regression method [1,5] which entails plotting, for example, ln(9x Mo/97 Mo)meas versus ln(90 Zr/91 Zr)meas and determining the ordinate distance between each sample and a reference line ¢tted to the in-run standards. On such a plot, samples and standards lie on parallel linear arrays whose o¡sets in the ordinate direction re£ect the di¡erence between their true Mo isotope ratios. Based on results from replicate analyses on multiple days over many months, external precision of N97=95 Mo on in-run standards is 6 þ 0.1x ( þ 2c) for the 97 Mo-enriched standard. This represents a factor of two improvement over our previously reported external precision [1] and is a direct result of the quality control criteria discussed above. However, N97=95 Mo of samples puri¢ed through chemistry have an external precision of 6 þ 0.25x ( þ 2c). The fact that the external precision on puri¢ed standard solutions is better than that for natural samples suggests that residual sample matrix has an important e¡ect on reproducibility at the þ 0.25x level. Therefore higher precision data will require more careful evaluation of matrix e¡ects. 3. Results Mo isotope data for anoxic sediments, ferromanganese nodules, continentally derived samples and seawater are presented in Table 1 and Fig. 1. The anoxic sediments include bulk sediments from the two uppermost units in the Black Sea [22]: i.e. the laminated coccolith oozes, characteristic of modern sedimentation (Unit 1) and the underlying sapropel (Unit 2), and a Devonian black shale (USGS Standard SDO-1 [23]). All have strongly positive N97=95 Mo (Table 1). Total

Fig. 1. Range of N97=95 Mo values observed in natural materials. The di¡erence between oxic and anoxic sediments suggests that signi¢cant Mo isotope fractionation occurs in the marine environment. The box indicates the range of model N97=95 Mo values for the continental Mo input to the oceans (see Table 2). Error bar is þ 2c external analytical precision on samples.

Mo concentrations and Mo/Al ratios in the Black Sea sediments and SDO-1 are very large relative to average upper crustal values. Assuming a detrital component with an average crustal Mo/Al, 96^99% of the Mo in these samples is authigenic. Consequently the N97=95 Mo values can be equated to an essentially pure authigenic fraction. The samples of Mo enriched under oxic conditions are ferromanganese nodule material. These may be representative of Mo in oxic sediments due to the general correlation of Mo and Mn concentrations observed in oxic marine sediments (e.g. [24,25]). The Paci¢c Ocean nodule material (USGS Standard: NOD-P-1) was collected from a water depth of 4340 m at 14³50PN, 124³28PW, and the Atlantic Ocean material (USGS Standard: NOD-A-1) from a depth of 788 m along the Blake Plateau [26]. In both these samples N97=95 Mo is substantially negative relative to Mo in the anoxic sediments and somewhat negative relative to our standard (Table 1). Continental Mo is represented by molybdenite (MoS2 ) ore samples from the Nambucca, Deepwater and Wallaroo deposits in Australia. These range from slightly positive to slightly negative N97=95 Mo values (Table 1) intermediate between

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the N97=95 Mo values of the anoxic sediments and the oxic samples (ferromanganese nodules). The N97=95 Mo value for Paci¢c seawater Mo falls within the range of values for anoxic sediments (Table 1). The total range of natural Mo isotope variations reported here is less than 10 times our present analytical precision. Thus improvements in analytical precision would help in developing the full potential of the Mo isotope system. Despite its complexity, the double spike method offers promise in this regard [27]. 4. Discussion 4.1. Fractionation processes Our data demonstrate that the isotopic composition of Mo is variable in nature. The clear di¡erence in N97=95 Mo of the anoxic and oxic sediments (Fig. 1) indicates that fractionation of Mo isotopes occurs in the marine environment. It is reasonable to infer therefore, that this relative fractionation is a result of di¡erences in the chemical behavior of Mo under contrasting environmental redox conditions. There are a variety of processes by which Mo is transferred between aqueous solutions and nonaqueous environments, both geological and biological. Depending on the mechanism, which may be physical, chemical or biological, some of these processes are capable of fractionating Mo isotopes. Of these, mass balance constraints dictate that only those processes for which the transfer of Mo is ine¤cient have the potential to produce materials with variable N97=95 Mo. Thus, whether the observed fractionation in the marine environment occurs during removal of Mo from seawater to anoxic sediments and/or during scavenging of Mo by ferromanganese phases will depend on the e¤ciency of the processes involved and on the mechanism of removal. The mechanism by which Mo is removed to Mn oxide bearing sediments is not fully understood. Indeed, the presence of Mo in non-detrital oxic sediments is somewhat surprising given the stability of the molybdate ion in oxic waters. It is well

453

known that hydrated oxides of a number of metals, including Fe and Mn, are e¤cient scavengers of trace metals from solution [28]. This ability has been used as a method for the quantitative extraction of trace metals from natural waters both for concentration determination (e.g. [29,30]) and water puri¢cation purposes (e.g. [31]). Experimental studies have demonstrated that Mo scavenging by Mn oxides is quantitative only at low pH (pH = 1.3 to 5.5 [30]). At the pH of seawater (pH = 7.8 to 8.3) scavenging e¤ciency is much lower, on the order of 40^70%. In a complex solution such as seawater, as opposed to optimized laboratory experiments, the e¤ciency of the scavenging process is likely to be lower still. Mo scavenging by Mn oxide cannot be explained simply by charge balance, and is most likely understood in the context of surface complexation modeling [25,32]. By analogy with studies of Fe isotopes, which show fractionation associated with changes in metal bonding environment [9,11,12], we postulate that an equilibrium or kinetic e¡ect could fractionate Mo isotopes during scavenging. From the point of view of a Mn oxide particle settling through an oxic water column, seawater, with its globally uniform Mo concentration under oxic conditions, can be viewed as an in¢nite Mo reservoir. Therefore the scavenging process seems to have the potential to fractionate Mo isotopes as Mo is removed from seawater. In addition, a biological contribution to fractionation cannot be excluded. It has been demonstrated that the oxidation of Mn(II) in the marine environment is microbially mediated (e.g. [33]) and that some elements that co-vary with Mn are involved in this process, for example Ce [34] and Co [35]. If there is a biotic component to the process by which Mn oxides scavenge Mo from the marine environment, this may well in£uence the N97=95 Mo of scavenged Mo. In contrast to the moderate e¤ciency of Mo scavenging by Mn oxides, it is well known that under anoxic bottom waters Mo is very e¤ciently removed from the water column at or near the sediment^water interface [19,36]. This removal is facilitated by intermediary oxothiomolybdate ions which form when molybdate ions di¡use from oxic waters higher in the water column and react

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with reduced sulfur under anoxic^sul¢dic conditions below the redoxcline. These oxothiomolybdate ions are highly particle-reactive and readily scavenged by iron sul¢des and organic particulates causing their co-precipitation as insoluble sul¢des [16^18]. Hence, a priori, mass balance dictates that there should be little or no fractionation between Mo in anoxic sediments and Mo in seawater, even if there is a sizeable isotope fractionation between molybdate and oxothiomolybdate ions, or between molybdate ions and reduced Mo sul¢de minerals. While this removal process is probably not 100% e¤cient, any residual fractionated Mo escaping extraction and returning to the oxic seawater reservoir would be (at the present day) a tiny fraction of the total oxic seawater Mo budget and thus unlikely to result in a detectable shift in the isotopic composition of seawater Mo. Based on this assessment of the relative e¤ciency of the processes involved, substantial fractionation of Mo isotopes on removal from the marine environment seems most likely to occur as a result of the scavenging by ferromanganese phases. The similarity of the N97=95 Mo of seawater to that of the anoxic^sul¢dic sediments supports this hypothesis. Scavenging of Mo by Mn oxides under oxic conditions may also contribute to the o¡set between the N97=95 Mo of seawater and continental material. The ferromanganese nodule data are identical within our þ 2c external analytical precision. However, the two samples are di¡erent at the þ 1c level, raising the possibility that the scavenging process does not result in globally uniform N97=95 Mo in ferromanganese nodules despite the long ocean residence time of Mo. A number of factors might in£uence the extent of fractionation during oxic scavenging. These include proximity to exotic sources of Mo (e.g. hydrothermal vents), water depth, primary Mn oxide phase and mechanism of growth.

reduced or partially reduced forms (Mo(IV) and Mo(V)). Under oxidizing surface conditions today, this Mo is gradually oxidized to Mo(VI), and transported in solution as MoO3 4 . This oxidation, possibly microbially mediated [37], results in the ready transfer of Mo from the crust to the hydrosphere and the wide bioavailability of Mo in modern terrestrial and oceanic environments. The e¤ciency with which Mo is lost from solution in sul¢dic waters results in anoxic^sul¢dic sediments being a disproportionate Mo sink. A number of estimates have been made of the global sources and sinks of Mo in the ocean (e.g. [15,24]). These data indicate that Mo burial in anoxic sediments accounts for 15^53% of the removal £ux of Mo from the oceans, even though such sediments account for only V0.3% of the modern sea £oor. The correlation between Mn and Mo concentration in oxic marine sediments (e.g. [24,25]) suggests that scavenging by Mn oxides accounts for the remainder of the Mo removal £ux although only around 10% of the oxic Mo removal £ux is actually attributed to ferromanganese nodule formation (e.g. [24,38]). Based on these considerations, it is reasonable

4.2. Ocean Mo isotope budget The global geochemical cycle of Mo is strongly in£uenced by environmental redox conditions. Much of the Mo in the continental crust is in

Fig. 2. A simple box model for the Ocean Mo isotope budget. Fx = £ux of x, Nx = Mo isotopic composition of x.

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to interpret the N97=95 Mo data in terms of the mass balance between the Mo £ux into the oceans from the continents and the Mo £ux out of the oceans into oxic and anoxic sediments (Fig. 2). In doing this, we assume a steady-state between the £ux of Mo entering the ocean from the continents and the £ux of Mo leaving the ocean to oxic and anoxic sediments. This common assumption may not be strictly correct. Given the long residence time of Mo in the oceans [15], the time constant for re-establishment of steady-state conditions following a mass-£ux perturbation must be long. Consequently, if as Emerson and Huested [19] suggest, glacial/interglacial transitions are associated with changes in deep-sea redox conditions, then Mo sources and sinks probably remain out of steady-state today. However, the extent of imbalance is likely to be small [19] and therefore a steady-state model is a reasonable ¢rst-order assumption. Taking the N97=95 Mo values of modern Black Sea sediments (Unit 1) and oceanic ferromanganese nodules as representative of anoxic and oxic end-members, we can estimate N97=95 Mo of the continental £ux into the oceans (Table 2). The range of calculated N97=95 Mo values overlaps the measured N97=95 Mo values for continental material (Fig. 1), and is distinct from either the oxic or anoxic values. The consistency of the Mo isotope data with independent knowledge of the Mo ocean budget strongly suggests that our model captures the essential features of the global Mo isotope budget. 4.3. Potential paleoredox applications The global geochemical cycle of Mo has almost certainly been perturbed as the result of changes

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in ocean redox conditions through Earth history. Prior to the development of an O2 -rich atmosphere at ca. 2.2 Ga (e.g. [39,40]), Mo would have been largely retained in crustal rocks during weathering. Once a signi¢cant partial pressure of O2 developed in the atmosphere, the abundance of Mo in the oceans would have been controlled by the extent of sea£oor anoxia. During extended periods of widespread sea£oor anoxia, such as have been suggested for much of the mid-Proterozoic [41], the end-Permian (e.g. [42]) and the Cenomanian^Turonian Boundary Event (e.g. [43,44]), it is likely that any Mo reaching the oceans would have been rapidly removed from solution into bottom sediments, resulting in Mo concentrations signi¢cantly lower than at present. Smaller changes in the seawater abundance of Mo may also have occurred during glacial/interglacial transitions if these glacial epochs are associated with expanded bottom water anoxia [19]. Our observations and initial interpretation of the Mo isotope system suggest that the Mo isotopic composition of seawater would have co-varied with the extent of ocean anoxia because the relative £uxes of Mo to anoxic^sul¢dic and oxic sediments control the fractionation of seawater Mo relative to the isotopic composition of the continent-derived Mo entering the oceans. Given the high e¤ciency with which Mo is removed from sul¢dic waters, N97=95 Mo of anoxic sediments may provide a record of the isotopic composition of seawater Mo through time. Assuming a constant mean isotopic composition for the continental Mo input, then by extension, such data could be used to constrain the extent of sea£oor anoxia in the past. Although enrichment of Mo in modern sediments has been used

Table 2 Calculated N97=95 Mo (x) for continental source (bold) Assumed oxic sink 97=95

Paci¢c nodules: N Mo = 30.42 Atlantic nodules: N97=95 Mo = 30.63

Fanoxic 0.15

0.25

0.40

0.50

30.13 30.31

0.07 30.09

0.36 0.23

0.55 0.45

The Black Sea, Unit 1 sample is selected as most representative of the modern anoxic sink. We therefore use the mean N97=95 Mo for this sample (1.52x) as the assumed anoxic sink in the model.

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as a sensitive indicator of anoxic conditions [15,19,36,45], it only provides information about local or, at best, regional redox conditions. In contrast, the Mo isotope system potentially provides information about the extent of anoxia on an oceanic scale. While speculative, the potential utility of this application justi¢es further study of Mo isotope variations in nature and in the laboratory. Clearly, a larger sample database in general is required to verify the global Mo isotope budget proposed here. Two areas of investigation are particularly important. Firstly, any interpretation of N97=95 Mo in ancient marine sediments requires knowledge of N97=95 Mo of continental sources, and their change with time. Hence, there is need for a broader survey of the natural variation in the isotopic composition of Mo in continental rocks and, since fractionation of Mo isotopes may also occur during terrestrial weathering and transport, it is also necessary to characterize Mo in soils, rivers and estuaries. Secondly, it is unknown whether N97=95 Mo in marine ferromanganese nodules is representative of Mo in more typical oxic sediments, such as pelagic red clays. Although the Mo concentration in such sediments is far lower than in ferromanganese nodules, they account for the bulk of the Mo removal £ux to oxic sediments [24]. Further study of Mo isotopes in oxic sediments, and of fractionation during laboratory scavenging experiments, would help clarify this question. Note added in proof It has come to our attention that the Isotope Geology Group in Bern, Switzerland has recently obtained results similar to those reported here, using MC^ICP^MS and an Mo isotope double spike. They ¢nd that N97=95 Mo of high Mo concentration Black Sea sediments approaches values 1.7x heavier than Mo from Atlantic, Paci¢c and Indian Ocean Fe^Mn crusts. N97=95 Mo of seawater is 1.7^2.0x heavier than Fe^Mn crusts, and is uniform in the Atlantic, Paci¢c and Indian Oceans (Siebert, Na«gler, von Blanckenburg and Kramers, in prep).

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