Redox speciation and early diagenetic behavior of dissolved molybdenum in sulfidic muds

Marine Chemistry 125 (2011) 101–107

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Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m

Redox speciation and early diagenetic behavior of dissolved molybdenum in sulfidic muds Deli Wang a,⁎,1, Robert C. Aller a, Sergio A. Sañudo-Wilhelmy b a b

School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA Department of Biological Sciences and Department of Earth Sciences, University of Southern California, Los Angeles, CA, 90089, USA

a r t i c l e

i n f o

Article history: Received 16 May 2010 Received in revised form 15 March 2011 Accepted 16 March 2011 Available online 22 March 2011 Keywords: Molybdenum Redox speciation Sediment porewater Early diagenesis

a b s t r a c t In order to further elucidate the early diagenetic behavior of Mo, we examined the redox speciation of dissolved Mo in organic rich sediment from a back-barrier salt marsh environment in eastern Long Island (Flax Pond, NY). Total dissolved Mo (ΣMo) was ~ 80 nM in surface nonsulfidic porewater, dominantly as Mo (VI). ΣMo increased up to 150 nM at a depth of 3.5 cm with low sulfide content (ΣH2S: 25–100 μM), and Mo (V) reached 20 nM. ΣMo decreased to ~ 70 nM at a depth of 7.5 cm in highly sulfidic deep sediment porewater (ΣH2S: N100 μM) with Mo(V) accounting for ~ 10%. Mo(VI) dominated residual ΣMo, likely as MoS2− 4 . Averaged in situ Mo speciation patterns are complicated by mixed redox conditions created by biogenic structures and reworking. Serial anoxic incubation of surface sediments revealed reductive redox reaction progression without complications from transport and biogenic microenvironments: Mo(VI) dominated initially, followed by increases in ΣMo (dMo/dt ~ 7 nM/h) and production of Mo(V) under low sulfide conditions (ΣH2S: 25–100 μM; Mo(V) as high as 160 nM). Mo(V) was subsequently lost rapidly from solution (dMo(V)/dt ~−5 nM/h) and residual ΣMo, presumably a mixture of Mo(VI) and a small percentage of Mo(IV), was gradually reestablished under highly sulfidic conditions (ΣH2S N 100 μM). Mo(V) is clearly produced as a transient dissolved intermediate during reductive redox reaction succession. Mo(V) may react with particles or disproportionate in the presence of polysulfides into Mo(IV), which likely rapidly adsorbs–precipitates as pyritic Mo–Fe–S, sulfidized organic complexes, or perhaps MoS2. Mo(VI), which remains, at least temporarily in solution as thiomolybdate is removed more slowly. In contrast to reductive reactions, reoxidation of reduced sediment results in rapid release of Mo dominated by Mo(VI). Dynamic diagenetic cycling and the existence of Mo(V) as a dissolved reaction intermediate must be accounted for in models of Mo fixation and associated isotopic fractionation in sulfidic deposits. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Molybdenum (Mo) is the most abundant transition element in the ocean, and has an almost conservative behavior under oxygenated conditions (mean concentration ~ 107 nM) (e.g., Collier, 1985). However, Mo is removed from solution and enriched in anoxic, sulfidic sediments (e.g., Bertine and Turekian, 1973; Emerson and Huested, 1991; Crusius et al., 1996; Morford et al., 2009; PoulsonBrucker et al., 2009). Thus, Mo enrichment and its isotopic composition in sediments and sedimentary rocks have been used extensively as paleoceanographic indicators of reducing depositional conditions (e.g., Nijenhuis et al., 1998; Dean et al., 1999; Adelson et al., 2001; Wilde et al., 2004; Lyons et al., 2009; Scheiderich et al., 2010).

⁎ Corresponding author. Tel.: + 86 592 2182682; fax: + 86 592 2184101. E-mail address: [email protected] (D. Wang). 1 Present address: State Key Laboratory of Marine Environmental Science, Xiamen University, 422 Siming Nanlu, Xiamen, 361005, China. 0304-4203/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2011.03.002

Mo(VI) may accumulate in oxic surface sediments in host phases such as Mn and Fe oxides, biomass, or in both organic and inorganic components of recycled sedimentary debris (e.g., Bertine and Turekian, 1973; Crusius et al., 1996; Sundby et al., 2004; Chappaz et al., 2008). Under reducing conditions dissolved Mo(VI) is regenerated during organic matter remineralization or reductive dissolution of Fe, Mn-oxides (Morford and Emerson, 1999; Adelson et al., 2001; Schlieker et al., 2001; Dalai et al., 2005; Brumsack, 2006). Further Mo reduction is generally thought to be necessary for Mo fixation, either in association with scavenging onto particles in euxinic water columns or during precipitation–adsorption processes within anoxic, sulfidic deposits (Emerson and Huested, 1991; Huerta-Diaz and Morse, 1992; Calvert and Pedersen, 1993; Crusius et al., 1996; François, 1988; Zheng et al., 2000). Under sulfidic conditions (H2S N 11 ± 3 μM or ΣH2S N 25 μM at pH ~ 8, as defined by Helz et al., 1996 and Erickson and Helz, 2000), Mo(VI) can form a series of 2− 2− thiomolybdates, e.g., MoO3S2−, MoO2S2− 2 , MoOS3 , and MoS4 , which can be partially co-precipitated with or adsorbed onto Fesulfides, or become associated with organic material, the latter often

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containing Fe and S (Helz et al., 1996; Vorlicek et al., 2004; Algeo and Lyons, 2006). The likely involvement of Mo(V) in Mo fixation has been proposed at various times but was never explicitly demonstrated (e.g., Szilágyi, 1967; Bertine, 1972; Vorlicek et al., 2004). As a transient intermediate, Mo(V) could directly react with organic matter, or be further reduced to Mo(IV) and precipitated as Mo–Fe–S pyrite surface associations, MoS2, or scavenged by organic matter, particularly under sulfidic conditions in the presence of elemental S and polysulfides (Vorlicek et al., 2004). The reduction processes: Mo(VI) → Mo (V) → Mo(IV), mediated by SO2− reducing bacteria and directly 4 producing MoS2, were also reported under sulfidic conditions in the laboratory (Sugio et al., 1988; Tucker et al., 1997; 1998). Although Mo enrichment and isotopic compositions in sediments have been used as diagnosis of reducing depositional conditions, limited information is available regarding dissolved Mo redox speciation and associated diagenetic reaction mechanisms. In this contribution, we measured Mo redox-speciation, using the analytical method recently developed by Wang et al. (2009), in porewater from a back-barrier salt marsh environment. The field study was complemented with manipulative laboratory experiments that allow us to infer speciation changes of dissolved Mo in response to different redox conditions in the absence of transport or bioturbation. We show that Mo(V) is produced during early diagenesis, that Mo concentrations and speciation are potentially highly dynamic in bioturbated deposits, and infer that Mo(V) is a likely significant intermediate in the anoxic removal of ΣMo from solution.

2. Materials and methods 2.1. Sediment porewater and surficial sediment sample collections Sediment cores were collected at intertidal sites in Flax Pond (FP), Long Island during low tide, in June 2007. FP is connected to Long Island Sound through a single channel, is tidally exchanged with seawater, has a small drainage basin and therefore has no large input of water or nutrients from the uplands (Woodwell and Whitney, 1977). FP sediments are organic-rich (1–6% Corg; Mackin, 1986; Montluçon and Lee, 2001), and organic inputs to the sediments in FP include marsh detritus, terrigenous material, and plankton debris (Swider and Mackin, 1989; Wang and Lee, 1994). Dissolved Mn(II) and Fe(II) typically reach maxima, ~10 and 50–150 μM respectively, in the upper 0.5 to 1 cm and attenuate sharply by 2 cm (Swider and Mackin, 1989; Montluçon and Lee, 2001). Sulfate reduction rates in the upper few centimeters range from 0.1 to 5 mmol L− 1 d− 1 (Swider and Mackin, 1989; Mackin and Swider, 1989; Wang and Lee, 1994), typical of reactive nearshore deposits (Burdige, 2006). During the sampling period, the salinity of the overlying bottom water was ~28, the temperature was 21 °C, pH = 7.6 and DO was 50 μM for water close to the sediment surface. Reported sediment pH over the upper ~ 15 cm is 6.5–7 (Swider and Mackin, 1989; Zhu et al., 2006). Core samples composed of sandy mud were collected with a handheld acrylic box corer (165 cm2 of cross-sectional area) and were extruded at 1 cm intervals (down to 7 cm depth) into 250 mL clean polypropylene bottles under nitrogen. The porewater was separated by centrifugation at 4000 rpm for 10 min. Supernatant samples were filtered through 0.2 μm Nuclepore membrane filters in plastic holders, and separated for different chemical redox species of Mo, Mo(V) and Mo(VI) immediately. The chemical speciation of Mo was quantified using the method of Wang et al. (2009). Filtered porewater was also collected in Teflon bottles and analyzed for dissolved sulfide (ΣH2S) by standard spectrophotometric methods (Cline, 1969). The detection limit for ΣH2S was ~ 10 μM. All sampling materials used in this study were prepared using trace metal clean techniques and all of the sample manipulations and separation procedures were carried out inside a nitrogen-filled glove bag (Bray et al., 1973; Troup et al., 1974). For the laboratory incubation experiments, we collected surficial (0–

1 cm) and deep (4–5 cm) sediments using a hand-held acrylic box corer.

2.2. General description of the method for separating Mo(V) Mo(V) was separated from water samples using the method of Wang et al. (2009). Mo(V) was first selectively complexed with tartrate solution under neutral conditions, and then the Mo(V)–tartrate complexes were removed by passing solutions through poly-prep columns with Amberlite XAD 7HP resins. Mo(V) was eluted off the column with acidic acetone and analyzed using GFAAS. The remaining Mo (represented as Mo(VI) in this research) was reduced to Mo(V) using stannous chloride solution, and analyzed according to the above steps for Mo(V). Total Mo (ΣMo) was obtained by summing concentrations of Mo obtained by these two steps. In order to check the speciation method for possible interferences from Mo(IV), MoS2 was leached with H2SO4 + H2S under N2, pH adjusted, and the solution analyzed as outlined (Wang et al., 2009). Approximately 6% of the Mo (IV) in solution was complexed by tartrate (e.g., 0.16 nM of a total 2.66 nM Mo(IV)), indicating minimal contribution from any dissolved Mo(IV) to the analytical determinations. All of the chemicals used were of analytical reagent grade or the highest purity available. Milli-Q water (18.2 MΩ, Millipore) and HPLC-grade absolute acetone were used throughout. 2.3. Anoxic incubation of oxidized surface sediment Sediment incubations are often used to reveal biogeochemical behaviors during aerobic and/or anaerobic processes (e.g., Martens and Berner, 1974; Aller and Yingst, 1980; Elsgaard and Jørgensen, 1992; Kristensen et al., 1999; Hansen et al., 2000). In this study, anoxic incubation of initially oxic surface sediments was carried out by homogenizing surface sediments (collected at a depth of 0–1 cm in FP during low tide) under N2 in a glove bag and distributing portions into a set of incubation bottles (200 mL wide-mouth HDPE). These bottles were sampled serially with time. All sediment-filled bottles were sealed with no gas space, and all sample bottles were buried in anoxic sediment to maintain anoxic conditions. The time series for sampling was: 0, 3, 12 and 24 h, 2, 3, 4, 6 and 7 d. Once a bottle was taken out of the incubation container, it was centrifuged at 4000 rpm for 10 min, supernatant removed into gas tight syringes, and filtered through inline 0.2 μm filters. Porewater was processed immediately and analyzed for total sulfide (ΣH2S), Mo(V) and Mo(VI). All bottles, filters and sampling apparatus were maintained under nitrogen for at least 12 h before the experiments (Bray et al., 1973; Troup et al., 1974).

2.4. Oxidation of sulfidic sediment Particle reworking and bioirrigation typically result in exposure of reduced sediment and reoxidation. Dissolved Mo speciation dynamics during reoxidation was examined in a simple oxidation experiment. Subsurface, reduced sediment (4–5 cm depth interval) was taken from a sandy mud core and ~ 219.5 g (dry weight equivalent) was immediately placed unamended into a 2 L plastic beaker. Two liters of seawater were added into the beaker, and the seawater–sediment slurry was vigorously aerated with a gas frit attached to an aquarium pump and stirred continuously with a Teflon coated magnetic stirring bar. Water samples (100 mL) were removed at 0 h, 3 h, 12 h, 24 h, 2 d, 3 d, 4 d, 6 d, and 7 d. The samples were immediately filtered through a 0.2 μm pore filter, and measured for S, Mn, Fe, Mo(V) and Mo(VI). All filters and water sampling apparatus were maintained under nitrogen for at least 12 h before sample handling (Bray et al., 1973; Troup et al., 1974).

D. Wang et al. / Marine Chemistry 125 (2011) 101–107

3. Results and discussion 3.1. Behavior of dissolved Mo species in sediment porewater Vertical porewater profiles showed that sulfide was not detected until 0.5–1 cm, where it had a concentration of ~0.3 mM, and thereafter increased with depth to the highest level of ~2.1 mM at 6.5 cm (Fig. 1). These sulfide distributions were consistent with extensive previous sedimentary porewater and SO2− 4 reduction rate data at Flax Pond (FP) (e.g., Novelli, 1987; Swider and Mackin, 1989; Mackin and Swider, 1989). Swider and Mackin (1989) showed that low concentrations of reactive Fe in the deeper sediments likely limited the formation of pyrite and therefore led to high levels of dissolved sulfide. Total dissolved Mo (ΣMo = Mo(VI)+ Mo(V)) was 82 nM in the overlying water, and a similar level (~80 nM) was observed in surface porewater at 0.5 cm (Fig. 1). Total dissolved Mo increased gradually with depth until a peak of ~144 nM at 3.5 cm, and thereafter dropped to ~71–100 nM at 6.5–8 cm (Fig. 1). Similar patterns (increase to a subsurface maximum) and total concentration ranges have been widely observed, for example: continental margin sediments (Zheng et al., 2000; McManus et al., 2002), in the freshwater reaches of the Gironde Estuary (Audry et al., 2006), in the Bay of Biscay (Chaillou et al., 2002), and in an intertidal sand flat of the Wadden Sea (Beck et al., 2008). A common interpretation is that higher levels of dissolved ΣMo below the surface are mainly due to release from decomposing carrier phases, such as organic matter and Fe, Mn-oxides (Zheng et al., 2000; Reitz et al., 2007; Poulson-Brucker et al., 2009). Dissolved Mo(VI) in the sediment porewater of FP was the dominant Mo species (range: 71–144 nM, typically accounting for N90% of ΣMo) throughout the vertical profile (Fig. 1). Because our analytical technique does not effectively detect Mo(IV) species, ΣMo are minima. Mo(IV) species are particle reactive and quickly precipitated under natural conditions in the presence of SO2− 4 reducing bacteria (Sugio et al., 1988; Tucker et al., 1997), however, although possible, we think it is unlikely that the ΣMo as measured are substantial underestimates of total dissolved Mo. Mo(VI) in surface oxic sediments of FP presumably exists initially as molybdate MoO2− 4 , with subsequent formation of a series of thiomolybdates in highly sulfidic deeper sediments (Helz et al., 1996; Erickson and Helz, 2000). The pore water ΣMo distributions demonstrate progressive mobilization and net loss of ΣMo from porewater during early diagenesis. The decrease of ΣMo with depth below the subsurface maximum in the mud flat core could reflect either net precipitation or nonlocal transport sinks due to biogenic irrigation. Primary observations are that a significant concentration of ΣMo is maintained throughout the surface sediment,

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there is a gradient toward the sediment–water interface and a concentration gradient into sediment below ~4 cm. These relatively modest concentration gradients imply diffusive fluxes of dissolved Mo into overlying water and authigenic mineral formation at depth. Diagenetic mobilization of Mo may contribute to elevated Mo in overlying water and groundwater (Fox and Doner, 2003; Dalai et al., 2005), however, despite elevated average pore water concentrations, the net flux of Mo may also be from overlying water into bioturbated, anoxic deposits (Morford et al., 2007; 2009). The relatively high ΣMo maintained at depth in the presence of substantial ΣH2S, is consistent with the formation of thiolmolybdates (Helz et al., 1996) and/or a balance between sets of production–consumption reactions. Our study showed that there is a small but not negligible amount of dissolved Mo(V) in the sediment porewater profile. Dissolved Mo(V) ranged from 0 to 20 nM, reached a peak concentration between 2 and 3 cm, and accounted for up to ~20% of the total dissolved Mo in porewater (Fig. 1). A role for Mo(V) during reductive diagenesis has been hypothesized since the earliest studies of sedimentary Mo cycling but to our knowledge its presence in natural pore water has not been directly demonstrated previously (e.g., Bertine, 1972). Because transport–reaction conditions in the tidal flat deposit are complicated by bioturbation and redox patterns associated with biogenic structure, it is not possible to simply interpret the averaged vertical pore water speciation distributions in terms of a simple unidirectional reaction sequence.

3.2. Mo speciation changes during anoxic incubation of surface sediments Serial incubation experiments allow examination of natural reduction reaction sequences independent of complex redox patterns (e.g., heterogeneous biogenic oxic–anoxic zonations), transport processes such as biogenic particle reworking and irrigation, or diffusive loss into overlying water. In the present case, our anoxic incubation of surface sediments provides direct and clear evidence that Mo is successively mobilized, subject to redox speciation changes, and precipitated in conjunction with changing sulfide levels (Fig. 2). From initially partially oxic sediment (Mo: 80 nM), sulfide increased slowly within 24 h (ΣH2S: ≤100 μM; defined as low sulfide), but then increased rapidly at an overall rate of ~2.5 mM/d through day 4, leveling off at ~10 mM ΣH2S by day 7. These temporal patterns and eventual high levels of sulfide are attributable to an abundance of reactive organic matter, rapid SO2− 4 reduction, initial uptake of ΣH2S by reactive Fe-oxides (~1 d), and eventual build up of ΣH2S due to the small pool of reactive Fe in FP intertidal deposits (e.g., Jacobson et al., 1987; Swider and Mackin, 1989).

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Fig. 1. Vertical profiles of ΣMo (●), Mo(VI) (□), Mo(V) (○) and ΣH2S (sulfide) (×) dissolved in the sediment porewater at Flax Pond (nM = nmol/L porewater).

Fig. 2. Variation of ΣH2S (sulfide), Mo(V), and Mo(VI) concentrations in porewater with time during anoxic incubation of surface sediments. The sediment was collected at a depth of 0–1 cm in Flax Pond and was initially partially oxic.

D. Wang et al. / Marine Chemistry 125 (2011) 101–107

Both the concentration and speciation of Mo varied dynamically and regularly as ΣH2S increased (Fig. 2). Initially the total dissolved Mo (ΣMo) was ~ 82 nM (0 h) and dominated by Mo(VI). Following anoxic enclosure, ΣMo increased rapidly (initial rate was ~ 7.2 nM/h, or ~ 0.011 nmol/g sed/h (assuming sediment porosity = 0.8, particle density = 2.6 g/cm3; Mackin and Swider, 1989) to a peak concentration of ~190 nM at 24 h. After 1 d, ΣMo decreased as ΣH2S continued to increase, eventually reaching concentrations of ~100 nM Mo at ~ 10 mM ΣH2S. Similar dissolved ΣMo patterns were reported in reductive incubations of soil samples (Bennett and Dudas, 2003). These patterns represent the net result of changing balances of the production–consumption reactions governing Mo. As Mo carrier phases are consumed, we expect the release rate of ΣMo to first increase and then decrease. The eventual build up of ΣH2S to ~ 10 mM is an evidence that reactive Fe-oxides are depleted (Goldhaber and Kaplan, 1974; Canfield, 1989), thus we infer that the production rate of ΣMo during Fe reduction, and presumably Mn reduction, is minimal during the later period of the incubation when ΣH2S is highest. The speciation patterns of Mo during the incubation are similar overall in time to those observed with depth in the sediment core, although Mo(V) is far more dramatically expressed as a major species of ΣMo during the first 24 h of incubation when ΣH2S is relatively low than observed in the core profile (up to ~89% of ΣMo) (Fig. 2). As the net rate of ΣMo production decreases and ΣH2S increases, the relative dominance of Mo(V) rapidly decreases and by day 4, as ΣH2S increases above 2 mM, measured Mo is exclusively Mo(VI). After day 3, ΣMo (~Mo(VI)) decreases to ~100 nM, attaining a similar concentration to that observed in basal regions of the mud flat core (Figs. 1 and 2). The experiment was not long enough to determine if Mo(VI) concentrations would continue to decrease. It is possible that dissolved Mo(IV) also builds up following the loss of Mo(V) but is essentially undetectable by our technique. However, as noted earlier, we think that under natural conditions dissolved Mo(IV) is sufficiently particle reactive and rapidly precipitated or scavenged in sulfidic sediment that it is unlikely to significantly accumulate (Sugio et al., 1988; Tucker et al., 1997; Vorlicek et al., 2004). A primary conclusion of the anoxic incubation experiment is that Mo(V) is a major intermediate during unidirectional reductive reaction sequences in marine sediments. The rapid rate of transition and virtually complete succession between Mo(VI) and Mo(V) species during the incubation experiment suggest that a thin zone of Mo(V) dominance in the mud flat core may have been present just below the sediment–water interface near the average redoxcline but was obscured during our sampling by concentration averaging in relatively coarse sampling intervals. Elevated Mo(V) is presumably also present throughout the bioturbated zone around irrigated biogenic structures and associated redox microenvironments with oxic–anoxic transitions. We infer that biogenic microenvironments account for the small quantities of Mo(V) detected at depth throughout the spatially averaged pore water profile (Fig. 1). 3.3. Dissolved Mo speciation during sediment reoxidation In addition to diagenetic reduction reactions, sediment and pore water solutes are subject to reoxidation during particle reworking, bioirrigation, or diffusion from anoxic into oxic regions of deposits. The oxidation experiment demonstrates that dissolved Mo is rapidly remobilized into solution as Mo(VI) at rates of ~7.7 nM/h, or, when normalized to sediment mass: ~0.07 nmol/g/h (Fig. 3). No dissolved Fe, Mn and H2S were detected in these sediment re-oxidation experiments. Although concentrations of Mo were as high as 1919 nM, no dissolved Mo(V) was detected during re-oxidation, implying that any Mo(V) intermediates are quickly oxidized or disproportionated under the experimental conditions. Thus, we infer here that reductive reaction sequences may serve as the primary source of dissolved Mo(V) observed during diagenesis. Together with the anoxic incubation series, these results show that Mo diagenetic behavior can be highly dynamic, with

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Time (hours) Fig. 3. Variation of dissolved Mo(VI) with time during oxidation of reduced sediment (4–5 cm) in seawater–sediment slurry. Dissolved Fe, Mn, ΣH2S, or Mo(V) were not detected. The plotted least squares fit line is: Mo = 7.7 t + 213 (r2 = 0.77).

Mo release into solution during both oxidative and reductive reaction sequences, and with significant differences in net Mo speciation patterns. Mo behavior is likely particularly complex in bioturbated deposits where redox conditions are heterogeneous and oscillate continuously, and in deposits where boundary oxygen conditions are variable (e.g., Morford et al., 2009). 3.4. Early diagenetic behavior of dissolved Mo This study demonstrates that ΣMo can be rapidly released into solution from host phases during reduction (dΣMo/dt ~7 nM/h; or ~0.01 nmol/g sed/h) or reoxidation (dΣMo/dt ~0.07 nmol/g sed/h). Although the anoxic incubation experiment data could be consistent with initial release of Mo(V) present as such, based on the occurrence of Mo(VI) in oxidized minerals such as Fe, Mn-oxides (e.g., Kuhn et al., 2003), we infer that Mo(VI) is initially introduced into solution and then a portion is rapidly reduced to Mo(V), or that Mo(V) is formed at the mineral surface during the process of reductive dissolution of a carrier. The dissolved Mo(V) could exist as a monomer or a dimer resulting from association of Mo(IV), formed by S° mediated reduction of Mo(VI), with residual dissolved Mo(VI) (Vorlicek et al., 2004). Regardless of the exact mechanism, an important observation is that Mo(V), which has been largely discounted as a diagenetic intermediate in many recent studies (e.g., Erickson and Helz, 2000; Dalai et al., 2005), is indeed present at FP. Mo(VI) can be reduced to Mo(V) by organic matter (e.g., polysaccharides, humic acid) or elemental S°-polysulfides (Goodman and Cheshire, 1982; Vorlicek et al., 2004). An intermediate speciation step of this kind, along with oscillating transitions between oxic and anoxic conditions in surface sedimentary deposits, may complicate Mo isotope fractionation mechanisms (Tossell, 2005) and interpretations of Mo isotopic patterns in sediments (Poulson-Brucker et al., 2009). The formation of thiomolybdates can potentially stabilize relatively high dissolved concentrations of Mo(VI) under sulfidic conditions in the absence of reactive particles (H2S N 11 μM; Helz et al., 1996; Erickson and Helz, 2000; Zheng et al., 2000). In sediments, however, interactions with elemental S°-polysulfides, further reduction to Mo(IV), and adsorption and incorporation of dissolved Mo onto minerals such as pyrite, can take place (Huerta-Diaz and Morse, 1992; Chaillou, et al., 2002; Vorlicek et al., 2004; Sundby, et al., 2004). Mo can also strongly associate with organic material and Fe-organic complexes during anoxic sulfidic diagenesis, presumably formed through thiomolybdate intermediates (Helz et al., 1996; Algeo and Lyons, 2006). Substantial Mo precipitation, however, is also observed in anoxic coastal sediments in the absence of any detectable dissolved H2S (e.g., Buzzards Bay, Massachusetts, ΣH2S b 2 μM; Morford et al., 2009), and the exact processes of Mo removal from solution and fixation in natural sediments remain poorly constrained.

D. Wang et al. / Marine Chemistry 125 (2011) 101–107

An anoxic reaction progression scenario consistent with our experimental and field results would be: initial release of Mo(VI) from an oxidized host (e.g., Fe, Mn minerals), formation of Mo(V) monomer or dimer intermediates mediated by S° and polysulfides, and disproportionation of Mo(V) into thiomolybdate forms of Mo(IV) and Mo(VI) (Rollinson, 1973; Pope et al., 1980) as ΣH2S builds up. Possible reactions in this hypothesized scheme are (reactions after Coucouvanis, 1988; Vorlicek et al., 2004): VI

2−

n

2Mo OS3 þ VI 2−

2Mo S4 þ VI

2−

Mo OS3 þ VI 2−

Mo S4 þ V

n

þ

2−

þ

−

−

V 2−

ð1Þ

n

=8 S8 ðaqÞ þ H → Mo2 S7þn þ HS

ð2Þ

n

=8 S8 ðaqÞ → Mo OS3þn

IV

ð3Þ

2−

IV 2−

=8 S8 ðaqÞ → Mo S4þn −

2−

Mo2 O2 S5þn þ HS V 2−

V

=8 S8 ðaqÞ þ H → Mo2 O2 S5þn þ HS

−

Mo2 S7þn þ HS

VI

2−

ð4Þ IV

2−

→ Mo OS3 þ Mo OS3þn þ H VI 2−

IV 2−

→ Mo S4 þ Mo S4þn þ H

þ

þ

ð5Þ ð6Þ

MoV dimers used for illustration here and observed experimentally at high Mo concentrations (mM), may not be stable in solution at low natural concentrations of Mo (nM–μM). As noted previously, dissolved Mo(IV) may be relatively rapidly incorporated into Fe-sulfides (Crusius et al., 1996; Vorlicek et al., 2004), may react with organic matter (Helz et al., 1996; Algeo and Lyons, 2006), or may precipitate as insoluble molybdenum disulfide (MoS2) (Ksp of 10− 43, Garrels and Christ, 1965; Reid, 1979; Tucker et al., 1997), the latter being least likely during low temperature diagenesis. The residual dissolved thiomolybdates (Mo (VI)) are presumably more slowly consumed by adsorption–precipitation onto pyrite and organic matter (Zheng et al., 2000; Bostick et al., 2003; Vorlicek et al., 2004; Algeo and Lyons, 2006). Because the anoxic incubation experiment is a unidirectional reaction sequence, the initial oxidized carriers are consumed and cannot be reconstituted by diagenetic recycling into an oxic zone at the surface or along macrofaunal burrows. Thus, as the production rate of dissolved Mo(VI) decreases and

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ΣH2S builds up during incubation, nearly complete net consumption of the intermediate Mo(V) occurs (dMo(V)/dt ~−5 nM/h), and residual thiomolybdates (Mo(VI)) presumably characterize the solution. Assuming reduction of Fe-oxides by ΣH2S is an important pathway of S° formation, the loss of reactive Fe-oxide with time will decrease S° availability, perhaps further inhibiting Mo(V) formation (i.e., an intermediate below detection) while the progressively higher ΣH2S at least temporarily stabilizes Mo(VI) thiomolybdates. Although dissolved ΣMo can be sustained in pore water at concentrations in the range of ~50 nM when H2S exceeds ~100 μM, as shown for the Santa Barbara Basin (Zheng et al., 2000), it seems likely, based on other nearshore pore water Mo profiles (Morford et al., 2007), that at ΣH2S N 1 mM, such as achieved in the anoxic incubations (Fig. 2), Mo(VI) is eventually depleted (e.g., a lower Mo(VI) concentration would have been attained had the anoxic incubation experiment been extended). The proposed overall diagenetic reactions influencing Mo cycling in sulfidic sediments underlying oxygenated waters are depicted in Fig. 4. Mud flat sediments differ from the anoxic incubation series in that they are open to diffusion and particle transport, have oxic regions at the sediment surface and along macrofaunal burrows, and are subject to dynamic biogenic reworking and reoxidation (e.g., 45–76% reoxidation of sulfide occurs in Flax Pond deposits; Swider and Mackin, 1989). Reactions associated with transfer from anoxic to oxic as well as oxic to anoxic conditions therefore take place. Diagenetically formed Mosulfides and organic complexes, for example, will be subject to reexposure to oxygen during particle mixing and Mo will be released and oxidized (e.g., Fig. 3; Amrhein et al., 1993; Morford et al., 2009). We propose that Mo(V) may also be produced as a transient intermediate during sedimentary Mo oxidation processes although it may be rapidly oxidized to Mo(VI), perhaps biogenically (Sugio et al., 1992).

4. Summary By applying the analytical methods developed by Wang et al. (2009), we separated different redox species of Mo: Mo(VI) and Mo (V), in sediment porewater. Mo(VI) dominated pore water in the upper ~10 cm of mud flat sediment, initially as MoO2− and subse4 quently as thiomolybdates, reaching concentrations of ~ 140 nM.

Fig. 4. Proposed overall reaction patterns during diagenesis of Mo in organic rich sediments underlying oxygenated water. Mo(VI) is released to solution during reduction of Fe, Mnoxide and oxidation of Corg. Mo(V) is formed as an intermediate in the presence of S° and ΣH2S (e.g., H2S N 11 μM). Mo(V) may react directly with Corg or Fe-sulfides, or may disproportionate into Mo(IV) and Mo(VI). Mo(IV) rapidly precipitates, for example as Mo–Fe-sulfides, or is scavenged by Corg. Mo(VI), is temporarily retained in solution as thiomolybdates at high ΣH2S and is subject to relatively slow loss during incorporation into Fe-sulfides and Corg (example: Mo–Fe–S–Corg complex) Mo(VI) is also released to solution during reoxidation of reduced sediment. In bioturbated deposits, these reaction patterns can occur in associated with irrigated burrow microenvironments and reoxidation of excavated material (e.g., feces) throughout surface sediment, as shown schematically on the right side of the figure.

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However, Mo(V) also was present and accounted for up to ~20% of ΣMo under low sulfide levels, with typical concentrations of 5–20 nM. The addition of dissolved ΣMo was attributed to release from Fe, Mn-oxides and organic matter, during reduction under low sulfide conditions. Anoxic incubations demonstrated that Mo is rapidly mobilized to solution (~7 nM/h, or ~0.011 nmol/g sed/h) during transitions from oxic to anoxic conditions. Mo(V) formed at an equivalent rate and dominated redox species for ~1–2 d. As the net release of ΣMo slowed, Mo(V) was quickly lost from solution (dMo(V)/dt ~−5 nM/h) leaving, at least temporarily, residual Mo(VI) under high ΣH2S conditions (10 mM). The overall patterns can be explained by the initial release of Mo(VI), rapid formation of Mo(V) as a transient intermediate, and disproportionation of Mo(V) to thiomolybdate forms of Mo(IV) to Mo (VI). Mo(IV) (and Mo(V)) are presumed to be relatively rapidly removed onto reactive particles (Fe-sulfides, organic matter) whereas Mo(VI) (as thiomolybdate) reacts more slowly. Mo is also rapidly released during reoxidation of reduced sediment (~0.07 nmol/g sed/h), appearing in solution as Mo(VI) under the experimental conditions. Thus, Mo is rapidly mobilized into solution during both oxic to anoxic and anoxic to oxic redox reactions. The oscillation of such redox transitions commonly characterizes bioturbated sediments or deposits with unsteady oxygen boundary conditions. Mo(V) is a significant transient intermediate during reductive diagenesis, and possibly during reoxidation. The presence of relatively stable residual Mo(VI) under high ΣH2S supports the inference that thiomolybdates are a diagenetically important species in sulfidic sediments. The dynamic cycling and multiple oxidation states of Mo must be considered in mechanistic models of authigenic mineral formation and isotopic fractionation. Acknowledgments The authors would like to thank Dr. Qingzhi Zhu for his kind help in collecting and processing sediment porewater samples, and Dr. Christina Heilbrun for help in analyzing the samples for sulfides. We are grateful for the helpful critical reviews from Profs. Silke Severmann and Tim Lyons. This research was supported in part by NSF DEB 0841911 (S.A. Sañudo–Wilhelmy) and in part by NSF OCE 0851207 (R.C. Aller). Appendix A

Appendix Table 1 Variation of ΣH2S, Mo(VI) and Mo(V) with time during anoxic incubation of surface sediments. Time

ΣH2S (mM)

Mo(V)

Mo(VI)

0h 3h 12 h 24 h 2d 3d 4d 6d 7d

0.031 0.039 0.172 0.166 2.15 5.54 8.88 8.88 10.07

Undetectable 30 96 161 149 27 Undetectable Undetectable Undetectable

83 83 77 29 36 154 170 107 105

Appendix Table 2 Variation of dissolved Mo(VI) with time during oxidation of reduced sediment (4–5 cm) in seawater–sediment slurry. Time

Mo(VI)

0h 3h 12 h

84 159 615

Appendix Table 2 (continued) (continued) Time

Mo(VI)

24 h 2d 3d 4d 6d 7d

375 677 682 920 837 1919

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