Mo Isotope Composition Research Articles

The riverine flux of molybdenum and its isotopes to the ocean: Weathering processes and dissolved-particulate partitioning in the Amazon basin

Molybdenum (Mo) abundances and isotopes in marine sedimentary rocks have become important tools for understanding the past redox state of the ocean-atmosphere system. Their use depends critically on the size and isotope composition of the dissolved riverine input to the oceans. Previous studies have demonstrated that rivers are isotopically heavier than the upper continental crust, but the reasons why are debated. The debate is important to the question of how the riverine input might change through Earth history, for example in response to tectonic- and climate-driven changes in weathering regime. Here, we present a comprehensive study of Mo and its isotopes in multiple tributaries of the Amazon Basin, across seasons, with the aim of understanding both the controls on riverine transport of Mo and its isotopes and the input to the oceans.For all Amazonian rivers, the dissolved load dominates over the particulate load for Mo, whether the size of the total suspended load is quantified by in-situ filtration or from approaches using cosmogenic data. This finding is common to other highly soluble elements like strontium (Sr), and is very different from published compilations of global rivers, where particulate/dissolved ratios of Mo and Sr have been reported to be an order of magnitude higher than found here for the Amazon. Mo isotope data for the dissolved phase (<0.45 μm) of Amazon rivers (δ98Mo=+0.52 to +1.46, relative to SRM NIST 3134 = +0.25) show substantially less variation than rivers globally (−0.2 to +2.4‰), but Mo concentrations vary over two orders of magnitude (0.06-6.2 nmol kg−1). There is systematic variability between river types, with black and clear water rivers like the Negro and the Tapajós-Trombetas having much lower concentrations and higher Mo isotope ratios than white water rivers. Low water season (November) concentrations are always greater, and Mo isotope compositions lighter, than high water season (June). A small number of analyses shows that the colloidal phase (<0.45 μm, 1 kDa) represents about 20-30% of the total dissolved load, is broadly similar in isotope composition, and invariant in size between seasons. Thus, the greater dissolved concentration and lighter isotope composition in November must predominantly be driven by changes in the “truly-dissolved” fraction.We find little evidence for lithological or mineralogical controls on the Mo isotope composition of Amazonian rivers. Dissolved Mo concentrations are well-correlated with other highly soluble elements found in major minerals, be they silicates, carbonates or sulphides. Rather, molybdenum isotope variations across tributaries and season are best explained by processes related to the weathering regime, including preferential mobilisation of heavy isotopes due to sequestration of the light isotopes to secondary phases in soils. In more detail, soil pH is suggested to play a secondary, but significant, role. The assessment of the global dataset for Mo in rivers in terms of these processes suggests that there could be significant temporal variability in the riverine source of Mo to the oceans, controlled by tectonics and climate and their impact on weathering regime.

Pulsed oxygenation events drove progressive oxygenation of the early Mesoproterozoic ocean

The Mesoproterozoic era has long been considered a time of relative environmental and biological stasis. However, emerging insight suggests that this period may have been more dynamic than previously considered, both in terms of oxygenation and potential consequences for biological evolution. Nevertheless, our understanding of this immense period of time remains limited. To provide more detailed constraints on oxygenation dynamics, we report a multiproxy geochemical study of an early Mesoproterozoic (∼1600–1540 million years ago, Ma) carbonate-dominated succession from the North China craton. We include inorganic carbon isotope (δ13Ccarb), iron-speciation, and major and trace element data, in addition to molybdenum isotopic compositions (δ98/95Mo). These geochemical data support previous inferences of persistent anoxic and ferruginous deeper water conditions in the earliest Mesoproterozoic ocean, with limited oxygenation of surface waters. However, the behaviour of these redox-sensitive geochemical proxies reveals pulsed oxygenation events, with each event increasing the maximum depth of oxygenation, leading to overall progressive oxygenation of the ocean. During these pulsed oxygenation events we find the lightest Mo isotope signatures ever measured in the rock record, which we attribute to initial drawdown of isotopically light Mo in association with extensive Mn and Fe (oxyhydr)oxide precipitation, followed by diagenetic recycling. However, shallower water sediments deposited after the pulses of deeper water oxygenation more faithfully record the Mo isotopic composition of coeval seawater. For these samples, we utilise a single reservoir Mo cycling model, constrained by an updated estimate of Mesoproterozoic seawater Mo concentration, and scaled using a function associated with differential organic carbon flux between the shelf and basin. When scaled to modern rates of Mo accumulation under variable marine redox conditions, our modelling estimates suggest a minimum oxic seafloor area of ∼30% of the total seafloor area at ∼1540 Ma. It remains unclear whether the oxygenation observed across this ∼60 million year interval represents a progressive transition to a more persistently oxygenated ocean, or whether oceanic oxygen levels fluctuated considerably through the later Mesoproterozoic.

Unusually heavy stable Mo isotope signatures of the Ottawa River: Causes and implications for global riverine Mo fluxes

The accurate use of Mo isotope mass balance modelling of ancient oceans relies on the assumption that the δ98Mo of modern riverine inputs represents a reasonable estimate of the past. A growing number of studies of global rivers have demonstrated significant variation in δ98Mo from the bedrock sources of Mo. The Ottawa River, Canada, was previously identified as having an anomalously heavy Mo isotope composition, with a δ98Mo signature close to seawater (2.3‰), for a seemingly pristine natural river. To further explore this unusual signature, we collected and filtered 29 water samples from the Ottawa River, tributaries, and small lakes, and analysed them for Mo isotopes as well as major and trace elements. Here, we fully document heavy δ98Mo signatures throughout the Ottawa River and its absence in surrounding areas of the wider Ottawa River basin (ORB). Our results reveal a progressive upstream increase in both Mo concentration and δ98Mo signatures in the Ottawa River towards values even heavier than seawater, up to 3.13‰ – the heaviest Mo isotopic signature of river water measured to date. In contrast, the tributaries and lakes display far lighter and more consistent δ98Mo signatures within the range previously found in other rivers. Weathering of an isotopically heavy bedrock source, fractionation during weathering and retention of light isotopes in soils have all been proposed as sources of heavy δ98Mo in rivers; however, none of these mechanisms can satisfactorily explain our new observations. Colloidal and particulate processes that remove elements downstream, as inferred from some trace element proxies, also cannot explain the decreasing δ98Mo, since the preferential removal of light Mo isotopes is predicted from these processes. Similarly, the downstream trends show no apparent relationships with constructed dams or known potential industrial sources. Therefore, our findings from the Ottawa River are best explained as the dilution of a yet unidentified point source of heavy Mo upstream of sampling, or a significant permanent sink for light isotopes existing only in the upper reaches of the catchment. In both cases, anthropogenic contribution from a large mining district in the headwaters of the river must be considered and should be explored further. Fractionation of Mo in waste rock storage facilities has been previously identified and may provide an unnatural sink for isotopically light Mo through the Rayleigh-type fractionation of dissolved Mo on oxyhydr(oxide) mineral surfaces. The implied anthropogenic alteration of the natural Mo cycle highlights the significant and wide-reaching effects of unnatural point sources of Mo on the cumulative δ98Mo signatures of the catchment, and emphasises the necessity for detailed geochemical screening of anomalous river water isotope signatures before natural isotope compositions are inferred.

Geochemistry and molybdenum isotopes of the basal Datangpo Formation: Implications for ocean-redox conditions and organic matter accumulation during the Cryogenian interglaciation

The basal Datangpo Formation in South China deposited in the Cryogenian interglacial period has gained increasing interest with the well development of organic-rich black shale and its hosted manganese (Mn) deposits (Mn carbonate), which contain important clues to the paleoclimate and ocean redox of this critical interval in Earth's history. The depositional environment of Mn deposits is still uncertain, as suggested by different suites of redox-indicating parameters. In addition, the Mn deposits are notable for their enrichment in organic matter (OM), despite the fact that the diagenetic origin for the Mn deposits would result in large consumption of OM. To gain insight into these issues, we present the geochemical and molybdenum (Mo) isotopic data of black shale and Mn carbonate samples from a new section. The Datangpo Mn carbonates are characterized by low Mo content (mostly <3.2 ppm) and slightly light Mo isotopic composition (mostly from +0.07‰ to +0.52‰), suggesting that Mo, once adsorbed in Mn-oxide particles, was released in suboxic or anoxic (non-sulfidic) pore water due to Mn-oxide reduction and then diffused back into the water column. Hence the extremely light Mo isotopic signatures might be rarely preserved in sediments. The Mn carbonate formation accompanied by OM re-mineralization within sediments may have caused lower total organic carbon (TOC) content relative to that in black shales. However, elevated nutrient input related with hydrothermal fluid activity probably enhanced primary productivity during the Mn deposition period. It is evidenced by: (1) the Mn-TOC-total inorganic carbon covariation displayed by the Datangpo section and (2) a positive correlation between TOC and Mn content when Mn content is higher than 10% in a large compiled dataset. A compilation of Mo concentration and Mo isotope data suggests that widespread anoxia prevailed during the Cryogenian interglaciation. Instead, Neoproterozoic ocean oxygenation was postponed to the Ediacaran.

Molybdenum Isotope Constraints on the Origin of Vanadium Hyper-Enrichments in Ediacaran–Phanerozoic Marine Mudrocks

Vanadium is an important redox-sensitive trace metal for paleoenvironmental reconstructions. Modern organic-rich sediments persistently contain sediment V enrichments &lt;500 μg/g, but many ancient marine organic-rich mudrocks record enrichments &gt;500 μg/g. Previous studies propose that ancient V enrichments of these magnitudes (“V hyper-enrichments”) were deposited from hyper-sulfidic bottom-waters with higher H2S levels (≥10 mM) than observed in modern euxinic basins. To test the importance of hyper-sulfidic conditions for generating V hyper-enrichments, we compare V concentrations with Mo isotope (δ98Mo) compositions from mudrock samples ranging in age from Ediacaran to Pleistocene. In the modern ocean, sediments deposited from strongly euxinic bottom waters ([H2S]aq &gt; 11 μM) closely record global seawater δ98Mo because conversion of molybdate to tri- and tetra-thiomolybdate is quantitative. By contrast, large Mo isotope fractionations occur during Mo adsorption to Fe-Mn particulates or because of incomplete formation of the most sulfidic thiomolybdates in weakly euxinic settings ([H2S]aq &lt; 11 μM), which both favor removal of lighter-mass Mo isotopes to sediments. We find multiple examples when mudrocks with V hyper-enrichments are associated with a wide range of δ98Mo for a single time interval, including values at or below oceanic input δ98Mo (0.3–0.7‰). This observation suggests significant isotopic offset from reasonable seawater values (typically ≥1.0‰). Thus, we conclude that hyper-sulfidic conditions were not responsible for many V hyper-enrichments in Ediacaran–Phanerozoic mudrocks. Instead, sediment V hyper-enrichments can be explained by high Fe-Mn particulate fluxes to weakly euxinic sediments or by moderately restricted euxinic settings with strongly euxinic ([H2S]aq &gt; 11 μM but not necessarily &gt; 10 mM) or weakly euxinic (with slow clastic sedimentation rates and high organic carbon fluxes) bottom waters where vigorous water exchange provides a continuous V supply from the open ocean.

Tracking the evolution of seawater Mo isotopes through the Ediacaran–Cambrian transition

The rapid increase in animal diversity and abundance over the Ediacaran–Cambrian boundary has been attributed to, among other parameters, pronounced Earth surface oxygenation at that time. However, whether atmospheric oxygen elevated to modern levels, or whether the extent was smaller and oxygen remained relatively muted until the late Paleozoic is currently debated. Here, we present new Mo isotope datasets of drill core material for two key black shale units (the Ediacaran Doushantuo Formation and the lower Cambrian Jiumenchong Formation), located on the Yangtze Block. These data capture a 0.7‰ isotope fractionation between sediments experiencing quantitative and non-quantitative Mo removal. By identifying this offset, we provide integrated estimates for global seawater Mo isotope compositions during the deposition of these two formations (~1.2–1.4‰ and ~1.8–2‰, respectively). When paired with a mass balance model, our results indicate that a large proportion of the early Cambrian ocean was bathed under oxic waters. We also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans.

Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1

Authigenic uranium isotope compositions of Holocene sapropel S1 (δ238Uauth = +0.10 to +0.52 ‰; ODP core 967, 2550 mbsl) are significantly higher than the proposed upper boundary (+0.2 ‰) associated with the transport-porewater diffusion model for sediment uranium uptake. It is shown that these high δ238Uauth values are compatible with rapid initial slowdown of thermohaline overturning and the development of an anoxic water column. These conditions would favour U uptake in an organic-rich floccule layer overlying the sediment-water interface. The high δ238Uauth values correlate with low δ98Moauth values (+0.02 to −0.88 ‰), interpreted to reflect weakly euxinic conditions controlled by thiomolybdate–molybdate solution equilibria. The S1 data contrast markedly with published data from last interglacial sapropel S5 from the same core, which show δ238Uauth and δ98Moauth characteristics compatible with a restricted euxinic basin due to progressive slowdown in the thermohaline circulation. The U-Mo isotope data for S1 are similar to a range of published palaeo-settings. Sapropels are therefore shown to be useful templates for the unravelling of the interplay between productivity and deep water renewal times in ancient settings.

Molybdenum isotope ratios in Izu arc basalts: The control of subduction zone fluids on compositional variations in arc volcanic systems

Molybdenum isotope variations in mafic arc lavas have mainly been attributed to the influence of slab-derived components, such as subducted sediment melts and aqueous fluids. The latter have been hypothesised to fractionate Mo isotopes through interaction with the oceanic crust and carry an isotopically heavy signal that is transferred to the source of arc magmas. Thus, understanding Mo isotope systematics in subduction zones requires characterising the Mo isotope composition of slab-derived fluids and their influence on the Mo isotope budget of arc magmas. However, Mo isotope data reported to date show a considerable influence from subducted sediments that complicate accurate constraints being placed on the fluid contribution. We present Mo isotope data for mafic lavas from the Izu arc, a highly depleted oceanic island arc whose magma compositions show a dominant control from slab-derived fluids. The lavas from the Izu volcanic front are isotopically heavier than MORB and the depleted mantle. Their δ98/95Mo (the relative difference in measured 98Mo/95Mo to NIST 3134) systematically varies with indicators for fluid-mobile element enrichment, suggesting that slab-derived fluids in the Izu arc have heavy Mo isotope compositions. Additionally, co-variations with radiogenic 143Nd/144Nd and 176Hf/177Hf point to a relationship between the addition of aqueous fluids and compositional heterogeneity of the sub-arc mantle. We present mass balance models that show that the influence of subduction zone fluids on the trace element pattern of arc magmas is more dominant when these are added to a more depleted and refractory sub-arc mantle, which preferentially melts due to a relatively higher fluid flux. The mass balance of Mo in the Izu arc predicts a light Mo isotope composition for the residual oceanic crust as a result of the preferential removal of isotopically heavy Mo during slab dehydration, consistent with previous suggestions for the Mariana arc and isotopically light Mo previously reported for eclogites.

Effect of Fe–Ti oxides on Mo isotopic variations in lateritic weathering profiles of basalt

This study aimed to improve our understanding of processes fractionating Mo isotopic compositions during extreme weathering of basalt and to identify a long-term surface reservoir for light δ98Mo signatures, which are key to constraining the isotopic composition of riverine inputs to the oceans. A lateritic weathering profile of basalt from Hainan Island, South China, was studied by determining bulk δ98Mo values as well as δ98Mo values of extracted Fe–Mn (hydro) oxides and residual phases, and parent-rock minerals. δ98Mo values relative to NIST SRM 3134 = +0.25‰ in the lateritic weathering profile were in the ranges of −0.28‰ to +0.48‰, with the δ98Mo values of the upper and middle sections being heavier, and those of the lower section lighter than that of the parent basalt. Rainwater input and changing redox conditions are likely to be the essential factors controlling variations in bulk δ98Mo values in the upper and middle sections, respectively, resulting in their heavier δ98Mo values. In contrast, the lighter values in lower section are likely due to the leaching of primary minerals (e.g., pyroxene and plagioclase) with some heavier δ98Mo being released to aqueous media. The Fe–Mn (hydro) oxides phase in the sequential extraction procedure is thus characterized by heavy δ98Mo values in the lateritic weathering profile, which may be responsible for heavy Mo isotopic composition in bulk samples. Light δ98Mo signatures are preserved in the residual phase, where Mo hosted in Fe–Ti oxides is inherited mainly from parent basalt rock, possibly accounting for light Mo isotopic composition in bulk samples. Owing to their high resistance to weathering and chemical erosion, Fe–Ti oxides might be a potential long-term surface reservoir for light δ98Mo signatures. The difference in degrees of Mo isotopic fractionation between basalt and granite lateritic weathering profiles may reflect variations in Mo behavior in parent-rock host minerals during weathering. The results elucidate mechanisms of Mo isotopic fractionation during chemical weathering and improve understanding of the riverine input of Mo isotopes to oceans.

High-precision molybdenum isotope analysis of low-Mo igneous rock samples by MC–ICP–MS

Recent advances in analytical techniques have achieved high-precision (2σ ≤ 0.06‰) determinations of Mo isotopic ratios in silicate rocks but few such data are as yet available, especially for low-Mo (<50 ppb) rocks such as leucogranites. Here we present two low-blank, double-spike, high-yield, ion-exchange chromatographic methods (single- and two-column) for precise determination of Mo isotopic ratios in low-Mo rock samples (10–500 ppb) using multicollector–inductively coupled plasma–mass spectrometry (MC–ICP–MS). The two-column technique involved AG 1-X8 anion-exchange (100–200 mesh) and BPHA chelatin resins, with the latter also used in the single-column procedure. Two USGS reference materials (basalt BIR-1a and diabase W-2a) were analyzed for quality-control purposes. Their δ98/95Mo values determined using the two methods are consistent within uncertainties, and with those determined by previously published methods. We provide the first Mo isotopic data for the reference materials of dunite DTS-2b (USGS), GPt-3 (NRCG) and peridotite WPR-1 (CCRMP). δ98/95Mo values for DTS-2b and WPR-1 by using two-column procedure are in good agreement with those obtained by single-column separation. Repeated analyses of these standards indicate a long-term external reproducibility of better than ±0.05‰. However, the δ98/95Mo values of GPt-3 show poor reproducibility due to either the sample heterogeneity or existing undigested minerals (e.g., spinel). The methods were applied to leucogranites with low Mo contents of 10–74 ppb from the Himalaya orogen and southern Lhasa terrane of Tibet. The isotopic analyses yielded δ98/95Mo values of −0.2‰ to 0.34‰ and −0.41‰ to −0.68‰ (against NIST SRM 3134), respectively, within the range of values for igneous crust. Their contrasting Mo isotopic compositions may indicate compositional diversity in their sources or differences in magma evolution.

Molybdenum isotopic behavior during intense weathering of basalt on Hainan Island, South China

The behavior of molybdenum (Mo) isotopes during chemical weathering was investigated through analysis of saprolite and soil samples from two weathering profiles developed on Neogene basalts of Hainan Island in southern China. The Penglai and Nanyang profiles exhibit strong (mean ∼60 wt.% kaolinite) and extreme (mean ∼85 wt.% kaolinite) degrees of weathering, respectively. Our results show that light Mo isotopes are selectively retained in residual minerals, including kaolinite and hematite, whereas heavy Mo isotopes are preferentially leached out. Relative to the Mo-isotopic composition of the underlying unaltered parent basalt –0.38 ± 0.05 ‰ (2σ); vs. NIST 3134), the δ98/95Mo values of the overlying saprolite and surface soil at Penglai are modestly lower (–0.81 to –0.39‰) and higher (–0.35 to –0.12‰), respectively. In this profile, both δ98/95Mo and total organic carbon (TOC) decrease with depth, and their positive correlation in the soil layer (r = +0.99, p(α) < 0.01, n = 6) implies that heavier Mo isotopes are fractionated into organic matter. In the more intensely weathered Nanyang profile, the δ98/95Mo of the underlying unaltered parent basalt is lower –0.56 ± 0.05‰ (2σ) than that at Penglai, and the weathered samples show a larger range of relatively lower values (–1.39 to –0.33‰), which may reflect adsorption of Mo by Fe oxides. Throughout the weathering profile, Fe2O3 correlates positively with Mo concentration (r = +0.86, p(α) < 0.01, n = 32) but negatively with δ98/95Mo (r = –0.69, p(α) < 0.01, n = 32), implying that adsorption onto Fe-oxides played a key role in Mo mobility and isotopic fractionation. Extremely negative δ98/95Mo values (to –1.39‰) in the middle saprolite were likely linked to hydrothermal activity, as indicated by unusual enrichments of trace metals (Ni, Cu, Mo), low Hf/Y ratios, and a positive Eu/Eu* anomaly. Furthermore, the overlying soil layers of both profiles are characterized by enrichment of heavier Mo isotopes and higher Mo concentrations relative to their parent rocks. This may have been caused by additional inputs of Mo from atmospheric precipitation (average δ98/95Mo = +0.94 ± 0.14‰, 1σ, n = 3). Based on mass balance modeling of Mo isotopes and concentrations through the integrated profile, we infer Mo inputs from volcanic ash in the basalt-derived soils of northern Hainan Island. This study highlights the range of potential factors controlling Mo retention and isotopic fractionation as a function of variable weathering intensity, which may help to explain the large range of Mo isotopic compositions observed in river catchments at a global scale.

New implications for the origin of the IAB main group iron meteorites and the isotopic evolution of the noncarbonaceous (NC) reservoir

The origin of the IAB main group (MG) iron meteorites is explored through consideration of 182W isotopic compositions, thermal modeling of 26Al decay, and mass independent (nucleosynthetic) Mo isotopic compositions of planetesimals formed in the noncarbonaceous (NC) protosolar isotopic reservoir. A refined 182W model age for the meteorites Campo del Cielo, Canyon Diablo, and Nantan suggests that the IAB-MG parent body underwent some form of metal-silicate segregation as early as 5.3 ± 0.4 Myr after calcium-aluminum rich inclusion (CAI) formation or as late as 13.8 ± 1.4 Myr after CAI formation. If melting of the IAB-MG occurred prior to 7 Myr after CAI formation, it was likely driven by 26Al decay for a parent body radius >40 km. Otherwise, additional heat from impact is required for melting metal this late in Solar System history. If melting was partially or wholly the result of internal heating, a thermal model of 26Al decay heat production constrains the accretion age of the IAB-MG parent body to ∼1.7 ± 0.4 Myr after CAI formation. If melting was, instead, dominantly caused by impact heating, thermal modeling suggests the parent body accreted more than 2 Myr after CAI formation. Comparison of Mo mass independent isotopic compositions of the IAB-MG to other NC bodies with constrained accretion ages suggests that the Mo isotopic composition of the NC reservoir changed with time, and that the IAB-MG parent body accreted between 2 to 3 Myr after CAI formation, thus requiring an origin by impact. The relationship between nucleosynthetic Mo isotopic compositions and accretion ages of planetesimals from the NC reservoir suggests that isotopic heterogeneity developed from either addition of s-process material to, or removal of coupled r-/p-process material from the NC reservoir.

Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: Towards a new nuclear forensic tool

The ability to identify the sources of nuclear materials using various analytical methods has become a major target of nuclear forensics science. However, despite the emergence of novel tools developed in recent years, it has become obvious that accurate identification of sources requires multiple tools. Here, we have developed a new isotope tool based on Mo isotopes that could be used in nuclear forensics. Molybdenum is a trace element that is found in significant levels in uranium ores (0.5–4800 ppm), uranium minerals (0.02–6000 ppm) and uranium ore concentrates (UOCs, from 0.7 to 1400 ppm). The Mo isotope composition, reported as δ98Mo was analyzed in these materials and shown to have a significant variability from −2.62 to +0.30‰ in uranium ores and from −3.50 to +0.46‰ in UOCs. In uranium ores, the largest Mo isotope fractionation is found in deposits of sedimentary origin while the few ores with a magmatic origin show limited Mo isotope fractionation. Thus, the Mo isotope ratios may ultimately be used to distinguish high temperature from low temperature uranium ores. The δ98Mo values in UOCs also show a significant range that not only originate from variations in ore compositions but can also be attributed to ore processing such as leaching, solvent extraction, resin extraction and UOC precipitation. Laboratory experiments based on existing protocols of uranium ore separation demonstrate the existence of sizeable Mo isotope fractionation associated with these processes. Experiments with solvent extraction and precipitation of ammonium diuranate and peruranate show that the refined uranium fraction is enriched in light Mo isotopes in both cases. In the case of a uranium ore (SOMAÏR, Niger) and associated UOC, the observed fractionation can possibly be assigned to the combined effect of solvent extraction and precipitation, although Mo isotope variability in ores and UOCs may make such a diagnostic difficult. Ultimately, these experiments can be used to reconstruct the Mo isotope composition of the ore, if there is sufficient knowledge about the ore processing flowsheet or to provide information about the type of uranium ore. Alternatively, it could be used to identify the process used for manufacturing a given UOC, if one can make an estimate about the composition of the ore, probably in conjunction with other tools. Our study demonstrates the potential of Mo isotopes as a useful tool in the field of nuclear forensics.

Molybdenum Mass Fractions and Stable Isotope Compositions of Sedimentary Carbonate and Silicate Reference Materials

A double‐spike method in combination with MC‐ICP‐MS was applied to obtain molybdenum (Mo) mass fractions and stable isotope compositions in a suite of sedimentary silicate (marine, lake, stream, estuarine, organic‐rich sediment, shales, slate, chert) and carbonate reference materials (coral, dolomite, limestones, carbonatites), and a manganese nodule reference material, poorly characterised for stable Mo isotope compositions. The Mo contents vary between 0.076 and 364 μg g−1, with low‐Mo mass fractions (&lt; 0.29 μg g−1) found almost exclusively in carbonates. Intermediate Mo contents (0.73–2.70 μg g−1) are reported for silicate sediments, with the exception of chert JCh‐1 (0.24 μg g−1), organic‐rich shale SGR‐1b (36.6 μg g−1) and manganese nodule NOD‐A‐1 (364 μg g−1). The Mo isotope compositions (reported as δ98Mo relative to NIST SRM 3134) range from −1.77 to 1.03‰, with the intermediate precision varying between ± 0.01 and ± 0.12‰ (2s) for most materials. Low‐temperature carbonates show δ98Mo values ranging from 0.21 to 1.03‰ whereas δ98Mo values of −1.77 and −0.17‰ were obtained for carbonatites CMP‐1 and COQ‐1, respectively. Silicate materials have δ98Mo values varying from −1.56 to 0.73‰. The range of δ98Mo values in reference materials may thus reflect the increasingly important relevance of Mo isotope investigations in the fields of palaeoceanography, weathering, sedimentation and provenance, as well as the magmatic realm.

Precise determination of the molybdenum isotopic composition of urine by multiple collector inductively coupled plasma mass spectrometry.

Molybdenum (Mo) is predominantly expelled from the human body in urine. Consequently, urinary variability in the concentration and isotopic composition of Mo may encode valuable clinical information. To access this information, however, it is first necessary to develop and demonstrate a rapid, accurate and precise methodology capable of concentrating Mo from urine for isotope analysis. The utility of N-benzoyl-N-phenylhydroxylamine (BPHA) to effectively separate and purify Mo from urine samples without the need for acid digestion was tested. Following this approach, applying a double-spike mass bias correction, we determined the Mo isotopic compositions of a set of urine samples by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Based on replicate analyses of an in-house urine standard, this approach demonstrates an external precision on δ98/95 Mo values of better than 0.08‰ (2SD, n = 15). Application to a sample set collected from healthy individuals in Guangzhou, China, provides the first suite of δ98/95 Mo measurements from urine samples. Samples from the female participants show δ98/95 Mo (‰) values (1.31 ± 0.19‰, Ave ± 2SD, n = 14) that are consistently lower than those from the male participants (1.55 ± 0.16‰, Ave ± 2SD, n = 17). The employed methodology is suitable for rapid, low-blank and high-throughput Mo isotope analysis of urine samples. Although resolvable δ98/95 Mo variability is seen in this preliminary dataset, the mechanism driving this variability is unknown. High-precision Mo isotopic analysis might be added to the urinalysis tool-kit, with the potential to provide valuable clinical information in the future.

Inverse correlation between the molybdenum and uranium isotope compositions of Upper Devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations

Coupled Mo-U isotope data from the Upper Devonian Kettle Point Formation (Ontario, Canada) provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales. In the Gore of Chatham core, the Kettle Point black shales have high Mo concentrations (48–473 μg/g) and consistently high Mo/U ratios (≥3 times the Mo/U ratio of modern seawater), suggesting a euxinic depositional environment. These shales yield an inverse correlation (r = 0.89, p < 0.001) between authigenic U isotope (δ238U = −0.3‰ to +0.6‰ relative to CRM145 = 0‰) and Mo isotope (δ98Mo = +0.5‰ to +2.0‰ relative to NIST SRM 3134 = 0.25‰) compositions and a stratigraphic trend upsection towards lower δ98Mo and higher δ238U. These stratigraphic trends were probably not caused by global ocean redox variations, which should shift both isotope systems in the same direction. Instead, the inverse correlation between the δ98Mo and δ238U of the black shales indicates that changes to local depositional conditions in the Chatham Sag affected both isotope systems, consistent with the lithological and elemental evidence for variable paleohydrographic conditions (paleosalinity and sea-level). Black shales with the highest δ98Mo and lowest δ238U likely capture the most quantitative removal of Mo and U from more intensely sulfidic bottom waters, and thus most closely approximate global seawater isotope compositions. Lower δ98Mo and higher δ238U in the black shales likely record less quantitative Mo and U removal from weakly euxinic bottom waters and thus capture greater sediment-seawater isotope fractionation (lighter Mo isotopes and heavier 238U were preferentially removed from seawater to the sediments). Some samples with low δ98Mo (0.5–1.0‰) have high V and Mo enrichments, suggesting delivery of V and isotopically light Mo to the sediments by Fe-Mn particulates, but this process does not significantly affect U isotopes (because of weak adsorption of U onto Fe-Mn particulates) and is thus not the major driver of the inverse correlation. The highest δ98Mo (2.0‰) and lowest δ238U (−0.3‰) from the lower Kettle Point Formation represent minimum and maximum estimates of the Mo and U isotope compositions, respectively, of early Famennian global seawater. The slope of the inverse Mo-U isotope correlation for all Kettle Point black shales broadly parallels data from modern euxinic basins but is shifted to slightly lower isotopic compositions. Hence, Famennian seawater likely had δ98Mo (about 2.0–2.2‰) and δ238U (about −0.7‰ to −0.4‰) values that were slightly lower than modern global seawater (δ98Mo = 2.3‰; δ238U = −0.4‰), suggesting a mildly greater extent of euxinia (up to 5% of the seafloor) in the Famennian oceans. The opposite stratigraphic trends in δ98Mo and δ238U through the Kettle Point Formation may have caused misinterpretation of global ocean redox conditions if either isotope system was used individually. Hence, paired Mo and U isotope analyses are recommended to determine if stratigraphic trends through euxinic black shales are caused by local depositional changes or global ocean redox variations.

Heterogeneous accretion of Earth inferred from Mo-Ru isotope systematics

The Mo and Ru isotopic compositions of meteorites and the bulk silicate Earth (BSE) hold important clues about the provenance of Earth's building material. Prior studies have argued that non-carbonaceous (NC) and carbonaceous (CC) meteorite groups together define a Mo-Ru ‘cosmic’ correlation, and that the BSE plots on the extension of this correlation. These observations were taken as evidence that the final 10–15% of Earth's accreted material derived from a homogeneous inner disk reservoir with an enstatite chondrite-like isotopic composition. Here, using new Mo and Ru isotopic data for previously uninvestigated meteorite groups, we show that the Mo-Ru correlation only exists for NC meteorites, and that both the BSE and CC meteorites fall off this Mo-Ru correlation. These observations indicate that the final stages of Earth's accretion were heterogeneous and consisted of a mixture of NC and CC materials. The Mo-Ru isotope systematics are best accounted for by either an NC heritage of the late veneer combined with a CC heritage of the Moon-forming giant impactor, or by mixed NC-CC compositions for both components. The involvement of CC bodies in the late-stage accretionary assemblage of Earth is consistent with chemical models for core-mantle differentiation, which argue for the addition of more oxidized and volatile-rich material toward the end of Earth's formation. As such, this study resolves the inconsistencies between homogeneous accretion models based on prior interpretations of the Mo-Ru systematics of meteorites and the chemical evidence for heterogeneous accretion of Earth.

Influence of particulate versus diffusive molybdenum supply mechanisms on the molybdenum isotope composition of continental margin sediments

The sedimentary concentration and stable isotope composition of molybdenum (Mo) is widely used as a proxy for paleo redox conditions in the marine environment. However, the behavior of Mo during early diagenesis is still not fully understood, which complicates the application of the Mo proxy in ancient continental margin environments. Here, we present Mo concentrations and isotope compositions of sediment and pore water samples from the Guaymas Basin in the Gulf of California. Our sample set covers a broad range of depositional environments, including sediments from within the eastern equatorial Pacific oxygen minimum zone (OMZ), from a semi-restricted oxic graben, and from near a hydrothermal vent-field. By investigating Mo cycling in these different settings, we provide new insights into different modes of Mo fixation and the associated isotope fractionation.Sediments from the OMZ have authigenic Mo concentrations (Moauth) between 3.3 and 17.2 µg/g and δ98Mo between +1.64 and +2.13‰. A linear decrease in pore water Mo concentrations to the depth where hydrogen sulfide accumulates, along with sedimentary authigenic δ98Mo values (δ98Moauth) close to seawater, indicate diffusion of Mo from the bottom water into the sediment with little isotope fractionation during quantitative Mo removal. Sediments from the site with oxic bottom water within the basin reveal Moauth concentrations ranging from 1.2 to 14.7 µg/g and δ98Moauth signatures between –1.39 to +2.07‰. Pore water Mo concentrations are generally higher than ambient bottom water concentrations and the light δ98Mo signatures of the pore waters between +0.50 and +0.80‰ and δ98Moauth signatures of the sediments indicate continuous Mo exchange between the pore water and Mn and Fe oxides during early diagenesis. Sediment samples from the vent field mainly consist of black smoker debris and are characterized by Moauth concentrations ranging from 8.6 to 33.2 µg/g and δ98Moauth values as high as +2.20‰. The relatively high Mo concentrations and seawater-like δ98Mo can be explained by near-quantitative Mo scavenging from hydrothermal solutions with little isotope fractionation at high temperatures. Comparison of our new data for the OMZ sediments in the Gulf of California with previously published data for sediments from the Peruvian OMZ highlights that Mo isotope compositions in this kind of setting strongly depend on how Mo is delivered to the sediment. If diffusive Mo delivery from the bottom water into the sediment significantly contributes to Mo accumulation in the solid phase, as is the case in the Guaymas Basin, sedimentary Moauth concentrations are relatively low but the isotope values are close to the δ98Mo signal of seawater. If Mo is exclusively delivered by particles, like on the Peruvian shelf, much higher sedimentary Moauth concentrations can be attained. In the latter case, Moauth isotope values will be lighter because the sediments preserve the isotopic offset that was generated during adsorption or uptake of Mo by particles. Our findings de-emphasize the role of dissolved Mo speciation in pore waters but highlight the importance of the mode of Mo delivery for the Mo concentration and isotope composition preserved in the paleo-record.

Investigating the molybdenum and uranium redox proxies in a modern shallow anoxic carbonate rich marine sediment setting of the Malo Jezero (Mljet Lakes, Adriatic Sea)

The molybdenum (Mo) and uranium (U) isotope compositions recorded in carbonate rich sediments are emerging as promising paleo-redox proxies. However, the effects of early diagenetic effects within the sediments on these isotope systems are not well constrained. We examined the Mo and U isotopic systematics in anoxic carbonate rich sediments in a semi enclosed karstic marine lake (Malo Jezero) of the Island of Mljet, Adriatic Sea.Measurements of water column redox behavior in the lake since the 1950s, have shown a transition from anoxic-sulfidic conditions in the deeper water column to more oxic conditions and anoxia refined to the sediment and pore-waters. A 50 cm long sediment core from the deepest part of the lake, show a transition from moderate to high authigenic Mo and U accumulation with depth, consistent with the changing lake redox environment in the past. In the deep euxinic part of the core, the authigenic Mo and U are isotopically lighter and heavier, respectively, than seawater, following similar systematics as observed in other modern euxinic basins, with high, but non-quantitative, Mo and U uptake into the sediments.Based on Bahamas bank carbonate sediments, it has been suggested that the 238U/235U ratio is ~+0.25‰ higher compared to seawater from the effects of early carbonate sediment diagenesis and this carbonate vs. seawater off-set is applicable to carbonate rich sediments across the geological past. The shallower part of lake sediment core was deposited under similar redox conditions as the Bahamas sediments, and these sediments show an average 238U/235U ratio +0.31 ± 0.01‰ (2SE) higher than seawater. Although the average 238U/235U ratios for these two carbonate rich settings are similar, caution is necessary when inferring seawater 238U/235U compositions from such sediments, as they contain U from different sources (e.g. diagenetic uptake and carbonate-bound). The Mo isotope compositions within the same Malo Jezero sediments are variable but approaches the seawater composition at low pore-water H2S concentrations. This show the potential of using the Mo isotope composition from carbonate rich sediments to infer the seawater composition, however, further work is required to establish the link between the Mo isotope composition and the chemistry of the pore water environment.

The influence of thermal maturity on the stable isotope compositions and concentrations of molybdenum, zinc and cadmium in organic-rich marine mudrocks

The concentrations and isotopic compositions of molybdenum (Mo), zinc (Zn) and cadmium (Cd) in organic-rich marine mudrocks may be used to characterize ocean chemistry in the geological past. These approaches rely on the rarely tested assumption that the geochemical signatures of these metals are not affected by the thermal maturation of the organic matter with which they are associated. We have conducted a series of artificial maturation experiments on two well-known immature organic-rich mudrocks, the Kimmeridge Blackstone Band (Late Jurassic age), and the Posidonia Shale (Early Jurassic age). These pyrolysis experiments allow us to trace changes in the composition of organic matter through varying stages of maturation, and the concentration and isotopic compositions of metals in rock residues and evolved organic fluids. Our results indicate that the thermal maturation of organic matter does not result in significant alteration of the isotopic compositions of Mo, Zn and Cd in the rock residues, which thus retain primary palaeodepositional information. Systematic increases in the concentrations of Mo, Zn and Cd in rock residues with progressively higher thermal maturity are attributed to the loss of substrate mass in the form of fluids released during pyrolysis-induced cracking of kerogen, and to the relatively low concentrations of Mo, Zn and Cd in these fluids. The Mo-isotope compositions of fluids produced during pyrolysis are isotopically similar to the bulk rock; in contrast the isotopic composition of Zn in organic fluids is ∼0.4–0.6‰ lighter than the bulk rock. The progressive loss of organic matter from rock residues during maturation coupled with the increases in metal concentrations leads to an increase of metal/TOC ratios, which may be up to double their original (syn-depositional) value in thermally mature rocks. This observation must be taken into account when using metal/TOC ratios as proxies for oceanic metal inventories throughout geological time. Finally, calculations using the mass of asphaltenes recovered during the pyrolysis experiments suggest that organically bound Mo, Zn and Cd account for several percent of the total rock metal inventory.