Molybdenum isotope fractionations between ferromanganese oxides and seawater influenced ancient sedimentary isotopic signatures

Abstract

The use of molybdenum isotopes (δ⁹⁸Mo) as a paleo-redox proxy requires quantifications of fluxes and isotopic fractionations in each molybdenum sink. However, no systematic tests on δ⁹⁸Mo variations in different oxic sediments have been made due to limitations of analytical precision and sample number in pioneering studies. Here, we present the δ⁹⁸Mo data of ferromanganese nodules sampled at various global basins, which exhibit large variations from −0.95‰ to −0.51‰ with appreciable negative correlations between δ⁹⁸Mo and manganese–iron ratios. Diagenetic nodules, experiencing dissolution and re-precipitation of manganese oxides in marine sediments, have lower δ⁹⁸Mo by ~0.3‰ and higher molybdenum contents relative to hydrogenetic nodules. We suggest that changes in relative proportions of manganese and iron oxides and early diagenesis of marine sediments likely influence overall molybdenum isotope fractionations in oxic sinks. Hence, evolution of ancient marine manganese and iron cycles can influence sedimentary δ⁹⁸Mo signatures, especially in low-oxygen Precambrian oceans.

Introduction

Molybdenum (Mo) and its isotope compositions (δ⁹⁸Mo, modified relative to NIST SRM 3134 = +0.25‰) have been widely used to reconstruct the average oxygenation level of global ocean through Earth’s history, attributed to the long resident time of Mo in the global ocean (e.g., 0.44–0.7 Myr in modern oceans)¹ and large Mo isotope fractionations occurring between oxic-suboxic sediments and seawater^2,3,4. Although δ⁹⁸Mo of global seawater may have been homogeneous throughout most of geological time, the records in sedimentary rocks (e.g., shales) are highly variable due to the different isotopic fractionations between sediments and seawater, controlled by local seawater redox and depositional environments^5,6,7,8,9. Depending on the distinct isotopic fractionations and fluxes, at steady-state, δ⁹⁸Mo and Mo concentration of global seawater can be determined by the relative proportions of Mo sinks in different types of sediments, when the fluxes and isotopes of Mo source are relatively constant¹⁰. Due to limited Mo isotopic fractionations and high Mo burial rates in strongly euxinic sediments, high δ⁹⁸Mo and Mo concentration of global seawater are ultimately caused by abundant Mo burial in oxic sediments, reflecting decreases in global euxinic seafloor area^11,12. For instance, the rise to modern seawater δ⁹⁸Mo signatures (~+2.34‰) in sedimentary rocks was proposed to represent the establishment of modern-like oxygen levels in early Cambrian oceans¹¹. However, such high δ⁹⁸Mo values have been occasionally documented in sedimentary rocks through the Proterozoic^13,14,15, during which other geochemical indices suggest extensive anoxic conditions, and instead interpreted as changes in local marine Mo cycles. Meanwhile, decreases in ancient seawater δ⁹⁸Mo can be induced by lower Mn burial rates, instead of expanded euxinic seafloor, for instance, during the Cretaceous OAE2 (ref. ¹⁶). Taken together, there are still large debates on whether changes in seawater δ⁹⁸Mo indicates substantially varying proportions of global seafloor redox conditions.

Compared to studies of anoxic Mo sinks^{5,6,7,8,17,18,19,20}, the kind and extent of Mo isotopic fractionations in oxic sinks and their effects on global marine sedimentary δ⁹⁸Mo signals are relatively understated in previous work due to the limited data for modern Fe-Mn oxides^5,6. The pioneering studies concentrated on investigations of the surface layer of ferromanganese crusts and bulk nodules that potentially represent modern deposits of oxides and have relatively uniform δ⁹⁸Mo values with a small sample number (δ⁹⁸Mo = –0.7‰ ± 0.1‰, 1 SD, n = 8)^5,6. Additionally, the interpretations of the controlling factors on δ⁹⁸Mo signatures in Fe-Mn oxides and their potential influence on marine Mo cycles have not been fully tested in natural sedimentary records, due to the lack of ample samples and systematic investigations of their geochemical compositions. A recent study analyzed different types of hydrothermal oxides, showing large δ⁹⁸Mo differences in Fe-oxides and Mn-oxides²¹, which is consistent with the results from synthetic experiments^22,23,24. Nevertheless, uncertainties remain when considering the source of Mo in hydrothermal systems and its reliability for representing Mo sink in oxic pelagic sediments²¹. Here, we revisit Mo isotope compositions of marine oxic Mo sinks and their effects on global marine Mo isotopic mass balance by systematically investigating the elemental and isotopic signatures of ferromanganese nodules—one of the major Mn-Fe precipitates in oxic marine seafloor. Ferromanganese nodules in different oceanic basins are studied to uncover the depositional processes that affect Mo isotope fractionations between oxic sediments and seawater (Fig. 1). Comparing δ⁹⁸Mo data from ferromanganese nodules in this study and other types of oxides (e.g., ferromanganese crust, hydrothermal oxides), we suggest that the Mo burial flux and isotopic fractionation between oxic sediments and seawater are not invariant, but closely related to the evolution of marine Fe and Mn cycles. In this view, the overall extent of global marine oxygenation based on Mo isotope records should be re-examined, especially during the Precambrian with low baseline of atmospheric and marine oxygen levels.

Fig. 1: Locations of sites for the studied ferromanganese nodules in different oceanic basins.

The background map of global ocean is from Ocean Data View (https://odv.awi.de). The tan points denote the hydrogenic origin and the green points denote the diagenetic origin for the studied nodules.

Large Mo isotope variations of hydrogenetic and diagenetic nodules

Both full digestion and partial leaching protocols (see Materials and methods) have been applied for geochemical analyses of ferromanganese oxides in previous studies for different metal isotope systems. For instance, full digestion for Mo, V and Ce isotopes^5,25,26, and partial leaching using HCl for Mo, Tl, and U isotopes^6,27,28. The studied ferromanganese nodules contain appreciable detrital precipitates (e.g., aluminosilicates) based on XRD analysis²⁹, which is also consistent with high Al contents of bulk samples (3.2% ± 0.7%, 1σ) in this study (Supplementary Fig. 1). In contrast, although HCl can likely dissolve parts of detrital materials, leached samples exhibit appreciably decreased Al contents (1.8% ± 0.5%, 1σ) relative to bulk samples (Supplementary Fig. 1). In general, the ferromanganese nodules after full digestion and partial leaching show similar δ⁹⁸Mo compositions due to low Mo contents in continental detrital minerals relative to ferromanganese oxides⁶, however, the δ⁹⁸Mo_bulk values of a few samples are appreciably higher than δ⁹⁸Mo_leach in this study (Supplementary Fig. 1). Given high Mo contents of the studied nodules (~260 ppm to 470 ppm), a considerable amount of additional Mo with high δ⁹⁸Mo (e.g., 30 ppm, +1.0‰) is likely required to explain an offset of ~0.1‰ between δ⁹⁸Mo_bulk and δ⁹⁸Mo_leach. With a few uncertainties, we suggest that this difference may be owing to the preferential dissolution of seafloor materials marked by high Mo contents and δ⁹⁸Mo values⁶, such as organic matter-rich marine sediments or phosphates in bulk samples^30,31,32. Meanwhile, the full digestion of bulk samples would also lead to more additions of Fe, along with other metal elements, from silicate minerals (e.g., Fe-bearing clays), hampering the investigations of relationships between δ⁹⁸Mo and these elements. Hence, in order to better extract the seawater information, all the geochemical data discussed below are derived from HCl leaching results, which are also comparable to previous study of ferromanganese crusts⁶.

Marine ferromanganese nodules precipitating throughout the oceans are commonly identified as hydrogenetic, diagenetic, or hydrothermal in origin, reflecting their depositional environments and modes of accretion^33,34,35. The hydrogenetic nodules are generally formed via the direct oxidation of dissolved Fe²⁺ and Mn²⁺ in oxygen-sufficient seawater, and then the accretion of amorphous or cryptocrystalline Fe-oxyhydroxide and Mn-oxide colloids around a nucleus³⁵. By contrast, the formation of diagenetic nodules occurs within the sediment upper porewater or at the sediment-water interface on the deep seafloor, reflecting the dissolution of Mn-oxides and re-precipitation of more crystallized phyllomanganates³⁵. In this study, multiple geochemical approaches were used to discriminate the types of the studied nodules, including rare earth elements (REE) and high-field strength elements (Fig. 2)^33,34. Most of the studied nodules are hydrogenetic with relatively low Mn/Fe ratios and Cu, Ni, Co contents, and high REE contents along with a significantly positive Ce anomaly (Ce/Ce* defined as the relative enrichments or depletions in Ce relative to other REE). In contrast, other five nodules are marked by considerably high Mn/Fe ratios, low REE contents, seawater-like REE patterns, indicative of a diagenetic origin following dissolution and reprecipitation of oxides through the suboxic sediment column, and elemental exchanges with bottom seawater³³ (Fig. 2). Despite minor discrepancies among different geochemical classification approaches (Fig. 2), the compositions of these five nodules are clearly distinct from those of typical hydrogenetic and hydrothermal nodules, reliably indicating diagenetic overprinting on their geochemical signatures. Overall, we conservatively identify the origins for different nodules based on combined results from major/trace elements and rare earth elements. Additionally, the diagenetic nodules have more phyllomanganate minerals relative to hydrogenetic nodules, based on XRD analysis of the identical sample set in previous study²⁹. No hydrothermally originated nodules are identified in this study. Hence, Mo isotope compositions of different types of nodules are individually discussed below.

Fig. 2: Discrimination of nodule genesis using high field strength elements and rare earth elements.

a, b using metal element indicator³⁴; c, d using rare earth elements and Y indicator³³. The Ce anomaly (Ce/Ce*) is calculated using the equation Ce/Ce* = Ce_n/(Pr_n²/Nd_n), where n denotes normalization of each concentration against the post-Archean Australian shale (PAAS)⁵¹.

Previous studies of surface layers of ferromanganese crusts and bulk nodules showed overall Mo isotope fractionations of –3.0‰ between oxides and recent seawater (δ⁹⁸Mo_oxides ≈ –0.7 ± 0.1‰, 1 SD, n = 8)^5,6. Considering that external analytical reproducibility in the early studies was ~0.1‰, changes in δ⁹⁸Mo values of ferromanganese crust surface layers are relatively limited with a small sample number⁶. Although the whole profile of an Atlantic ferromanganese crust shows appreciably lower δ⁹⁸Mo signatures (–0.9‰ to –0.8‰), such low values only occur during ca. 62–7 Ma, while δ⁹⁸Mo values of sub-surficial layers (–0.6‰ ~ –0.5‰) are close to those of Atlantic nodule standard (Nod-A-1), surface layers of other crusts, and the Pacific crust profile⁶ (Fig. 3a). By comparison, the hydrogenetic and diagenetic nodules in this study exhibit appreciably larger δ⁹⁸Mo ranges from –0.95‰ to –0.51‰ and a clear negative correlation between δ⁹⁸Mo and Mn/Fe (r = –0.76 for all the nodules and r = –0.63 for only hydrogenetic nodules in this study). Further, the δ⁹⁸Mo of diagenetic nodules (–0.95‰ to –0.71‰) are systematically lower than those of hydrogenetic nodules (–0.77‰ to –0.51‰) (p value = 2.37 × 10⁻⁷ in Student t-test), and previously reported data for crusts (Fig. 3a). The notable correlation between δ⁹⁸Mo_leach and Mn/Fe observed in this study suggest that Mo isotope compositions of ferromanganese nodules are not homogeneous and likely affected by their mineralogy. The hydrogenetic nodules, marked by relatively low Mn/Fe, comprise larger proportions of primary Fe-(oxyhydr)oxides (e.g., amorphous ferrihydrite and cryptocrystalline Fe-vernadite), whereas diagenetic nodules are dominated by phyllomanganates due to recycling of Mn with sediment piles³⁵. Previous studies have demonstrated that Fe-(oxyhydr)oxides can substantially adsorb isotopically light Mo from seawater but show distinctly smaller isotopic fractionations (Δ⁹⁸Mo_{Fe-oxide-seawater} = –0.8‰ to –2.2‰) and lower Mo concentrations relative to Mn-oxides^9,22,36. Hence, the correlation between δ⁹⁸Mo and Mn/Fe of the studied samples is suggested as a result of mixing Fe-(oxyhydr)oxide and Mn-oxide minerals in nodules. Further, diagenetic nodules show appreciably larger Mo isotope fractionations than experimentally constrained fractionations in synthetic Mn oxides, with higher Mo contents relative to hydrogenetic nodules (314.6 ± 34.1 ppm for hydrogenetic and 437.9 ± 29.4 ppm for diagenetic nodules, 1σ) (Fig. 3b), likely reflecting substantial adsorption of additional Mo with light isotopes from suboxic pore water in deeper sediment profiles, more rapid growth rates of diagenetic nodules^35,37, or larger distribution coefficient of Mo in Mn-oxides compared with Fe-(oxyhydr)oxides³⁶.

Fig. 3: Cross-plots of δ⁹⁸Mo vs. Mn/Fe ratio and Mo content for different types of ferromanganese oxides.

In a we plot the nodule samples and USGS nodule standards (Nod-A-1 and Nod-P-1) in this study. As previous studies did not present Mn/Fe data, the dashed lines and shadowed areas represent the average Mo isotopic fractionations and their ranges between ferromanganese crusts or nodules, and seawater^6,23. In c, d the δ⁹⁸Mo data of modern hydrothermal oxides, Mn-rich continental anoxic sediments, and the Archean iron formations are compiled from refs. ^9,21,40. The r values in a, b denote the trimmed coefficients of determination of the correlations (exclusion of outliers in the dashed squares).

Heterogeneous Mo isotope signatures in different types of ferromanganese oxides

In this study, the ferromanganese nodules from different basins in modern global ocean exhibit large ranges of δ⁹⁸Mo (–0.95‰ to –0.51‰) and Mo content (260.0–471.3 ppm), which are highly related to the Mn/Fe ratios (i.e., mineralogy and precipitating environments) of nodules. The pioneering work has observed appreciable variations of δ⁹⁸Mo in surface layers of ferromanganese crusts, while variations of Mo isotope fractionation are interpreted to be related to the species of Mo⁶⁺ in the oceans and then the adsorption efficiency of Mo in Fe- and Mn- oxides^5,6. As the studies of surface layers of ferromanganese crusts did not present Mn/Fe data of the samples⁶, the effects of relative proportions of Fe- and Mn-oxides on their δ⁹⁸Mo signatures have not been examined. Nevertheless, the observation of larger δ⁹⁸Mo ranges and close correlations between δ⁹⁸Mo and Mn/Fe in the studied ferromanganese nodules bolsters the different roles of Fe- and Mn-oxides in regulating overall Mo isotope fractionations between oxides and seawater, which is supported by the results of laboratory synthetic experiments^22,23,36. Additionally, analogous to the formation of diagenetic nodules, the dissolution of pre-formed Mn oxides in suboxic deeper pore waters, and subsequent re-oxidation after the dissolved Mn diffuses upwards to relatively oxic upper sediments (i.e., recycling of Mn oxides through the sediment column) can lead to the accumulation of more isotopically lighter Mo in diagenetic Mn oxides near the bottom water-sediment interface. This process is likely pervasive under fluctuating redox conditions of bottom seawater ranging between suboxic ([O₂] <10 μM) and hypoxic ([O₂] <61 μM) levels, during which only recycling of Mn oxides remains active^38,39. We further compiled δ⁹⁸Mo and elemental data of hydrothermal oxides and the Archean iron formations in previous studies^21,40, and compared them to ferromanganese nodules in this study (Fig. 3c, d). Despite the hydrothermal oxides showing appreciable correlations between δ⁹⁸Mo and Mn/Fe, their δ⁹⁸Mo values and Mo contents are systematically higher than those of ferromanganese crusts and nodules (Fig. 3c, d), even considering those oxides with significantly high Mn/Fe ratios (>100). This observation may suggest that the hydrothermal oxides, especially Mn-oxides may not represent typical open-marine depositional environment, or adsorb non-marine sourced Mo in hydrothermal systems²¹. In contrast, the Archean iron formations exhibit much lower Mn/Fe and Mo contents, but high δ⁹⁸Mo values (Fig. 3c, d), indicative of less Mo removal via Mn oxides, and overall low Archean oxygenation levels. The correlations between δ⁹⁸Mo values vs. Mn/Fe ratios and Mo contents in these Archean iron formations also suggest that the Mo isotope fractionations in ancient oceans are determined by formation of Mn oxides.

Despite the uncertainties on whether ferromanganese crusts and nodules are sufficiently large in mass to be the dominant Mo sink, authigenic Fe- and Mn-oxides in marine pelagic sediments can be a significant sink for Mo via the association with grain coatings and micronodules in these sediments^6,41,42. The Mn/Fe ratios of authigenic oxides in their associated marine sediments can be comparable to those of ferromanganese nodules, and diagenetic remobilization within the sediments can increase Mn/Fe ratios and Mo contents in the oxides⁴². In this view, the relative proportions of co-precipitating Fe- and Mn-oxides in marine pelagic sediments may have influenced the average Mo isotope fractionations between the oxic sink and seawater on a global scale. Additionally, during the early diagenetic remobilization (i.e., recycling of Mn oxides within sediment piles), increased Mn/Fe ratios of authigenic oxides in sediments may remove more Mo from the bottom seawater, supported by the dominant role of Mn-oxide in binding Mo in pelagic sediments^42,43. Thus, the Mo isotope offsets between oxides in marine pelagic sediments and bottom seawater are likely larger than those of hydrogenous crusts and nodules.

The potential effects of Mo isotope compositions in ferromanganese oxides on sedimentary δ⁹⁸Mo records in deep time

Oceanic Mo isotope mass balance suggests gradual increases in seawater δ⁹⁸Mo accompanying with decreased Mo burial in euxinic sinks, and namely elevated Mo burial in the oxic sinks (i.e., Fe- and Mn-oxides in marine sediments), which is used to estimate the area of oxic versus euxinic seafloor areas in the global ocean¹⁰. However, when the seafloor areas of each Mo sink under different redox conditions are invariant, seawater δ⁹⁸Mo can also be influenced by changes in isotopic fractionations in each sink. For instance, decreased burial flux of Mn-oxides when bottom seawater is suboxic may diminish Mo burial proportions in oxic marine sediments at the global scale, and drive global seawater δ⁹⁸Mo towards to low values without expansion of euxinic seafloor during Cretaceous OAE2¹⁶. Similarly, the transition from Fe oxide to Mn oxide formation dominated in global marine sediments, meaning more Mo burial in Mn-oxide minerals may have significantly increased seawater δ⁹⁸Mo without requirements of decreased euxinic seafloor. Here, we use a Mo isotope mass balance model to reproduce the seawater δ⁹⁸Mo variations as the function of the seafloor areas with different redox states (Fig. 4) (see Supplementary Methods for model descriptions and parameters). In the model, we compare the effects of Mo burial rates and isotopic fractionations during the formation of different oxides on global seawater δ⁹⁸Mo. The average isotopic fractionation associated with the natural oxic Mo sink in the modern ocean is generally assumed as –3.0‰, based on previous assessments of global Mo burial with Fe and Mn oxides on the oxic seafloor^6,7,8. Building on the observed relationship between δ⁹⁸Mo and Mn/Fe from the studied ferromanganese nodules, we adopt a fractionation value of –2.8‰ to represent Fe oxide-dominated conditions in suboxic marine sediments (i.e., low Mn/Fe ratios of the oxic sediments), and –3.3‰ to represent Mn oxide-dominated conditions (i.e., high Mn/Fe ratios of the oxic sediments). It should be noted that the value of –3.3‰ is appreciably larger than experimentally observed Mo isotope fractionations between Mn oxide and solution (–2.4‰ to –2.9‰)^23,24. This discrepancy is proposed to result from recycling of Mn oxides through the sediment piles during early diagenesis, a process analogous to the formation of diagenetic nodules^39,41,42. This interpretation is consistent with the observation that δ⁹⁸Mo values of surface layers of ferromanganese crusts remain slightly lower than those of synthetic δ-MnO₂ under the seawater condition^6,23,24, partly suggesting the potential diagenetic effects during the deposition and burial of ferromanganese crusts. Additionally, in the model (Fig. 4), the burial rates of Mo in Mn oxide-rich marine sediments are assumed to be higher than those in Fe oxide-rich sediments, as Mn oxides play a dominant role in removing Mo in global marine sediments^41,42,43. The results of the mass balance models show clear shifts of seawater δ⁹⁸Mo accompanying the variations of isotopic fractionations between oxic sediments and seawater. A notable increase in seawater δ⁹⁸Mo from 1.8‰ to 2.34‰ can be observed when global oxide-dominated Mo sink transfers from Fe-oxide to mixing Fe- and Mn-oxides (e.g., modern ocean conditions), even with no changes in anoxic and euxinic seafloor areas (Fig. 4a). Moreover, our model results suggest that the amplification of seawater δ⁹⁸Mo increases is more prominent when Mn-oxides become to dominate the oxic sink of Mo (from 1.6‰ to 2.34‰ in Fig. 4b).

Fig. 4: Mo mass balance model showing changes in global seawater δ⁹⁸Mo as functions of seafloor areas with different redox conditions.

a Comparisons of modern ocean (solid lines with Δ_oxic = –3.0‰) and estimated Fe cycling active ocean (dashed lines with Δ_oxic = –2.8‰) with different Mo isotope fractionations and fluxes in the oxic sink. b Comparisons of estimated Mn cycling active (solid lines with Δ_oxic = –3.3‰) and Fe cycling active oceans (dashed lines with Δ_oxic = –2.8‰). Under the typical framework of Mo isotope budgets in the modern ocean, the removal of Mo from seawater mainly occurred under strongly euxinic conditions ([H₂S] > 11 μM) with the lowest isotopic fractionation but high burial flux (subscripted euxinic). In contrast, the removal of Mo in oxic sediments occurred under oxic conditions with [O₂] higher than 10 μM associated with high isotopic fractionation but low burial flux, following the formation of Mn oxides in global pelagic sediments (subscripted oxic). The intermediate sink for Mo includes a large range of redox conditions [i.e., suboxic (0 <[O₂] <10 μM), anoxic ([O₂] = 0 μM, [H₂S] = 0 μM), and weak euxinic conditions ([O₂] = 0 μM, 0 <[H₂S] <11 μM)], which is uniformly defined as a “anoxic” condition (subscripted anoxic) in this study. The detailed descriptions of model and selected parameters are presented in the Supplementary Methods and Table 1.

Changes in the proportions of the burial of Fe- and Mn-oxides, and enhanced recycling of Mn oxides under suboxic–hypoxic redox conditions may have affected the Precambrian δ⁹⁸Mo records when baseline of atmospheric and marine oxygenation levels was substantially lower than present time, especially in the late Archean and late Neoproterozoic (Fig. 5a). Due to the differences in the redox potential for Fe and Mn, Fe²⁺ is preferentially oxidized in the ocean prior to Mn²⁺, when marine oxygen gradually increases^38,44. The Archean oceans were dominated by overall anoxic and ferruginous conditions, thus the formation of Fe oxides in marine sediments was more prevalent relative to Mn oxides when oxygenation levels of global ocean gradually increased^45,46, leading to persistently low δ⁹⁸Mo signals in marine sedimentary records during the peak of banded iron formations (Fig. 5a). Accordingly, increased sedimentary δ⁹⁸Mo signals during the Archean–Paleoproterozoic transition can be attributed to enhanced burial of Mn oxides in global pelagic sediments (Fig. 5a), without the requirement of decreased anoxic and euxinic seafloor area. The temporal distributions of banded iron formations and sediment-hosted Mn ores from the late Archean to early Paleoproterozoic also support the conversions from Fe active to Mn active cycles following the stepwise oxidation of Precambrian oceans (Fig. 5b). The transitions to substantial formation of Mn oxides are also observed from the late Neoproterozoic to early Paleozoic due to the accumulation of dissolved Mn in seawater under suboxic conditions (i.e., [O₂] <10 μM defined as a “manganous condition”)⁴⁷. Thus, the substantially increased sedimentary δ⁹⁸Mo signals from the latest Neoproterozoic can be amplified due to increases in Mn oxide formation as a dominant Mo sink on the seafloor, when global seawater started to mark with hypoxic conditions ([O₂] > 10 μM) (Fig. 5a). In this light, the preservation of near-modern seawater δ⁹⁸Mo signals during the Neoproterozoic and Paleozoic transition does not necessarily indicate a rise in global ocean oxygenation to near-modern levels. Although transitions of marine Fe and Mn cycles imply the gradual increases in seawater oxygen, the average extent to global marine oxygenation, evaluated from high δ⁹⁸Mo values in sedimentary records, could still be low (e.g., hypoxic conditions). Conversely, oceanic deoxygenation during the Phanerozoic may also reduce global seawater δ⁹⁸Mo by decreasing Mn oxide formation and more Mo removal via Fe oxide in marine sediments, which do not necessarily require the expansion of euxinic conditions³. Additionally, analogous to the case on more negative δ⁹⁸Mo in diagenetic nodules, active marine Mn cycles when seawater redox states vary between suboxic and hypoxic conditions³⁸ may also account for more distributions of negative δ⁹⁸Mo signatures in sedimentary records from the early Paleoproterozoic and late Neoproterozoic (Fig. 5a), due to the recycling of isotopically light Mo from Mn-oxides during early diagenesis. Thus, more distributions of negative δ⁹⁸Mo in shales may reflect the expansion of suboxic–hypoxic conditions, rather than euxinic conditions.

Fig. 5: Mo isotope records in shales and abundance distributions of banded iron formations and sedimentary Mn deposits through geological time.

a Marine sedimentary records of Mo isotopes throughout geological time. b Temporal changes in distributions of banded iron formations and sedimentary Mn deposits. In a the dashed line in represents the δ⁹⁸Mo values of modern seawater and average rivers¹⁰. Red arrows denote the increased occurrence of positive and negative δ⁹⁸Mo signatures following the enhanced marine Mn cycle. The δ⁹⁸Mo data of shales are modified from ref. ¹⁴. The distributions of banded iron formations and sedimentary Mn deposits are modified from refs. ^45,46.

Conclusions

This study provided a systematic investigation of δ⁹⁸Mo signatures in the marine ferromanganese nodules from different oceanic basins to better explore Mo isotope fractionations between oxic sediments and seawater and their effects on ancient marine Mo isotope cycles. The δ⁹⁸Mo values of nodules exhibit considerably larger variation ranges relative to those of surface layers of ferromanganese crusts in the pioneering research, which are tightly related to Mn/Fe ratios of nodules. Additionally, diagenetic nodules, comprising more phyllomanganates, have lower δ⁹⁸Mo values and higher Mo contents relative to hydrogenetic nodules, suggesting that dissolution and re-precipitation of Mn-oxides within marine sediments may lead to larger Mo isotope fractionations between oxides and seawater. Accordingly, we propose that changes in relative proportions of authigenic Fe- and Mn-oxides preserved marine pelagic sediments may have influenced overall Mo isotope fractionations in oxic sink of global ocean. In this view, sedimentary δ⁹⁸Mo evolution through Earth’s history, especially large δ⁹⁸Mo shifts during the Precambrian, is closely associated to changes in global marine Fe and Mn cycling.

Methods

Sampling

Thirty-nine nodule samples were collected from 18 sites in 6 basins in the Pacific, Indian, and Atlantic oceans (Fig. 1)²⁹, with detailed location information in the Supplementary Table 1. The collected nodules were formed with water depths ranging from 4271 m to 5629 m, whose structures were mainly spheroidal, joined spheroidal, and cauliflower-shaped. Based on previous XRD analysis, the minerals in the collected nodules are mainly composed of todorokite, phyllomanganate, vernadite, ferrihydrite, quartz, and phillipsite²⁹. The samples for geochemical analyses were scraped from the surface of each nodule to a depth of 1–2 mm and then ground into 200-mesh powders.

Geochemical analyses

The sample powders were analyzed for elemental and isotopic data using full digestion (with a subscript of “bulk”) and partial leaching approaches (with a subscript of “leach”), respectively. For full digestion, mixing acids of HF + HNO₃ + HCl was used to dissolve both ferromanganese oxides and potential aluminosilicates in nodules. For partial dissolution of ferromanganese oxides, the bulk samples were leached with 6 M HCl overnight at room temperature following the previous studies of ferromanganese crusts and nodules^6,27. The USGS geostandard Nod-A-1 was also treated using the 6 M HCl leaching approach. The above sample solutions were measured for major and trace elements using a Thermo Element XR ICP-MS at the Metal Isotope Geochemistry Lab in the Centre for Research and Education on Biological Evolution and Environment (CREBEE), Nanjing University. The OSIL seawater and USGS Nod-A-1 standards were analyzed to monitor the long-term instrument precision, which is typically better than 5% in this study.

For Mo isotope analysis, a ⁹⁷Mo–¹⁰⁰Mo double spike was used to calibrate the potential Mo isotope fractionation during the Mo separation and isotopic measurement. The sample-spike mixtures were purified using 0.8 mL TRU resin in an Eichrom 2 mL column, modified from the procedure in a previous study⁴⁸. The purified Mo solution was analyzed for isotopic ratios using a Thermo Scientific Finnigan Neptune XT MC-ICP-MS associated with a CETAC Aridus III desolvating system at CREBEE, Nanjing University. The Mo isotope ratios are ultimately reported in standard delta (δ⁹⁸Mo) notation relative to the NIST SRM 3134 Mo standard using the equation below:

$${\delta }^{98}{{{\rm{Mo}}}}=[({\scriptstyle{{98}}\atop} {{{{\rm{Mo}}}}{/}^{95}{{{\rm{Mo}}}}})_{{{{\rm{sample}}}}}/(({\scriptstyle{{98}}\atop} {{{{\rm{Mo}}}}{/}^{95}{{{\rm{Mo}}}}})_{{{{\rm{NIST}}}}\,3134}\times 0.99975)-1]\times 1000$$

Here, the δ⁹⁸Mo value of NIST SRM 3134 is defined as +0.25‰ to better compare with previously published Mo isotope data⁴⁹. The long-term external reproducibility of analysis was assessed to be better than ±0.05‰ (2σ), based on repeated measurements of NIST SRM 3134 (δ⁹⁸Mo₃₁₃₄ = +0.25‰ ± 0.03‰ n = 30), OSIL seawater standard (δ⁹⁸Mo_seawater = +2.34‰ ± 0.04‰, n = 4) and USGS Nod-A-1 and Nod-P-1 manganese nodule standards (δ⁹⁸Mo_Nod-A-1 = –0.48‰ ± 0.05‰, n = 2 and δ⁹⁸Mo_Nod-P-1 = –0.62‰ ± 0.03‰, n = 4), consistent with recently published data^48,50.