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# Mackinawite partial oxidation to green rust produces a large, abiotic uranium isotope fractionation

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Romain Guilbaud<fnref n="1" />✉, Morten B. Andersen©², Helen M. Freeman³, Jeffrey Paulo H. Perez©⁴, David Uhlig©⁵ & Liane G. Benning©⁴,⁶

Low-oxygen settings and transition zones between strictly anoxic and oxic conditions may have characterised large expanses of Precambrian continental margins, where oxygen-breathing, complex life emerged and diversified. Accurate reconstructions of oxygen levels in such conditions are therefore required, but current geochemical proxies fail in identifying transitional redox conditions. Uranium isotopes are an emerging palaeoredox proxy, as large isotope fractionations are recorded during the reduction of uranium into anoxic sediments. Their potential application to transitional conditions is, however, unclear, because the redox regulating mineralogy of such environments and the associated isotope fractionations are poorly constrained. Here, we explore the mineralogical transformations occurring during the partial oxidation of mackinawite. We show that green rust, a key mineral of Precambrian oceans, forms as a by-product of mackinawite oxidation, along with uraninite and polysulphide. We also demonstrate that this mechanism records a large abiotic uranium isotope fractionation during its reaction with aqueous iron sulphide.

The isotopic variability of uranium (U) isotopes is being increasingly used on ancient sedimentary rocks to reconstruct palaeo-redox conditions<fnref n="1" />⁻⁶ in Proterozoic (2.5 to ~0.54 billion years ago, Ga) oceans. Their use has been applied predominantly to carbonates and black shales. In the first instance, this is due to limited U isotope fractionation during U incorporation into carbonates, which may, in some instances, record the seawater isotopic composition with limited diagenetic alterations²,⁷,⁸. In the case of black shales, U isotopes fractionate (by up to ~1.2‰) during U reduction and fixation into anoxic sediments<fnref n="1" />,³,⁹,¹⁰. However, Precambrian oceans were characterised by a range of oxygen-depleted conditions, from low-oxygen ('suboxic', here referred to as <100 µM [O₂]) levels to ferruginous (anoxic, ferrous iron-containing and non-sulphidic) conditions, and to euxinic (anoxic and sulphide-containing) conditions. All these conditions likely fractionate U isotopes in different ways. Hence, interpreting variations in sedimentary U isotope compositions requires a detailed understanding of the redox context in which U isotopes where transported and archived into anoxic sediments.

Throughout most of the Proterozoic, oceans were dominantly ferruginous<fnref n="1" />,¹², with oxic shallow waters and euxinic conditions spanning along productive continental margins¹²,¹³. From ~1 Ga, the spread of euxinic

margins became severely limited¹⁴,¹⁵, with expansive ferruginous conditions until the late Proterozoic¹⁶,¹⁷. Dynamic fluctuations between oxic and ferruginous water masses¹⁸,¹⁹ eventually led to the establishment of modern-like oxygen minimum zones at the Proterozoic-Cambrian transition, with punctual euxinic conditions in basins with high organic influxes²⁰⁻²². This long-term evolution of ocean redox chemistry suggests that the application of the U isotope proxy to Precambrian sedimentary rocks demands enhanced knowledge on how U isotopes may fractionate in these distinct redox contexts.

Under ferruginous conditions, the formation of iron minerals such as ferrihydrite²³ and green rust²⁴ (a mixed Fe(II)/Fe(III) hydroxide, naturally occurring as either Fe₆(OH)₁₂SO₄ or Fe₆(OH)₁₂CO₃) in the water column may fractionate U isotopes through co-precipitation or adsorption processes, although the data is limited to few experimental studies²⁵,²⁶. It has been suggested that U reduction by mixed Fe(II)/Fe(III) minerals such as magnetite and green rust lead to the reduced U(IV) products being isotopically light²⁶. This is contrasting with other field and experimental observations²⁷,²⁸, which indicate that U(IV) phases incorporate preferentially heavy U isotopes, as is the case for mass independent isotope fractionation effects such as nuclear field shift fractionation. Unfortunately,

<footnote n="1">¹Centre National de la Recherche Scientifique (CNRS), Géosciences Environnement Toulouse (UMR 5563), 31400 Toulouse, France. ²School of Earth and Environmental Sciences, Cardiff University, Cardiff, CF10 3AT, UK. ³Priestley Centre for Climate Futures, University of Leeds, Leeds, LS2 9JT, UK. ⁴GFZ Helmholtz Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany. ⁵Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany.
⁶Department of Earth Sciences, Freie Universität Berlin, 12249 Berlin, Germany. ✉e-mail: romain.guilbaud@cnrs.fr</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1 | Characterization of initial non-U reacted FeS.** a TEM image of FeS sheet aggregates. b SAED patterns of (a) with peaks at 1.72 Å (201), 1.57 Å (211), 1.85 Å, 2.31 Å (111), 2.62 Å (110) and 3.03 Å (101), confirming that the particles only consist of mackinawite¹⁷, c, d High resolution image and FFT showing *d*-spacing at 0.5 and 0.3 nm, supportive of nanoparticulate mackinawite¹⁷.</figcaption></figure>

natural observations in modern ferruginous analogues, which could potentially corroborate these experimental data, show a high variability in the U isotope compositions, presumably due to a range of isotope fractionation effects occurring during various biogeochemical pathways²⁹. Under euxinic conditions, U reduction is accompanied by a large isotope fractionation, which favours the incorporation of isotopically heavy U into the reduced phase and the sediments³,²⁷. In these euxinic settings, U reduction and removal are likely biologically mediated by, for example, sulphate-reducing bacteria³⁰. Mackinawite (here nominally labelled as FeS) occurs as the preponderant nanoparticulate Fe mineral within euxinic water columns, as its precipitation is kinetically favoured over pyrite (FeS₂)³¹⁻³³. While abiotic reduction of U(VI) to U(IV) on FeS surfaces is well known³⁴,³⁵, contradictory experimental datasets suggest equivocal effects on the associated U isotope fractionations²⁶,³⁶. In particular, the roles of adsorption versus reduction on the overall isotope fractionation have been notoriously difficult to distinguish in experimental set-ups³⁶.

In addition, dynamic shifts from euxinic to ferruginous or oxic conditions during the Precambrian may have resulted in the establishment of transient redox transitional zones. Despite the significance of such redox transitional zones for the Precambrian biota on continental margins, they have received relatively little attention. The study of such transitions in modern analogues (e.g., Arvadi Spring, Switzerland) suggests that in ferruginous-to-oxic and euxinic-to-oxic transition zones, the mineralogy of Fe- and S-bearing phases is dominated by green rust and elemental S, respectively³⁷. The formation of elemental S during the oxidation of FeS has also been documented experimentally³⁴,³⁵,³⁸. In these experiments, reported Fe-bearing products include either greigite (Fe₃S₄) as an intermediate phase during pyrite formation³³,³⁹, or a range of iron oxide assemblages, such as hematite and magnetite⁴⁰, goethite and lepidocrocite⁴¹, and amorphous Fe(OH)₃ [refs. 42,43]. While ‘green rust-like’ precipitates have also been suggested to form under acidic pH during oxidation by selenate⁴¹ and chromate⁴⁴, green rust has never been reported as a Fe-bearing product of an FeS oxidation reaction by uranium at seawater pH.

Here, we explore experimentally the mechanism of FeS oxidation by U(VI) at seawater pH. Using high resolution transmission electron

microscopy (TEM) imaging and analyses, we report the end-product mineralogy for both Fe and U phases, and we track the evolution of U isotope compositions in both the dissolved and the solid phases. Ultimately, we provide a new key insight to interpret U isotope signatures in ancient anoxic transition zones, revisiting earlier views on abiotic U isotope fractionations.

## Results

### Characterization of initial conditions

Mackinawite precipitated by mixing of equimolar Fe(II) and S(-II) solutions, following previously published protocols⁴⁵. Immediately after mixing, well-developed nanoparticles with Fe-S composition and typical plate-structured aggregates⁴⁶,⁴⁷ formed (Fig. 1). This phase was identified as mackinawite through selected area diffraction (SAED, Fig. 1b) and fast Fourier transforms (FFT, Fig. 1d), and through the Fe/S ratios from cross-correlated X-ray photoelectron spectroscopic analyses (XPS, Fig. 2, Tables S1–S4).

This freshly precipitated FeS slurry was aged in its supernatant for one hour in order to allow for particle aggregation and equilibration⁴⁸. In a second step, a U(VI) solution (NaHCO₃-buffered at pH 8) was injected into this 1 h aged FeS slurry, and the mixture was further aged for various experimental durations (up to 1.17 h; Table 1). At set time points the resulting solids were separated from the supernatants through filtration (see the Supplementary methods 1 for full details on the protocols) and each component characterized. For longer-term experiments (up to 144 h), the uranium solution was added directly after FeS precipitation with no additional aging. Due to the bicarbonate buffer, dissolved U(VI) was largely dominated by the uranyl carbonate complex UO₂(CO₃)₂²⁻, as confirmed by aqueous speciation calculations (Figs. S1 and S2). The experimental setup was therefore comparable to previous studies²⁶,³⁶, with a major difference being the higher pH (pH 8 compared to pH 6.8 and pH 7.1 in refs. 26,36, respectively). This higher pH was chosen to minimize the adsorption of negatively charged uranyl carbonate species onto mackinawite surfaces, which become positively charged at pH<7.5 [ref. 49], inducing additional U isotope fractionations⁵⁰. However, whilst this efficiently diminishes outer-
<figure><img src="image_4.png" /><figcaption>Fig. 2 | High resolution XPS spectra for the initial FeS, with calculated Fe/S of 0.76, as reported for mackinawite$^{48}$. Spectra shown are for Fe 2p$_{3/2}$ (a), S 2p (b), and O 1s (c).</figcaption></figure>

sphere adsorption, inner-sphere adsorption may still occur, as well as U(VI) entrapping during FeS particle aggregation$^{21}$. Yet, this would also apply to the setup of previous work, and our work diverges from previous studies in that adsorption effects are limited.

## Mackinawite oxidation to green rust

Subsequent to U injection into the FeS slurry and during the first hours of the experiment, U was removed from solution following first-order kinetics (Fig. 3). The rates of U removal are very similar to previous work, with faster rates when no Ca is introduced to the system$^{36}$. Uraninite (UO$_2$) is commonly invoked as a solid U$^{IV}$ end-product of U reduction by FeS, based on spectrometric methods$^{26,34,36}$, but it has only been observed by TEM imaging in two studies$^{35,38}$. Both our XPS and TEM results confirm the formation of UO$_2$ nanoparticles as the final U(IV) phase in our experiments (Fig. 4d-e), with d-spacings in agreement with the presence of uraninite nanoparticles$^{25,52}$. We also explored the products of FeS oxidation by U(VI). Our XPS results suggest that S species include mono- and polysulphides but exclude pyrite or other disulphides as end-products (Fig. 4b). This supports the formation of elemental S and polysulphide during the FeS oxidation reaction, again confirming results reported in both experimental$^{34,35,38}$ and natural$^{37}$ studies.

The calculated Fe$^{II}$-S/Fe$^{III}$-S ratio of 0.28 on the Fe 2p$_{3/2}$ XPS spectrum (Fig. 4a) suggests the presence of oxidized Fe$^{III}$-S species following reaction with U(VI). Such a ratio would correspond stoichiometrically to an intermediate phase between mackinawite and greigite. However, whilst greigite has been observed as a solid intermediate phase during the formation of pyrite$^{33,39}$, we found no TEM evidence for its presence in our experiments. Any Fe$^{III}$-S species could potentially be a consequence of alteration phases at the surface of mackinawite aggregates; but again, there was no TEM evidence supporting alteration patterns or oxidation products and the FeS aggregates persisted until the end of the reaction (Fig. 5a). Furthermore, while we cannot exclude inner-sphere complexation, outer-sphere adsorption through electrostatic interactions is not favoured in our experimental setup, with pH >pH$_{PZC}$ of mackinawite$^{53}$, and we presume that extensive U(VI) adsorption is hindered. Instead, we propose that FeS oxidation likely occurred via solubility-controlled dissolution.

At a pH of 8 and for millimolar S(-II) concentrations, the solubility of mackinawite is dominated by the formation of aqueous FeS$_{aq}$ clusters, contrasting with the formation of Fe$^{2+}$ under more acidic conditions$^{54}$. Therefore, in our experiment and in most FeS-rich seawater environments, reduction of uranium is likely to occur via the reaction with aqueous FeS$_{aq}$. Hence, we argue that Fe$^{II}$-S/Fe$^{III}$-S species are unlikely to form at the surface of solid FeS aggregates following U(VI) adsorption. Instead, our TEM results clearly document the presence of well-developed hexagonal structures (Fig. 5a-c), that were also Fe phases (as confirmed by EDX, Fig. S3). The

SAED point towards a mixed Fe$^{II}$/Fe$^{III}$ hydroxide green rust, (Fig. 5d), and we therefore suggest that that the mixed Fe$^{II}$-S/Fe$^{III}$-S XPS spectra may include, at least partly, some green rust component. Thermodynamic calculations (using PHREEQC/PhreePlot and the phreeqc.dat and wateq4f.dat database for uranium) further support the stability of green rust under our experimental Eh-pH conditions (Fig. S4). Because the starting Fe phase was FeS, it is reasonable to expect that the product consists of sulphate green rust (Fe$_6$(OH)$_{12}$SO$_4$), following the partial oxidation of both Fe and S, which is consistent with the presence of polysulphide (Eq. 1). However, the d-spacing measured by SAED (Fig. 5d) suggests carbonate green rust (Fe$_6$(OH)$_{12}$CO$_3$), which is smaller than the d-spacing of sulphate green rust$^{55}$. Our data confirm previously documented sulphate to carbonate green rust transformation by substitution$^{56}$ due to the high bicarbonate concentration in solution (Eq. 2). We also note that partial green rust oxidation during electron irradiation may have resulted in diminished d-spacings$^{57}$.

$$6\text{Fe}^{\text{II}}\text{S}_{\text{aq}} + 9\text{UO}_{2}(\text{CO}_{3})_{2}^{2-} + 16\text{H}_{2}\text{O} = \text{Fe}^{\text{II}}_{4}\text{Fe}^{\text{III}}_{2}(\text{OH})_{12}\text{SO}_{4} + \text{S}_{5}^{2-} + 2\text{H}^{+} + 9\text{UO}_{2} + 18\text{HCO}_{3}^{-} \quad (1)$$

$$\text{Fe}^{\text{II}}_{4}\text{Fe}^{\text{III}}_{2}(\text{OH})_{12}\text{SO}_{4} + \text{HCO}_{3}^{-} = \text{Fe}^{\text{II}}_{4}\text{Fe}^{\text{III}}_{2}(\text{OH})_{12}\text{CO}_{3} + \text{SO}_{4}^{2-} + \text{H}^{+} \quad (2)$$

## Large U isotope fractionation during green rust formation

We measured the isotope composition of U (i.e., the $^{238}$U/$^{235}$U relative variation in permil compared to the starting composition, expressed as $\delta^{238}$U) of both dissolved U(VI) and solid U$^{IV}$O$_2$ (Supplementary methods 3 and 4). Our work extends on the findings in earlier reports$^{26,36}$, which primarily focussed on the isotopic evolution of the dissolved pool. Based on our experimental protocol, we can infer that U adsorption to the solid mackinawite nanoparticles was insignificant, because the negatively charged mackinawite surfaces at our experimental pH of our experiments will prevent substantial U adsorption$^{53}$. The subsequent formation of green rust as an oxidation product of the initial FeS may potentially induce U adsorption. However, at pH 8, the system is also close to the pH$_{PZC}$ of green rust$^{58}$, limiting the extent of U adsorption onto green rust surfaces. Similarly, the rapid transformation of sulphate to carbonate green rust would have negligible effects, as both species have a very similar pH$_{PZC}$$^{55}$. Hence, and as supported by our spectroscopic and high resolution microscopic and analytical results, we conclude that any U that was associated with the solid particles in our experiments predominantly consisted of uraninite (Fig. 4d and e), and not surface-sorbed U. Yet, it is possible that an additional U isotope fractionation occurs upon reaction with neoformed green rust.
<table><caption>Table 1 | Elemental and isotope data for the entire set of experiments</caption><thead><tr><th colspan="10">Short term experiments &lt;2 h</th></tr><tr><th>solids<br>sample</th><th>time (h)</th><th>[Fe] ppm</th><th>[U] ppm</th><th>δ238U (‰)</th><th>2σ (‰)</th><th>Solutions<br>sample</th><th>time (h)</th><th>[Fe] ppm</th><th>[U] ppm</th><th>δ238U (‰)</th><th>2σ (‰)</th><th>Δ238U U(VI)-U(IV) (‰)</th><th>mass balance (‰)</th><th>2σ (‰)</th></tr></thead><tbody><tr><td>FeS-1-S</td><td>0.00</td><td>56.0</td><td>0.0</td><td></td><td></td><td>FeS-1-D</td><td>0.00</td><td></td><td>16.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FeS-11-S</td><td>0.08</td><td>37.2</td><td>5.8</td><td>0.9</td><td>0.2</td><td>FeS-11-D</td><td>0.08</td><td>3.6</td><td>8.5</td><td>-1.0</td><td>0.2</td><td>-1.8</td><td>-0.2</td><td>0.3</td></tr><tr><td>FeS-12-S</td><td>0.08</td><td>38.2</td><td>5.9</td><td>0.7</td><td>0.2</td><td>FeS-12-D</td><td>0.08</td><td>3.9</td><td>9.7</td><td>-0.4</td><td>0.2</td><td>-1.1</td><td>0.0</td><td>0.3</td></tr><tr><td>FeS-13-S</td><td>0.25</td><td>37.7</td><td>6.3</td><td>0.7</td><td>0.1</td><td>FeS-13-D</td><td>0.25</td><td>3.1</td><td>7.2</td><td>-0.7</td><td>0.1</td><td>-1.5</td><td>-0.1</td><td>0.1</td></tr><tr><td>FeS-14-S</td><td>0.25</td><td>31.1</td><td>5.3</td><td>0.4</td><td>0.1</td><td>FeS-14-D</td><td>0.25</td><td>3.2</td><td>7.8</td><td>-0.5</td><td>0.1</td><td>-0.9</td><td>-0.1</td><td>0.1</td></tr><tr><td>FeS-15-S</td><td>0.50</td><td>39.4</td><td>7.1</td><td>0.6</td><td>0.1</td><td>FeS-15-D</td><td>0.50</td><td>3.8</td><td>8.2</td><td>-0.8</td><td>0.1</td><td>-1.4</td><td>-0.1</td><td>0.1</td></tr><tr><td>FeS-16-S</td><td>0.50</td><td>39.9</td><td>7.1</td><td>0.6</td><td>0.1</td><td>FeS-16-D</td><td>0.50</td><td>3.8</td><td>8.7</td><td>-0.7</td><td>0.1</td><td>-1.3</td><td>-0.1</td><td>0.1</td></tr><tr><td>FeS-17-S</td><td>1.17</td><td>38.2</td><td>7.4</td><td>0.7</td><td>0.1</td><td>FeS-17-D</td><td>1.17</td><td>3.7</td><td>8.2</td><td>-0.5</td><td>0.1</td><td>-1.2</td><td>0.0</td><td>0.1</td></tr><tr><th colspan="10">Long term experiments &gt;2 h</th></tr><tr><th>solids<br>sample</th><th>time (h)</th><th>[Fe] ppm</th><th>[U] ppm</th><th>δ238U (‰)</th><th>2σ (‰)</th><th>Solutions<br>sample</th><th>time (h)</th><th>[Fe] ppm</th><th>[U] ppm</th><th>δ238U (‰)</th><th>2σ (‰)</th><th>Δ238U U(VI)-U(IV) (‰)</th><th>mass balance (‰)</th><th>2σ (‰)</th></tr><tr><td>FeS-1-S</td><td>0</td><td>56.0</td><td>0.0</td><td></td><td></td><td>FeS-1-D</td><td>0</td><td></td><td>16.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FeS-2-S</td><td>2.5</td><td>49.0</td><td>12.3</td><td>0.7</td><td>0.1</td><td>FeS-2-D</td><td>2.5</td><td>0.6</td><td>4.6</td><td>-1.4</td><td>0.1</td><td>-2.1</td><td>0.1</td><td>0.1</td></tr><tr><td>FeS-7-S</td><td>5</td><td>47.5</td><td>13.9</td><td>0.5</td><td>0.1</td><td>FeS-7-D</td><td>5</td><td>5.6</td><td>0.6</td><td>-2.9</td><td>0.1</td><td>-3.4</td><td>0.4</td><td>0.1</td></tr><tr><td>FeS-3-S</td><td>5</td><td>48.3</td><td>13.6</td><td>0.3</td><td>0.1</td><td>FeS-3-D</td><td>5</td><td>0.3</td><td>0.7</td><td>-2.8</td><td>0.1</td><td>-3.1</td><td>0.2</td><td>0.1</td></tr><tr><td>FeS-4-S</td><td>26</td><td>46.9</td><td>16.2</td><td>0.2</td><td>0.1</td><td>FeS-4-D</td><td>26</td><td>0.4</td><td>0.1</td><td>-0.7</td><td>0.1</td><td>-0.9</td><td>0.2</td><td>0.1</td></tr><tr><td>FeS-5-S</td><td>48</td><td>51.1</td><td>15.5</td><td>0.0</td><td>0.1</td><td>FeS-5-D</td><td>48</td><td>0.2</td><td>0.1</td><td>-0.8</td><td>0.1</td><td>-0.8</td><td>0.0</td><td>0.1</td></tr><tr><td>FeS-9-S</td><td>48</td><td>43.0</td><td>13.1</td><td>0.0</td><td>0.2</td><td>FeS-9-D</td><td>48</td><td>0.2</td><td>0.1</td><td>-0.6</td><td>0.2</td><td>-0.6</td><td>0.0</td><td>0.3</td></tr><tr><td>FeS-6-S</td><td>144</td><td>52.0</td><td>15.3</td><td>0.1</td><td>0.1</td><td>FeS-6-D</td><td>144</td><td>0.1</td><td>0.3</td><td>-0.8</td><td>0.1</td><td>-0.9</td><td>0.1</td><td>0.1</td></tr><tr><td>FeS-10-S</td><td>144</td><td>44.8</td><td>13.1</td><td>0.2</td><td>0.2</td><td>FeS-10-D</td><td>144</td><td>0.2</td><td>0.3</td><td>-0.5</td><td>0.2</td><td>-0.7</td><td>0.2</td><td>0.3</td></tr></tbody></table>

Also shown are mass balance calculations, which suggest full isotope recovery from both solids and solutions.
However, the effects of this mechanism would remain anecdotal, as there is
no evidence for green rust oxidation products, and the main observed
process is the oxidation of FeS to green rust.

Our results from the long-term experiments (Fig. 6a) showed a large U
isotope fractionation favouring the incorporation of heavy U isotopes into
the solid phase (UO₂), resulting in the formation of an isotopically light
dissolved pool³⁶. The incorporation of heavy U isotopes in the solid phase
contrasts with kinetic fractionation predictions, whereby solid products
would incorporate the kinetically more labile, lighter isotopes. However, this
is in agreement with nuclear field shift effects, where isotope fractionations
are governed by changes in the nuclear charge density, as expected for
heavier elements such as uranium²⁷,²⁸. Yet, unlike in a previous study³⁶, our
results indicate that the isotope fractionation between dissolved and solid U
decreased with time. This suggests kinetic effects during the first hours of the
experiment, prior to reaching a steady state between dissolved U(VI)
and UO₂, where the isotope difference between U(VI) and U(IV)

(Δ²³⁸U<sub>U(VI)-U(IV)</sub>) ~0.8‰ (Table 1, Fig. 6a). Indeed, initial Δ²³⁸U<sub>U(VI)-U(IV)</sub> reaches ~3.4‰ during the first hours of experiment, when the U solution was injected immediately after FeS precipitation. This is in sharp contrast with the results in our short-term experiments (Table 1 and Fig. 6b), where FeS was first aged in its supernatant solution before initiating the reaction with U, and where initial Δ²³⁸U<sub>U(VI)-U(IV)</sub> remained closer to the final Δ²³⁸U<sub>U(VI)-U(IV)</sub> of ~1.2‰ (Table 1). Previous isotope⁵⁶ and small-angle X-ray scattering⁵⁷ studies have identified aggregation growth as the mechanism leading to mackinawite assembly in the earliest steps of FeS precipitation. We therefore suggest that the large apparent isotope fractionation in the long-term experiments are the result of rapid FeS nanoparticle growth via oriented aggregation mechanism²¹,⁵⁹, which may record the highest expression of U isotope fractionations. During subsequent isotope exchange, the system quickly reaches a steady state, which resembles the short-term experiment steady state (Δ²³⁸U<sub>U(VI)-U(IV)</sub> ~1‰). In both cases, the evolution of the isotope composition scales logarithmically with the removal of U(VI)aq from solution (Fig. 7), confirming earlier studies²⁶,³⁶, with a substantial U-ray scattering shift when data include initial, short term particle growth, or when data solely reflect longer-term isotope fractionations.

Our results, pointing towards a large, apparent isotope fractionation
recorded as steady state sharply contrasts with previous studies in the
absence of calcium<sup>26,36</sup>. We envisage that this discrepancy is due to (1) the
minimisation of adsorption effects, which may fractionate U isotopes in
opposite direction to reduction effects<sup>50</sup>, in turn diminishing the extent of
apparent fractionation; (2) the fundamentally different mechanism for FeS
dissolution at higher pH, involving FeS<sub>aq</sub> clusters over Fe<sup>2+</sup>, and the partial
oxidation of FeS<sub>aq</sub> to green rust. Hence, we propose that in most FeS-rich
seawater environments with pH >7, the reduction of uranium by FeS likely
occurs via the formation of FeS<sub>aq</sub> and green rust phases, and that this
reaction is indeed accompanied by a large U isotope fractionation, which
concentrates the heavier isotopes in the reduced (solid) phase, with U iso-
tope compositions approximately 1‰ more positive than the residual
solution.

<figure><img src="image_5.png" /><figcaption>**Fig. 3 |** *Linearized dissolved U concentrations through time, during U<sup>V1</sup> removal from solution.* Our study compares well to previous work<sup>34,36</sup>, suggesting 1<sup>st</sup> order kinetics for the rate of U removal with no Ca present in solution.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4 |** *High resolution XPS spectra of the mineralogical products after one day of experiment.* Spectra are shown for Fe 2p<sub>3/2</sub> (a), S 2p (b), O 1s (c) and U 4f (d), and TEM image with FFT patterns of nanocrystalline uraninite (e).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5** | TEM data supporting green rust production. (a) TEM image of FeS aggregates and green rust hexagons (highlighted by dotted white lines) after one day of experiment, as confirmed by the SAED peaks in (b). (c) Visible hexagonal structures (highlighted by dotted white lines) where an EDX analyses was carried out (supplementary information Fig. S3), and (d) typical green rust hexagonal patterns identified by SAED, with the outer ring corresponding to the 116 lattice plane of the carbonated species (in green)$^{55}$.</figcaption></figure>

**Discussion: Implications for Precambrian redox transitions.** The highly dynamic nature of ocean redox chemistry in the Precambrian highlights the significance of redox transitional zones for the reconstruction of ancient biogeochemical cycles. In modern environments, the mineralogy of such transitions can be explored through the study of redoxclines, or more generally at the interface between different redox settings. For instance, in modern ferruginous analogues, the redox interface between shallow, oxygenated waters and anoxic, Fe-containing deeper waters, is characterized by the precipitation of ferrihydrite and its partial reduction to green rust$^{23,24,60}$. Magnetite also plays a major role as an end-product of the rapid transformation of metastable green rust species$^{24,58,61}$, or potentially as a primary precipitate at the oxic-ferruginous interface$^{62}$.

At the redoxcline of euxinic environments, the release of Fe(II) upon reduction of Fe (oxyhydr)oxides by H₂S may promote the formation of an anoxic and non-sulphidic layer. An eloquent example for such non-sulphidic interface above euxinic deeper waters is the Black Sea, with several meters of Mn-rich water between the strictly oxic and strictly euxinic water masses$^{63}$, but thinner anoxic and ferruginous interfaces have also been reported in euxinic lakes$^{64}$. Modern oxygenated ferro-euxinic springs, considered analogous to ancient anoxic-oxic transitions, have reported elemental sulphur and green rust as dominant mineral phases$^{37}$. Such environments may be characteristic of suboxic settings, where low levels of oxygen dominate, or at the interfaces between strictly oxic and strictly anoxic conditions. Our study supports the natural observations, and further hints at the fact that the partial oxidation of FeS, a preponderant metastable mineral in strictly euxinic systems$^{65}$, indeed leads to the formation of green rust. We showed that 9 moles of U(VI) are sufficient to partially oxidise 6 moles of FeS into 1 mole of green rust and polysulphide (Eq. 1). We note that polysulphide may further react with FeSaq to nucleate pyrite under euxinic conditions$^{65,66}$, as a competing mechanism for FeS partial oxidation. Although we did not observe pyrite formation in our experiments, this could

be kinetically hindered, and potential U isotope fractionations during U(VI) reduction by pyrite have never been determined. Assuming oxygen is the only oxidant source in the system (as opposed to uranium), the reaction would be equivalent to the oxidation of 6 moles of FeS by 4.5 moles of oxygen (Eq. 3). From our experimental set up, where we started with ~ 70 µM U(VI), we infer that oxygen concentrations of 35 µM are sufficient to drive the partial oxidation of FeS into green rust and polysulphide.

$$6\text{Fe}^{\text{II}}\text{S}_{\text{aq}} + 4.5\text{O}_2 + 7\text{H}_2\text{O} = \text{Fe}_4^{\text{II}}\text{Fe}^{\text{III}}_2(\text{OH})_{12}\text{SO}_4 + \text{S}_5^{2-} + 2\text{H}^+ \quad (3)$$

In such environments, U isotopes may fractionate substantially during the partial oxidation to green rust, with the preferential incorporation of $^{238}$U into uraninite. Therefore, our study implies that chemocline fluctuations between strictly euxinic (containing FeS suspensions) and ‘suboxic’ conditions along Proterozoic continental margins$^{67}$ are likely to have led to the formation of green rust minerals in the water column, and to the deposition of sediments being dominantly enriched in heavy U isotopes. An abiotic $\Delta^{238}U_{\text{sediment-water}}$ of ~1‰ is substantially larger than the typical level of $\delta^{238}U_{\text{sediment-water}}$ observed in modern euxinic basins (~0.5‰)$^{9,10,27}$. While U reduction driven by bacterial sulphate-reduction within sediments is expected to dominate U uptake under strongly euxinic conditions, it may be diminished under less extreme, transitional redox settings$^{27}$. Therefore, abiotic U reduction during partial FeS oxidation to green rust may have played a prominent role for authigenic U enrichments within zones of transitional redox conditions in the Precambrian oceans. This reinforces previous views$^{24,61}$ that green rusts are key minerals for the reconstruction of Precambrian environments, due to their ubiquity in ferruginous, ‘suboxic’ and transitional redox settings. Such sedimentary systems should be detected by substantial $\Delta^{238}U_{\text{sediment-water}}$ (~1‰) and a high authigenic $\delta^{238}U$ component in anoxic sediments. High authigenic $\delta^{238}U$ compared to seawater has been observed in sediments from the chemocline of the Black Sea$^{68}$, as well as in Mediterranean sapropels characterised by weakly or temporal euxinic
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6 |** Isotopic evolution of dissolved U<sup>V1</sup> and solid UO<sub>2</sub> through the experiment. Heavy U isotopes are preferentially incorporated into uraninite, as predicted for nuclear field shift effects. (a) Long-term experiments, where U injection was immediately following FeS precipitation; (b) short-term experiments, where U injection was done only after 1 h of aging the precipitated FeS in its supernatant.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7 |** Evolution of δ<sup>238</sup>U in solution as a function of U remaining in the solution, for the experimental dataset <48 h in red, and >48 h in grey. C stands for U concentration at any given time, and C<sub>0</sub> stands for initial U concentrations. The data best fit with a logarithmic trend, with R<sup>2</sup> = 0.97 for <48 h experiments, and R<sup>2</sup> = 0.82 for >48 h experiments. The substantial shift between both datasets is best explained by initial particle growth effects (in red) versus longer term isotope fractionations at steady state (in grey).</figcaption></figure>

conditions<sup>68,69</sup> and in a range of Paleoproterozoic to early Phanerozoic sediments<sup>1,70–72</sup>. While there are different ways to explain higher δ<sup>238</sup>U than observed in modern euxinic sediments<sup>3,68,73</sup>, we suggest that abiotic U reduction during partial FeS oxidation to green rust may have contributed a substantial part of authigenic U enrichments in such sediments.

## Data availability

All data are included in present paper.

Received: 26 April 2024; Accepted: 31 January 2025;

Published online: 08 February 2025

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## Acknowledgements

This work was funded by a Helmholtz Centre/GFZ short-term stay, a Royal Society Research Grant (RSG\R1\180326) and ANR grant agreement ANR-21-CE49-0007-01 to RG, and a Royal Society Research Grant (RSG\R1\160890) to MA. JPHP and LGB acknowledge the funding from the European Union's Horizon 2020 Marie Skłodowska-Curie Innovative Training Network METAL-AID (grant no. 675219), and the Helmholtz Recruiting Initiative (award no. I-044-16-01) awarded to LGB. JPHP is in part funded by the GFZ Discovery Fellowship (grant no. P-032-45-002). ICP-OES analyses were carried out at the Helmholtz Laboratory for the Geochemistry of the Earth Surface (HELGES) at GFZ Potsdam. XPS data were collected at the BAM Federal Institute for Materials Research and Testing, and the authors thank Dr. Jörg Radnik for the help. TEM analyses were carried out at the German Research Centre for Geosciences in the Potsdam Imaging and Spectral Analyses (PISA) facility that is partly funded by the European Regional Development Fund and the State of Brandenburg. We thank 3 anonymous reviewers for their constructive comments.

## Author contributions

RG designed and performed the experiments. MBA analysed the uranium isotope composition of the samples. HMF performed the TEM analysis with RG and interpreted the TEM data with RG and LGB. JPHP performed the XPS analysis, and interpreted the XPS data with RG and LGB. DU performed the elemental analysis. The manuscript was initially written by RG, with significant contribution from MBA, HMF, JPHP, DU and LGB.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s43247-025-02088-5.

**Correspondence** and requests for materials should be addressed to Romain Guilbaud.

**Peer review information** *Communications Earth & Environment* thanks Abu Saeed Baidya and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Carolina Ortiz Guerrero. A peer review file is available.

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