Alain Manceau
email: alain.manceau@univ-grenoble-alpes.fr

Alexandre Simionovici

Nathaniel Findling

Pieter Glatzel

Blanka Detlefs

Anna V Wegorzewski

Kira Mizell

James R Hein

Andrea Koschinsky
email: a.koschinsky@jacobs-university.de

Crystal Chemistry of Thallium in Marine Ferromanganese Deposits

Keywords: EXAFS, XANES, X-ray diffraction, Raman spectroscopy, birnessite, vernadite, todorokite

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Thallium (Tl) is enriched in ferromanganese (FeMn) crusts and nodules that cover the surface of seamounts and seafloors throughout the oceans. [START_REF] Koschinsky | Uptake of Elements from Seawater by Ferromanganese Crusts: Solid-Phase Associations and Seawater Speciation[END_REF][START_REF] Rehkamper | Cenozoic Marine Geochemistry of Thallium Deduced from Isotopic Studies of Ferromanganese Crusts and Pelagic Sediments[END_REF][START_REF] Rehkamper | The Mass Balance of Dissolved Thallium in the Oceans[END_REF][START_REF] Hein | Deep-Ocean Mineral Deposits as a Source of Critical Metals for High-and Green-Technology Applications: Comparison with Land-Based Resources[END_REF] The average Tl contents of nine major regions of the Pacific, Atlantic, and Indian oceans range from 129 to 347 ppm (mg/kg) in nodules and from 41 to 160 ppm in crusts. [START_REF] Hein | Deep-Ocean Ferromanganese Crusts and Nodules[END_REF] Taking a mean dissolved Tl concentration of 65 pmol/kg in the global oceans, [START_REF] Rehkamper | The Mass Balance of Dissolved Thallium in the Oceans[END_REF] the partitioning of Tl between seawater and FeMn crusts and nodules is as high as 10 7 . The enrichment of Tl in marine FeMn deposits makes them a useful archive of Tl fluxes over the Cenozoic for which Tl isotopes have been interpreted as a proxy for organic carbon burial in the oceans. 6 In addition, FeMn minerals may be considered economically interesting as a resource for Tl. Thallium is classified as a strategic element because of its low natural abundance and use in advanced technologies, including electronics, optics, and HT-superconductors. Currently, the main source of mineable Tl is the trace amounts occurring in Cu (chalcopyrite), Pb (galena), Zn (sphalerite), and other sulfide ores. The concentration of Tl in commonly exploited sulfide ores is about 100 ppm (mg/kg), 7 which is met or surpassed by the mean concentrations in marine FeMn minerals. Determining the enrichment mechanisms, phase associations, and coordination chemistry of Tl in FeMn crusts and nodules has implications for both paleoceanographic and resource evaluations.

State of knowledge for thallium enrichment in manganese oxides. It is considered that Mnassociated Tl enrichment in marine deposits and soils results from the oxidative scavenging of Tl(I) to Tl(III) in contact with phyllomanganates of the birnessite group (MnO2-x). [START_REF] Marenich | Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions[END_REF][START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Hlawatsch | Fast-Growing, Shallow-Water Ferro-Manganese Nodules from the Western Baltic Sea: Origin and Modes of Trace Element Incorporation[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF][START_REF] Manceau | Estimating the Number of Pure Chemical Components in a Mixture by X-Ray Absorption Spectroscopy[END_REF][START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF][START_REF] Li | Geochemical and Molecular Characteristics of Ferromanganese Deposits and Surrounding Sediments in the Mariana Trench: An Implication for the Geochemical Mn Cycle in Sedimentary Environments of the Trench Zone[END_REF] A single micro-XANES study of a hydrogenetic crust from the Pacific Ocean performed in 2012 by Peacock and Moon [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF] showed that Tl, associated spatially with turbostratic (i.e., c-disordered [START_REF] Manceau | Determination of Mn Valence States in Mixed-Valent Manganates by XANES Spectroscopy[END_REF] ) birnessite (δ-MnO2, named vernadite [START_REF] Glans | Experimental and Theoretical Study of the Electronic Structure of HgO and Tl2O3[END_REF][17][18][START_REF] Hochella | Environmentally Important, Poorly Crystalline Fe/Mn Hydrous Oxides: Ferrihydrite and a Possibly New Vernadite-like Mineral from the Clark Fork River Superfund Complex[END_REF][START_REF] Lee | The Structure and Crystal Chemistry of Vernadite in Ferromanganese Crusts[END_REF] ) and with todorokite, [START_REF] Post | Refinement of the Todorokite Structure[END_REF] occurred in both 1+ and 3+ oxidation states. As much as 58 ± 3% of Tl was 1+ despite the oxic formation environment of FeMn crusts. This conundrum is informed by laboratory experiment, which shows that the Tl(I)(aq) → Tl(III)(s) oxidation occurs only when birnessite is depleted in Mn 3+ , and does not occur at all on todorokite, [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] because this tectomanganate is naturally rich in Mn 3+ . [START_REF] Post | Synchrotron X-Ray Diffraction of the Structure and Dehydration Behavior of Todorokite[END_REF][START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF] These findings suggest that the Tl(III)/Tl(I) ratio could be a powerful tracer of the Mn(IV)/Mn(III) ratio and vice versa, the manganate mineralogy, and the redox conditions of the geochemical setting. Thus, precise knowledge of the oxidation states of Mn and Tl and identity of the Tl-carrying phases is essential to advance research on geochemical processes responsible for the enrichment of Tl in marine deposits.

Understanding the structure of the Tl(III) surface complex on vernadite is also a necessary prerequisite to model the Tl(I)(aq) → Tl(III)(s) redox reaction and to understand the key role of Mn 3+ in this reaction. Extended X-ray absorption fine structure (EXAFS) studies of Tl sorption on δ-MnO2 showed that Tl(I) is oxidized to Tl(III) primarily on vacancy sites of the MnO2 layer as a tridentate corner-sharing (TCS) complex with a Tl-Mn distance of 3.60-3.63 Å and secondarily on the edges of the layer as a double edge-sharing (DES) complex with a Tl-Mn distance of 3.27-3.28 Å (Figure 1). [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF][START_REF] Manceau | Estimating the Number of Pure Chemical Components in a Mixture by X-Ray Absorption Spectroscopy[END_REF] The coordination of the Tl(III)-TCS complex derived from EXAFS has not been fully resolved. Two studies reported a split O coordination shell with d(Tl-O) = 2.15-2.18 Å and 2. 23-2.26 Å interpreted as an elongated tetrahedral coordination over the Mn vacancies, [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Manceau | Estimating the Number of Pure Chemical Components in a Mixture by X-Ray Absorption Spectroscopy[END_REF] and one study reported a single bond distance of 2.20 Å interpreted as an octahedral coordination on both the vacancy and edge sites. [START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] Considerations for thallium X-ray absorption spectroscopy in marine ferromanganese minerals. The Tl concentration in the Peacock and Moon 9 study was as high as 2000 ppm, and even higher at the points of interest (n = 4) measured by micro-XANES spectroscopy. There is a clear need to extend this pioneering research in the range of a few hundred ppm down to several ppm that represent the typical concentrations of Tl in FeMn deposits from the global ocean. It is also desirable to obtain the oxidation state of Tl in whole samples to ensure that observations made at the micrometer scale with point analyses are representative of the bulk. EXAFS data on marine vernadite is lacking, therefore there is no evidence that Tl-sorbed δ-MnO2 surrogates represent how Tl(III) is taken up in FeMn marine deposits. We show below that information on the oxidation state and bonding environment of Tl is difficult to obtain on natural materials with standard XANES and EXAFS spectroscopy because of analytical challenges posed by the multi-elemental composition of Earth materials.

X-ray absorption spectra of elements in low concentrations are measured usually in fluorescenceyield detection mode using an energy-dispersive detector (EDS). An EDS has an energy bandwidth of about 150-200 eV at 10,000 eV, not high enough to separate the X-ray fluorescence lines for two or more elements in multi-element samples. This is particularly striking for Tl because of the proximity of the Tl L3 (12,658 eV) and the As K (11,867 eV) absorption edges, and the resulting overlap of the Tl Lα1 (10,269 eV) and As Kα1,2 (10,508 -10,543 eV) fluorescence lines. The Tl/As atomic ratio of the nine main FeMn deposits ranges between 1.09 and 0.06 (mean = 0.39 ± 0.38). [START_REF] Hein | Deep-Ocean Mineral Deposits as a Source of Critical Metals for High-and Green-Technology Applications: Comparison with Land-Based Resources[END_REF] At [Tl] = 283 ppm and [As] = 151 ppm ((Tl/As)at = 0.69, sample MP23-2, Table 1), the Tl Lα1 line is half as intense as the Kα1,2 line (Figure 2a). Thus, the measurement is feasible with an EDS albeit with a degraded signalto-noise ratio due to the high background signal from the parasitic As Kα1,2 fluorescence. In contrast, at [Tl] = 2 ppm and [As] = 302 ppm ((Tl/As)at = 0.002, sample BS-surface, Table 1), the Tl Lα1 line is totally shadowed by the intense As Kα1,2 line (Figure 2b), hampering the measurement of X-ray absorption spectra at low Tl concentration in natural FeMn oxides. Still, when measurement is feasible the EXAFS spectrum is limited to kmax = 9.9 Å -1 by the Pb L3-edge situated 377 eV above the absorption edge of Tl (Figure S1). The resolution in distance is ΔR = π/2kmax = 0.16 Å, [START_REF] Lee | Extended X-Ray Absorption Fine Structure-Its Strengths and Limitations as a Structural Tool[END_REF] clearly not enough to detect a possible split of the oxygen shell.

Here, this analytical challenge was tackled by selecting the Tl Lα1 line using a wavelength dispersive spectrometer (WDS) consisting of five high-luminosity analyzer crystals. [START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] A crystal analyzer acts both as a low-pass and high-pass filter, eliminating the undesired fluorescence lines at lower and higher energies (e.g., the As Kα1,2 and Pb Lα lines), and also the elastic and inelastic rays at higher energy.

Notably, it filters the intense Fe and Mn Kα1,2 lines from the matrix (5900 and 6400 eV), which otherwise would saturate the detector. This is shown in Figure S2 with the BS-surface nodule which has 15.6 wt % Mn and 14.7 wt % Fe, and the MP23-2 crust which has 28.9 wt % Mn and 7.3 wt % Fe.

The Mn and Fe Kα1,2 fluorescence lines are between two and three orders of magnitude more intense than the Tl Lα1 line. Filtering the Pb Lα line allows extending the k-range of EXAFS beyond the Pb L3-edge (Figure 2c and S1). [START_REF] Glatzel | Range-Extended EXAFS at the L Edge of Rare Earths Using High-Energy-Resolution Fluorescence Detection: A Study of La in LaOCl[END_REF] Another benefit of a spectrometer to study Tl is that the width of the diffracted Lα1 line is about four times lower than the core hole lifetime of the photoelectric X-ray absorption event (~1.5 eV vs ~6.0 eV). [START_REF] Campbell | Widths of the Atomic K-N7 Levels[END_REF] As a consequence, the XANES structures are finely resolved when measured at high energy-resolution (HR-XANES) with a WDS, [START_REF] Hämäläinen | Elimination of the Inner-Shell Lifetime Broadening in X-Ray-Absorption Spectroscopy[END_REF][START_REF] Atkins | Release of Ni from Birnessite during Transformation of Birnessite to Todorokite: Implications for Ni Cycling in Marine Sediments[END_REF] whereas they are featureless when measured with an EDS. 9,30 Thus, HR-XANES provides better precision on the Tl(III)/Tl(I) ratio.

In the field of mineralogy, HR-XANES (also designated HERFD-XANES [START_REF] Glatzel | Hard X-Ray Photon-in Photonout Spectroscopy[END_REF] ) has been applied previously to study the crystal chemistry of gold, arsenic, and selenium in pyrite and arsenopyrite, 32- 35 uranium in phosphates, [START_REF] Pape | HERFD-XANES Spectroscopy at the U M4-Edge Applied to the Analysis of U Oxidation State in a Heavily Contaminated Wetland Soil[END_REF] and cobalt in Mn and Fe (oxyhydr)oxides and olivine. [START_REF] Dublet | Vertical Changes of the Co and Mn Speciation along a Lateritic Regolith Developed on Peridotites (New Caledonia)[END_REF] Organization of the paper. First, we present the composition, mineralogy and average Mn valence of 17 crusts and 11 nodules from the global ocean obtained by chemical analysis, X-ray diffraction (XRD), Raman spectroscopy, and Mn K-edge XANES spectroscopy. Then, we compare the XANES and HR-XANES spectra of a Tl(I) (TlNO3) and a Tl(III) (Tl2O3) reference, and we assign the Tl2O3 spectral features with ab initio FEFF9.6 [START_REF] Rehr | Parameter-Free Calculations of X-Ray Spectra with FEFF9[END_REF] calculations. Next, the HR-XANES spectra of the 28 crusts and nodules are presented, analyzed by principal component analysis (PCA) and iterative target factor analysis (ITFA), [START_REF] Ressler | Quantitative Speciation of Mn-Bearing Particulates Emitted from Autos Burning (Methylcyclopentadienyl)Manganese Tricarbonyl-Added Gasolines Using XANES Spectroscopy[END_REF][START_REF] Rossberg | Complexation of Uranium(VI) with Protocatechuic Acid-Application of Iterative Transformation Factor Analysis to EXAFS Spectroscopy[END_REF][START_REF] Manceau | Quantitative Speciation of Heavy Metals in Soils and Sediments by Synchrotron X-Ray Techniques[END_REF][START_REF] Manceau | Estimating the Number of Pure Chemical Components in a Mixture by X-Ray Absorption Spectroscopy[END_REF] and the Tl(III)/Tl(I) ratios calculated using least-squares fitting (LSF) of the experimental HR-XANES spectra with linear combinations of the Tl(III) and Tl(I) endmember spectra obtained by ITFA. Finally, we describe and fit the EXAFS data of one crust to confirm the coordination environment of thallium in marine FeMn oxides. These data provide novel insights regarding (1) the coordination chemistry of Tl(III) on vernadite and the molecular model of the surface complexes, (2) the relationship between the oxidation states of Mn and Tl, (3) the identity of the Tl(I) carrier phase, and (4) the geochemical significance of the spectroscopic results with regard to sequestration of Tl in ferromanganese oxides over their million-year growth history and implications for the deposit model.

MATERIALS AND METHOD

FeMn oxide samples. Samples were recovered from a wide range of depositional settings from the Pacific, Atlantic, Indian, and Arctic oceans and the Baltic Sea to determine if Tl geochemistry and crystal chemistry vary regionally due to different geographic or oceanographic parameters (Tables 1 andS1). They include hydrogenetic FeMn crusts (i.e. precipitated from the water column) and diagenetic FeMn nodules (i.e. formed from biogeochemical processes in surface and buried sediments), as well as mixed hydrogenetic-diagenetic nodules from the Clarion Clipperton Zone (CCZ) in the northeast Pacific. The hydrogenetic crusts formed at extremely slow growth rates of only a few mm/Ma (million years) over periods of up to 70 Ma 5 while the fast growing shallow-water nodule from the Baltic Sea formed at a rate of 0.2 mm/y and is only a few thousand years old. [START_REF] Hlawatsch | Fast-Growing, Shallow-Water Ferro-Manganese Nodules from the Western Baltic Sea: Origin and Modes of Trace Element Incorporation[END_REF] The deepest crust and nodule samples were retrieved from >4000 m and the shallowest was collected from 20 m depth in the Baltic Sea. Most samples used here have already been investigated in previous studies and are hence rather well characterized with respect to formation and chemical and mineralogical compositions, [START_REF] Koschinsky | Sequential Leaching of Marine Ferromanganese Precipitates: Genetic Implications[END_REF][START_REF] Bau | Comparison of the Partitioning Behaviours of Yttrium, Rare Earth Elements, and Titanium between Hydrogenetic Marine Ferromanganese Crusts and Seawater[END_REF][START_REF] Kuhn | Origin of Negative Ce Anomalies in Mixed Hydrothermal-Hydrogenetic Fe-Mn Crusts from the Central Indian Ridge[END_REF][START_REF] Schmidt | Fractionation of the Geochemical Twins Zr-Hf and Nb-Ta during Scavenging from Seawater by Hydrogenetic Ferromanganese Crusts[END_REF][START_REF] Wegorzewski | The Influence of Suboxic Diagenesis on the Formation of Manganese Nodules in the Clarion Clipperton Nodule Belt of the Pacific Ocean[END_REF][START_REF] Wegorzewski | Mineralogical Characterization of Individual Growth Structures of Mn-Nodules with Different Ni+Cu Content from Central Pacific Ocean[END_REF][START_REF] Koschinsky | Platinum Enrichment and Phase Associations in Marine Ferromanganese Crusts and Nodules Based on a Multi-Method Approach[END_REF][START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF] although Tl concentrations were not reported for all samples.

Hydrogenetic crusts from the Pacific were collected from water depths between about 1100 and 3049 m from various seamounts mostly in the central North and South Pacific Ocean. [START_REF] Hein | Deep-Ocean Polymetallic Nodules as a Resource for Critical Materials[END_REF] While most hydrogenetic crusts were not affected by diagenetic phosphatization, three crust samples are included in which the older crust generation was phosphatized via impregnation and partial replacement by carbonate fluorapatite (CFA) under suboxic conditions. [START_REF] Koschinsky | Effects of Phosphatization on the Geochemical and Mineralogical Composition of Marine Ferromanganese Crusts[END_REF] The two crusts from the NE Atlantic are from different seamounts, the well-characterized Tropic Seamount, located approximately 470 km off Cape Blanc, West Africa, where thick crusts were influenced by terrigenous input [START_REF] Koschinsky | First Investigations of Massive Ferromanganese Crusts in the NE Atlantic in Comparison with Hydrogenetic Pacific Occurrences[END_REF][START_REF] Koschinsky | Ferromanganese Crusts as Indicators for Paleoceanographic Events in the NE Atlantic[END_REF][START_REF] Yeo | Distribution of and Hydrographic Controls on Ferromanganese Crusts: Tropic Seamount, Atlantic[END_REF] with ages up to 75 Ma, [START_REF] Josso | Late Cretaceous and Cenozoic Paleoceanography from North-East Atlantic Ferromanganese Crust Microstratigraphy[END_REF] and an unnamed seamount south of Tropic Seamount for which recovered crusts had not yet been described. The Arctic Ocean crusts show atypically high aluminosilicate detritus, Fe/Mn ratios, and growth rates compared to crusts formed elsewhere. [START_REF] Hein | Arctic Deep Water Ferromanganese-Oxide Deposits Reflect the Unique Characteristics of the Arctic Ocean[END_REF] The Indian Ocean crusts have growth rates comparable to Atlantic and Pacific ocean crust growth rates, but maximum crust ages are generally less than for thick Pacific and Atlantic crusts. One FeMn crust (SO92-74GTV) was collected at the Green Rock Hill in 2808 m water depth close to the Rodriguez Triple Junction in the central Indian Ocean. This crust sample formed on serpentinized ultramafic substrate as fallout from hydrothermal plumes at a high growth rate of about 20 mm/Ma and reduced contents of elements which are typically enriched in hydrogenetic crusts by oxidative scavenging such as Co and Ce. [START_REF] Kuhn | Origin of Negative Ce Anomalies in Mixed Hydrothermal-Hydrogenetic Fe-Mn Crusts from the Central Indian Ridge[END_REF] The hydrothermal plume probably derived from the recently detected Kaimana vent field located about 17 km distance from the sample site (Schwarz-Schampera & Gibson, in prep).

The fast-growing shallow Baltic Sea nodule was collected from the southwestern part of the Baltic Sea, where nodules occur in about 20 m water depth as coatings up to several cm thick on pebbles. [START_REF] Hlawatsch | Fast-Growing, Shallow-Water Ferro-Manganese Nodules from the Western Baltic Sea: Origin and Modes of Trace Element Incorporation[END_REF][START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF] A part of the selected sample was crushed as a bulk sample and another part only the upper mm of the surface was scraped off. The nodule SO79 KG1313 was recovered from an extended nodule field in the Peru Basin [START_REF] Von Stackelberg | Manganese Nodules of the Peru Basin[END_REF] at the southern rim of the equatorial high-productivity zone in the eastern Pacific, which explains the high growth rates of up to 250 mm/Ma and the strong diagenetic component compared to the nodules from the central Pacific Ocean within the CCZ which are mostly of mixed hydrogenetic-diagenetic origin. The CCZ nodules are growing on the sediment surface, the surface of nodules is in contact with the near bottom sea water and the bottom side is buried within the upper few cm of the sediment. Those nodules are mainly mixed type nodules consisting of hydrogenetic as well as diagenetic origin. [START_REF] Wegorzewski | The Influence of Suboxic Diagenesis on the Formation of Manganese Nodules in the Clarion Clipperton Nodule Belt of the Pacific Ocean[END_REF][START_REF] Halbach | The Manganese Nodule Belt of the Pacific Ocean[END_REF] The hydrogenetic growth structures form due to metal precipitation from the oxic near-bottom sea water as well as pore water and diagentic growth structures form due to metal precipitation from pore water under suboxic conditions. Furthermore, nodules buried within the sediment column down to even 10 m were found and included into this study (Table 1). Those nodules are diagenetically overprinted and differ in chemistry as well as mineralogy from those forming on the sediment surface. [START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF] Chemical analysis. Crusts samples DODO 232D, RRKNOX06 D20-1A and D20-1B, HLY0905 DS4-003A, and AVON02 11-A were analyzed by SGS, Canada laboratory using XRF borate-fused disks for major elements; inductively coupled plasma mass spectrometry (ICP-MS) using lithiummetaborate fused disks for Cu, Mo, Ni, V, Zr, and Rare Earth elements plus Y (REY); ICP-MS following four-acid digest for all other trace elements. Further information regarding chemical analyses for the RRKNOX06 samples can be found in Hein et al., [START_REF] Hein | Controls on Ferromanganese Crust Composition and Reconnaissance Resource Potential, Ninetyeast Ridge, Indian Ocean[END_REF] and for the HLY0905 sample in Hein et al. [START_REF] Hein | Arctic Deep Water Ferromanganese-Oxide Deposits Reflect the Unique Characteristics of the Arctic Ocean[END_REF] All other samples were analysed in the geochemistry laboratory at Jacobs Univeristy Bremen using acid-pressure digestion and ICP analysis. Ground samples of FeMn crusts were dried at 105 °C for 24 h and then 0.05 g of sample powder was weighed into Teflon digestion vessels. The acid pressure digestion in a PicoTrace system was done using 3 mL HCl, 1 mL HNO3 and 1 mL HF (all Merck suprapure quality). After evaporation of the liquid and twofold evaporation of 5 mL HCl, the residue was taken up in 50 mL of 0.5 M HNO3/0.5% HCl. Major and minor elements in the FeMn nodules and crusts were measured using a Spectro Ciros Vision SOP ICP-OES, whereas trace elements including Tl were measured with a Perkin-Elmer Elan drc-e quadrupole ICP-MS. Arsenic and V were measured in a reaction cell in collision cell mode (KED mode) with helium gas, to eliminate interferences. Details of the digestion method, the analytical procedure and analytical uncertainties with common certified reference materials (CRM) are outlined by Dulski (2001) [START_REF] Dulski | Reference Materials for Geochemical Studies: New Analytical Data by ICPMS and Critical Discussion of Reference Values[END_REF] and Alexander (2008). [START_REF] Alexander | Trace Element Analyses in Geological Materials Using Low Resolution Inductively Coupled Plasma Mass Spectrometry[END_REF] Method reliability was verified by analysis of the CRM FeMn-1, JMn-1, and NODP1, which were digested and analyzed along with the FeMn crust and nodule samples. The analytical precision is better than ±5% of published reference values, except Al, Ba, V, some REY, Zr and W with an analytical precision of up to ±10%. Elemental compositions are reported in Table S2.

X-ray diffraction. 23 samples were characterized by XRD in this study and 5 were characterized previously (Table 1). [START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF] The new XRD patterns were recorded using a Bruker D8 Discover diffractometer with Cu Kα radiation (wavelength = 1.54 Å) from 5 to 71° 2θ, 0.06° step size, and 16 s counting time. The samples with a basal reflection near 10 Å at ambient condition were dehydrated at 120 °C in vacuum during ~12 hours, then a new diffractogram was measured at ambient condition from 7 to 27° 2θ. This treatment allows differentiating 10 Å vernadite from todorokite. [START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF][START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF][START_REF] Usui | Geochemistry and Mineralogy of a Modern Buserite Deposit from a Hot Spring in Hokkaido, Japan[END_REF][START_REF] Manceau | Natural Speciation of Ni, Zn, Ba and As in Ferromanganese Coatings on Quartz Using X-Ray Fluorescence, Absorption, and Diffraction[END_REF] Raman spectroscopy. The black (i.e., manganese-rich) particles of hydrogenetic Fe-vernadite were subjected to Raman analysis to identify the mineralogical nature of the Fe-component. The analyses were performed using a Horiba XploRA plus (Jobin Yvon) confocal microscope equipped with a 532 nm laser. The spectra were acquired with 10x magnification objective lens at 1 mW laser power, using a 600 lines/mm grating and 30 s integration time. 10 spots were measured per sample to verify the repeatability of the data and were summed to enhance the signal to noise ratio. The background fluorescence was subtracted by linear interpolation of the intensity at 100 cm -1 and 1200 cm -1 .

Mn-XANES. The Mn K-edge XANES spectra were measured at room temperature in transmission mode on pressed pellets at the Swiss Norwegian BM31 beamlines [START_REF] Van Beek | S BM31 at ESRF beyond 2020 -Combined XRD-PDF-XAS[END_REF] at the European Synchrotron Radiation Facility (ESRF) using the new extremely brilliant X-ray source (EBS). The flux on the sample was approximately 8 × 10 9 photon/s in a beam footprint of ~0.6 (V) x 5 (H) mm² FWHM. The photon energy was calibrated by assigning the maximum of the derivative of the Mn foil to be 6537.7 eV. [START_REF] Kraft | High Resolution X-Ray Absorption Spectroscopy with Absolute Energy Calibration for the Determination of Absorption Edge Energies[END_REF] The stability in energy was monitored by measuring simultaneously to the samples the absorption signal of a Mn foil with an additional ion chamber on the downside of the experiment. The incident energy of the XANES spectra was scanned continuously from 6450 eV to 7000 eV in 0.2 eV steps and the spectra were normalized with Athena. [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT[END_REF] The average oxidation state (AOS) of Mn was determined by LSF to an extended database of reference spectra under the constraint of non-negativity in the loadings (Combo fit [START_REF] Manceau | Determination of Mn Valence States in Mixed-Valent Manganates by XANES Spectroscopy[END_REF] ). The reference spectra were measured in transmission mode on diluted pellets during the same experimental session as the FeMn samples. The database is a transmission version of the one measured previously in total electron yield (TEY) mode. [START_REF] Manceau | Determination of Mn Valence States in Mixed-Valent Manganates by XANES Spectroscopy[END_REF] It includes four Mn(IV) standards, five Mn(III) standards, and six Mn(II) standards (Table S3). The consistency between the transmission and TEY measurements was checked by remeasuring three mixed-valent tectomanganates, hollandite, psilomelane, and todorokite. The new AOS are systematically higher than the older ones by 0.04 valence unit (v.u.) for hollandite, 0.05 v.u.

for psilomelane, and 0.06 v.u. for todorokite (Table 2). The accuracy of the Mn AOS was estimated to be equal to the variation of their best-fit values when the fit residual (NSS) was increased by 20%. NSS is the normalized sum-squared difference between the data and fit expressed as Σ[(ydata-yfit) 2 ]/Σ(ydata 2 ).

The precision on the variation of the Mn AOS between two samples is about one half as good as the accuracy of estimation (Figure S3).

Tl-XANES and EXAFS. The Tl L3-edge HR-XANES and EXAFS spectra were measured on pressed pellets at the ID26 beamline of the ESRF using the EBS. The flux on the sample was over 10 13 photon/s in a beam footprint of ~80 (V) x 80 (H) µm² FWHM. The Tl Lα1 (3d5/2 → 2p3/2) fluorescence line was selected using the 555 reflection of five Si crystals of 100 mm diameter and spherically bent to a radius of 0.5 m. [START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] The Bragg angle was 74.3° in a vertical Rowland geometry. The diffracted intensity was measured with a Si drift diode detector (SDD) in single photon counting mode. The effective energy resolution, obtained by convoluting the total instrument energy bandwidth (spreads of the incident and emitted rays) and the 3d5/2 core-hole width from the Lα1 line, was about 3.0 eV, compared to an intrinsic line broadening of about 6.1 eV with an energy-dispersive detector.

The photon energy was calibrated by assigning the maximum of the derivative of the L3-edge spectrum for elemental Pb to be 13,035.0 eV. The stability in energy (~0.1 eV) was monitored with the Tl2O3 reference. The HR-XANES spectra were measured from 12,630 eV to 12,760 eV in continuous scan mode with 0.2 eV step size, and normalized to unity at E = 12,760 eV. A fixed number of 200 scans was collected per sample regardless of the Tl, Fe, and Mn concentrations (1h40 acquisition time). Therefore, the signal-to-noise ratio is proportional on a first order to the Tl concentration (Figure S4a). Data were reduced with the PCA and the LSF programs from beamline 10.3.2 at the Advanced Light Source. [START_REF] Manceau | Quantitative Speciation of Heavy Metals in Soils and Sediments by Synchrotron X-Ray Techniques[END_REF][START_REF] Marcus | Beamline 10.3.2 at ALS: A Hard X-Ray Microprobe for Environmental and Materials Sciences[END_REF] The accuracy of the fit components (i.e., fraction of Tl(III) and Tl(I)) was estimated to be equal to the variation of their best-fit values when the fit residual (NSS) was increased by 20%. The EXAFS spectrum of crust MP23-2 was measured from 12,500 to 13,380 eV in continuous scan mode with 2 eV step size. 1300 scans of 60 s were acquired and averaged to improve statistics. EXAFS data were reduced with the Athena software [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT[END_REF] and fit with Winxas. [START_REF] Ressler | WinXAS: A Program for X-Ray Absorption Spectroscopy Data Analysis under MS-Windows[END_REF] Theoretical amplitude and phase shift functions were generated with FEFF8.2 [START_REF] Ankudinov | Relativistic Calculations of Spin-Dependent X-Ray-Absorption Spectra[END_REF] using the structure of the Tl(III)-TCS DFT complex modeled in this study. The amplitude reduction factor (S0 2 ) was fixed to 0.9.

All measurements were performed at a temperature of 10-15 K using a helium flow cryostat and pellets were moved horizontally and vertically after every scan to access unexposed material. No changes in spectral features were noted during the course of data collection that would indicate radiation damage. Marafatto et al. [START_REF] Li | Geochemical and Molecular Characteristics of Ferromanganese Deposits and Surrounding Sediments in the Mariana Trench: An Implication for the Geochemical Mn Cycle in Sedimentary Environments of the Trench Zone[END_REF] observed that Tl(III) is reduced to Tl(I) at 10 K with a photon flux density of 10 10 -10 11 ph/μm 2 /sec and is stable at room temperature with a photon density of 0.4-1 × 10 8 ph/μm 2 /sec. No radiation damage was observed here at 10-15 K and a photon density of about 10 9 ph/μm 2 /sec.

RESULTS

Thallium concentration. The Tl concentration ([Tl]

) ranges from 1.5 to 319 ppm in crusts with a mean of 146 ± 93 ppm (n = 17), and ranges from 2 to 112 ppm in nodules with a mean of 19 ± 32 ppm (n = 12) (Table 1). The high variability of [Tl] in the crusts coincides with that reported previously. [START_REF] Rehkamper | Cenozoic Marine Geochemistry of Thallium Deduced from Isotopic Studies of Ferromanganese Crusts and Pelagic Sediments[END_REF][START_REF] Hein | Deep-Ocean Mineral Deposits as a Source of Critical Metals for High-and Green-Technology Applications: Comparison with Land-Based Resources[END_REF] Three crusts from the Pacific Ocean have [Tl] close to, and one above, 300 ppm, which is the highest concentration reported to date. Two out of three of these high-Tl crusts are phosphatized crusts. At least one crust sample from each of the oceans contains >120 ppm Tl, except the Arctic, suggesting that Tl enrichment may not be constrained geographically. The concentration of Tl is also highly variable in nodules, and is on average much lower than that reported previously because the majority of the nodules studied here were buried in marine sediments (Table 1). Nodules at the sediment surface contain 2-112 ppm Tl, those at the subsurface contain 5-15 ppm, and those at >500 cm depth contain 2 ppm. Figure 3a Mineralogy. The mineralogy of manganese in FeMn marine oxides is dominated by different types of vernadite (Fe-vernadite, 7 and 10 Å vernadite) and todorokite in proportion that depends on the genetic type of the deposit. Hydrogenetic crusts and nodules formed in contact with oxic seawater contain Fe-vernadite, which consists of birnessite nanosheets exfoliated with feroxyhite (δ-FeOOH) nanoparticles, 17,18,[START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF][START_REF] Burns | Mechanism for Nucleation and Growth of Manganese Nodules[END_REF][START_REF] Golden | Synthesis of Todorokite[END_REF][START_REF] Varentsov | Me-Fe Oxyhydroxide Crusts from Krylov Seamount (Eastern Atlantic): Mineralogy, Geochemistry and Genesis[END_REF] perhaps as a result of the surface catalyzed oxidation of aqueous Fe(II). [START_REF] Krishnamurti | The Catalytic Role of Birnessite in the Transformation of Iron[END_REF][START_REF] Postma | Concentration of Mn and Separation from Fe in Sediments-I. Kinetics and Stoichiometry of the Reaction between Biiessite and Dissolved Fe(H) at 10°C[END_REF][START_REF] Gao | Interaction Mechanisms and Kinetics of Ferrous Ion and Hexagonal Birnessite in Aqueous Systems[END_REF] Fe-vernadite has no basal XRD reflection due to the exfoliation of the Mn particles into individual MnO2 layers. [START_REF] Song | Porously Assembled 2D Nanosheets of Alkali Metal Manganese Oxides with Highly Reversible Pseudocapacitance Behaviors[END_REF] Otherwise a faint 001 reflection is observed when the Mn particles have two or more layers separated by a constant interlayer spacing. [START_REF] Manceau | Short-Range and Long-Range Order of Phyllomanganate Nanoparticles Determined Using High Energy X-Ray Scattering[END_REF] The diffraction pattern of Fe-vernadite is characterized by two broadened and asymmetrical hk0 peaks at 2.45 Å (100 reflection) and 1.42 Å (110 reflection), also called hk bands. [START_REF] Glans | Experimental and Theoretical Study of the Electronic Structure of HgO and Tl2O3[END_REF][17][18][START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF] Fe-vernadite starts to dissolve at the early stage of suboxic diagenetic environment and the vernadite component reprecipitates into 10 Å vernadite, which has two hydration layers and a MnO2-MnO2 interlayer spacing of 10 Å. The loss of one hydration layer during diagenesis yields 7 Å vernadite. The 10 Å to 7 Å transformation can be performed in the laboratory upon heating to 105-110 °C in air or under vacuum. [START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF][START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF][START_REF] Usui | Geochemistry and Mineralogy of a Modern Buserite Deposit from a Hot Spring in Hokkaido, Japan[END_REF][START_REF] Manceau | Natural Speciation of Ni, Zn, Ba and As in Ferromanganese Coatings on Quartz Using X-Ray Fluorescence, Absorption, and Diffraction[END_REF] When diagenesis evolves under more reducing conditions, 10 Å vernadite transforms into todorokite. [START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF][START_REF] Bodei | Formation of Todorokite from Vernadite in Ni-Rich Hemipelagic Sediments[END_REF][START_REF] Wu | Transformation of Co-Containing Birnessite to Todorokite: Effect of Co on the Transformation and Implications for Co Mobility[END_REF][START_REF] Liang | Coupled Morphological and Structural Evolution of δ-MnO2 to α-MnO2 through Multistage Oriented Assembly Processes: The Role of Mn(III)[END_REF] Todorokite also has a reflection near 10 Å, but which does not collapse to 7 Å in contrast to 10 Å vernadite. Reduction of insoluble Fe(III) to soluble Fe(II) under O2-deficiency increases the Mn/Fe ratio. A Mn/Fe wt ratio of about 2.5 -3.0 divides purely hydrogenetic precipitates from those that experienced the beginning of early diagenetic transformation. [START_REF] Halbach | Geochemical and Mineralogical Control of Different Genetic Types of Deep-Sea Nodules from the Pacific Ocean[END_REF][START_REF] Li | Critical Metal Enrichment Mechanism of Deep-Sea Hydrogenetic Nodules: Insights from Mineralogy and Element Mobility[END_REF][START_REF] Li | Geochemical and Molecular Characteristics of Ferromanganese Deposits and Surrounding Sediments in the Mariana Trench: An Implication for the Geochemical Mn Cycle in Sedimentary Environments of the Trench Zone[END_REF] The XRD patterns are shown in Figure S5 and the abundances of the Mn phases are reported in Table 1 together with the Mn/Fe wt ratios. Fe-vernadite is distinctive of all crusts. No 7 Å and 10 Å vernadites were detected in any crust and minor todorokite is observed in one phosphatized crust (MP23-2) out of three. The three phosphatized crusts contain CFA. Evidence for a vernadite-feroxyhite mixing at the nanoscale is provided by Raman spectroscopy. Synthetic δ-MnO2 has three vibrational modes near 500, 570, and 630 cm -1 , [START_REF] Post | Raman Spectroscopy Study of Manganese Oxides: Layer Structures[END_REF] whereas feroxyhite has a single broad mode in this wavenumber region pointing at 690 cm -1 (Figure 4a). [START_REF] Bellot-Gurlet | Raman Studies of Corrosion Layers Formed on Archaeological Irons in Various Media[END_REF] The three modes of δ-MnO2 are also observed in Fevernadite, but this spectrum also has a shoulder in the 670-720 cm -1 interval attributed to feroxyhite (Figure 4b andS6a). The intensity of the feroxyhite mode varies from crust to crust, being maximum in 74GTV (Figure 4c). Examination of the Raman spectrum for Fe-rich vernadite layers of FeMn nodules from the Mariana Trench shows that the Fe phase is also feroxyhite (Figure S6b). [START_REF] Li | Geochemical and Molecular Characteristics of Ferromanganese Deposits and Surrounding Sediments in the Mariana Trench: An Implication for the Geochemical Mn Cycle in Sedimentary Environments of the Trench Zone[END_REF] Raman spectroscopy is a surface technique. The penetration depth of the laser source is a complex function of the refraction index, conductivity, and permittivity of the material and varies for example between 0.8-1 μm for silicon and 20-50 nm for germanium at λ = 532 nm. [START_REF] Shur | Physics of Semiconductor Devices[END_REF]91 Because the analyzed black spots of Fe-vernadite are close in opacity to Ge, one can estimate that the Raman spectra reflect the composition of the uppermost atomic layers, and therefore confirm the intimate mixing of vernadite and feroxyhite at the nanometer scale.

The mineralogical signature for the hydrogenetic nature of the crusts is backed up by the Mn/Fe wt ratios which range from 0.26 to 2.55 in non-phosphatized crusts and from 1.92 to 3.95 in the three phosphatized crusts. The nodules cover a large range of Mn/Fe wt ratios, extending from 0.41 (hydrogenetic EBS8-6 nodule) up to 30.3 (diagenetic KG1313 nodule). Todorokite occurs in all nodules analyzed here, except in hydrogenetic EBS8-6. Mixed nodules at the sediment surface contain 7 Å and 10 Å vernadite, and minor amounts of todorokite, whereas the buried nodules have exclusively todorokite. The vernadite to todorokite transformation with sediment depth is accompanied by a loss of Tl. Nodules in which only todorokite is detected by XRD have only 2-11 ppm Tl. We shall see below that the remaining Tl pool is dominantly 1+.

Oxidation state of manganese. The Mn K-edge XANES spectra form two groups from visual inspection of their derivatives (Figure S7a). The first group comprises 19 spectra and includes all FeMn crusts (both phosphatized and non-phosphatized), one hydrogenetic nodule (EBS8-6) and one diagenetic nodule (09KL-44-49cm). Mn is speciated essentially as Fe-vernadite, todorokite being present only in 09KL-44-49cm and a phosphatized crust (MP23-2) (Table 1). The spectra from the first group were averaged to obtain one spectrum representative of the FeMn crusts for use in XANES fitting and further comparisons (Figure 5a). The second group includes all todorokite-containing diagenetic nodules, except 09KL-44-49cm. Their XANES derivatives have a lower amplitude at 6555.7 eV and are shifted to lower energy in the 6547-6562 eV interval (Figure 5b). Comparison of the Mn standards shows that the two spectral features fingerprint higher amounts of Mn(III) and Mn(II) (Figure S7b). The spectra from the second group were not averaged because there is a gradient in the amount of reduced Mn with the sediment depth.

Results from the Combo fits are detailed in Table S4, summarized in Table 2, and two representative fits are shown in Figures 5c and5d. Mn has an average oxidation state (AOS) of 3.95 ± 0.05 (mean ± accuracy) for the averaged spectrum of all FeMn crusts and a Mn 3+ fraction lower than 5 %, on average. The oxidation state of Mn in nodules decreases from 3.88 ± 0.08 at the sediment surface (44KG-2) to 3.77 ± 0.08 at 530 cm depth (22KL-1-530cm) and to 3.74 ± 0.07 at 985 cm depth (65SL-985cm). Mn 2+ makes up <4% of total Mn at all depths, whereas Mn 3+ increases from 4% at the surface (44KG-2) up to 22% at 985 cm below the surface.

Oxidation state of thallium. Tl references. The Tl L3-edge XANES spectra arise from the photoelectric ejection of a core 2p3/2 electron to unoccupied electronic states of s and d symmetry (dipole-allowed Δl = ± 1 transitions). Tl(I) has an atomic orbital configuration of [Xe]6s 2 4f 14 5d 10 6p 0 , and therefore only 2p3/2 transitions to unbound nd (n ≥ 6) states nominally occur. In contrast to Tl(I), the unoccupied 6s 0 valence states of Tl(III) make the 2p3/2 → 6s transition possible at low kinetic energy of the photoelectron. Figure 6a shows that the differences between the TlNO3 (Tl(I)) and Tl2O3 (Tl(III)) spectra are subtle in the rising edge where electronic transitions to 6s valence states occur. [START_REF] George | The Speciation of Thallium in (Tl,Sb,As)-Rich Pyrite[END_REF] TlNO3 has no structure, in agreement with the nominally occupied 6s states, whereas Tl2O3 exhibits a low-edge shoulder at approximately 12,650 eV and a second weaker shoulder in the mid-edge at approximately 12,665 eV. The two features are enhanced at high energy-resolution (Figures 6b), while the TlNO3 low-and mid-edge remain featureless (Figure 6c). Thus, the rising edge of HR-XANES serves as powerful tool to differentiate Tl(I) and Tl(III) by virtue of probing the electronic occupation of the 6s states. In contrast to the low-edge region, the top-and trailing-edge region at 12,670-12,760 eV is sensitive to the local structure of Tl, and therefore is less diagnostic of its oxidation state.

Tl occupies two crystallographic sites in Tl2O3, a symmetrical octahedral site (Tl(1)) with 6 O at 2.36 Å, and a distorted octahedral site (Tl(2)) with 2 O at 2.16 Å and 4 O at 2.28 Å (Figure 7). [START_REF] Gomis | High-Pressure Structural and Elastic Properties of Tl2O3[END_REF] Ab initio FEFF9.6 [START_REF] Rehr | Parameter-Free Calculations of X-Ray Spectra with FEFF9[END_REF] calculation shows that the low-edge peak observed at high energy-resolution in Tl2O3 arises from a transition to 6s and 5d states and the mid-edge bump arises from a transition to higher d states (Figure 7a,b). A nominally dipole forbidden weak 2p3/2 → 5d transition is actually observed as a result of the partial mixing of the Tl 6s and 5d states, as shown by valence-band X-ray photoemission and band-structure calculation, [START_REF] Glans | Experimental and Theoretical Study of the Electronic Structure of HgO and Tl2O3[END_REF] and by empty density of states (DOS) calculations (Figure S8). The Tl 6s-5d mixing is higher at the Tl(2) than the Tl(1) site, and the 6s states are less occupied (i.e., the empty density is higher) at the Tl(2) site (Figure S9). These two factors increase the probability of the 2p3/2 → 6s5d transition, which is manifested in the low-edge region as a more intense peak at the Tl(2) site than the Tl(1) site (Figure 7a,b). Therefore, the intensity of the low-edge peak provides a clue to the distribution of the Tl-O distances in a pure Tl(III) compound, and hence of the coordination of Tl (Figure 7c). The low-edge intensity is, however, less diagnostic in mixed-valent Tl(I)-Tl(III) compounds, as Tl(I) has no low-edge structure and, therefore, produces the same effect of decreasing the intensity of the low-edge peak as does the symmetrical Tl(1) site of Tl2O3.

FeMn crusts and nodules. The Tl HR-XANES spectra from the marine samples are represented in Figure S9. Investigating all spectra using PCA, we found that only two endmembers were required to account for the variance in the data set (Figure 8a). The first component (PCA) resembles Tl2O3 and the second component (PCA2) resembles TlNO3 (Figure 8b). PCA1 has a sharper low-edge peak than Tl2O3, which suggests that the Tl(III) site is distorted. The proportions of Tl(III) and Tl(I), as obtained by least-squares fitting with linear combinations of the PCA1 and PCA2 endmember spectra, are given in Tables 1 andS5, and the fits are shown in Figure S9.

The proportion of Tl(III) is >60% in FeMn crusts, and no correlation was observed between the proportion of Tl(III) in the crust and the geographic location or with phosphatization. For nodules, the proportion of Tl(III) is between 0% and 100% (Figure 9, Table S5). Two nodules sampled at the The other nodules have <80% Tl(III) and are severely depleted in Tl, with explains the noisy shape of the Tl(I) endmember spectrum calculated by PCA (component PCA2, Figure 8a). A prototypical example is the surface layer of the FeMn nodule from the Baltic Sea (BS-surface), which contains only 2 ppm Tl speciated exclusively as Tl(I) and a Tl/As atomic ratio as low as 0.002 (Figure 2b). The three deepest buried nodules (≥ 530 cm), which contain only todorokite, 2 ppm Tl, and have 3.74 ± 0.07 ≤ Mn AOS ≤ 3.77 ± 0.08, have 50-69% Tl(I). Overall, the higher percentage of Tl(III) in Fe-vernadite crusts and 10 Å vernadite surface nodules (> 60%) and the higher percentage of Tl(I) in deeply buried todorokite nodules (≥ 50%) are consistent with (1) uptake experiments of Tl on manganates, which showed that Tl(I) is oxidized on phyllomanganates and not on tectomanganates, [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] and (2) the monovalent state of Tl in the tunnels of hollandite. [START_REF] Gołębiowska | TlMn 4+ 7.5 Cu 2+ 0.5 O16[END_REF] There is also experimental evidence that the Tl oxidation is hindered on Mn(III)-rich phyllomanganate. 10 Å vernadite has more Mn(III) than Fevernadite (Table 2), [START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF] and therefore is not expected to have up to 96-100% Tl(III), as observed in KG1313 and 21KG-1(n). We will return to this point when discussing the results.

Bonding environment of Tl(III).

The EXAFS spectrum and radial structure function (RSF, Fourier transform) of crust MP23-2 show considerable degrees of similarities with published data for Tl(III)sorbed δ-MnO2 (Figure 11). [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF][START_REF] Manceau | Estimating the Number of Pure Chemical Components in a Mixture by X-Ray Absorption Spectroscopy[END_REF] The RSF exhibits a first peak corresponding to the nearest Tl-O shell, and a double second peak corresponding to the nearest and second-nearest Mn shells from the MnO2 layer. Best fit results were obtained with 3.6 O at 2.18 Å (σ = 0.084 Å), 2.4 O at 2.33 Å (σ = 0.084 Å), 0.9 Mn at 3.22 Å (σ = 0.10 Å), and 3.1 Mn at 3.66 Å (σ = 0.10Å) (Table S6). The coordination number of Tl was enforced to 6 and the mean displacements of bond lengths (σ) were kept identical for the two O sub-shells and for the two Mn shells to decrease correlation between parameters. This highly constrained fit calculation provides an excellent visual description of the experimental data over the entire data range. Fitting the O shell with a single distance resulted in an average Tl-O bond length of 2.21 Å with a large σ value of 0.12 Å and a fit quality degraded by 27%.

DISCUSSION

Sensitivity of HR-XANES to the oxidation state of Tl. The results underscore the enhanced sensitivity of XANES spectroscopy performed at high-energy resolution for the quantitative determination of the oxidation state of Tl in compositionally complex materials down to the ppm level.

The precision on the determination of the fraction of Tl(I) and Tl(III) depends on the data quality and how well the Tl(I) and Tl(III) references used to fit data represent the unknown mixed-valent samples.

The precision is between 2% and 6% of total Tl for [Tl] > ~10 ppm, and between 3% and 21% below 10 ppm (Figure S4b). The % error and the fit residual (NSS parameter) increase abruptly at low Tl concentration where the signal-to-noise ratio deteriorates rapidly (Figure S4b,c). This is because the total acquisition time was the same for all samples (1h40), not because of an inadequate fit model at low Tl concentration. Error bars can be tightened by increasing the counting time. We conclude that it is possible with HR-XANES to obtain a precision better than 10% on the oxidation state of Tl in natural FeMn oxides down to the ppm level.

Structure of the Tl(III) surface complex on vernadite.

We show that HR-XANES is also sensitive to the coordination of Tl(III). Unequal Tl-O distances in Tl2O3 are phenomenologically reflected in a single, strong low-edge peak, whereas this peak is a weak doublet when the Tl-O distances are identical. The single peak shape of all Tl(III)-rich samples suggests that the coordination environment of Tl(III) is asymmetric at the vernadite surface, which EXAFS confirmed with a split of the O coordination shell into two sub-shells at 2.18 Å and 2.33 Å. The ambiguity on the tetrahedral or octahedral coordination of the TCS complex (Figure 1) was addressed by the density functional theory (DFT) following a modeling scheme described previously for the complexation of Ni, Cu, Zn, and Pb on birnessite nanolayers (PBE0/def2-TZVP-ecp, Supplementary Information, SI), [START_REF] Bellot-Gurlet | Raman Studies of Corrosion Layers Formed on Archaeological Irons in Various Media[END_REF] and by FEFF calculation of the HR-XANES spectra.

The geometry-optimized Tl(III)-TCS complex bonds the three surface oxygen atoms coordinated to two Mn atoms bordering a layer vacancy (O2Mn) at 2 × 2.19 Å + 2.21 Å in octahedral coordination and at 2.10-2.12 Å in tetrahedral coordination, and bonds the water molecules (O2H) at 3 × 2.29-2.30 Å in the first configuration and 2.17 Å in the second (Figure 1). The bond distances are more unequal in octahedral coordination and the predicted d( VI Tl-O) values are close to the crystallographic distances for the Tl(2) site of Tl2O3 and to the EXAFS distances of 2.18 Å and 2.33 Å. The Tl-O2Mn distances are much shorter than the Tl-O2H distances, as also observed with 3d transition metals. [START_REF] Post | ZnMn3O7:3H2O: New Crystal-Structure Determination[END_REF][START_REF] Manceau | Structure of Heavy Metal Sorbed Birnessite. Part III. Results from Powder and Polarized Extended X-Ray Absorption Fine Structure Spectroscopy[END_REF][START_REF] Toner | Zinc Sorption to Biogenic Hexagonal-Birnessite Particles within a Hydrated Bacterial Biofilm[END_REF][START_REF] Kwon | Zinc Surface Complexes on Birnessite: A Density Functional Theory Study[END_REF][START_REF] Kwon | Understanding the Trends in Transition Metal Sorption by Vacancy Sites in Birnessite[END_REF][100] The water molecules are oriented such that one H from H2O forms a H-bond with the nearest O3Mn atom, further stabilizing the octahedral complex.

In keeping with the DFT distances, the FEFF-calculated HR-XANES spectrum for the VI Tl(III)-TCS complex exhibits a sharp near-edge peak (Figure 10a). This peak is also observed for the VI Tl(III)-DES complex because the predicted Tl-O distances are distributed from 2.21 Å to 2.36 Å (Figure 10a).

In contrast, the more regular coordination of the IV Tl(III)-TCS complex manifests itself in a blunt nearedge structure unfit to data (Figure 10b). A third result in support of a VI Tl(III)-TCS complex is the Tl(III)-Mn distance. The octahedral complex has a DFT-calculated d( VI Tl-Mn) = 3.59-3.65 Å, consistent with the EXAFS distance of 3.66 Å, and the tetrahedral complex has d( IV Tl-Mn) = 3.46-3.55 Å, inconsistent with the EXAFS distance. Therefore, geometry optimization of the Tl(III) surface complexes on a MnO2 layer based on DFT, together with EXAFS spectroscopy and FEFF calculation of the HR-XANES spectra for the DFT structure models, demonstrate that Tl(III) is octahedrally coordinated on vernadite. HR-XANES cannot differentiate the VI Tl(III)-TCS and VI Tl(III)-DES surface complexes, in contrast to EXAFS which identified the two complexes on the basis of their distinct Tl-Mn distances equal to 3.66 Å and 3.22 Å, respectively. The EXAFS value of 3.22 Å coincides to within ~0.02Å with the DFT value of 3.19-3.20 Å (Figure 1).

Tl uptake in FeMn crusts and nodules. The linear correlation between [Tl] and [Mn]/[Fe] in

FeMn crusts is well explained by the oxidative scavenging of thallium by interlayers of Fe-vernadite (Figure 3a). Then, one would expect the proportion of Tl(III) to be also correlated with [Mn]/[Fe], which is not the case since it varies irregularly between 62% and 100% (Figure 3b, Table 1).

Hydrogenetic crusts form in oxidizing environments as supported by the 3.95 ± 0.05 average oxidation state of Mn, regardless if the crust is phosphatized (Table 2). Tl(I) sorption experiment on synthetic vernadite (δ-MnO2) predicts that all the Tl should be trivalent under these conditions. [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] Wick et al. 11 observed that Tl(I) sorbed on Mn(III)-free δ-MnO2 was totally oxidized to Tl(III) up to a Tl/Mn atomic ratio of ~0.02, whereas only 12% Tl(I) was oxidized at Tl/Mn = 0.0023 on δ-MnO2 containing 17% Mn(III). The Tl loading in the crusts is 10-fold to 500-fold lower (0.05 × 10 -4 ≤ Tl/Mn ≤ 2.91 × 10 -4 , Table 1) than in the sorption experiment, which added to the mostly fourvalent state of Mn, is incompatible with several tens of percent Tl(I) in some of the crusts. A case in point is sample 232D-B 0-78, which has 21 ± 3% Tl(I) despite having only 1.5 ppm Tl and a Tl/Mn atomic ratio of 0.05 × 10 -4 (Table 1). The incomplete oxidative uptake of Tl(I) in crusts cannot be simply explained by the saturation of redox reactive Mn sites due to a surface excess of Tl nor by the abundance of Mn(III).

There are potentially several reasons why some of the Tl(I) is recalcitrant to oxidation in crusts.

Three hypotheses can be advanced to explain the occurrence of Tl(I) in crusts that formed in oxygen-rich environments. One is the high affinity of Tl(I) for phyllosilicates. Tl(I) is a chemical analog to K and can replace it in the interlayer of micaceous clay minerals as does Cs. 101 The Tl(I)clay association is well documented in soils. [START_REF] Hlawatsch | Fast-Growing, Shallow-Water Ferro-Manganese Nodules from the Western Baltic Sea: Origin and Modes of Trace Element Incorporation[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF][START_REF] Wegorzewski | Mineralogical Transformations in Polymetallic Nodules and the Change of Ni, Cu and Co Crystal-Chemistry upon Burial in Sediments[END_REF][START_REF] Li | Geochemical and Molecular Characteristics of Ferromanganese Deposits and Surrounding Sediments in the Mariana Trench: An Implication for the Geochemical Mn Cycle in Sedimentary Environments of the Trench Zone[END_REF] This explanation seems unlikely, however, because phyllosilicates were detected by XRD in only two crusts, DS4-003A B08-28 and 122K, out of 17 and both contain little Tl(I). The first crust has 94 ± 5% Tl(III) and the second, which is phosphatized, has 79 ± 3% Tl(III). Another possibility is to consider that Tl(I) is associated with feroxyhite in Fe-vernadite since Tl(I) is not oxidized by Fe oxyhydr(oxides). This second hypothesis is incompatible with the results of Peacock and Moon 9 on the 237KD hydrogenetic crust from the Pacific Ocean. The crust had a concentric growth microstructure with alternating Fe-rich and Mn-rich growth layers and element imaging by X-ray micro-fluorescence showed that Tl occurred in the Mn layers, not in the Fe-rich layers.

Using Tl-XANES and Mn-EXAFS spectroscopy, Peacock and Moon 9 also observed that 58 ± 3% of Tl was monovalent in the Mn layers and that 79 ± 10 % of Mn was speciated as hexagonal birnessite and 21 ± 10% as todorokite. Some of the Tl(I) was assumed to be sorbed on birnessite without oxidation, perhaps because the Mn vacancy sites were capped by other cations, such as trace metals, Mn(III) and possibly Fe(III) from feroxyhite. In contrast to transition metals which sorb above octahedral layer vacancies, K sorbs with high affinity at the surface of the MnO2 layer above tetrahedral cavities formed by three edge-sharing octahedra (Figure 12). This crystallographic position has been described in well-crystallized birnessite synthesized at high temperature 102,103 and in nanoparticulate birnessite (δ-MnO2) synthesized at ambient condition by Pseudomonas putida. 104 Barium also sorbs on this site in natural vernadite, [START_REF] Manceau | Natural Speciation of Ni, Zn, Ba and As in Ferromanganese Coatings on Quartz Using X-Ray Fluorescence, Absorption, and Diffraction[END_REF] and the coordination of K and Ba on this site coincides with that in the tunnels of tectomanganates. 105 The DFT geometry-optimized Tl(I)-O and Tl(I)-Mn distances are 2.95-3.10 Å and 3.75-3.95 Å, respectively, much larger than the EXAFS distances (Figure 12). Tl(I) does not occupy the center of the cavity but is shifted in direction of one of the three edge-sharing octahedra, as also observed for K and Ba. [START_REF] Manceau | Natural Speciation of Ni, Zn, Ba and As in Ferromanganese Coatings on Quartz Using X-Ray Fluorescence, Absorption, and Diffraction[END_REF]102,103,105 The offset in position depends on the layer charge and is larger when one of the three underlying Mn cations is Mn(III). [START_REF] Manceau | Natural Speciation of Ni, Zn, Ba and As in Ferromanganese Coatings on Quartz Using X-Ray Fluorescence, Absorption, and Diffraction[END_REF] In this case, the Tl(I)-O distance with the Mn(III)-bonded oxygen atom undersaturated by the Jahn-Teller distortion is 2.66 Å and the two other oxygen atoms are at 3.03 Å and 3.28 Å from Tl(I). The Tl(I)-Mn distances are between 3.70 Å and 3.87 Å Therefore, we hypothesize that at least part of Tl(I) is taken up on the Ba site of Fe-vernadite in hydrogenetic crusts. This sorption site is abundant since the 17 crusts contain 1523 ± 558 ppm Ba.

Thallium exhibits pronounced depletion at the onset of diagenesis and is as low as 2 ppm in nodules buried at ≥ 530 cm depth (Figure 3a, Table 1). Almost all Tl is released to pore waters under the reducing conditions that prevail at this depth. Todorokite is the only Mn species detected by XRD and has an average Mn oxidation state of 3.74-3.77 ± 0.07, compared to 3.95 ± 0.05 in crusts (Table 2).

From 50% to 69% of Tl is monovalent (Table 1), and is likely taken up by todorokite and/or phyllosilicate. Yet, one third to one half of the Tl pool is trivalent. Because todorokite does not oxidize Tl(I), [START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF][START_REF] Rovezzi | High Energy-Resolution x-Ray Spectroscopy at Ultra-High Dilution with Spherically Bent Crystal Analyzers of 0.5 m Radius[END_REF] the Tl(III) present in trace amounts may be associated with some residual phyllomanganate (< ~ 2-3 mineral %) that did not transform into todorokite during diagenesis. [START_REF] Manceau | Mineralogy and Crystal Chemistry of Mn, Fe, Co, Ni, and Cu in a Deep-Sea Pacific Polymetallic Nodule[END_REF][START_REF] Bodei | Formation of Todorokite from Vernadite in Ni-Rich Hemipelagic Sediments[END_REF] It is unlikely an oxidized porewater species sorbed on todorokite, because Tl(III) species are sparingly soluble and Tl 3+ is a highly hydrolyzable metal ion.

The identity of the Tl(I) carrier phase is less certain in nodules located at the sediment surface and subsurface. The percentage of Tl(I) varies between 0% (21KG-1(n)) and 100% (BS-surface) and does not covary with the [Mn]/[Fe] ratio (Figure 3b) nor the AOS of Mn (Table 2). For example, thallium is 96% Tl(III) in KG1313 with a Mn AOS of 3.76 ± 0.07 and is 82% Tl(I) in BS-bulk with a similar Mn AOS of 3.79 ± 0.08 (Figure S3) and in addition the two nodules have practically identical mineralogy (Figure S5). Tl(I) can be hosted by vernadite and todorokite, and also phyllosilicate when present. Nodules found at the sediment surface and subsurface are subject to partial diagenetic alteration, therefore have more chemical and structural inhomogeneity than hydrogenetic crusts and fully altered buried nodules. The large variability in the percentage of Tl(I) reflects this complexity and also the larger-scale geochemical diversity of the depositional settings from the global ocean.

The Baltic Sea nodule which was separated into a bulk sample and the <1mm surface layer scraped off with a knife (representing the most recent precipitate of the last decades) has lowest Tl concentrations of 4 ppm and 2 ppm, respectively, which reflects the very high growth rate of about 20 µm year -1 compared to deep-sea nodules. [START_REF] Hlawatsch | Fast-Growing, Shallow-Water Ferro-Manganese Nodules from the Western Baltic Sea: Origin and Modes of Trace Element Incorporation[END_REF] The surface layer is the only sample that has an exclusive Tl redox speciation of +1, with the bulk nodule having the second highest Tl(I) fraction of 82%. This nodule can be considered as the FeMn nodule endmember on the low-oxygen side, as it forms under highly variable redox conditions reflected by a distinct early diagenetic Fe-Mn banding. 106 This is related to the seasonal thermohaline stratification in the western Baltic Sea during summer (June to September), which reduces the oxygen flux from the surface to the bottom water. Enhanced decay of organic matter results in oxygen depletion at the bottom which exposes the nodules to suboxic or even anoxic conditions, preventing further oxide precipitation and potentially even remobilizing part of the precipitate diagenetically. In fall and winter when wind and waves replenish oxygen in the deeper water layers, FeMn oxide growth may resume. Apparently, redox conditions are never sufficiently oxidizing to allow substantial Tl(I) oxidation on the Mn oxide; alternatively, this could also be a kinetic effect because the prevailence of more oxidizing conditions is only a few months.

CONCLUSION

The results provoke a revisitation of the enrichment process of Tl in marine ferromanganese deposits.

The scavenging of Tl is not solely an oxidative process but also a highly efficient sorption mechanism of Tl(I), probably on the high affinity K/Ba crystallographic sites to balance the charge deficit created by a Mn(III) for Mn(IV) substitution in the manganate structure. 103 Under this premise, the Tl(I)/Tl(III) ratio depends, among other factors, on the density of the vacancy sites and the Mn(III)/Mn(IV) ratio, and therefore on the defective structure and stoichiometry of the Tl host manganate. [START_REF] Hlawatsch | Fast-Growing, Shallow-Water Ferro-Manganese Nodules from the Western Baltic Sea: Origin and Modes of Trace Element Incorporation[END_REF][START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF] measured at 13,000 eV. (c) Spectrum of crust MP23-2 measured at 13,500 eV, above the Pb L3-edge (13,035 eV). The Tl concentrations and the Tl/As atomic ratios are 283 ppm and 0.69 in the crust and 2 ppm and 0.002 in the nodule (Table 1 andS2). Linear scale. S4 and Ref. [START_REF] Manceau | Determination of Mn Valence States in Mixed-Valent Manganates by XANES Spectroscopy[END_REF] ). The precision on the variation of the AOS between two samples is about one half as good as the reported accuracy of estimation (Figure S3 and Table S4). b The NSS values of the Combo fits for the normalized XANES spectra are in the 10 -5 range. The formulae used to calculate the signal-to-noise ratios are given in the caption of Figure S4 Table S6. Structural parameters of the Tl(III)-TCS and Tl(III)-DES complexes on the MP23-2 crust derived from the Tl L3-edge EXAFS analysis

  shows [Tl] against the [Mn]/[Fe] weight (wt) ratio. Two data fields can be identified in this figure, one corresponding to the crusts in which [Tl] and [Mn]/[Fe] are correlated (R = 0.92), and the other corresponding to nodules which lack correlation. The positive relationship between [Tl]and [Mn]/[Fe] observed in crusts suggests that Tl is dominantly 3+ and oxidatively scavenged by vernadite, as documented by Peacock and Moon.[START_REF] Hlawatsch | Trace Metal Fluxes to Ferromanganese Nodules from the Western Baltic Sea as a Record for Long-Term Environmental Changes[END_REF]

  sediment surface have only Tl(III). 21KG-1(n) has 100% Tl(III), [Tl] = 44 ppm, [Mn]/[Fe] = 7.4, Mn AOS = 3.83, and KG1313 has 96 ± 4% Tl(III), [Tl] = 22 ppm, [Mn]/[Fe] = 30.3, Mn AOS = 3.76, and they both contain 10 Å vernadite and todorokite, with minor, residual Fe-vernadite. Although the Mn AOS, [Mn]/[Fe] ratios, and mineralogy indicate that the two nodules were affected by suboxic diagenesis, the O2-depletion is not reflected in the oxidation state of Tl which is 3+.

(

Figure 1 .

 1 Figure 1. Geometry-optimized surface complexes of Tl(III) on a phyllomanganate layer. (a) Tridentate corner-sharing (TCS) octahedral complex over a Mn(IV) layer vacancy. (b) Double edgesharing (DES) octahedral complex on a layer edge. (c) TCS tetrahedral complex over a vacancy site. PBE0/def2-TZVP-ecp modeling scheme (SI). Mn octahedron = gray; Tl(III) octahedron = dark red; oxygen = red, proton = gray.

Figure 2 .

 2 Figure 2. X-ray fluorescence spectra in the energy range of the Tl Lα1 fluorescence line measured with an energy dispersive detector and fit with PyMCA. 107 (a,b) Spectra of crust MP-23-2 and the surface layer of the Baltic Sea nodule 43,59 measured at 13,000 eV. (c) Spectrum of crust MP23-2 measured at

Figure 3 .

 3 Figure 3. Thallium concentration (a) and percentage of Tl(III) (b) as a function of the ratio of the Mn to Fe concentrations on a weight basis. Black points: FeMn crusts; purple diamonds: nodules.

Figure 4 .

 4 Figure 4. Raman spectra for δ-MnO2 and feroxyhite (δ-FeOOH) (a), for δ-MnO2 and FeMn crusts (b), and for δ-MnO2 and the 74GTV FeMn crust. The feroxyhite spectrum is from Bellot-Gurlet et al. (2009), 89 δ-MnO2 is from Manceau et al. (2013),[START_REF] Manceau | Short-Range and Long-Range Order of Phyllomanganate Nanoparticles Determined Using High Energy X-Ray Scattering[END_REF] , and the FeMn crusts spectrum is the average spectrum of the 10 FeMn crust spectra shown in FigureS6. The peak broadening observed near 700 cm -1 in FeMn crusts is attributed to feroxyhite.

Figure 5 .

 5 Figure 5. Representative Mn K-edge XANES absorption spectra (a) and first derivative (b) of hydrogenetic and diagenetic FeMn deposits. The FeMn crust spectrum is the average of 17 spectra from crusts and two spectra from nodules with an hydrogenetic XRD signature (Fe-vernadite). The 65SL-985cm spectrum is from a buried nodule of the Clarion Clipperton Zone (CCZ) in the central equatorial Pacific with a diagenetic XRD signature (todorokite). (c, d) Combo fits to a database of reference spectra (TableS4and Ref.[START_REF] Manceau | Determination of Mn Valence States in Mixed-Valent Manganates by XANES Spectroscopy[END_REF] ).

Figure 6 .

 6 Figure 6. Tl L3-edge XANES spectra of TlNO3 and Tl2O3 measured at standard resolution with an energy dispersive detector (a) and at standard and high ernergy-resolution (HR-XANES) with a wavelength dispersive detector (five-crystal analyzer) (b, c).

Figure 7 .

 7 Figure 7. (a,b) Calculated L3-edge HR-XANES spectra of Tl2O3 for dipole-allowed transitions to final states with s symmetry, d symmetry, and s + d symmetry. (c) Calculated L3-edge HR-XANES spectra of the Tl(1) and Tl(2) sites for Tl2O3.

Figure 8 .

 8 Figure 8. (a) Pure Tl(I) and Tl(III) component spectra obtained by principal component analysis of the 23 HR-XANES spectra of crusts and nodules. (b) Comparison of the two component spectra with the experimental TlNO3 and Tl2O3 spectra.

Figure 9 .

 9 Figure 9. Percentage of Tl(III) as a function of the thallium concentration.

Figure 10 .

 10 Figure 10. FEFF-calculated HR-XANES spectra of the VI Tl(III)-TCS and VI Tl(III)-DES surface complexes (a,b) and the VI Tl(III)-TCS and IV Tl(III)-TCS surface complexes.

Figure 11 .

 11 Figure 11. k 3 -weighted Tl L3-edge EXAFS spectrum of crust MP23-2 (a) and radial structure function (b) with fit.

Figure 12 .

 12 Figure 12. Geometry-optimized structural position of Tl(I) above a tetrahedral cavity of the MnO2 layer formed by three edge-sharing Mn octahedra. Mn octahedron = gray; Tl(II) = cyan; oxygen = red, proton = gray.

Figure 3 Figure 5

 35 Figure 3

40

 40

Figure 6 Figure 7

 67 Figure 6

Figure 10

 10 Figure 10

Figure S9 .

 S9 Figure S9. Plot of the 29 Tl L3-edge HR-XANES spectra measured for this study with the best-fit spectra calculated with linear-combinations of the Tl(I) and Tl(III) endmembers, and with the signal-to-noise ratios.

  b -6.1 b a Coordination number fixed to 6. b Parameters constrained to suppress correlations between σ and CN. c Res = [Σ(| χ exp -χ fit |)/Σ(| χ exp |] x 100. Accuracy: R = 0.02 Å, CN = 20%.

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Table 1 .

 1 Origin, depositional setting, Mn oxide, percentage of Tl(III) and Tl(I), Tl concentration, and Mn/Fe and Tl/Mn weight ratios of the 29 FeMn samples A portion of the 1mm surface layer was sampled with a drill. M=major, m=minor. c Hydrogenetic growth structures are observed by EPMA.

	Sample name	Source	Setting	Mn oxide a	%Tl(III) %Tl(I) [Tl] ppm [Mn]/[Fe] wt	[Tl]/[Mn] at x 10 4
	232D-B 0-78	Eastern Indian Ocean	Hydrogenetic crust	Fe-vernadite	79	21	1.5	0.37	0.05
	D20-1A	Indian Ocean	Hydrogenetic crust	Fe-vernadite	100	0	190	1.77	1.87
	D20-1B	Indian Ocean	Hydrogenetic crust	Fe-vernadite	100	0	151	1.24	1.73
	DS4-003A B08-28 Arctic	Hydrogenetic crust	Fe-vernadite	94	6	47	0.26	2.36
	D11-A	South Pacific	Hydrogenetic crust	Fe-vernadite	66	34	157	1.22	1.27
	37KD-3	CCZ, Pacific Ocean 48	Hydrogenetic crust	Fe-vernadite	62	38	103	0.92	1.35
	122DK	Northeast Atlantic, seamount	Phosphatized crust	Fe-vernadite	79	21	46	0.53	1.17
	DS835-1	Northeast Atlantic, seamount	Hydrogenetic crust	Fe-vernadite	74	26	127	0.95	1.60
	10DSR young	Central South Pacific	Hydrogenetic crust	Fe-vernadite	88	12	252	1.92	2.32
	10DSR old	Central South Pacific	Phosphatized crust	Fe-vernadite	91	9	319	3.32	2.92
	39DK	Central North Pacific	Hydrogenetic crust	Fe-vernadite	77	23	278	2.55	2.30
	40DS	Central North Pacific	Hydrogenetic crust	Fe-vernadite	91	9	135	1.26	1.57
	74GTV	Central Indian ridge	Hydrogenetic/hydrothermal crust	Fe-vernadite	79	21	39	0.71	0.59
	MP-1	Manihiki Plateau, South West Pacific	Hydrogenetic crust	Fe-vernadite	85	15	97	0.92	1.27
	MP-14	Manihiki Plateau, South West Pacific	Hydrogenetic crust	Fe-vernadite	88	12	157	1.04	2.02
	MP-27-3	Manihiki Plateau, South West Pacific	Phosphatized crust	Fe-vernadite	75	25	97	1.03	1.62
	MP-23-2	Manihiki Plateau, South West Pacific	Phosphatized crust	Fe-vernadite (M); todorokite (m)	95	5	283	3.95	2.64
	BS-surface b	Baltic Sea	Nodule	Not measured	0	100	2	1.06	0.03
				10 Å and 7 Å
	BS-bulk	Baltic Sea	Nodule	vernadite (M);	19	82	4	2.61	0.04
				todorokite (m)
				Fe-vernadite (m); 10
	KG1313	Peru Basin	Nodule, sediment surface	Å and 7 Å vernadite	96	4	22	30.33	0.13
				(M); todorokite (M)
				Fe-vernadite (M); 10
	EBS8-6	CCZ, Verma zone	Hydrogenetic nodule	Å vernadite (M); 7 Å	77	23	10	0.41	0.27
				vernadite (m)
				Fe-vernadite (M); 10
	44KG-2	CCZ, Pacific Ocean 48,51	Nodule, sediment surface c	Å vernadite (M); 7 Å vernadite (m);	74	26	112	6.26	0.91
				todorokite (m)

a XRD Abundance. b

Table 2 .

 2 Fractional and average valence states of Mn obtained from the Combo fit of XANES derivatives in the 6535-6570 eV interval.

		Mn(IV) Mn(III) Mn(II)	Average a	NSS b
	Hollandite
	Manceau et al. (2012) 0.82	0.16	0.02	3.81 ± 0.04
	This study	0.873	0.110	0.018	3.85 ± 0.06 9.5 × 10 -3
	Psilomelane
	Manceau et al. (2012) 0.81	0.16	0.04	3.77 ± 0.04
	This study	0.858	0.106	0.036	3.82 ± 0.07 9.4 × 10 -3
	Todorokite-Japan
	Manceau et al. (2012) 0.78	0.21	0.01	3.78 ± 0.04
	This study	0.881	0.076	0.043	3.84 ± 0.10 14.7 × 10 -3
	(Fe)-vernadite_average	0.975	-	0.025	3.95 ± 0.05 6.3 × 10 -3
	BS-bulk	0.855	0.079	0.066	3.79 ± 0.08 9.3 × 10 -3
	KG1313	0.810	0.144	0.046	3.76 ± 0.07 9.6 × 10 -3
	44KG-2	0.918	0.045	0.037	3.88 ± 0.08 7.7× 10 -3
	21KG-1(n)	0.868	0.095	0.036	3.83 ± 0.08 9.9 × 10 -3
	107KG-1-16cm	0.897	0.078	0.025	3.87 ± 0.07 7.1 × 10 -3
	107KG-36cm	0.870	0.113	0.018	3.85 ± 0.08 8.6 × 10 -3
	22KL-1-530cm	0.801	0.166	0.033	3.77 ± 0.08 9.9 × 10 -3
	22KL-2-801cm	0.790	0.177	0.033	3.76 ± 0.07 9.1 × 10 -3
	65SL-985cm	0.759	0.219	0.022	3.74 ± 0.07 7.5 × 10 -3
	a

Table S1 .

 S1 Location of the 29 FeMn samples

	SO193	MP-27-3	8º56.97'S 162º20.62'W	1982 -2455
	SO193	MP-23-2	9º30.02S 162º4.42'W	1222 -1222
	-	BS-surface	ca. 54°10'N ca. 11°10'E	20
	-	BS-bulk	ca. 54°10'N ca. 11°10'E	20
	SO79	KG1313	7°00'S	88°35'W	4150
	SO237	EBS8-6	11°40.73`N -11°40.45`N	47°58.03`W 47°59.00`W -	4986
	SO205	44KG-2	11°57.4`N 116°57.2`W	4188
	SO240	21KG-1(n)	13°10.5´N 118°08.2`W	4288
	SO240	107KG-1-16cm	11°50.1`N 116°32.9`W	4351
	SO240	107KG-36cm	11°50.1`N 116°32.9`W	4351
	SO240	09KL-44-49cm	13°10.5`N 118°10.1`W	4335
	SO240	22KL-1-530 cm	13°10.5`N 118°08.2`W	4302
	SO240	22KL-2-801 cm	13°10.5`N 118°08.2`W	4303
	SO240	65SL-985cm	12°56.1`N 119°08.9`W	4293
	Cruise ID	Sample name	Latitude	Longitude Water depth (m)
	DODO	232D-B 0-78	05˚23.0'S	97˚29.0'E	4119-3558
	RRKNOX06 D20-1A	13° 5' 60'' S	87° 50' 60'' E	3283
	RRKNOX06 D20-1B	13° 5' 60'' S	87° 50' 60'' E	3283
	HLY0905	DS4-003A B08-28	78° 39.6' N 158° 15.0' W	2200
	AVON02	D11-A	0° 11.28' S 173° 39.6' E	1399
	SO205	37KD-3	11°40.6`N 116°38.3`W	4006
	SO83	122DK	24°53'N	21°42'W	1700-1800
	POS235	DS835-1	30°09'N	14°444'W	1111-1230
	SO66	10DSR young	4°10.10'S 174°51.20'W	1600
	SO66	10DSR old	4°10.10'S 174°51.20'W	1600
	SO33	39DK	09º11'N	164º50'W	1217
	SO37	40DS	15º38'N	169º05'W	3049
	SO92	74GTV	25°23.73'S 069°45.61'E	2808
	SO193	MP-1	10º48.52'S 168º44.75'W	3672 -4063
	SO193	MP-14	13º41.76'S 161º27.28'W	3979 -4237
					S13

Table S3 .

 S3 List of pure-valence references used in Combo fits The TEY spectrum was used in the Combo fits, because this reference was unavailable.

	Standard			Formula		Mn valence
	Pyrolusite	β-MnO 2				4.0
	Ramsdellite	MnO 2				4.0
	Ca 2 Mn 3 O 8	2/3 Ca 2 Mn 3 O 8 + 1/3 CaMnO3		4.0
	KBi a	K	0.296	4+ 0. 926 (Mn	7 0 . 0 4 	2 )O .4H 2 0 ⋅	O	4.0
	Feitknechtite	β-MnOOH				3.0
	Groutite/manganite	1/3 α-MnOOH + 2/3 γ-MnOOH	3.0
	Manganite	γ-MnOOH				3.0
	Mn 2 O 3	Mn 2 O 3				3.0
	MnPO 4	MnPO 4				3.0
	Hureaulite	Mn 5 (PO 4 ) 2 (PO 3 OH) 2 •4H 2 O		2.0
	Rhodochrosite	MnCO 3				2.0
	Manganosite	MnO					2.0
	Pyroxmangite	(Mn,Fe)SiO 3				2.0
	Tephroite	Mn 2 SiO 4				2.0
	MnSO 4(s)	MnSO 4 •xH 2 O				2.0

a S14

Table S5 .

 S5 Tl concentration, percentage of Tl(III) and Tl(I) and precision, fit residual, and signal to noise ratio of the HR-XANES spectra

	ID	[Tl] ppm %Tl(III) %Tl(I) Precision NSS x10 5 (s/n) 1 a (s/n) 2 a
	232D-B 0-78	1.5	79	21	3	6.6	177	122
	D20-1A	190	100	0	3	11.2	361	266
	D20-1B	151	100	0	3	8.3	283	169
	DS4-003A B08-28	47	94	6	5	22.0	188	79
	D11-A	157	66	34	5	17.4	204	178
	37KD-3	103	62	38	6	26.2	111	86
	122DK	46	79	21	3	7.7	202	100
	DS835-1	127	74	26	3	6.0	279	141
	10DSR young	252	88	12	3	5.5	391	234
	10DSR old	319	91	9	3	7.5	481	107
	39DK	278	77	23	4	17.2	340	234
	40DS	135	91	9	6	26.7	235	111
	74GTV	39	79	21	3	8.3	159	68
	MP-1	97	85	15	3	4.2	248	182
	MP-14	157	88	12	2	3.2	321	241
	MP-27-3	97	75	25	3	5.5	256	165
	MP-23-2	283	95	5	3	5.6	517	221
	BS-surface	2	0	100	9	82.1	47	13
	BS-bulk	4	19	82	9	63.9	51	14
	KG1313	22	96	4	4	14.5	128	36
	EBS8-6	10	77	23	6	29.7	104	21
	44KG-2		74	26	3	5.6	355	174
	21KG-1(n)	44	100	0	5	20.8	111	40
	107KG-1-16cm	11	79	21	6	42.5	61	16
	107KG-36cm	5	77	23	11	88.3	45	18
	09KL-44-49cm	15	78	22	9	71.0	53	20
	22KL-1-530 cm	2	39	61	15	213.0	28	10
	22KL-2-801 cm	2	49	50	21	433.0	19	8
	65SL-985cm	2	31	69	18	266.0	22	11
	a

ACKNOWLEDGMENTS

Thomas Kuhn is acknowledged for providing sample SO92 74GTV and information on it, Wouter van Beek, BM31, is acknowleged for support with the Mn-XANES measurements, Alexander Sobolev is acknowleged for providing access to the RAMAN microscope at ISTerre, and four anonymous reviewers are acknowledged for constructive comments on the manuscript. This equipment has been funded by ERC Synergy 856555 MEET project and by Isere Department, France. Support was provided to A. Simionovici by the Institut Universitaire de France.

Our findings illuminate significant aspects of thallium crystal chemistry and geochemistry in marine ferromanganese deposits. They also give rise to challenging questions and suggest future research directions. Additional studies will be required to investigate how Tl(I) is oxidized mechanistically to Tl(III) by Mn(IV) on layered manganese oxides. EXAFS spectroscopy showed that Tl(III) is bonded to the edge and vacancy sites of Fe-vernadite, but only one sample was analyzed (MP23-2) with a rather high Tl concentration (283 ppm). Questions remain as to which of the two sites is most reactive and which dominates the partitioning of Tl at low concentration. The sorption sites of Tl(I) on (Fe)vernadite and todorokite also remain unknown. Perhaps Tl(I) is substituted for K + and/or Ba 2+ in the interlayers of (Fe)-vernadite and in the tunnels of todorokite, but this hypothesis needs to be tested.

All these important crystal chemical questions underlying the geochemical partitioning of Tl between seawater and FeMn crusts and nodules, on the one hand, and in coexisting manganese minerals, on the other hand, are now leading to investigations in natural marine deposits by HR-XANES and EXAFS spectroscopy down to the ppm level.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: DFT method, supplementary figures S1-S9 and tables S1, S3, S5, S6 (PDF)

Supplementary tables S2 and S4, and Cartesian coordinates of the surface complexes (xlsx)

AUTHOR INFORMATION

Corresponding Authors E-mail: alain.manceau@univ-grenoble-alpes.fr; a.koschinsky@jacobs-university.de ORCID Blanka Detlefs : 0000-0002-9768-647X Pieter Glatzel : 0000-0001-6532-8144 James Hein: 0000-0002-5321-899X Andrea Koschinsky: 0000-0002-9224-0663 Alain Manceau: 0000-0003-0845-611X Kira Mizell: 0000-0002-5066-787X Alexandre Simionovici: 0000-0001-5349-6482

Notes

The authors declare no competing financial interests. Corresponding Authors : alain.manceau@univ-grenoble-alpes.fr; a.koschinsky@jacobs-university.de

DFT modeling of the surface complexes

The equilibrium geometries of the Tl(III)-DES and Tl(III)-TCS complexes on the Mn18 nanosheets were calculated using ORCA [START_REF] Koschinsky | Uptake of Elements from Seawater by Ferromanganese Crusts: Solid-Phase Associations and Seawater Speciation[END_REF] and with the same high-level quantum chemical methods as described in detail previously for the sorption of 3d transition metals and Pb(II) on δ-MnO2. [START_REF] Rehkamper | Cenozoic Marine Geochemistry of Thallium Deduced from Isotopic Studies of Ferromanganese Crusts and Pelagic Sediments[END_REF] Briefly, the electronic exchange-correlation energy was calculated with the Perdew Burke and Ernzerhof functional PBE0 (also called PBE1PBE) [START_REF] Rehkamper | The Mass Balance of Dissolved Thallium in the Oceans[END_REF][START_REF] Hein | Deep-Ocean Mineral Deposits as a Source of Critical Metals for High-and Green-Technology Applications: Comparison with Land-Based Resources[END_REF] and atom-pairwise dispersion interactions were described with the D3BJ method [START_REF] Hein | Deep-Ocean Ferromanganese Crusts and Nodules[END_REF] applied to all atoms. The valence electronic configurations of all atoms were treated with the all-electron polarized def2-TZVP basis sets of triple ζ quality, 6 in combination with the Coulomb fitting auxiliary def2 /J basis sets. 7 The scalar relativity of Tl was accounted for by replacing the 60 innermost electrons with the def2-ECP effective core potential (ECP), as obtained from the Stuttgart pseudopotential library. The atom-pairwise dispersion correction D3BJ [START_REF] Hein | Deep-Ocean Ferromanganese Crusts and Nodules[END_REF] was applied to all atoms. The solvation model based on density (SMD) [START_REF] Marenich | Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions[END_REF] with water as the solvent was applied to all calculations. The accuracy of the Mn AOS given after the mean value in parenthesis was estimated to be equal to the variation of the best-fit AOS value when the fit S4 residual (NSS) was increased by 20%. We show with this example that this criterion is most conservative and that the precision on the variation of the AOS is actually better, on the order of 0.03 to 0.04 v.u. [START_REF] Glans | Experimental and Theoretical Study of the Electronic Structure of HgO and Tl2O3[END_REF] The density of empty 6s states is higher at the Tl(2) site, which means that the probability of 2p3/2 → 6s transitions is higher, and therefore the low-edge-peak more intense. In addition, the 6s states of Tl(2) are shifted to higher energy and as a result the 6s-5d hybridization is stronger, which also contributes to enhance the low-edge peak.

Supplementary References

Supplementary Figures