A negative test of orbital control of geomagnetic reversals and excursions

A ~41 Kyr periodic component has been reported in some sedimentary paleointensity records, allowing speculation that there may be some component of orbital control of geomagnetic field generation such as by obliquity modulation. However, no discernable tendency is found for astronomically-dated geomagnetic reversals in the Plio-Pleistocene (0 to 5.3 Ma) or excursions in the Brunhes (0 to 0.78 Ma) to occur at a consistent amplitude or phase of obliquity cyclicity, nor of orbital eccentricity. An implication is that paleointensity lows which are characteristically associated with these features are not distributed in a systematic way relative to obliquity and eccentricity, supporting the idea that orbital forcing does not power the geodynamo

Sub-Milankovitch cycles in periplatform carbonates from the early Pliocene Great Bahama Bank

High-resolution bulk sediment (magnetic susceptibility and aragonite content) and δ18O records from two different planktonic foraminifera species were analyzed in an early Pliocene core interval from the Straits of Florida (Ocean Drilling Program site 1006). The δ18O record of the shallow-dwelling foraminifera G. sacculifer and the aragonite content are dominated by sub-Milankovitch variability. In contrast, magnetic susceptibility and the δ18O record of the deeper-dwelling foraminifera G. menardii show precession cycles. The relationship between the aragonite and the paleoproxy data suggests that the export of sediment from the adjacent Great Bahama Bank was triggered directly by atmospheric processes rather than by sea level change. We propose a climate mechanism that bears similarities with the semiannual cycle component of eastern equatorial Pacific sea surface temperatures under present-day conditions

Impact of sulfur on the fate of platinoids in hydrothermal fluids

Le but de ce travail de thèse est de mieux quantifier le rôle des fluides hydrothermaux porteurs de soufre dans le transport de métaux critiques chalcophiles comme les platinoïdes (PGE). La connaissance des complexes chimiques qu'ils forment avec les ligands soufrés est essentielle pour prédire la solubilité et le transport de ces métaux en fonction de la composition chimique du fluide (e.g., concentration et spéciation de soufre, pH, fO2) dans une large gamme de température et pression de la croûte terrestre. Toutefois cette connaissance est encore très incomplète et parfois contradictoire. En particulier, l'impact sur la mobilisation de ces métaux de nouvelles formes de soufre, les ions radicalaires trisulfures S3•-, récemment découverts dans les conditions hydrothermales, demeure inconnu. Pour combler cette lacune, dans ce travail nous avons mis en œuvre trois approches complémentaires : i) expériences de solubilité en réacteur hydrothermal en conditions contrôlées; ii) mesures par spectroscopie d'absorption de rayons X in situ (XAS) sur synchrotron; et iii) modélisation de structure et de stabilité de complexes par méthodes thermodynamiques et de dynamique moléculaire. Le principal résultat de ce travail est la mise en évidence, pour la première fois, de la formation de deux complexes majeurs responsables du transport de Pt et Pd, (Pt/Pd)(HS)42- dans des solutions sulfurées et (Pt/Pd)(HS)2(S3)22- dans des solutions contenant sulfate et sulfure, à des températures entre 50 et 300 °C et à des pressions jusqu'à 1 kbar. Ces nouveaux résultats ont permis de générer les constantes thermodynamiques de réactions de formation de ces complexes. Ces constantes ont été utilisées pour prédire la solubilité des sulfures de platinoïdes (PtS, PtS2, PdS). Nos prédictions montrent que le transport des platinoïdes a été largement sous-estimé, d'un facteur au moins 1000 à 10 000, dans les travaux antérieurs qui ont tous ignoré le rôle de l'ion S3•-. En résumé, nos résultats ont permis d'élucider, pour la première fois et de manière systématique, la nature et la stabilité des principales formes du platine et du palladium dans les fluides géologiques, et de mieux quantifier l'action du soufre et de ses radicaux sur les transferts de matière dans la lithosphère. Ces résultats ont des applications à la fois pour la métallogénie et les ressources minérales, le traçage géochimique ou encore les nanotechnologies utilisant ces métaux.The goal of this thesis is to better quantify the role of sulfur-bearing hydrothermal fluids in the transport of critical chalcophile metals such as platinoids (PGE). Knowledge of the chemical complexes formed between these metals and sulfur ligands is essential to predict the solubility and transport of these metals as a function of fluid composition (e.g., sulfur concentration and speciation, pH, fO2) over a wide range of temperatures and pressures in the Earth's crust. However, this knowledge yet remains elusive. In particular, the exact impact of both traditional major sulfide ligands (HS-/H2S) and the trisulfur radical ion S3•- recently discovered at hydrothermal conditions on the mobilization of these metals, remains both inconsistent and controversial. To fill this gap, in this work we use three complementary approaches: i) hydrothermal reactor solubility experiments under controlled conditions; ii) in situ synchrotron X-ray absorption spectroscopy (XAS) measurements; and iii) structure and stability modeling of complexes by thermodynamic and molecular dynamics methods, to quantify, systematically for the first time, the speciation and solubility of Pt and Pd in H2S/HS/S3--bearing hydrothermal fluids. The major results of our work is the demonstration for the first time of the formation of two key complexes responsible for the transport of Pt and Pd: (Pt/Pd)(HS)42- in sulfide solutions and (Pt/Pd)(HS)2(S3)22- in sulfide-sulfate solutions, at temperatures between 50 and 300 °C and pressures between up to at least 1 kbar. These new results allowed us to generate thermodynamic stability constants for these complexes, and to use them to predict the solubility of Pt- and Pd-bearing sulfide solid phases (PtS, PtS2, PdS). Our predictions show that the transport of platinoids has been largely underestimated, by a factor of at least 1000 to 10000 in previous studies because they all ignored the role of the S3•- ion. In summary, our results have elucidated, for the first time and in a systematic manner, the nature and stability of the major forms of platinum and palladium in S-bearing geological fluids, and have better quantified the action of sulfur and its radicals on critical metals transfers in the lithosphere. The results of our work have applications for metallogeny and mineral resources, geochemical tracing and noble metal based nanotechnologies

The effect of the trisulfur radical ion on molybdenum transport by hydrothermal fluids

International audienceKnowledge of molybdenum (Mo) speciation under hydrothermal conditions is a key for understanding the formation of porphyry deposits which are the primary source of Mo. Existing experimental and theoretical studies have revealed a complex speciation, solubility and partitioning behavior of Mo in fluid-vapor-melt systems, depending on conditions, with the (hydrogen)molybdate (HMoO4-, MoO42-) ions and their ion pairs with alkalis in S and Cl-poor fluids [1-3], mixed oxy-chloride species in strongly acidic saline fluids [4, 5], and (hydrogen)sulfide complexes (especially, MoS42-) in reduced H2S-bearing fluids and vapors [6-8]. However, these available data yet remain discrepant and are unable to account for the observed massive transport of Mo in porphyry-related fluids revealed by fluid inclusion analyses demonstrating 100s ppm of Mo (e.g., [9]). A potential missing ligand for Mo may be the recently discovered trisulfur radical ion (S3•-), which is predicted to be abundant in sulfate-sulfide rich acidic-to-neutral porphyry-like fluids [10]. We performed exploratory experiments of MoS2 solubility in model sulfate-sulfide-S3•--bearing aqueous solutions at 300°C and 450 bar. We demonstrate that Mo can be efficiently transported by S3•--bearing fluids at concentrations ranging from several 10s ppm to 100s ppm, depending on the fluid pH and redox, whereas the available data on OH-Cl-S complexes cited above predict negligibly small (<100 ppb) Mo concentrations at our conditions. Work is in progress to extend the experiments to wider T-P-composition range of porphyry fluids and to quantitatively assess the role of S3•- in Mo transport by geological fluids.1. Kudrin A.V. (1989) Geochem. Int. 26, 87-99. 2. Minubayeva Z. and Seward T.M. (2010) Geochim. Cosmochim. Acta 74, 4365-4374. 3. Shang L.B. et al. (2020) Econ. Geol. 115, 661-669. 4. Ulrich T. and Mavrogenes J. (2008) Geochim. Cosmochim. Acta 72, 2316-2330. 5. Borg S. et al. (2012) Geochim. Cosmochim. Acta 92, 292-307. 6. Zhang L. et al. (2012) Geochim. Cosmochim. Acta 77, 175-185. 7. Kokh M.A. et al. (2016) Geochim. Cosmochim. Acta 187, 311-333. 8. Liu W. et al. (2020) Geochim. Cosmochim. Acta 290, 162-179. 9. Kouzmanov K. and Pokrovski G.S. (2012) Soc. Econ. Geol. Spec. Pub. 16, 573-618. 10. Pokrovski G.S. and Dubessy J. (2015) Earth Planet. Sci. Lett. 411, 298-309

Platinum group elements and sulfur in hydrothermal fluids: a love story told by in situ spectroscopy, molecular dynamics, and thermodynamics

International audienceKnowledge of Platinum Group Elements (PGE) speciation in hydrothermal fluids is essential to better understanding the transport of these metals in the Earth's crust and to identifying potential hydrothermal deposits where PGE may be present in economic grades. Existing data on aqueous chloride, sulfate, and hydroxide complexes of PGEs indicate extremely low metal contents (< ppt to ppb) in fluids from most geological settings [1-3] that cannot explain multiple instances of PGE concentration and mobilization in hydrothermal systems, thus appealing to an important role of sulfide (HS–) and, potentially, trisulfur (S3–) ligands [4] in PGE transport. To quantify the effect of sulfur on the solubility of platinum and palladium in hydrothermal fluids, we combined in situ solubility (Fig. 1A) and X-ray absorption spectroscopy (XAS; Fig. 1B) measurements with molecular dynamics (MD; Fig. 1C) and thermodynamic (TD) simulations [5]. Our results show that two main complexes transport these metals in hydrothermal fluids across a wide pH range (4–8), temperature and pressure (up to at least 350 °C and 1000 bar): Pt(HS)42– and Pd(HS)42– in H2S/HS– solutions, Pt(HS)2(S3)22– and Pd(HS)2(S3)22– in H2S/SO42–/S3– solutions. The role of the trisulfur ion in PGE hydrothermal transport thus appears to be particularly crucial [5,6], with solubilities (10s ppm Pt, Pd) up to 10,000 times higher than those of the ‘traditional’ complexes with H2S/HS–. Our results offer perspectives for the exploration of new PGE resources, their extraction and recycling, and hydrothermal synthesis of PGE-based nanomaterials. Future research on other metals in fluid-mineral systems will benefit from the combined approach implemented in this study, as well as an open-access database of XAS spectra of Pt reference compounds acquired in our work [7].[1] Bazarkina et al. (2014), GCA 146, 107–131; [2] Kokh et al. (2017), GCA 197, 433–466; [3] Tagirov et al. (2019), GCA 254, 86–101; [4] Pokrovski and Dubessy (2015), EPSL 411, 298–309; [5] Laskar et al. (2022), GCA 336, 407–422; [6] Pokrovski et al. (2021), PNAS 118, e2109768118; [7] Laskar et al. (2022), Minerals 12, 1602

Forcing of wet phases in southeast Africa over the past 17,000 years

Intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast Africa since the Last Glacial Maximum, about 20,000 years ago. In particular, the relative importance of atmospheric and oceanic processes is not firmly established. Southward shifts of the intertropical convergence zone (ITCZ) driven by high-latitude climate changes have been suggested as a primary forcing whereas other studies infer a predominant influence of Indian Ocean sea surface temperatures on regional rainfall changes. To address this question, a continuous record representing an integrated signal of regional climate variability is required, but has until now been missing. Here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro-climatology in southeast Africa during rapid climate changes over the past 17,000 years. Our results are based on a reconstruction of precipitation and river discharge changes, as recorded in a marine sediment core off the mouth of the Zambezi River, near the southern boundary of the modern seasonal ITCZ migration. Indian Ocean sea surface temperatures did not exert a primary control over southeast African hydrologic variability. Instead, phases of high precipitation and terrestrial discharge occurred when the ITCZ was forced southwards during Northern Hemisphere cold events, such as Heinrich stadial 1 (around 16,000 years ago) and the Younger Dryas (around 12,000 years ago), or when local summer insolation was high in the late Holocene, that is, during the past 4,000 years