NANOSCALE MINERAL CHEMISTRY AT A MODERN EUXINIC SPRING AND ITS IMPACT ON TRACE METAL MOBILITY

Abstract

Euxinic environments, defined by the absence of oxygen and the presence of elevated concentration of sulfide (H₂S), are some of the most extreme yet ecologically significant habitats on Earth. These systems occur in diverse settings, from the depths of the Black Sea to some hydrothermal environments. Their study provides critical insights into solid and aqueous phase mineral biogeochemical cycling on early Earth, and even the potential for life on extra-terrestrial bodies such as Mars1–7. In marine systems, restricted oxygen availability coupled with high organic input leads to microbial sulfate reduction. This is a process where sulfate-reducing bacteria (SRB) convert sulfate into sulfide8–10, leading to the formation of euxinic environments such as the permanently stratified Black Sea. The sharp transition from oxygenated surface waters to sulfidic and anoxic conditions at depth creates a unique ecosystem where anaerobic microorganisms dominate11. Similarly, hydrothermal vents produce sulfide-rich fluids that sustain chemosynthetic communities, that rely on symbiotic sulfur-oxidizing bacteria12. Also, terrestrial habitats where sulfide migrates upward from buried hydrocarbon rich formations can generate euxinic conditions when sulfide mixes with groundwater in the subsurface13. This study specifically explores nanoscale mineral chemistry at Zodletone spring, one such modern terrestrial environment with an oxic-euxinic interface. One of the importance of modern euxinic environments is that they serve as windows into the early Earth. Before the Great Oxidation Event (~2.4 billion years ago), most especially early Proterozoic, euxinic conditions dominated the earth’s oceans and lasted for about 1 Ga14,15. These early oceans hosted microbial communities similar to those found in modern analogs such as the black sea and the Cariaco basin16. Modern euxinic systems are natural laboratories that broadens our understanding of ancient biogeochemical processes, habitability on Mars, the evolution of early life, metal mobility and mineral reactivity under these extreme conditions. Mineral reactivity is one of the driving forces of geochemical processes in euxinic environments. Apart from bulk minerals, another major contributor to the geochemical processes in euxinic environments are nanoparticles. Nanoparticles and mineral phases at the nanoscale (1–100 nm) exhibit properties that differ markedly from their bulk counterparts, including increased surface area, enhanced reactivity, and unique electronic configurations17. In euxinic environments, such nanoscale minerals play a pivotal role in controlling the speciation and mobility of iron, sulfur, and metals. Smectite clays falls into the category of such nanoscale minerals that are important for controlling the fate of metals in the environment. Smectite clays play critical roles in regulating elemental cycling, contaminant mobility, and geochemical processes in subsurface environment18. Their reactivity under anoxic, sulfidic conditions is of particular importance for understanding natural biogeochemical cycles in reducing environments such as marine sediments and euxinic basins19. Although many studies explored the reactivity of clays with sulfide in laboratory environments, few have directly investigated the evolution of clay mineral chemistry as a function of exposure to sulfide-rich waters in low-temperature field environments. Therefore, this dissertation investigates mineral reactivity in a modern euxinic spring system from bulk to nanoscale level. Field and laboratory-based investigations were used including X-ray diffraction (XRD), Transmission and Scanning Transmission Electron Microcopy coupled with energy dispersive spectroscopy (S/TEM-EDX), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Chapter 1 focuses on understanding the impact of modern euxinic conditions on bulk sediment and clay fractions. The study demonstrates that (1) XRD-detectable sulfur bearing minerals in a soil profile exposed to euxinic groundwater are present outside the zone with visible sediment iron reduction and (2) Al-smectite nanoparticles primarily respond to euxinic conditions by immobilizing cations via adsorption or inter-layer cation exchange rather than significantly changing their octahedral sheet cation composition. Chapter 1 has been published in The American Chemical Society (ACS) journal ACS Earth and Space Chemistry (Apalara et al., 2025). Chapter 2 investigates how smectite clay octahedral sheet composition influences smectite interaction with a modern euxinic spring in terms of morphological changes and secondary mineral formation. Two different smectites with differing octahedral sheet compositions; SWa-1 (Fe-rich) and SAz-1 (Al-rich) were reacted at Zodletone spring for 504 hours. The study shows that SWa-1 underwent progressive reductive dissolution of Fe, accompanied by color change from yellow to black. The dissolution was followed by secondary Fe-sulfide and S0 precipitation. However, SAz-1 showed remarkable stability of its aluminosilicate framework despite minor Fe loss. In addition, the interlayer of both clays responded differently to the euxinic condition, in that Ca dominated the interlayer of SWa-1 while Na dominated the interlayer of SAz-1 by the end of the experiment. In Chapter 3, trace element mobility in a modern euxinic environment was studied in the presence of smectites with differing octahedral sheet composition. The Fe-rich and Al-rich clays from chapter 2 were studied, with three samples collected over the 504 hours total reaction time in the euxinic spring. The study demonstrates that, after 504 hours, reduced SWa-1 showed the highest trace element enrichment, particularly Ba, Rb, Sr and Cs, correlating with increasing Fe(III)-reduction, while SAz-1 showed similar trend but the enrichment of Ba, Rb, Sr and Cs are not as high as seen in SWa-1. Also, a reverse trend in the accumulation of chalcophilic trace elements, lithophilic trace elements and REEs in SWa-1 was seen in SAz-1. In the reacted Fe-rich SWa-1, chalcophiles are enriched at the start of the experiment but depleted after 8 hours, whereas in the reacted Al-rich SAz-1, chalcophiles are only enriched after 504 hours in the spring. Furthermore, In SWa-1, lithophiles are enriched after 504 hours in the spring while in SAz-1, only Cr, Ta, Ga and Th show short term enrichment after 120 hours followed by depletion after 504 hours in the spring. The enrichment of REEs was seen in reacted SWa-1 while no REE enrichment was observed in SAz-1 throughout the reaction time. Overall, this dissertation demonstrated new findings regarding the interactions between Fe- versus Al-rich smectites that reacted with euxinic spring waters for thousands of years, and their nanoscale interactions with iron and sulfur, major elements, and trace elements. References (1) Jørgensen, B. B.; Findlay, A. J.; Pellerin, A. The Biogeochemical Sulfur Cycle of Marine Sediments. Front. Microbiol. 2019, Volume 10-2019. https://doi.org/10.3389/fmicb.2019.00849. (2) Sperling, E. A.; Melchin, M. J.; Fraser, T.; Stockey, R. G.; Farrell, U. C.; Bhajan, L.; Brunoir, T. N.; Cole, D. B.; Gill, B. C.; Lenz, A.; Loydell, D. K.; Malinowski, J.; Miller, A. 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