Sedimentary pyrite is becoming one of the most promising and reliable archives for biogeochemical processes and environmental evolution of the Earth’s surface today. It represents a major reservoir of sulfur within the global sulfur cycle, with most of its formation taking place in organic-rich sediments along continental margins. Authigenic pyrite typically forms through microbial sulfate reduction coupled to organic matter remineralization or anaerobic oxidation of methane in sediments. Pyrite formation in marine sediments influences global seawater sulfate concentrations and sulfur isotope patterns, reflecting local microbial activities or environmental change, and tracking past seawater chemistry. Applications as a paleoenvironmental proxy rely on characteristic geochemical signatures archived in pyrite, including its sulfur isotopic and trace element compositions. Therefore, a comprehensive understanding of the controls on pyrite geochemistry is critical for the effective application of this proxy in studying the Earth system.

Marine methane-rich sediments alone continental margins, such as seeps, are excellent natural laboratories to study mineral authigenesis, while also being global hotspots of sulfate consumption and authigenic pyrite formation. We present various geochemical datasets including multiple sulfur (³²S, ³³S, ³⁴S, ³⁶S), iron (⁵⁴Fe, ⁵⁶Fe), and molybdenum (⁹⁵Mo, ⁹⁸Mo) isotopic compositions, along with trace element patterns of authigenic pyrite from modern and ancient methane-rich sediments deposited along continental margins. Our results highlight the potential of pyrite geochemistry as a tool to distinguish and characterize different modes and intensities of microbial sulfate reduction during early diagenesis. Furthermore, this study reveals that the trace element inventory of pyrite formed during early diagenesis is affected by sediment composition rather than by seawater. A comprehensive understanding of early diagenetic processes improves our understanding of pyrite formation and its geological implications.

How to cite: Lin, Z., Strauss, H., and Peckmann, J.: Authigenic pyrite in marine sediments: Geochemical insights from present and past , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-256, https://doi.org/10.5194/egusphere-egu25-256, 2025.

Nature of sulfidization in shallow shelf sediments: Influence of organoclastic sulfate reduction and anaerobic oxidation of methane

Anaerobic oxidation of methane is an important biogeochemical process in marine sediments responsible for methane consumption, significantly influencing the atmospheric methane budget, the marine carbon cycle, and sediment pore fluid chemistry. Sulfate-driven anaerobic oxidation of methane (SO₄^2--AOM) and organoclastic sulfate reduction (OSR) in marine sediments commonly lead to the precipitation of authigenic pyrite with characteristic sulfur isotopic compositions. In the present study, we have investigated the nature of pyrite and C–Fe–S geochemistry in a sediment core collected from a water depth of ~30 m off the West Coast of India, Eastern Arabian Sea, which represents shallow shelf sediments rich in methane and characterized by high carbon sulfur burial rates.  Our goal was to assess the sulfidization patterns to understand the past variation in methane fluxes within these sediments. Porewater geochemical profiles provide evidence for the combined influence of OSR and AOM on the sediment fluid chemistry. The sediment core is characterized by a shallow sulfate-methane transition zone (SMTZ) between 263 and 303 cmbsf. The Chromium reducible sulfur  (CRS) content and sulfur isotopic composition of pyrite (δ³⁴S_CRS) shows high variability throughout the core, with the upper sedimentary layers (from sediment-water interface to 2.7 mbsf) characterized by relatively low CRS content (0.7 to 3.93 wt %) and low δ³⁴S_CRS values (-37.53 to -25.94 ‰ VCDT). This pattern is interpreted to reflect the dominance of OSR in shallow sediments. In the deeper sediment layers (below ~2.7 mbsf), CRS contents (1.9 to 10.2 wt %) are enriched and δ³⁴S_CRS values show an overall trend towards positive values, suggesting that sulfide minerals are primarily linked to SO₄^2--AOM. The enrichment trend in δ³⁴S_CRS values corresponds to zones affected by ΣHS^- diffusion from relict SMTZs. The evidence for paleo-SMTZs, indicated by enriched δ³⁴S_CRS values and the presence of large framboids, framboid clusters, and rod-like aggregates at multiple depths underscores episodic upward methane flux events. Future research should focus on high-resolution geophysical and geochemical investigations to elucidate the mechanisms driving methane migration, sulfidization variability, and their implications for global carbon and sulfur cycling in these coastal marine systems.

How to cite: Sivan, K., Mazumdar, A., Peketi, A., Mishra, S., Steinhöfel-Sasgen, G., and Henkel, S.: Nature of sulfidization in shallow shelf sediments: Influence of organoclastic sulfate reduction and anaerobic oxidation of methane , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19809, https://doi.org/10.5194/egusphere-egu25-19809, 2025.

Pyrite plays an important role in the sulfur cycle, reflecting changes in both global and local redox conditions within sedimentary environments. The grain size of framboidal pyrite is an effective indicator of the redox state of the sedimentary water column, while its sulfur isotope characteristics provide insights into early diagenetic history. However, variations in water column hydrodynamics can diminish the reliability of framboidal pyrite grain size distribution as an indicator of redox conditions. Additionally, bulk sulfur isotope measurements of pyrite are often influenced by later diagenetic processes. In this study, we investigated the redox sensitive elements content, morphology and in-situ sulfur isotopic characteristics of pyrite in the Wufeng (Ordovician)-Longmaxi (Silurian) Formation shales in South China. The results indicate that bottom currents, by altering the hydrodynamic conditions of the sedimentary water column, leads to larger and more dispersed grain sizes of framboidal pyrite formed in anoxic water column. Moreover, framboidal pyrite formed during the Late Ordovician and Early Silurian exhibits distinctly different sulfur isotope distribution characteristics at the particle scale, which appears to reflect the response of sedimentation rate changes to sea level fluctuations. Ultimately, we systematically reconstructed the redox evolution of the sedimentary water column during the Ordovician-Silurian transition in South China, dividing it into five stages: (1) The upper Wufeng Formation experienced increasingly reducing conditions, culminating in euxinia at the top. (2) Oxidizing conditions briefly prevailed at the base of the Longmaxi Formation. (3) Oxygen levels in the sedimentary waters of the lower Longmaxi Formation decreased, s stabilizing in a prolonged dysoxic to euxinic state. (4) The middle-lower Longmaxi Formation experienced a gradual increase in the oxidative state of the sedimentary waters, transitioning to an oxic water column. (5) The middle Longmaxi Formation sustained a long-term dysoxic to oxic water column.

How to cite: Ji, S., Liang, C., Liu, K., Cao, Y., and Tang, Q.: Morphology and in-situ sulfur isotope characteristics of pyrite across the Ordovician-Silurian boundary marine shale in South China: Indicative significance for sedimentary environment, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18, https://doi.org/10.5194/egusphere-egu25-18, 2025.

The formation of gold deposits may involve multiple stages of gold enrichment, which makes it difficult to differentiate the potential multistage processes of ore material enrichment. Determining whether these events represent the remobilization of gold from pre-existing deposits or the introduction of new gold during a distinct epigenetic event is often challenging. The Balong gold deposit is a representative lode gold deposit in the East Kunlun metallogenic belt in China. Gold mineralization is hosted in Triassic granitoids and is characterized by multi-stage quartz-sulfide veins. Pyrite is the most abundant sulfide in the ore and is also the most important host for gold. Three types of pyrite have been identified. The porous Py1 exhibits low trace element content, with an absence of gold. Subhedral Py2-1 contains various Cu-Pb-Zn-Ag mineral inclusions. Py2-2 shows a significant increase in As (median 17, 073 ppm) and Au (median 3.79 ppm), exhibiting obvious distinctions between Py2-1 and Py2-2.

Gold in the Balong deposit consists of both visible and invisible gold. Visible gold is found within the micro-fractures of pyrite and arsenopyrite, appearing as irregular inclusions or infillings. In addition to visible gold grains, the majority of the invisible gold in Py2-2 exists as solid solutions (Au). Backscattered Electron imaging and trace-element analyses show that invisible gold occurs only in the As-rich bands. Pyrite records a narrow range of δ⁺³⁴S values from -1.6 to 5.4‰, reflecting sulfur from a deep magmatic source. In conjunction with fluid inclusion studies and the estimated age of the related magmatic activity, our results point to magmatic-hydrothermal fluids as the main contributors of ore materials. Coupled dissolution-reprecipitation reactions of early pyrite are a key factor for visible gold precipitation and later invisible gold enrichment. Our pyrite data constrain the evolution of ore-forming processes and offer new perspectives on zonal pyrite formation.

How to cite: Zhao, Y.: Pyrite textures and trace element compositions from the Balong gold deposit in the Eastern Kunlun Orogenic Belt, Northern Tibetan Plateau: Implications for gold mineralization processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4409, https://doi.org/10.5194/egusphere-egu25-4409, 2025.

        Pyrite is an important component in reconstructing the biogeochemical conditions and sedimentary environments of the earth's surface in the past. The study of Cretaceous pyrite in the Songliao Basin not only helps to clarify the formation mechanism of high-quality source rocks in the area, but is also important for reconstructing the Cretaceous paleoenvironment. Pyrite in the Qingshankou Formation includes euhedral pyrite, anhedral pyrite, fine-grained pyrite aggregates, pyrite framboids and polyframboids. According to this genetic division, the euhedral pyrite can be divided into "authigenic type" and "secondary type". The "authigenic type" euhedral pyrite is directly precipitated from solution, while "secondary type" euhedral pyrite is formed by recrystallization of pyrite framboids. The "secondary type" can be further divided into "compaction type" and "cementation type" type, indicating that the transformation of pyrite framboids into secondary euhedral pyrite is controlled by compaction and cementation, respectively. Anhedral pyrite is usually precipitated on the surface of iron-rich clay minerals (e.g., chlorite), or by metasomatism of other minerals, biological skeletons, and microorganisms. Pyrite framboids are transformed from greigite during the syndiagenetic stage. Under the same redox conditions, higher water flow energy conditions enhance the abundance of pyrite framboids, increase the number of microcrystalline layers, and lead to larger diameter pyrite framboids. The sediments in the K₂qn¹ Formation were deposited in a semi-arid to semi-humid climate, in an anoxic and reducing environment. The sedimentary lacustrine basin was a highly restricted environment with brackish to saline water. In this environment, circulation was weak, resulting in fewer pyrite framboids with fewer microcrystalline layers and smaller diameters. The restricted environment resulted in abnormally high δ³⁴S_py values. The enhanced development of euhedral pyrite with heavier sulfur isotope values and the low occurrence of pyrite framboids with lighter sulfur isotope values is also an important reason for the abnormally high δ³⁴S_py values. This study provides a new understanding of the genetic mechanism of different types of pyrite.

How to cite: Wu, Y. and Wang, M.: Genesis and geological significance of pyrite in the Cretaceous shale of Songliao Basin, NE China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2302, https://doi.org/10.5194/egusphere-egu25-2302, 2025.

Pyrite (FeS₂) is the end-product of microbial sulfur cycling in reduced environments and is the main burial pathway of sulfur in marine sediments. Pyrite forms by a series of reactions between sulfide and Fe(II)/Fe(III), and sulfur-metabolizing microorganisms play an important role in mediating their formation. Here we tested microbial pyrite formation by the iron(III)- and sulfur-reducing bacterium Geobacter sulfurreducens in the presence of the Fe(III) (oxyhydr)oxide mineral ferrihydrite and elemental sulfur (S⁰). Over 6 months of incubation, two main stages were observed for the geochemical evolution of the system. In the initial ferruginous stage, rapid release of aqueous Fe(II) into the solution is accompanied by mackinawite (FeS) formation through the reaction between sulfide and ferrihydrite. In the second sulfidic stage, sulfide and polysulfides accumulate in solution, catalyzing mackinawite’s transformation to greigite (Fe₃S₄) and eventually to pyrite. Scanning electron microscopy demonstrated that individual spherulitic pyrites formed on the surfaces of elemental sulfur, eventually replacing it completely while still preserving the original shape of the sulfur particles. Hence, elemental sulfur is a significant reactant with key functions in polysulfide formation and templating effect on microbial pyrite formation. Therefore, our results suggest a mechanism for microbial pyrite formation in microenvironments in modern sediments and sulfate-poor ecosystems throughout time (e.g., Archean Earth). Future research will be focused on the bioavailability of microbial pyrite to have a complete picture of the role of pyrite in microbial sulfur cycle.

How to cite: Sekerci, F., Fischer, S., Joshi, P., Peiffer, S., Kappler, A., and Mansor, M.: Elemental Sulfur as a Key Intermediate for Microbial Pyrite Formation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15175, https://doi.org/10.5194/egusphere-egu25-15175, 2025.

Seafloor massive sulfide (SMS) deposits form on the modern ocean seafloor at active hydrothermal vent systems through mixing of mineral-rich, hydrothermal fluids with ambient oxygenated seawater. Once hydrothermal activity ceases, oxygenated seawater infiltrates these deposits, fostering to abiotic oxidative weathering. Microbial activity considerably accelerates this transformation, driving sulfide mineral breakdown, thus enhancing metal transport. Under conditions, restricting oxygen entrainment, low-oxygen zones form below the surface, shielding SMS deposits from oxidative weathering, potentially extending their preservation. SMS deposits are valuable sources of metals governing the interest of their lifespan.
In this study, we explore the impact of microbial activity on SMS transformation and mineral dissolution under oxic and low-oxygen conditions. We incubated sulfide minerals, i.e. pyrite and chalcopyrite for four years on the seafloor at active and inactive vent sites along the Indian Ridge. These sulfide minerals were then used for metagenomics, microscopy, microbial enrichment experiments, physiological studies, and geochemistry to identify the key microbial agents driving mineral transformation and metal release. Scanning electron microscopy (SEM) reveals diverse mineral structures, such as twisted stalks and nanowires, suggesting various Fe-oxidizing microbes as well as those involved in extracellular electron transfer. Preliminary metagenomic analyses provide insights into the presence of genes associated with iron oxidation and reduction. Laboratory cultivation experiments mimicked different temperature, oxygen, and pH conditions of hydrothermal vent fluids admixed to distinct degrees with ambient seawater and suggest faster microbially mediated mineral dissolution under oxic conditions and of pyrite as opposed to chalcopyrite. By assessing turnover rates of mineral transformations, we aim to predict how microbial activity affects SMS deposit longevity under varying oxygen conditions.

How to cite: Tecza-Wiezel, A., Laufer-Meiser, K., Solerbeck, C.-H., Schloesser, J., Sander, S., and Perner, M.: New Insights into SMS Deposits: How Microbial Activity and Oxygen Levels Shape Metal Preservation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8834, https://doi.org/10.5194/egusphere-egu25-8834, 2025.

Pyrite formation has been widely investigated because of its abundance and significance in the iron and sulfur cycles in many anoxic environments. The ferric-hydroxide-surface (FHS) pathway is an important pathway for rapid pyrite formation, relying on the generation of surface-bound precursor species >Fe^IIS^2-.^[1] However, ferric (oxy)hydroxides are often microbially produced and thus associated with organic matter (OM). Additionally, in natural environments, sulfide (S(-II)) supply rates are typically regulated by sulfate-reducing bacteria, providing a more continuous flux, in contrast to the single-pulse S(-II) additions commonly used in laboratory experiments.^[2] To our knowledge, the combined effect of surface coating and sulfide supply rates on pyrite formation and secondary iron mineral transformation remains unexplored. In this study, we therefore compared pyrite formation rates and reaction products by exposing 40 mM synthetic ferric (oxy)hydroxides (goethite and ferrihydrite) and biogenic Fe(III) (oxy)hydroxides (BioFe, which includes associated organic matter, cells and phosphate) to sulfide at pH 6. Sulfide was supplied under strictly anoxic conditions either as single-pulsed 10 mM S(-II) pulse or multiple 0.5 mM/d S(-II) pulses over 20 days (final Fe(III):S(-II) = 4:1). Aqueous- and solid-phase S and Fe speciation as well as changes in Fe mineralogy were tracked using wet chemistry techniques, Raman micro-spectroscopy and X-ray diffraction. Our results show that ferrihydrite was transformed mostly into lepidocrocite, goethite and pyrite after single-pulsed S(-II) addition, and to goethite and pyrite in the multiple-pulsed S(-II) treatment. Rietveld quantitative phase analysis via XRD revealed that the multiple-pulsed S(-II) mode delayed pyrite formation. However, no pyrite was identified in the treatment with biogenic Fe(III) (oxy)hydroxides, where the added sulfide was instead converted to zero-valent sulfur, presumably due to occupation of the surface sites by OM and/or phosphate. Notably, phosphate from the bacterial growth medium was sequestered in vivianite. Our findings demonstrate that pyrite formation via the FHS pathway is strongly influenced by the presence of surface-active components (e.g., organic matter or PO₄^3-) and sulfide addition rates. [1] M. Wan et al., 2017, Geochim. Cosmochim. Acta, 217, 334–348. [2] Skyring, G.W., 1987, Geomicrobiol J 5: 295–374.

How to cite: Tang, X., Hockmann, K., Obst, M., ThomasArrigo, L. K., Lacina, M., Sekerci, F., Mansor, M., Kappler, A., and Peiffer, S.: Sulfide supply rate and organic surface coating affect pyrite formation during sulfidization of ferric (oxy)hydroxides, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8394, https://doi.org/10.5194/egusphere-egu25-8394, 2025.