Gas chimneys within marine sediments function as preferential conduits for focused methane migration, significantly altering early diagenetic stratification and subsequent porewater geochemistry. A critical locus for these biogeochemical transformations is the sulfate–methane transition zone (SMTZ), where the anaerobic oxidation of methane is stoichiometrically coupled with sulfate reduction, regulating sedimentary carbon cycling. This study investigates the regulatory role of chimney-enhanced methane flux and gas hydrate dynamics on SMTZ depth and microbial community architecture within deep-sea sediments (water depths >2,000 m). We combined detailed porewater chemistry measurements, including hydrogen and oxygen isotope ratios of water, with DNA-based community profiling, and compared two chimney cores with a distal non-chimney core. The non-chimney core did not show a clearly defined SMTZ within the recovered interval. In contrast, the chimney cores showed a shallower and narrower SMTZ, consistent with stronger upward methane transport and tighter coupling between methane consumption and sulfate use. At one chimney site, a strong decrease in chlorinity together with shifts in water isotope ratios suggested gas-hydrate dissociation within the sediment. Microbial communities in hydrate-affected sediments were dominated by groups often associated with methane-rich and low-oxygen conditions, and additional increases in taxa linked to diverse carbon use suggest that high methane flow can broaden available energy and carbon pathways. Overall, these results support a feedback pattern in which focused methane transport and hydrate instability change the SMTZ and redox structure, which then shapes microbial community composition and, in turn, the chemical signals preserved in deep-sea sediment records.

How to cite: Han, D., Jang, K., and Kim, J.-H.: Chimney-associated methane migration and hydrate dynamics influence SMTZ structure and microbial communities in deep-sea sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2270, https://doi.org/10.5194/egusphere-egu26-2270, 2026.

Magnetotactic bacteria have the ability to biomineralize intracellular magnetite (Fe₃O₄) nanoparticles. Resulting biomagnetite can be efficiently preserved in sedimentary rocks and represents past traces of biological activity that can be searched for paleontological and paleoenvironmental reconstructions. Recent work on trace-element incorporation into magnetite has shown that molybdenum exhibits a strong affinity for biomagnetite, with enrichments up to four orders of magnitude higher than in abiotic magnetite. This enrichment likely reflects molybdenum-dependent metabolic processes, such as nitrate reduction during denitrification, which support cellular energy production and contribute directly to magnetite biomineralization.

            Using a combination of molecular, chemical and magnetic approaches, we show that Mo availability directly stimulates growth and magnetite precipitation in the model microorganism Paramagnetospirillum (formerly Magnetospirillum) magneticum AMB-1 under environmental conditions favoring nitrate reduction. These findings demonstrate a functional link between molybdenum, nitrogen metabolism and biomineralization.

            Altogether, our results clarify the central metabolic role of molybdenum in magnetotactic bacteria and propose a mechanistic framework for interpreting the geochemical signatures of biomagnetite in ancient environments where nitrate-bearing oxidized species were present.

How to cite: Garry, M., Albalat, E., Touboul, M., Dumont, A., Egli, R., Thomazo, C., Balter, V., Modolo, L., Yvert, G., and Amor, M.: Molybdenum-dependent nitrogen metabolism drives magnetite formation in magnetotactic bacterium AMB-1., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1894, https://doi.org/10.5194/egusphere-egu26-1894, 2026.

The redox evolution of Earth and the evolution of life are tightly coupled through the progressive bioavailability of transition metals. As microbial metabolisms emerged and diversified, newly available metals were incorporated into oxydoreductase enzymes, reshaping global biogeochemical cycles and the redox state of the atmosphere and oceans. This evolutionary history is preserved in microbial metallomes, which record the metals integrated into metabolic nanomachinery over geological time and thus provide potential proxies for paleo-metabolic reconstructions.

Here, we imaged trace-metal distributions in commercial enzymes, modern carbonate spherules from microbial mats of the Dead Sea shores, and Archean mineralized biofilms from the 2.72 Ga Tumbiana formation using synchrotron-based XRF and particle-induced X-ray emission (PIXE), and integrate sedimentological, mineralogical, and geochemical constraints to infer the nature of the microbial metabolisms involved. Beyond this comparative approach, we aim to assess whether mineralized microbial systems retain diagnostic signatures of ancient metabolic pathways and redox conditions.

In practice, trace-metal measurements in enzymes are feasible, as demonstrated by our synchrotron-based analyses of carbonic anhydrase and associated calcium carbonate, which show systematic Zn enrichment. In modern arsenic-rich microbial mats from the Dead Sea, carbonate (aragonite) spherules and needles are enriched in Sr and Ni, likely linking carbonate precipitation to urease activity, which contains two Ni²⁺ ions per active site. Despite strong arsenic enrichment in the extracellular polymeric substances (EPS) driven by seasonal arsenic pulses in spring waters (Thomas et al., 2024), arsenic is excluded from the carbonate crystal lattice. In arsenic-rich Tumbiana stromatolitic laminae, PIXE analyses of layers containing nanopyrite and carbonaceous matter reveal complex but potentially syngenetic metal distributions. Multivariate discrimination identifies metal signatures in carbonaceous horizons dominated by As, Cu, and Mo. Taking into account both passive abiotic metal enrichment and previous interpreted metabolic signatures inferred for  the Tumbiana Formation stromatolites (i.e.  arsenic reduction and oxidation, nitrification and denitrification, sulfate reduction, anaerobic oxidation of methane ; Marin-Carbonne et al., 2018; Sforna et al., 2014; Thomazo et al., 2011) metallomic signatures may be in agreement with microbial arsenic and nitrogen cycling (Sforna et al., 2014). Given the complexity and different nature of metal accumulation in those enzymes, carbonates or modern and fossilized biofilms, extracting a metabolic signature associated to a metallome remains elusive without integrating lab-based approaches. Further work is therefore needed to constrain metal circulation and immobilization in organic matter (EPS, biofilm) and mineralizing phases to better assess biosignatures associated to metals and their isotopes in such objects.

Marin-Carbonne et al. (2018). Sulfur isotope’s signal of nanopyrites enclosed in 2.7 Ga stromatolitic organic remains reveal microbial sulfate reduction. Geobiology, 16(2), 121–138. 

Sforna et al. (2014). Evidence for arsenic metabolism and cycling by microorganisms 2.7 billion years ago. Nature Geoscience, 7(11), 811–815. 

Thomas et al. (2024). Combined Genomic and Imaging Techniques Show Intense Arsenic Enrichment Caused by Detoxification in a Microbial Mat of the Dead Sea Shore. Geochemistry, Geophysics, Geosystems, 25(3), e2023GC011239. 

Thomazo et al., (2011). Extreme 15N-enrichments in 2.72-Gyr-old sediments: Evidence for a turning point in the nitrogen cycle. Geobiology, 9(2), 107–120.

How to cite: Ariztegui, D., Thomas, C., Thomazo, C., Marin-Carbonne, J., Alleon, J., Agnon, A., Gedulter, N., Medjoubi, K., Sorieul, S., and Thaler, C.: Biofilm and carbonate trace metals as biomarkers : tentatively tracking enzymatic pathways in geobiological objects , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20427, https://doi.org/10.5194/egusphere-egu26-20427, 2026.

Numerous biological factors have been proposed to influence the formation of minerals under Earth-surface conditions, but the underlying concepts are often confused due to inconsistent terminology. The current systematics has largely developed historically, yet remains unclear because several terms have contrasting definitions or are not self-explanatory. Over time, the variety of processes proposed to explain biological effects on mineral formation has expanded, but the mechanisms often remain far from fully resolved and sometimes lack a proof of concept.

Here, a systematic framework of terms is proposed, requiring only slight modifications of the established terminology, primarily by removing some of the non-self-explanatory connotations. For example, the term ‘biologically influenced’ mineral formation better should represent a general ‘influence’ rather than a specific mechanism. In turn, ‘biologically induced’ should be used in its original meaning as ‘driven by supersaturation’. New terms such as ‘biologically nucleated’ and ‘biologically mediated’ precipitation would more precisely describe the specific mechanisms where organisms or biogenic organic substances act as a nucleation substrate or as a catalyst facilitating mineral growth from already supersaturated solution.

The proposed scheme would necessitate minimal intervention into existing terminology and at the same time become more user friendly for broad application in sedimentology and biogeosciences. Establishing a coherent and canonical terminology will not only improve clarity but also provide a common ground for future research on how biological and abiotic factors influence mineral formation under Earth-surface conditions.

How to cite: Meister, P. H. and Preto, N.: How life affects mineral formation: a reappraisal of concepts and terminology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4853, https://doi.org/10.5194/egusphere-egu26-4853, 2026.

Soft, unlithified sediments recovered from modern lakes rarely offer clear evidence of diagenetic alterations. Recent work has documented products of early diagenesis in the deep lacustrine setting of Lake Van. Lake Van, cored in 2010 in the frame of the ICDP PALEOVAN project, is a terminal, alkaline lake in Eastern Anatolia, Turkey (McCormack & Kwiecien, 2021). The lake carbonate inventory consists of (1) primary phases: inorganic calcite and aragonite precipitating in surface water, and low-Mg calcite ostracod valves formed at the sediment-water interface; and (2) secondary phases: early diagenetic dolomite forming in the sediment pores and aragonite encrustation of ostracod valves and organic remains.  Here we focus on aragonite encrustations.

Encrusted grains appear episodically in Lake Van sediments younger than 270 ka, and their occurrence is restricted to two lithologies; homogenous and banded muds, representing lake low-stands, reduced primary productivity/preservation and a well-ventilated water column. Although lake level changes occurred in the past, the water depth of the coring site – today at 350 m – unlikely fell below 200 m.

SEM and thin section analyses of the as yet enigmatic encrustations show two generations of aragonite crystals; larger (10 – 20 μm), columnar to blocky ones (inside the closed valves) and a magnitude smaller (1 – 2 μm), columnar ones (outside the valves) intercalated with clay minerals and probably organic matter. The isotopic composition of encrusted valves contrasts with that of inorganic carbonates precipitating in the water column; higher δ¹⁸O values support a formation in cold bottom water, higher δ¹³C values are likely related to microbial activity, however, the nature of this relation is yet unclear. Encrusted valves are often articulated but display different stages of opening. As ostracod valves usually disarticulate within hours to days after the animal’s demise, semi-open valves suggest that the early diagenetic process was – in geological terms – extremely rapid.

Our finding calls for care and attention analyzing even sub-recent biogenic carbonates. The episodic and facies-bound occurrence suggests that encrustation is ultimately controlled by environmental factors, yet so far, we were unable to pinpoint these factors or a mechanism responsible for this process. If you are intrigued just like us, do get in touch!  

References

McCormack & Kwiecien, 2021. Coeval primary and diagenetic carbonates in lacustrine sediments challenge palaeoclimate interpretations. Scientific Reports    

How to cite: Kwiecien, O. and McCormack, J.:  Did you say ‘fast’? Mysterious early diagenesis in sub-recent lacustrine sediments of Lake Van, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13965, https://doi.org/10.5194/egusphere-egu26-13965, 2026.

The diagenetic precipitation of barite (BaSO₄) in sediments requires the mobilisation and sources of dissolved barium and sulfate, the latter often limited in the sulfur cycling of lacustrine systems. In this study, we investigate the origin and proxy potential of barite that has crystallised in freshwater sediments of the Baltic Sea. Barite nodules with up to millimetre-scale grain sizes are found in the glacial varved clays of the limnic Baltic Ice Lake phase (>16 to 11.7 ka BP), underlying brackish Holocene muds. We have comprehensively analysed the solid phase of the host sediments and the barite, and the porewaters in the respective sediments, both, geochemically and isotopically for the signatures of sulphur, barium, oxygen, and also the related carbon cycling. The sulphur isotope signatures preserved in the barites display a remarkable downward gradient from the lithological boundary between the brackish Holocene sediments and the preceding limnic varved clay deposits. The sulphur isotope signature of different mineral components (marcasite, pyrite and barite) shows that the porewater sulphur reservoir was initially affected by microbial sulphate reduction. Aside from the smaller importance of bacterial activity in the glacial clays, the observed trend sustains an isotope discrimination upon solid phase formation, or minor fractions of isotopically light sulphur that may have been incorporated upon crystallisation at depth. It had been hypothesised that sulphate for barite precipitation originated from the postglacial connection of the Baltic Sea with the Atlantic Sea, that has led to brackish waters flowing into the different Baltic Sea basins and downward diffusion of sulphate and other dissolved constituents through the sediment column. Taken together, the observed changes in barite surface texture and Sr composition, as well as isotope signatures (Ba, S, O isotopes), indicate changes in the supersaturation and composition of the paleo-porewater fluids and the crystal growth rate, supporting the concept of a paleo-salinisation gradient that is geochemically imprinted in the barites up to date. Moreover, we explore the oxygen isotope signature in the barite as a proxy for the parent porewater fluids, and show that the pore waters at this site with low sedimentation rates have been completely modified to date by diffusional processes, in contrast to sites with higher sedimentation rates (IODP cores) that still retain the original porewater signature.

This investigation outlines that diagenetic barites in limnic sediments can evidence past salinization events, and furthermore, how the isotope signature of individual barite constituents can be used infer the parental fluid composition. This abstract summarises a detailed investigation recently published in a Special Publication (Roeser et al., 2025).

Roeser P., Böttcher M.E., Lapham L.L., Halas S., Pretet C., Nägler T., Prieto M., Struck U., Huckriede H. (2025) Barite in Baltic freshwater sediments crystallises in a diffusive salinisation gradient, 370-395; In: Nucleation and Growth of Sedimentary Minerals (Eds P.H. Meister, C. Fischer and N. Preto), International Association of Sedimentology, Special Publication, 50

How to cite: Roeser, P., Böttcher, M. E., Lapham, L., Halas, S., Pretet, C., Nägler, T. F., Prieto, M., Struck, U., and Huckriede, H.: Barite precipitation in freshwater limnic sediments: a proxy for salinization, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10930, https://doi.org/10.5194/egusphere-egu26-10930, 2026.

The chemical weathering of mafic magmatic rocks (e.g., basalt) is known to remove CO₂ from the atmosphere, transforming it into dissolved or solid inorganic carbon phases. Natural marine sediments contain a wide variety of organic and inorganic phases as well as microbial communities impacting the “submarine weathering engine”, e.g., increasing weathering potential by lowering ambient pH, or decreasing the CO₂ removal potential by forming authigenic clay minerals. Environments rich in reactive organic matter, mafic silicate minerals, and amorphous silica (e.g., ash, biogenic opal) reflect this natural complexity, and can serve as natural laboratories for understanding what controls submarine silicate weathering. Icelandic fjords with their high primary productivity and their mafic hinterland can serve as examples for these complex conditions. We present geochemical sediment and pore water data down to 5 m sediment depth from Hvalfjörður (SW Iceland) and Reyðarfjörður (SE Iceland) taken during the 2023 DEHEAT research cruise onboard RV Belgica. Our data show intense diagenesis that is both related to organic matter degradation and to submarine silicate weathering. The relatively uniform sedimentary material is fine-grained and particularly rich in iron, titanium and magnesium compared to average shale. Tentative sedimentation rates of about 0.5 cm/yr and organic carbon ranging between ~0.5 and 2.5 wt% with a dominantly marine origin based on TOC/TN ratios indicate an accumulation environment providing large amounts of highly reactive organic matter. Sulphate-methane transition zones are established at 75-100 cm sediment depth, but pore water alkalinity and DIC linearly increase to, and probably beyond, the deepest samples. Below the SMTZ, Ikaite crystals are found at various depths throughout the sediments of both fjords. Pore water profiles e.g. of dissolved silica and lithium show undulating downcore structures hinting both at silicate dissolution, but also at clay mineral formation. The data altogether provides insight into a complex interplay of dissolution and precipitation processes tied to the geology of the area, accumulation characteristics and the availability and respiration of organic matter.

How to cite: Wagner, K., März, C., van de Velde, S. J., Hylén, A., Arndt, S., Hall, P. O. J., Hidalgo-Martinez, S., Kononets, M., Meysman, F. J. R., Reyniers, P., Verweirder, L., and Hendry, K. R.: Diagenetic processes in fjord sediments of Southern Iceland – A complex interplay of organic matter respiration and submarine silicate weathering, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9849, https://doi.org/10.5194/egusphere-egu26-9849, 2026.

The Lower Cambrian Yurtus Formation in the Tarim Basin preserves important evidence of hydrothermal activity, microbial processes, and seawater chemistry that affected silica deposition and organic matter enrichment during the Ediacaran–Cambrian transition. Using field observations, petrography, redox-sensitive geochemical data, and biomarkers, this study examines how silica formed and what environmental conditions controlled the accumulation of black shales. The Yurtus Formation was deposited on a passive continental margin that was affected by extensional tectonism and occasional hydrothermal discharge. Geochemical data indicate that bottom waters were saline, acidic, and mainly anoxic, and that reducing conditions increased at times when hydrothermal H₂S and other reduced fluids entered the basin.

The siliceous layers show several ways through which silica was added or precipitated. Hydrothermal fluids supplied dissolved silica, while upwelling brought silica-rich deep water and nutrients into the basin. Microbial activity also contributed to silica precipitation. The presence of amorphous silica, barite nodules, and chert–mud alternations, together with microbial mats, radiolarians, and sponge spicules, shows strong interactions between microbes and minerals and the influence of early diagenesis. Acidification caused by hydrothermal gases and microbial metabolism played an important role in forming SiO₂ quickly. Differences between the siliceous units relate to changes in the balance between hydrothermal input and upwelling. Layers rich in phosphate and barite suggest increased nutrient supply and fluid mixing. Continuous barite beds and chert–mud layers also indicate silica delivery from distant volcanic and hydrothermal sources.

Organic-rich shales in the upper Yurtus Formation contain Type I–II kerogen from plankton, algae, and bacteria. Their biomarker features match those of Bashituo oils, showing that the Yurtus Formation is an important regional source rock. These results show that hydrothermal fluids were the main source of silica, and that microbial processes and upwelling influenced how silica and organic matter were preserved.

How to cite: Li, J., Kang, Z., and Zhang, X.: Siliceous deposition and hydrothermal contributions in the Lower Cambrian Yurtus Formation, Tarim Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1442, https://doi.org/10.5194/egusphere-egu26-1442, 2026.

Potassium (K) is ubiquitous in soils and has therefore received much less attention in modern edaphology compared with nitrogen (N) and phosphorus (P). However, the need to elucidate the phytoavailability of native soil K has recently been re-emphasized due to the rising cost of K fertilizers. Although native soil K largely occurs in minerals in immobile forms, biotite—a trioctahedral mica containing iron (Fe) and magnesium (Mg) in the octahedral sheet—can release K more rapidly than other K-bearing minerals. Octahedral Fe in biotite, originally present as ferrous iron (Fe²⁺), is oxidized to ferric iron (Fe³⁺). This Fe oxidation is hypothesized to cause two opposing effects on K retention. If the oxidized Fe³⁺ remains in the trioctahedral structure, the reduced layer charge may weaken K retention in the interlayer. Conversely, if part of the oxidized Fe³⁺ is released from the octahedral sheet, the structure shifts from a trioctahedral to a dioctahedral type, which may strengthen interlayer K retention. Although both mechanisms have been proposed, no direct evidence has been provided to date. The objective of this study was to determine how Fe oxidation in biotite influences K retention in the interlayer.

Biotite (2–50 µm) was first treated with sodium (Na) tetraphenylborate solution to replace most interlayer K with Na. The Na-biotite was then reacted with H₂O₂ at molar ratios of 0, 0.1, 0.5, and 10 relative to structural Fe, resulting in Fe³⁺ proportions of 6%, 30%, 69%, and 92%, respectively. These oxidized Na-biotite samples were subsequently washed several times with KCl solution to refill the interlayer with K, yielding biotite samples with varying degrees of Fe oxidation. Their atomic arrangements were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). Iron speciation was examined using selective dissolution analysis and Mössbauer spectroscopy. The release rate of interlayer K from biotite was evaluated using a resin extraction method.

XRD 060 reflections clearly showed a gradual shift from tri- to dioctahedral structures with increasing Fe³⁺ proportions, which was also supported by shifts in the OH absorption bands in the FTIR spectra. Although we initially assumed that this alteration would strengthen interlayer K retention, the oxidized and dioctahedral biotite released K more rapidly than the less oxidized samples. The weaker K retention after Fe oxidation could not be explained solely by changes in the octahedral sheet structure. TEM analysis revealed that highly oxidized biotite exhibited partially expanded interlayer spaces, which were likely filled with Fe hydroxides.

We concluded that Fe oxidation not only modifies the octahedral sheet structure but also promotes the formation of Fe hydroxides within the interlayer, leading to weakened K retention and enhanced K release from biotite.

How to cite: Nakao, A., Nakajima, A., Kogure, T., and Yanai, J.:   Iron oxidation and associated structural alterations in K-bearing minerals: How do they impact K phytoavailability in soils?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21542, https://doi.org/10.5194/egusphere-egu26-21542, 2026.

Microbially induced calcite precipitation (MICP) provides a low-carbon alternative to traditional soil stabilization methods. However, the coupled impact of key input biochemical parameters, namely biomass concentration, chemical reagent dosage, and initial pH, on this biocementation process remains largely unexplored, which in turn influences the precipitation pathway and crystal characteristics, such as quantity, size, and mineralogy, ultimately affecting the overall strength gain. The study conducts laboratory experiments using the Sporosarcina pasteurii bacterium with varying biomass concentrations, ranging from an optical density of 0.25 to 1.00, cementation reagent concentrations varying from 0.25 M to 1.00 M, and initial pH values changing from 7 to 9. This is followed by an optimization scheme aimed at achieving maximum strength gain. Urea hydrolysis and calcite precipitation were monitored through the release of ammonium amount and the concentration of dissolved calcium ions in the cementation solution, respectively. The precipitated biomineral was analyzed for microstructural and mineralogical attributes. Following this, soil biocementation experiments were conducted to arrive at optimized biochemical parameters using statistical regression analysis. Results show that higher biomass accelerates ureolysis, while final calcite quantity mainly depends on reagent availability. Yet, soil strength is not primarily dependent on biomineral quantity; instead, crystal size and morphology are decisive, which are strongly influenced by the coupled interaction of biochemical parameters. A lower biomass concentration, combined with an increased reagent amount, promotes crystal growth. However, an increase in the amount of cementation reagent becomes detrimental to crystal size at higher biomass levels. Moreover, lower pH provides some lag time to the reaction but can also accelerate bacterial growth, thereby altering the crystal size. Furthermore, stable calcite mineral is found to precipitate at lower biomass cementation due to the inhibition of bacterial enzymatic activity. Soil biocementation results revealed that larger crystals bridging the soil pores significantly increase strength, up to 10 MPa from 0.17 MPa, compared to abundant but small-sized crystals. Thus, reaction conditions that favour rapid precipitation can be mechanically ineffective without effective pore bridging, emphasizing that biocementation should focus not only on producing large amounts of biominerals but also on the size of the precipitated crystals. By identifying biochemical thresholds that promote stronger, more interlocked crystals, this work offers guidelines for achieving maximum strength gain with optimised biochemical parameters.

How to cite: Joshi, R., Nikitha, T. R., and Arnepalli, D. N.: Effect of Biochemical Parameters on Biomineral Formation and Soil Strength Development in Microbially Induced Calcite Precipitation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-679, https://doi.org/10.5194/egusphere-egu26-679, 2026.