Determining fault rheology is vital for assessing earthquake hazards, yet the mechanisms controlling fault strength, slip behaviour, and healing remain uncertain, especially in fluid-rich carbonate systems. Advances in high-resolution microscopy and isotope geochemistry reveal that crustal carbonate fault zone deformation occurs at the nanoscale, significantly impacting fault rheology throughout the seismic cycle. We integrate natural fault observations from Greece with high-velocity rotary-shear experiments to investigate nanoscale transformations in carbonate gouges. Under fluid-rich conditions and using ¹⁸O-enriched tracer fluids, experiments show near-instantaneous calcite decarbonation during slip, producing transient phases like CaO, amorphous carbon, and nanocrystalline calcite. These products govern fault strength evolution, enabling dynamic weakening independent of conventional frictional mechanisms. Natural mirror slip surfaces also display nanoscale coatings of amorphous carbon and secondary calcite nanograins, indicating similar processes occur in nature. Nanostructural evidence reveals a shift in deformation mechanisms, from grain-boundary sliding to crystal-plastic flow and dynamic recrystallisation, driving cyclic grain size reduction and strengthening. We propose that triboelectrochemical processes, triggered by slip-induced electrical potentials, may facilitate CO₂ reduction and amorphous carbon formation. These findings challenge traditional fault models by showing that weakening and healing are largely governed by chemically driven nanoscale phase changes, not just mechanical abrasion. Together, these nanoscale processes explain the formation of ultra-smooth fault surfaces and influence both co- and post-seismic fault behaviour. Our results emphasise the critical role of fluids and transient phases in seismic mechanics and advocate for integrating nano-analytics with geochemistry to better understand earthquake processes in carbonate-rich faults.

Mineral formation from hard water creeks is sensitive to variations of physico-chemical boundary conditions, biological factors, and climate change. Groundwaters are often equilibrated with aquifer CaCO₃, but supersaturated in CO₂ wrt. to the atmosphere emerge from springs near the German coastline. Upon contact with the Earth’s atmosphere, CO₂ starts to degas, absorbs O₂, and after exceeding a critical threshold carbonate starts to precipitate. Ferruginous groundwaters may be dominated by the precipitation of iron oxyhydroxides. Trace element and stable isotope partitioning upon crystallization are controlled by non-equilibrium processes. The spring composition characterizes element sources and subterrestrial water-microbe-rock interaction related to subterrestrial weathering. The spatial gradients display exchange with the atmosphere and mineral precipitation, controlled by morphology and season. The hardwater creeks form a source for carbon and will finally contribute to the coastal waters, modulating their buffer capacity, and, therefore, impact surface water acidification and their CO₂ release potential.

Examples for hard water systems were chosen from different aquifer lithologies at Gespensterwald, Rügen Island, and Meschendorf beach.

Besides in-situ parameters, the hydrogeochemistry (major and trace elements) and multiple stable isotope partitioning between dissolved and solid phase were analyzed. The aqueous solution was subjected to physicochemical PHREEQC analysis. Carbonate or iron oxyhydroxide precipitation and CO₂ degassing rates, are estimated based on different methods:

- Hydrochemical gradients along the flow path

- Direct measurements using a new in-situ placer technique (Peters, MSc thesis 2025)

- Comparison of field findings with lab-based trace element partitioning into the solid

A detailed diagenetic reconstruction of the Anisian Schaumkalk (c. 245 Ma) and the Oxfordian Korallenoolith (c. 163 Ma) in the Central European Basin was conducted by combining petrographic techniques (including cathodoluminescence microscopy) with fluid inclusion analysis and in situ U–Pb dating of carbonates. Pack- and grainstone lithofacies were sampled from outcrops and cores, representing the present-day range from surface exposures in Central Germany to maximum burial depth in North Germany. U–Pb dating of carbonate grains and cementations yielded ages in the range of 244–63 Ma that cluster in seven distinct age groups. The earliest age group of 244–232 Ma (I) is related to eogenetic modifications of carbonate mud and cements of the Schaumkalk in shallow burial depth. Cementation groups II–VII are related to basin-scale fluid migration as evidenced by fluid inclusions trapped at 90–310°C with salinities of 13–24 wt% NaCl+CaCl₂. Trapping of highly saline basinal fluids in authigenic cementations at 239–237 Ma (II), 227–223 Ma (III), 202–189 Ma (IV), 170–156 Ma (V), 102–99 Ma (VI) and 64–63 Ma (VII) correspond to phases of structural reorganization and tectono-magmatic activity in the Central European Basin. Comparing the intense diagenetic overprint of the Schaumkalk (all age groups present) with the incipient diagenesis of the Korallenoolith (age groups V–VII present) highlights the importance of fluid-driven diagenetic alteration. This refers in particular to wide-spread neomorphism of carbonate matrix in the Schaumkalk that cannot be explained by burial diagenesis alone.

Karst landscapes provide vital services to humans, but the rates and controls of carbonate rock weathering and erosion, in sum termed denudation (D), are not well constrained. To address this gap, we adapted a framework using the cosmogenic meteoric¹⁰Be/⁹Be ratio. Radioactive ¹⁰Beryllium (Be) traces atmospheric input, while stable ⁹Be indicates weathering. This method can be performed on water, soil, sediment, or travertine, does not rely on specific minerals like quartz or calcite, and D can be partitioned into weathering and erosion rates. Extensive testing showed that the method is suitable for both pure limestone and mixed carbonate-siliciclastic settings.

We employed the ¹⁰Be/⁹Be method to the temperate limestone-rich French Jura Mountains, obtaining rates of catchment-wide D (from sediment) and point source D (from soil). Average D range from 300-500 t/km²/yr, with denudation primarily driven by weathering (W/D>0.92), and a non-negligible contribution to weathering from deep (below soil). These rates are consistent within a factor of two when compared to decadal-scale D derived from combined suspended and dissolved fluxes, underscoring the substantial potential of this method for Earth surface research in karst landscapes.

On a global scale, our data provide the first cosmogenic nuclide‐based denudation rates for the mean annual precipitation (MAP) range of 1,200–1,700 mm/yr under dense vegetation cover. At similar MAPs, our rates exceed previous rates derived from e.g. in situ ³⁶Cl measured in calcite from less vegetated sites. Altogether, global D patterns suggest a control of precipitation on D and/or W that may be modulated by vegetation cover.

Ikaite (CaCO₃·6H₂O) is a metastable mineral, occurs in cold regions of Earth, and readily transforms into more stable minerals such as calcite or vaterite. The conditions for formation and transformation of ikaite, however, are insufficiently explored. Important implications of the occurrence of ikaite, such as its potentially large effect on the polar carbon cycle or its applicability as low temperature proxy, therefore, cannot be assessed adequately.

To contribute to a better understanding, three key aspects of ikaite formation and transformation were investigated: i) the effect of the presence of mineral surfaces on ikaite nucleation, ii) the effect of increasing temperatures (T = 0-20 °C) on ikaite stability in aqueous solutions, iii) the effect of phosphate on ikaite growth.

Our experiments revealed that mineral surfaces can significantly promote nucleation of ikaite vs. calcite and vaterite. Furthermore, the persistence of ikaite is reduced by the temperature dependent increase of vaterite and calcite precipitation. Inhibition of these minerals, therefore, is inevitable for an increased ikaite persistence. No significant incorporation of aqueous phosphate (an effective calcite inhibitor) into growing ikaite was detected.

The findings agree with an ikaite formation mechanism via an assembling of aqueous CaCO₃ complexes. Ikaite nucleation per se is possible at increased temperatures, if solution supersaturation and inhibitor concentration are sufficiently high. In natural environments, however, occurrence of such conditions becomes less likely with increasing temperature.