Geosciences Paris Sud

International audience

Abstract As larger particles require more energy for transport, grain size is a key indicator for the magnitude of geohazards and other depositional processes. However, sample size requirements and laborious laboratory procedures limit our ability to extract this information at human‐relevant (years to decades) timescales. The emergence of non‐destructive and high‐resolution core scanning techniques offer a solution to upscale measurements by mapping grain size‐sensitive parameters at μm instead of cm scales. These include X‐Ray Fluorescence (XRF)—tracking variations in elemental geochemistry that are often linked to mineral grain size, and Computed Tomography (CT)—capturing differences in density that control size sorting during deposition. Recent work demonstrates that these relations can be captured with linear regression fits, thus paving the way for predictive grain‐size modeling approaches. Here, we expand on this work by assessing the potential of CT greyscale density data as a predictor. To do so, we developed a controlled experiment using synthetic sediment records (phantoms)—varying grain size, geochemistry, as well as two major sources of noise: organic and water content. Our results show that CT data can be used as a sole predictor, especially in cases where a homogenous mineralogy limits the use of XRF geochemistry‐based approaches. Applications on natural sediment cores containing reworked volcaniclastics confirm these findings under real‐world conditions, and highlight the complementarity of CT and XRF data. Finally, we present a code‐free web‐based workflow to make the presented grain‐size prediction approach readily accessible to the wider geoscience community.

Enceladus is one of the most intriguing icy moons of the solar system, exhibiting active cryovolcanism and continuous plume activity at its south pole [1]. The surface in this region is geologically young and frequently resurfaced by plume fallout and tectonic activity [2]. While numerous studies have mapped compositional variations using Cassini-VIMS data [3,4], few have quantified the microphysical properties of water ice—including grain size, crystallinity and surface roughness—over the full VIMS spectral range. Understanding these parameters is critical for interpreting the thermal and geological evolution of the surface and for tracing exchanges between the subsurface ocean and the surface.Here, we present a comprehensive study in which we fit VIMS reflectance spectra across the 1.0–5 µm range to estimate key microphysical properties of Enceladus's surface. We apply a Hapke-based radiative transfer model [5,6] in a Bayesian Markov Chain Monte Carlo (MCMC) framework [7] to retrieve the posterior distributions of ice crystallinity, grain size and surface roughness for selected observations. We use the optical constants of Mastrapa et al. [8], which provide laboratory measurements of crystalline and amorphous water ice at temperatures from 20 to 150 K. The full-spectrum inversion enables robust fitting of broad and narrow features, including the diagnostic absorption band at 1.65 µm, which appears only in crystalline ice [9]. This band is not used as an isolated fitting constraint but is reproduced by the model if crystalline ice is present at the appropriate temperature. We find that its shape and central wavelength are well matched in many spectra, especially in the South terrains, where the inversion yields low temperatures (

The physical processes involved in the transition of a planet from a liquid magma ocean (‘MO’) to a convective solid mantle are still debated. Highly turbulent penetrative convection prevails when the MO is still liquid on the surface. But as the MO cools down in interaction with its atmosphere its upper surface thermal boundary layer (‘TBL’) will eventually first becomes partially molten, then solid. As soon as the rheological front, with a melt content less than 40%, reaches the surface, the upper part of the TBL could behave like a solid skin. This has led to suggest that MO cooling would always end up in a stagnant lid regime of convection, whereby mantle convection proceeds under a surface plate that remains stagnant, limiting the heat and volatile transfers to the atmosphere. This would help retaining water within the mantle, but would render the onset of subduction and plate tectonics more difficult (how to break a thick lid?). On the other hand, another family of cooling MO models suggests that the numerous impacts during the early stages of a planet would break repeatedly any floating skin on the MO, so that it would be difficult to establish a stagnant lid regime.Laboratory experiments of penetrative convection-evaporation using visco-elasto-plastic colloidal dispersions (Di Giuseppe et al, 2012) suggest that two other phenomena could also be at play to destabilize the first solid skin: (1) melt flowing through a porous skin would generate in-plane compression that could generate buckling, exceed the yield strength of the material and initiate subduction; (2) rapid thermal contraction due to large temperature gradients across the skin could generate stresses large enough to exceed the yield strength and initiate subduction. We use these insights to explore the growth and stability of the TBL at the surface of a cooling magma ocean which interacts with a H₂O-CO₂ atmosphere. Our results indicate that, while on Earth, thermal stresses due to cooling could easily exceed the early lithosphere yield strength, this might not have been the case on Venus. On Venus, this process is strongly influenced by atmospheric conditions. For a high albedo of 0.5, the upper TBL could yield as early as 1.5 million years after cooling begins, similar to Earth, and therefore the MO stage would end up directly into a convective regime with repeated breaking and foundering of the lithosphere (e.g. subduction). But for an albedo of 0.3, thermal stresses never overcome the TBL’s yield strength. In such a scenario, the MO stage would end in a stagnant lid regime, which could act as a barrier to heat transfer and potentially filter degassing.

Abstract The Col de Dio palaeontological site yields the first swimming trails of fishes from the continental Permian deposits of the Lodève Basin (southern France). Several morphologies of trails were distinguished, each of them including one, two or four thin, horizontal and sinusoidal grooves/ridges. Trails are ascribed to Undichna bina, Undichna britannica and Undichna unisulca. They co-occur with arthropod body remains (e.g. “conchostracans”, triopsids), invertebrate traces (cf. Cochlichnus, Diplichnites, and Diplopodichnus) and swimming tracks of tetrapods (Characichnos). The sedimentological analysis conducted at the Col de Dio tracksite indicates the presence of two orders of stratigraphic sequences that rule the alternation of humid and arid cycles in an endorheic terminal mud-dominated floodplain and playa-lake environment. Although body fossils of fishes remain unknown in the Salagou Formation, the presence of Undichna shows that ichthyofauna was an abundant component of the palaeoecosystem related to this formation. The diverse shapes of Undichna suggest a great diversity of fish morphologies and/or behaviours. This new discovery of fish trails co-occurring with tetrapod and arthropod tracks complements other coeval French tracksites, suggesting that such ichnofaunas were common in Permian terrestrial ecosystems from the Eastern Pangean intertropical zone.

Ices are widespread across the solar system, present on the surfaces of nearly all planets and moons. Icy moons, in particular, are of high interest due to their potential habitability, as they can harbor liquid water oceans beneath their icy crust making them prime targets for the upcoming JUICE (ESA) and Europa Clipper (NASA) missions. While space observations suggest that these surfaces are made of granular water ice, the fine-scale structure — such as the size, shape, and distribution of ice grains — remains poorly understood. This raises the question: What is the current state of the ice microstructure on these surfaces?Various interdependant surface processes interact over large timescales and together alter the microstructure of the icy surfaces. To adress this, we have developed an innovative multiphysics simulation tool, LunaIcy, which integrates the main physics that affect Europa’s ice microstructure and simulates their interactions. This model has already provided valuable insights into Europa's surface, helping to estimate the thermal dynamics, ice cohesiveness/sintering, and crystallinity.Space observations will greatly benefit from such modeling advancements, which will be essential for a better interpretation of data from the upcoming missions. Multiple other applications for different icy bodies are underway, as we expect that the study of planetary surfaces, much like General Circulation Models for climate science, can greatly benefit from such multiphysical approaches.

Abstract Fjords are increasingly recognized as hotspots for organic carbon (OC) burial. The OC buried in fjords is of both marine and terrestrial origin, with a predominance of terrestrial OC in fjords worldwide. The proportions of marine and terrestrial OC in fjords are traditionally calculated using end‐member modeling based on δ13C and/or N/C. However, characterizing the terrestrial end‐member remains a challenge, with authors inconsistently using measurements obtained on land plants, soils, and/or river sediments. Here, we analyzed the TOC, δ13C, and N/C composition of soil samples, suspended river sediments, and bulk and grain‐size fractions of river sediments from the main rivers discharging into the Patagonian fjords (44–48°S), to identify the processes that affect the biogeochemistry of the terrestrial organic matter reaching fjords via rivers. Radiocarbon measurements indicate that Patagonian rivers contain 0.18% petrogenic OC and variable concentrations of biospheric OC. Despite soil δ13C significantly decreasing with precipitation, δ13C in river sediments remains relatively stable around −27‰. In contrast, N/C in river sediments is highly variable, mostly due to a high contribution of petrogenic nitrogen in glacier‐fed rivers. Furthermore, N/C varies significantly with sediment grain size, making it virtually impossible to define a fixed N/C value to represent the terrestrial end‐member. By comparison, grain size has a limited influence on δ13C. Overall, our results support the use of riverine δ13C to define terrestrial OC in mixing models, regardless of the presence of glaciers in the watershed, and they suggest that the fraction of terrestrial OC buried in fjord sediments may have been underestimated.