Valérie Chavagnac

Continental duricrusts cover >10% of Earth's land surface and play a fundamental role in cratonic landscapes. Yet, competing models for their formation and diagenetic evolution have been difficult to test systematically. This work synthesizes current understanding of formation mechanisms through an integrated review and a new high-resolution revision of case studies from the Kalahari Basin, comprising 10-20% of the world's known duricrust deposits. By combining petrographic, geochemical, and chronological constraints, we evaluate end-member models and provide time constraints for the African Surface evolution in the Kalahari while developing a unified framework for duricrust evolution.Case studies from contrasting structural settings across the Okavango Rift Zone reveal diagenetic pathways spanning open-system meteoric precipitation, evaporitic concentration in closed basins, and repeated mineral replacement during climatic transitions. Chronological constraints demonstrate episodic formation and major unconformities linked to tectonic reorganization, highlighting the strong diachroneity of duricrust development across structural contexts.These observations show that neither purely in situ laterization nor simple hydrological solute-transport models adequately explain duricrust formation. Instead, duricrusts emerge from a dynamic continuum in which weathering on structural highs supplies solutes to adjacent basins above and underground, followed by repeated dissolution, reprecipitation, and progressive cementation into erosion-resistant surfaces. This diagenetic recycling operates much faster (103-104 years) than primary weathering (10⁵-10⁶ years), explaining how substantial transformation occurs during climatic transitions. The episodic reworking maintains chemical reactivity within apparently stable landscapes, demonstrating that cratonic landscape resilience arises not from long-term stability, but from continuous internal reorganization recorded in duricrust systems.

Models describing banded iron formation deposition beneath oxygenated Paleoproterozoic marine shelf waters assume that microaerophilic iron-oxidizing microorganisms were significant contributors. However, direct evidence for this mechanism is lacking, and it remains unclear how associated primary organic biomass and biogenic iron oxyhydroxides transformed into organic carbon-poor iron oxide/carbonate-rich bands in banded iron formations. Here we explore in situ modern Zetaproteobacterial iron-oxidizing mats thriving in the Lucky Strike hydrothermal vent field to develop an empirical model that explains the potential contribution of iron-based metabolisms to deposition of rocks resembling banded iron formations. Petrographic, mineralogical, and geochemical data suggest that abundant production of Zetaproteobacterial stalks nucleates and drives spontaneous biotic/abiotic ferrihydrite precipitation in the mats. Subsequent early diagenetic anaerobic microbial oxidation of dead biomass as the mats aged, coupled to microbial reduction of iron oxyhydroxides/oxides, generates ferrous carbonates and mixed valence magnetite. Collectively, this forms crystalline iron-rich organic carbon-lean ¹³C-depleted layers reminiscent of Precambrian banded iron formation microbands.

During the TONGA cruise (2019), seawater samples were collected to assess the effect of volcanic eruption versus submarine hydrothermal system on the water column. For this purpose, two locations were investigated, the first one located directly under the influence of the New Late’iki island (eruption in October 2019), and the second one showing ongoing submarine hydrothermal activity. At both locations, the total strontium (TSr) and lithium (TLi) concentrations vary between 94.4 and 152.3 µmol/L and 13.2 and 203.5 µmol/L, respectively. When combined, TSr and TLi concentrations of all samples in the water column are higher than those of the oligotrophic water. Both volcanic eruption and submarine hydrothermal activity (e.g. volcanic ashes, particles, gas condensate) can deliver substantial amount of TSr and TLi to the water column. The distribution of TSr versus TLi evidences linear trends either with a negative or positive slope. The negative correlation is observed in the water column at both sites, directly under the influence of the eruption and in the vicinity of the volcano with hydrothermal activity. The positive TSr versus TLi correlation is observed at site under submarine hydrothermal influence and is in line with black smokers related hydrothermal plumes. The ⁸⁷Sr/⁸⁶Sr ratios vary between 0.709147 and 0.709210 and δ⁷Li values vary between +10.1 and +37.6 ‰. While 92% of the measured ⁸⁷Sr/⁸⁶Sr ratios are in line with the mean value of oligotrophic waters, once combined with the δ⁷Li values, only 20% of them remains within this field. The wide range of δ⁷Li values decreases from sea-surface down to ~140 mbsl, before increasing at greater depth, while defining different linear trend according to the dissolved inorganic carbon concentrations. The variability of δ⁷Li values reflect hydrothermal contribution, mineral–seawater interaction and potentially biology–environment interaction. In the particular geological setting of the study, where both hydrothermal and volcanic activities were at play, disentangling both contributions on water column implies a combined use of elemental and isotopic signatures of Sr and Li tracers.

About 80% of Earth volcanic activity occurs underwater, releasing deep carbon to submarine environments and impacting Earth’s climate over geological timescales. The CO ₂ emitted during submarine eruptions and/or hydrothermal degassing creates local ocean acidification, affecting the seawater carbonate equilibrium and oceanic ecosystems at large regional scales. Here, we report for the first time the existence of a major CO ₂ hydrates field at the seafloor offshore Mayotte Island (Indian Ocean) associated with liquid CO ₂ venting, following the submarine eruption that occurred in 2018. Using detailed acoustic surveys and in situ Raman spectroscopy, we reveal multiple hydrate mounds and seep zones distributed over an area of 0.06 km². We show that the gas seeps are mainly composed of CO ₂ , with minor contributions of CH ₄ and H ₂ , with noble gas ratios and stable and radio-carbon isotopes clearly demonstrating their magmatic origin. Estimates of the CO ₂ emitted over the entire area represent about 0.5% of the global magmatic carbon flux. Our discovery also suggests that CO ₂ hydrates may potentially be stable at the seafloor at the right pressure-temperature conditions, bringing new prospects into CO ₂ sequestration and decarbonization pathways in the ocean, in particular regarding kinetics of hydrates dissolution and environmental impacts.

We report on a 3 years monitoring experiment of low to medium temperature diffuse venting at two vent sites (Tour Eiffel and White Castle) of the Lucky Strike, black smoker‐type hydrothermal field, Mid‐Atlantic Ridge. Diffuse vents account for a large part of the energy flux of mid‐ocean ridges hydrothermal fields and provide key habitats for the hydrothermal fauna. We document the time and space variability of diffuse venting temperature and chemistry, describe the effect of tidal loading and currents and discuss the extent of mixing, cooling of black smoker fluids, heating of entrained seawater and anhydrite precipitation/dissolution in the substratum. We emphasize the role of a thin (80°C) and contain >10% of the hot endmember fluid component, over distances of up to 25 m from the black smokers. These results provide a geologically integrated framework in which to study site‐scale, near seafloor hydrothermal circulation and associated vent habitats at Lucky Strike and other black smoker‐type hydrothermal fields. They suggest diffuse heat fluxes in the upper range of previously published estimates at the two studied Lucky Strike hydrothermal vent sites.

Methylmercury is a potent toxin threatening the global population mainly through the consumption of marine fish. Hydrothermal venting directly delivers natural mercury to the ocean, yet its global flux remains poorly constrained. To determine the extent to which anthropogenic inputs have increased oceanic mercury levels, it is crucial to estimate natural mercury levels. Here we combine observations of vent fluids, plume waters, seawater and rock samples to quantify the release of mercury from the Trans-Atlantic Geotraverse hydrothermal vent at the Mid-Atlantic Ridge. The majority (67–95%) of the mercury enriched in the vent fluids (4,966 ± 497 pmol l⁻¹) is rapidly diluted to reach background seawater levels (0.80 pmol l⁻¹). A small Hg fraction (2.6–10%) is scavenged to the Trans-Atlantic Geotraverse mound rocks. Scaling up our findings and previous work, we propose a mercury flux estimate of 1.5–64.7 t per year from mid-ocean ridges. This hydrothermal flux is small in comparison to anthropogenic inputs. This suggests that most of the mercury present in the ocean must be of anthropogenic origin and that the implementation of emissions reduction measures outlined in the Minamata Convention could effectively reduce mercury levels in the global ocean and subsequently in marine fish.

Iron (Fe) is an essential micronutrient for diazotrophs, which are abundant in the Western Tropical South Pacific Ocean (WTSP). Their success depends on the numerous trace metals, particularly Fe, released from shallow hydrothermal vents along the Tonga Arc. This study aimed to explore the spatio‐temporal impact of hydrothermal fluids on particulate trace metal concentrations and biological activity. To identify the composition of sinking particles across a wide area of the WTSP, we deployed sediment traps at various depths, both close and further west of the Tonga Arc. Seafloor sediments were cored at these deployment sites, including at a remote location in the South Pacific Gyre. The sinking particles were composed of a large amount of biological material (up to 88 mg d ⁻¹ ), indicative of the high productivity of the region. A significant portion of this material (∼21 ± 12 wt.%) was lithogenic of hydrothermal origin, as revealed through Al‐Fe‐Mn tracing. The sinking material showed similar patterns between lithogenic and biogenic fractions, indicating that hydrothermal input within the photic layer triggered surface production. A hydrothermal fingerprint was suggested in the sediments due to the high sedimentation rates (>47 cm kyr ⁻¹ ) and the presence of large, heterogeneous, metal‐rich particles. The presence of nearby active deep hydrothermal sources was suspected near the Lau Ridge due to the large particle size (1–976 μm) and the significant excess of Fe and Mn (2–20 wt.%). Overall, this study revealed that hydrothermal sources have a significant influence on the biogeochemical signature of particles in the region.

Iron (Fe) is an essential micronutrient for phytoplankton, particularly diazotrophs, which are abundant in the Western Tropical South Pacific Ocean (WTSP). Their success depends on the numerous trace metals, particularly iron, released from shallow hydrothermal vents along the Tonga Arc. This study aimed to explore the impact of hydrothermal fluids on particulate trace metal concentrations and biological activity. To identify the composition of sinking particles across a wide area of the WTSP, we deployed sediment traps at various depths, both close and further west of the Tonga Arc. Seafloor sediments were cored at these deployment sites, including at a remote location in the South Pacific Gyre. The sinking particles were composed of a large amount of biological material, indicative of the high productivity of the Lau Basin. A significant portion of this material was lithogenic of hydrothermal origin, as revealed through Al-Fe-Mn tracing. The sinking material showed similar patterns between lithogenic and biogenic fractions, indicating that hydrothermal input within the photic layer triggered surface production. A hydrothermal fingerprint was suggested in the sediments due to the high sedimentation rates and the presence of large, heterogeneous, trace metal-rich particles. The presence of nearby active deep hydrothermal sources was suspected near the Lau Ridge due to the large particle size and the significant enrichment of Fe and Mn. Overall, this study revealed that deep and shallow hydrothermal sources along with submarine volcanism have a significant influence on the biogeochemical signature of particles in the Lau Basin at large spatial and temporal scales.