Magali Ader

Based on an analysis of available data combined with new observations, we propose a steady-state model of the freshwater lens of Petite Terre island in Mayotte. The hydrological balance of the airport's meteorological station allows us to estimate the annual recharge using Turc formula. This recharge is on the order of two million cubic meters per year, a portion of which could be extracted for the drinking water supply. Simple modeling under the Dupuit Boussinesq approximation enables to characterise the form of the lens and estimate its volume. We show that the model accords with available data. These results suggest the existence of a significant resource and call for the implementation of further monitoring and analysis, coupled with a review of the groundwater extraction strategy by public authorities in a socially and politically tense island context.

The growing use of hydroacoustics to study magmatic degassing underwater is providing increasingly powerful insights in topics as varied hazard monitoring and microbial evolution. This talk will discuss the use of marine acoustics to study magmatic degassing and present two case studies, 1) Dziani Lake (Mayotte) a Precambrian analogue where variations in magmatic degassing may be driving microbial adaptation , and 2) Lake Kivu (Rwanda) where deadly limnic eruptions may be triggered by newly discovered gas blowouts. Hydrophones record the sound emitted by gas bubbles at the moment of their release into the water column. The frequency and power of these signals tells us about their number and size. By applying Passive Acoustic Flux Inversion techniques, bubble oscillation spectra can be inverted to passively quantify gas flux continuously, regardless of water quality or depth. This approach enables long-term monitoring of degassing dynamics that are inaccessible using traditional geochemical or visual methods.Dziani Dzaha Lake undergoes persistent magmatic degassing and is considered one of the best modern analogues for Precambrian environments. A better understanding of what drives adaption in the microbial population of Dziani provides a better understanding of what drove Precambrian evolution. A month long observation reveals strong temporal variability in gas flux, with rapid increases potentially preceding local seismic activity. These observations provide the first quantitative constraints on magmatic gas input to the lake and may suggest a tentative link between volcanic activity and early life.At Lake Kivu, a hydrophone deployed during the 2021 Nyiragongo dyke intrusion provided the first direct acoustic observations of lakebed degassing. Analysis reveals highly variable degassing behaviour, including pulsed bubble releases, long-period signals associated with subsurface gas migration, and previously undocumented explosive gas blowout events on the lakebed. These high-energy events have the potential to trigger limnic eruption but were not detected by the regional land-based seismic network, highlighting critical gaps in current monitoring strategies. Although no limnic eruption was triggered, the observations demonstrate that potentially hazardous degassing processes can occur silently and episodically, challenging assumptions of steady gas input used in existing limnic hazard forecasts.Together, these case studies demonstrate the growing potential of hydroacoustics to study magamtic degassing and will hopefully inspire future studies incorporating the use of hydrophones to study magmatic degassing.

Over geological times, the evolution of carbon isotope composition of carbonates (δ13Ccarb) in the sedimentary record is punctuated by numerous positive isotopic excursions (CIEs), which reflect significant perturbations of the carbon cycle on Earth surface environments. Such isotopic events are mainly interpreted as a consequence of an increase of organic carbon burial in sediments. However, the significant spatial and temporal variability observed in many Proterozoic sedimentary successions recording positive CIEs still challenge this postulate. Among others, the potential influence of methanogenesis has been raised to explain at least a part of the variability observed during CIEs.The Dziani Dzaha is a shallow tropical volcanic crater lake located on the Petite Terre Island of Mayotte (Comoros Archipelago, Indian Ocean). Its water most likely originate from the nearby ocean through bedrock seepage thanks to the fracturation associated with the phreatomagmatic eruption at that formed the crater 7 to 4 Kyr ago. Based on numerous analogies, this atypical modern lacustrine system was considered as an analogue of Proterozoic environments, and more specifically of those having recorded strongly positive δ13C values, such as for example the Lomagundi-Jatuli event. A previous study demonstrated the significant role of methane in the Dziani Dzaha carbon cycle through an intense organic matter degradation by methanogenesis associated with methane degassing into the atmosphere. Here, in order to investigate the processes responsible for the onset of this methanogenic activity, we present coupled C and S isotope records in a sediment core of the lake.Based on geochemical and sedimentological evidences, four different units have been identified in the sediment core. From the bottom to the top, carbon and sulfur isotopic signatures in the first unit are similar to modern oceanic values (δ13Corg ~ -25‰, δ13Ccarb ~ 0‰ and δ34Spy ~ -20‰), which is consistent with a marine origin of the lake water. In the second unit, the δ34Spy increases progressively from -20‰ to -10‰ while the δ13C of organic and inorganic carbon remain constant, which is consistent with a progressive consumption of the sulfate pool through the degradation of organic matter by sulfatoreduction in a restricted environment. The δ34Spy shifts sharply to 35‰ at the transition with the third unit where it remains constant up to the top, while both organic and inorganic carbon isotopic signatures increase progressively from -25‰ to -14‰ and from 0‰ to almost 20‰, respectively. This turning point most likely results from a depletion of the initial sulfate pool to a point allowing part of the organic matter to be remineralized through methanogenesis with a degassing of methane into the atmosphere. These results highlight the potential of C-S-isotope coupling to identify a potential impact of methanogenesis on the carbon-isotope signatures observed in the sedimentary record through geological times.

The nitrogen isotopic composition of sedimentary rocks (δ¹⁵N) can trace redox-dependent biological pathways and early Earth oxygenation^1,2. However, there is no substantial change in the sedimentary δ¹⁵N record across the Great Oxidation Event about 2.45 billion years ago (Ga)³, a prominent redox change. This argues for a temporal decoupling between the emergence of the first oxygen-based oxidative pathways of the nitrogen cycle and the accumulation of atmospheric oxygen after 2.45 Ga (ref. ³). The transition between both states shows strongly positive δ¹⁵N values (10–50‰) in rocks deposited between 2.8 Ga and 2.6 Ga, but their origin and spatial extent remain uncertain^4,5. Here we report strongly positive δ¹⁵N values (>30‰) in the 2.68-Gyr-old shallow to deep marine sedimentary deposit of the Serra Sul Formation⁶, Amazonian Craton, Brazil. Our findings are best explained by regionally variable extents of ammonium oxidation to N₂ or N₂O tied to a cryptic oxygen cycle, implying that oxygenic photosynthesis was operating at 2.7 Ga. Molecular oxygen production probably shifted the redox potential so that an intermediate N cycle based on ammonium oxidation developed before nitrate accumulation in surface waters. We propose to name this period, when strongly positive nitrogen isotopic compositions are superimposed on the usual range of Precambrian δ¹⁵N values, the Nitrogen Isotope Event. We suggest that it marks the earliest steps of the biogeochemical reorganizations that led to the Great Oxidation Event.

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ¹⁵N values found in well‐preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much‐needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ¹⁵N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non‐quantitative oxidation of ammonium via the Feammox pathway, a metabolic co‐cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ¹⁵N signatures in Archean sediments.

Editorial: Refining the Interpretation of Nitrogen Isotopes in Deep Time Systems

Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how modern microbialites form in relation with the bio-physico-chemical conditions of their environment. In this study, we investigated the formation of modern carbonate microbialites in the hydrothermal system of La Salsa in Laguna Pastos Grandes (Bolivia), which spans a wide range of physicochemical conditions and associated microbial communities. By combining dissolved inorganic carbon (DIC) isotope mass balance modeling, analysis of carbonates solubility diagram, and imaging of the microorganisms–mineral assemblages within microbial mats, we found that several modes of carbonate precipitation dominate in distinct portions of the hydrothermal system. (1) In high-[DIC] waters, undersaturated to slightly saturated with respect to calcite, cyanobacterial calcification is promoted by CO₂ degassing and photosynthetic activity within the microbial mats. (2) In alkaline waters undergoing sustained evaporation, the precipitation of an amorphous calcium carbonate phase seems to control the water a(Ca²⁺)/a(CO₃²⁻) ratio and to serve as a precursor to micritic calcite formation in microbial mats. (3) In saline ephemeral ponds, where the carbonate precipitation is the highest, calcite precipitation probably occurs through a different pathway, leading to a different calcite texture, i.e., aggregates of rhombohedral crystals.

Since May 2018, an unexpected long and intense seismic crisis started offshore Mayotte (Indian Ocean, France). This ongoing seismic sequence is associated with the birth of a newly discovered submarine volcano 50 km east of Petite Terre. Here, we investigate the indirect impact of this crisis on the stability of an atypical ecosystem located in Mayotte, the Dziani Dzaha Lake. This lacustrine system presented physical, chemical and biogeochemical characteristics that were distinct from those classically observed in modern lakes or seawater, e.g. high salinity (up to 70 psu), lack of nitrate, sulfate content below 3 mM, and permanent water column anoxia below ca. 1.5 m depth (2012–2017 period). Based on three surveys conducted in 2020 and 2021, we documented an episode of water column oxygenation, a significant pH decrease and an impressive change in the carbon isotope signatures. These preliminary data suggest that the functioning of biogeochemical cycles in the Dziani Dzaha Lake is impacted by increased CO ₂ inputs and the changes in the lake mixing dynamics, which is an indirect consequence of the ongoing seismo-volcanic crisis.

The Comoros archipelago is an active geodynamic region located in the Mozambique Channel between East continental Africa and Madagascar. The archipelago results from intra-plate volcanism, the most recent eruptions having occurred on the youngest island of Grande Comore and on the oldest one of Mayotte. Since 2018, the eastern submarine flank of Mayotte has been the site of one of the largest recent eruptive events on Earth in terms of erupted lava volume. On land, the most recent volcanic activity occurred in Holocene on the eastern side of Mayotte, corresponding to the small Petite Terre Island, where two main and persistent gas seep areas are present (Airport Beach, namely BAS, and Dziani Dzaha intracrateric lake). The large submarine eruption at the feet of Mayotte (50 km offshore; 3.5 km b.s.l.) is associated with deep (mantle level) seismic activity closer to the coast (5–15 km offshore) possibly corresponding to a single and large magmatic plumbing system. Our study aims at characterizing the chemical and isotopic composition of gas seeps on land and assesses their potential link with the magmatic plumbing system feeding the Mayotte volcanic ridge and the recent submarine activity. Data from bubbling gases collected between 2018 and 2021 are discussed and compared with older datasets acquired between 2005 and 2016 from different research teams. The relation between 3 He / 4 He and δ 13 C - CO 2 shows a clear magmatic origin for Mayotte bubbling gases, while the variable proportions and isotopic signature of CH 4 is related to the occurrence of both biogenic and abiogenic sources of methane. Our new dataset points to a time-decreasing influence of the recent seismo-volcanic activity at Mayotte on the composition of hydrothermal fluids on land, whose equilibrium temperature steadily decreases since 2018. The increased knowledge on the gas-geochemistry at Mayotte makes the results of this work of potential support for volcanic and environmental monitoring programs.