Hélène Angot

Fallout radionuclide (FRN) archived in soils are key tracers of soil redistribution, sediment dynamics, and associated carbon fluxes, yet their global application is limited by poorly constrained baselines in the Southern Hemisphere. Here, we present the first hemisphere-scale baseline and uncertainty maps of ¹³⁷ Cs and ^239+240 Pu soil inventories. Predictions exhibited lower spatial variability and relative uncertaintyfor ¹³⁷ Cs across most landscapes, whereas ^239+240 Pu showed greater spatial variability and larger relative uncertainty. The reconstructed inventories revealed strong latitudinal and topographic gradients, with the highest predicted fallout occurring in tropical and mountainous regions. We further show, under a standardized hypothetical 50% inventory-loss scenario, that uncertainty in low-inventory regions can propagate nonlinearly into soil erosion and carbon-budget estimates, producing carbon budget biases of up to 19.8 t C ha ⁻¹ . These findings provide a new benchmark for Southern Hemisphere fallout radionuclide applications and a scalable framework for integrating spatial uncertainty into sediment tracing, soil degradation assessment, and carbon cycle studies.

Mercury (Hg) is a highly toxic global pollutant that is transported throughout the atmosphere and transferred to terrestrial ecosystems through vegetation uptake and atmospheric wet and dry deposition. Owing to its strong association with organic carbon (OC) and long residence time, soils represent the largest Hg reservoirs. Environmental changes can, however, alter Hg stocks and fluxes, potentially enhancing Hg emissions to the atmosphere or export to riverine systems. Existing global models have yet to fully capture the dynamics of dissolved Hg and its transport through terrestrial hydrological pathways, as well as Hg storage and remobilization in deep soil layers, limiting their ability to represent terrestrial Hg responses to environmental change. Here, we present the first version of ORCHIDEE-Hg (ORCHIDEE-Hg v1.0), a global land-surface model of the terrestrial Hg cycle developed within the well-established ORCHIDEE framework. The model uniquely integrates explicit representations of dissolved carbon species and vertical discretization of soil across 15 plant functional types. ORCHIDEE-Hg represents the terrestrial Hg cycle, from atmospheric wet and dry deposition and vegetation uptake to soil storage, dissolved Hg dynamics, and atmospheric re-emissions. We evaluate ORCHIDEE-Hg against recent observations of vegetation Hg concentrations, litterfall Hg fluxes, and soil stocks across two temperate regions. The model successfully reproduces observed median Hg concentrations, stocks, and fluxes within and between key compartments. This evaluation provides a robust foundation for investigating the controls of terrestrial Hg cycling. A companion study will assess model sensitivity to key parameters and processes, providing a detailed understanding of model behavior before using ORCHIDEE-Hg to investigate changes in soil Hg stocks and fluxes under environmental perturbations, including climate change, land-use and land-cover change, and changes in atmospheric deposition.

In the central Arctic, warm and moist air intrusions (WAMIs) are increasingly prevalent during winter and spring, significantly affecting the near-surface energy budget. Here, we combine in situ observations from the 2019–2020 Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition with model simulations to investigate the role of WAMIs in transporting pollutants and enhancing black carbon (BC) wet deposition. Several WAMI episodes were associated with elevated ambient BC concentrations, and the model indicates that these events lead to an average of approximately 40% increase in BC wet deposition compared to typical winter–spring conditions. To assess the potential implications of this deposition for the cryosphere, we performed a set of sensitivity experiments using the SURFEX/ISBA-Crocus multilayer snowpack model. These experiments allowed isolating the impact of enhanced BC deposition, indicating that WAMI-driven inputs can increase solar energy absorption by 3–5 W/m2 on average and accelerate snowmelt rates by more than 20% during the following summer. These simulations offer process-based insights on the influence of episodic pollution transport events on snow and climate feedbacks. These findings underscore the need for targeted and spatially distributed measurements of BC in central Arctic snowpacks—away from potential local contamination sources (e.g., ship exhaust)—to better constrain the role of WAMIs in Arctic snow and climate processes.

Total ozone (O ₃ ) reactivity monitors (TORMs) were used to investigate biogenic volatile organic compound (BVOC) emissions from several ecosystems. Measurements were conducted in temperate deciduous North American forests, during the CABINEX and SOAS campaigns, on tundra vegetation emissions at Toolik lake field station (TFS) in Alaska, and in a boreal fen in Finland. Deciduous tree emissions produced up to six times higher O ₃ loss than emissions from conifers. Emissions from the fen resulted in maximum O ₃ loss values comparable to deciduous tree emissions, whereas emissions from Arctic tundra ground vegetation generated the smallest signals. Across all ecosystems, the O ₃ loss expected from the reactions with identified BVOCs was consistently lower than the directly measured O ₃ loss rate measured with TORM during midday emission peaks. On average, calculated losses from identified BVOCs accounted for 10–60% of the directly measured O ₃ loss, depending on vegetation type and time of day. Measurement uncertainties explain only part of this discrepancy, leaving a substantial fraction of the observed signal unaccounted for. Additional uncertainty may arise from oxidation chemistry not captured by the BVOC‐O ₃ reaction rate constants. These findings highlight the need for further research on O ₃ ‐BVOC reactivity across ecosystems to better characterize emissions, their impacts on atmospheric chemistry, and especially their chemical diversity that is rarely captured exhaustively due to limitations of analytical methods.

High-fidelity models are essential to constrain the role of the atmosphere in the global cycle of microplastics (MPs), including emissions, atmospheric transport, and deposition. Inconsistencies in the representation of MP particle size distribution (PSD) has blocked progress in the ability to understand the atmospheric lifecycle of MP. Observations report MP number concentrations and deposition across varying size ranges, while models simulate discrete and often small MP size ranges. As a result, data comparisons generally fail to account for mismatches in PSDs. In this study, we address these challenges using a top-down framework by size-harmonizing observational and modeled data and integrating observed PSDs in MP emissions. Our results reveal global atmospheric MP emissions of about 12 Gg yr ^− 1 over the 0.1–100 µm particle size range, a 27-fold reduction compared to previous global GEOS-Chem estimates due to physically consistent comparisons between observations and the model. Land emissions (8.7 Gg yr ^− 1 ) are higher than oceanic emissions (3.5 Gg yr ^− 1 ), but both are lower than prior top-down estimates. This work bridges the gap between top-down and bottom-up MP emissions estimates, converging on substantially lower MP emissions from both oceans and land.

Climatic changes in the physical environment modulate biogeochemical cycles, biodiversity, and trophic interactions in the Central Arctic Ocean (CAO). Physical processes and sea-ice conditions are highly seasonal in the CAO and dependent on interactions that occur throughout the evolution of the upper ocean–sea ice–lower atmosphere system. Understanding these seasonal interactions is critical to comprehending and predicting the long-term trends as the CAO moves towards ice-free summers and to informing future policy decisions at the core of ongoing discussions concerning the CAO Fisheries agreement, for example, at the Arctic Council and International Council for Exploration of the Sea working group on the CAO. Here, we review current knowledge of the physical environment, biogeochemical cycles, and biodiversity in the waters of the CAO, identify emerging research questions, and introduce the science plan for the first Tara Polaris drift onboard the Tara Polar Station to advance knowledge and address these questions. Despite increased observational programmes in the CAO over the past years, for example, the Nansen and Amundsen Basin Observational System (NABOS) and Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), extensive knowledge gaps remain in relation to ocean stratification, sea ice and lightscape, nitrogen fixation and nutrient fluxes, carbon export and transfer, sympagic-pelagic coupling, aerosol production, contaminant transport and transformation, chronobiology, and fish distribution. Further knowledge on overall CAO biodiversity, ecosystem functionality and interannual variability is also critically needed. We describe a way forward to address these knowledge gaps using ice-tethered and profiling instruments coupled with multi-omics, culturing, and imagery approaches deployed from Tara Polar Station during the first of ten Tara Polaris drifts designed to facilitate detection of interannual variability and change over time.

Arctic shipping is projected to increase as sea ice retreats, yet the impact of modern low-sulfur ship emissions on Arctic clouds and radiation remain poorly constrained. We use year-long in situ observations from the MOSAiC expedition to characterize ship-aerosol-cloud interactions for an icebreaker burning ultra-low sulfur fuel (0.1% mass per mass). Exhaust plumes were found to be strongly enriched in Aitken-mode particles, organic aerosol, and black carbon, but showed no detectable enhancement in particulate sulfate. Despite reduced hygroscopicity relative to ambient aerosols, ship emissions substantially increased local cloud condensation nuclei concentrations. A droplet activation parametrization was applied to quantify responses in cloud droplet number concentration (N _d ) to ship-induced perturbations in low-level Arctic clouds. In winter, abundant background accumulation-mode particles from Arctic haze supplied nearly all cloud droplets, while additional particles from ship emissions had little impact on N _d . In contrast, during summer months, when unperturbed background aerosol concentrations are low, ship emissions nearly doubled N _d compared to average background conditions and increased N _d by a factor of five compared to very clean background conditions (25 ^th percentile of background aerosol number concentrations). Longwave radiative transfer simulations for typical conditions of summer Arctic low-level clouds/fog suggest that these ship-induced increases in N _d locally (i.e., < 100 km downwind) lead to enhanced net surface longwave fluxes and consequent warming, primarily for optically thin clouds (liquid water path (LWP) ≤ 30 g•m ^-2 ). For LWP = 10 g•m ^-2 , ship emissions lead to an increase of 1 W•m ^-2 in cloud longwave forcing at the surface compared to average unperturbed conditions (+ 7% relative increase), and up to 4 W•m ^-2 when compared to very clean background conditions (+ 22% relative increase). Even ultra-low sulfur fuel emissions can therefore locally and episodically modify Arctic cloud microphysics and radiative properties, especially during summer, implying that future increases in Arctic shipping could have non-negligible regional climate impacts.

Mercury (Hg) atmospheric emissions in Europe peaked in the mid-20th century and have since declined due to environmental policies, reducing atmospheric deposition to ecosystems. However, regional disparities have emerged as climate change can remobilize Hg reservoirs, complicating efforts to assess environmental recovery. This study investigates Hg accumulation rates in lacustrine sediments from two neighboring high-altitude alpine lakes in the French Alps: Grand Lake (GDL), a rain-fed lake reflecting regional atmospheric deposition, and Eychauda Lake (EYC), a proglacial lake supplied by glacial meltwater and derived sediment. Sediment chronologies for both lakes were established using short-lived radionuclides, allowing for the reconstruction of Hg deposition over the past century. Comparison of the Hg records from both lakes revealed contrasting trends: GDL responds to regional anthropogenic emissions, while EYC shows a still increasing Hg accumulation rate over recent decades. This discrepancy and its correlation with erosion proxies suggest that glacier melting has released Hg stored in the ice, masking the expected trend of reduced regional emissions. These findings illustrate the importance of accounting for climate-mediated remobilization of Hg when evaluating the effectiveness of environmental policies as this process can impose a climate change-driven penalty that delays ecosystem recovery.

Le mercure (Hg) est un métal émis dans l’atmosphère par des sources naturelles et anthropiques. Il est préoccupant à l’échelle mondiale de par sa propagation atmosphérique sur de longues distances, loin des sources d’émissions, sa persistance dans l’environnement, son potentiel de bioaccumulation dans les chaînes alimentaires aquatiques et ses effets néfastes sur la santé humaine. Les modèles atmosphériques, utilisés pour retracer son cheminement depuis les sources d’émissions jusqu’aux dépôts au sein des écosystèmes, sont entachés de fortes incertitudes en raison notamment de notre compréhension partielle des processus atmosphériques (réactions d’oxydo-réduction, dépôts, réémissions) et du manque de données d’observations à l’échelle planétaire. L’objectif de ces travaux de thèse est d’améliorer notre compréhension du cycle atmosphérique du Hg en trois sites reculés de l’Hémisphère Sud : l’île d’Amsterdam (AMS) en plein océan Indien, Concordia (DC) sur la calotte glaciaire antarctique et Dumont d’Urville (DDU) sur la côte Est du continent. Les données acquises à AMS démontrent une réactivité atmosphérique limitée du Hg dans cette région du globe. L’île étant faiblement et rarement influencée par des masses d’air continentales polluées, il s’agit d’un site clé pour la surveillance, sur le long terme, du bruit de fond atmosphérique aux moyennes latitudes de l’Hémisphère Sud. Les données acquises en Antarctique démontrent l’existence de processus inédits en termes de réactivité dans l’atmosphère et à l’interface air-neige. Les processus observés sur la calotte glaciaire influent par ailleurs sur le cycle du Hg à l’échelle continentale du fait des forts vents catabatiques. Ces avancées scientifiques permettront, à terme, de contraindre et d’améliorer les modèles atmosphériques globaux.