Lara Hughes-Allen

Reconstitutions paléoenvironnementales et caractérisation des gaz à effet de serre dans les systèmes aquatiques pergélisolés de la Yakoutie centrale (Sibérie): La région de la Yakoutie centrale se caractérise par un pergélisol profond et riche en glace. Les changements climatiques passés et présents et d'autres perturbations peuvent avoir des effets importants sur les paysages de pergélisol et le bilan carbone mondial. Dans les zones de pergélisol riche en glace, la dégradation peut résulter en la formation de lacs thermokarst, qui sont des ‘hotspots' pour l'activité biologique et les émissions de gaz à effet de serre (GES). Les trois axes de cette thèse sont 1) la dynamique passée des lacs, 2) l'hétérogénéité spatiale et temporelle des concentrations de GES et des flux diffusifs des lacs thermokarst, et 3) l'analyse à grande échelle de la dynamique récente (depuis les années 1960) des lacs en réponse aux variations climatiques locales et aux activités humaines. Pour comprendre le développement des lacs, l'accumulation de sédiments et de carbone organique, et les changements de la productivité primaire, dans le contexte des changements climatiques du Pléistocène final et de l'Holocène, une analyse paléolimnologique multi-proxy d'une séquence de carottes de sédiments du lac Malaya Chabyda en Yakoutie centrale a été réalisée (max âge ~14 cal kBP). Les propriétés sédimentologiques et biogéochimiques dans la section la plus profonde de la carotte suggèrent un environnement lacustre principalement influencé par la végétation terrestre, où l'accumulation de carbone organique a pu être relativement faible, beaucoup plus élevée que la moyenne mondiale moderne. La section médiane de la carotte a été caractérisée par une productivité primaire plus élevée dans le lac, une sédimentation beaucoup plus forte et une grande augmentation de la livraison de carbone organique (OC). Les conditions dans la section supérieure de la carotte suggèrent une productivité primaire élevée dans le lac et des taux élevés d'accumulation de CO, avec des conditions environnementales stables. Les changements dans le caractère et la quantité de l'accumulation de CO peuvent avoir des implications importantes pour la libération future de GES. Ces résultats aident à comprendre l'histoire du développement des lacs dans les paysages de pergélisol dans le contexte des changements climatiques passés. Pour analyser l'hétérogénéité spatiale et temporelle des concentrations de GES et du flux diffus, les concentrations de CO₂ et de CH₄ dissous dans les lacs thermokarst d'un site d'étude en Yakoutie centrale ont été mesurées sur quatre saisons. Les lacs formés au cours de l'Holocène (lacs alas) sont comparés aux lacs qui se sont développés au cours des dernières décennies. Les résultats montrent des différences dans les gaz à effet de serre dissous entre les types de lacs et les saisons. Les lacs peu profonds situés dans des dépressions d'alas hydrologiquement fermées ont agi comme des réservoirs de CO₂ et de fortes sources de CH₄ diffusif pendant certaines saisons. Les lacs thermokarstiques récents étaient des sources extrêmement élevées de CO₂ et de CH₄ diffusifs, avec une accumulation considérable de gaz à effet de serre sous la couverture de glace (hiver) ou dans de l'eau les plus profondes (été). Les flux diffusifs mesurés dans les lacs thermokarst de ce paysage typique de taïga alas de la Yakoutie centrale sont parmi les plus élevés présentés dans les régions arctiques et subarctiques.Enfin, une analyse de télédétection a été menée pour étudier le développement récent (depuis les années 1960) des lacs et quantifier les différences dans l'histoire du développement entre les types de lacs. Cette étude a indiqué que certains types de lacs sont plus sensibles aux changements annuels des précipitations. Ces informations seront utilisées pour étendre les mesures in situ des gaz à effet de serre à une plus grande unité de paysage.

Global change poses “wicked problems” that have become ever more complex, pervasive, and damaging. Developing innovative solutions increasingly require diverse research approaches. The Franco-German Make Our Planet Great Again (MOPGA) program was designed to create a unique international network of top-level research, from fundamental to solution-oriented projects. MOPGA stands out from other large research initiatives by focusing not on a singular central research challenge but on facilitating multidisciplinary interactions between traditionally separated fields. MOPGA recognized that social, natural and engineering sciences share a unifying aim to address global change. In addition to addressing timely and innovative research questions within disciplines, MOPGA worked to improve communication across disciplines via annual meetings for all laureates and their research groups, scientific board exchanges, and public online seminars. Drawing on our MOPGA experiences, we discuss how such exchanges should be extended to meet the needs identified by the scientific community, international policy-makers, and regional stakeholders. In the current political landscape of scientific suppression and heightened mistrust in scientific expertise, the need for such bold, independent and collaborative scientific initiatives is greater than ever.

Eurasian permafrost soils contain large amounts of organic carbon (OC) and mercury (Hg), sequestered by vegetation during past and present interglacial periods. Lake sediment archives may help understand past OC and Hg dynamics and how they interact with climate‐related variables. We investigated Hg accumulation, OC dynamics, and Hg and OC stable isotopes in a 14,000‐year sediment record from Lake Malaya Chabyda (Central Yakutia, Russia). Sediment Hg was correlated to OC ( p value < 0.01), with lower OC and Hg accumulation rates (OCAR, HgAR) during the cold Younger Dryas (YD, 12,900–11,700 cal BP), when the lake level was low. Elevated sediment Δ ²⁰⁰ Hg (0.05‰ ± 0.11‰), representing dominant Hg ^II deposition, and low δ ¹³ C, indicates low lake primary productivity during the YD. During the early Holocene, Δ ²⁰⁰ Hg and Δ ¹⁹⁹ Hg decreased, while δ ¹³ C, δ ²⁰² Hg, OCAR, and HgAR increased, suggesting enhanced algal primary productivity in deeper, more turbid waters. From 4,100 cal BP to present, Hg/OC ratios and HgAR increased at constant OCAR, indicating an additional Hg source to the lake. Analysis of Hg isotopes suggests direct Hg ⁰ uptake into lake waters, potentially driven by primary production and efficient Hg burial. Our observations suggest that the gradual climate warming since the Last Glacial Termination and into the early Holocene led to enhanced OC and Hg burial in northern lakes and watersheds. Late Holocene enhanced Hg burial, but not OC, is possibly related to a renewed increase in lake primary productivity. Continued global warming may lead to further Hg sequestration in northern aquatic ecosystems.

Eurasian permafrost soils contain large amounts of organic carbon (OC) and mercury (Hg), sequestered by vegetation during past and present interglacial periods. Lake sediment archives may help understand past OC and Hg dynamics and how they interact with climate-related variables. We investigated Hg accumulation, OC dynamics, and Hg and OC stable isotopes in a 14,000 year sediment record from lake Malaya Chabyda (Central Yakutia, Russia). Sediment Hg was correlated to OC (p value <0.01), with lower OC and Hg accumulation rates (OCAR, HgAR) during the cold Younger Dryas (YD, 12,900­–11,700 cal BP), when the lake level was low. Elevated sediment Δ200Hg (0.05 ± 0.11‰), representing dominant HgII deposition, and low δ13C, indicates low lake primary productivity during the YD. During the early Holocene, Δ200Hg and Δ199Hg decreased, while δ13C, δ202Hg, OCAR, and HgAR increased, suggesting enhanced algal primary productivity, in deeper, more turbid waters. From 4,100 cal BP to present, Hg/OC ratios and HgAR increased at constant OCAR, indicating an additional Hg source to the lake. Analysis of Hg isotopes suggests enhanced direct Hg0 uptake into lake waters, driven by primary production and efficient Hg burial. Our observations suggest that the gradual climate warming since the Last Glacial Termination and into the late Holocene led to enhanced OC and Hg burial in northern lakes and watersheds. Early Holocene enhanced Hg burial, but not OC, is possibly related to a renewed increase in lake primary productivity. Continued global warming may lead to further Hg sequestration in northern aquatic ecosystems.

The current rate and magnitude of temperature rise in the Arctic are disproportionately high compared to global averages. Along with other natural and anthropogenic disturbances, this warming has caused widespread permafrost degradation and soil subsidence, resulting in the formation of thermokarst (thaw) lakes in areas of ice-rich permafrost. These lakes are hotspots of greenhouse gas emissions (CO₂ and CH₄), but with substantial spatial and temporal heterogeneity across Arctic and sub-Arctic regions. In Central Yakutia (Eastern Siberia, Russia), nearly half of the landscape has been affected by thermokarst processes since the early Holocene, resulting in the formation of more than 10,000 partly drained lake depressions (alas lakes). It is not yet clear how recent changes in temperature and precipitation will affect existing lakes and the formation of new thermokarst lakes. A multi-decadal remote sensing analysis of lake formation and development was conducted for two large study areas (~1200 km² each) in Central Yakutia. Mask Region-Based Convolutional Neural Networks (R-CNN) instance segmentation was used to semi-automate lake detection in Satellite pour l’Observation de la Terre (SPOT) and declassified US military (CORONA) images (1967–2019). Using these techniques, we quantified changes in lake surface area for three different lake types (unconnected alas lake, connected alas lake, and recent thermokarst lake) since the 1960s. Our results indicate that unconnected alas lakes are the dominant lake type, both in the number of lakes and total surface area coverage. Unconnected alas lakes appear to be more susceptible to changes in precipitation compared to the other two lake types. The majority of recent thermokarst lakes form within 1 km of observable human disturbance and their surface area is directly related to air temperature increases. These results suggest that climate change and human disturbances are having a strong impact on the landscape and hydrology of Central Yakutia. This will likely affect regional and global carbon cycles, with implications for positive feedback scenarios in a continued climate warming situation.

Corrigendum: 14,000-Year Carbon Accumulation Dynamics in a Siberian Lake Reveal Catchment and Lake Productivity

A multi-proxy paleolimnological analysis of a sediment core sequence from Lake Malaya Chabyda in Central Yakutia (Eastern Siberia, Russia) was conducted to investigate changes in lake processes, including lake development, sediment and organic carbon accumulation, and changes in primary productivity, within the context of Late Pleistocene and Holocene climate change. Age-depth modeling with ¹⁴C indicates that the maximum age of the sediment core is ∼14 cal kBP. Three distinct sedimentary units were identified within the sediment core. Sedimentological and biogeochemical properties in the deepest section of the core (663–584 cm; 14.1–12.3 cal kBP) suggests a lake environment mostly influenced by terrestrial vegetation, where organic carbon accumulation might have been relatively low (average ∼100 g OC m⁻² a⁻¹), although much higher than the global modern average. The middle section of the core (584–376 cm; 12.3–9.0 cal kBP) is characterized by higher primary productivity in the lake, much higher sedimentation, and a remarkable increase in OC delivery (average ∼300 g OC m⁻² a⁻¹). Conditions in the upper section of the core (−2 a⁻¹), with stable environmental conditions. The transition from organic-poor and mostly terrestrial vegetation inputs (TOC/TN_atomic ratios ∼20) to conditions dominated by aquatic primary productivity (TOC/TN_atomic ratios 30%) measured in the upper section of the core. Compact lake morphology and high sedimentation rates likely resulted in this lake acting as a significant OC sink since the Pleistocene-Holocene transition. Sediment accumulation rates declined after ∼8 cal k BP, however total OC concentrations were still notably high. TOC/TN_atomic and isotopic data (δ¹³C) confirm the transition from terrestrial-influenced to aquatic-dominated conditions during the Early Holocene. Since the mid-Holocene, there was likely higher photosynthetic uptake of CO₂ by algae, as suggested by heavier (isotopically enriched) δ¹³C values (>−25‰).

In the ice‐rich permafrost area of Central Yakutia (Eastern Siberia, Russia), climate warming and other natural and anthropogenic disturbances have caused permafrost degradation and soil subsidence, resulting in the formation of numerous thermokarst (thaw) lakes. These lakes are hotspots of greenhouse gas emissions, but with substantial spatial and temporal heterogeneity across the Arctic. We measured dissolved CO₂ and CH₄ concentrations in thermokarst lakes of Central Yakutia and their seasonal patterns over a yearly cycle. Lakes formed over the Holocene (alas lakes) are compared to lakes that developed over the last decades. The results show striking differences in dissolved greenhouse gases (up to two orders of magnitude) between lake types and seasons. Shallow lakes located in hydrologically closed alas depressions acted as CO₂ sinks and strong sources of diffusive CH₄ during some seasons (ebullition was not assessed). Recent thermokarst lakes were moderate to extremely high sources of diffusive CO₂ and CH₄, with considerable accumulation of greenhouse gas under the ice cover (winter) or in the deepest water layers (summer), highlighting the need to include spring and autumn as critical periods for integrated assessments. The water column was stratified in winter (all lake types) and especially in summer (recent thermokarst lakes), generating anoxia in bottom waters and favoring CH₄ production and storage, particularly in the most organic‐rich lakes. The diffusive fluxes measured from thermokarst lakes of this typical taiga alas landscape of Central Yakutia are among the highest presented across Arctic and subarctic regions.