Jean-Carlos Montero-Serrano

Harmful algal blooms (HABs) caused by the toxigenic dinoflagellate Alexandrium catenella recur regularly in the Lower St. Lawrence Estuary (LSLE, eastern Canada). A quantitative polymerase chain reaction (qPCR) assay detects A. catenella DNA which may originate from cysts, cells, or extracellular DNA. Here, A. catenella DNA is used as a proxy for cyst presence in the Pointe-des-Monts canyon system. We show that resting cysts are concentrated in nearshore sediments and on the canyon fan, but are depleted along the canyon axis. Sediment traps reveal that major cyst resuspensions occur exclusively during winter storms under ice-free conditions, with peaks in A. catenella DNA fluxes coinciding with turbidity currents and high total particulate matter export. These events effectively flush coastal cysts offshore, creating a negative feedback loop that reduces subsequent local bloom potential. Conversely, climate change-driven increases in summer-to-autumn storm frequency and intensity could resuspend cysts under optimal proliferation conditions. Sedimentary ancient DNA in a ²¹⁰ Pb-dated sediment core also demonstrates that qPCR reliably records known historical A. catenella bloom events. These results highlight the critical, yet underappreciated, role of submarine canyon systems and storm-driven sediment dynamics in modulating A. catenella bloom risks, and suggest that ongoing climate-driven shifts in sea-ice cover and storm seasonality are likely to influence future HABs in the LSLE and similar high-latitude systems.

Dynamique sédimentaire et paleoclimatique du Golfe du Mexique au cours du dernier cycle glaciaire: Les objectifs de cette thèse sont d’étudier la sédimentation dans des sous bassins privilégiés du Golfe du Mexique (Bassin de Pigmy et de La Salle) afin de reconstituer la variabilité des apports détritiques via le fleuve Mississippi au cours du dernier cycle climatique. Une approche intégrée couplant minéralogie, granulométrie, géochimie élémentaire et organique, dans un cadre chronostratigraphique précis, a permis de progresser dans la compréhension des relations entre les mécanismes climatiques de moyenne et haute latitude et l’hydrologie du domaine subtropical et de proposer des schémas d’interprétation du couplage océan-atmosphère. La minéralogie contrastée des zones en érosion drainées par le système fluviatile du bassin versant du Mississippi permet de contraindre les zones continentales qui vont successivement alimenter la sédimentation du Golfe du Mexique. Les modifications des sources détritiques ainsi mises en évidence au cours du dernier cycle climatique ont permis (1) de contraindre la localisation des zones de déstabilisation de la marge Laurentide et l’enchaînement des processus sédimentaires lors des décharges d’eau douce qui caractérisent la dernière déglaciation, (2) de mettre en évidence une modification du front de précipitations associée aux périodes de mégacrue du Mississippi lors des périodes interglaciaires et d’en déduire des informations sur le régime hydrologique et (3) de suggérer des schémas de transferts méridiens d’humidité contrastés lors des stades interglaciaires 1 et 5e, suggérant des configurations atmosphériques sensiblement différentes. La confrontation de ces résultats avec d’autres données paléoclimatiques a alors permis de proposer plusieurs modèles simples pour les interactions océan-atmosphère dans le Golfe du Mexique, basés sur les modifications des configurations atmosphériques (Jet Stream et ITCZ) et océaniques (extension de l’Atlantic Warm Pool) et confirme le rôle majeur de l’atmosphère comme vecteur de la variabilité climatique rapide.

Paleomagnetic and rock magnetic analyses were conducted on sediment cores from the northeastern Greenland Shelf and Young Sound along the western edge of Fram Strait. The paleomagnetic signal in all three sediment cores is characterized by a strong and stable single component magnetization carried by low coercivity ferrimagnetic single domain or vortex state grains, attesting to the quality of the recorded relative paleointensity (RPI) and paleomagnetic secular variation (PSV) signals. Comparison with other archives from the northern North Atlantic, northern Greenland, and northern Europe and with geomagnetic field model outputs indicates broad similarities between the records, particularly with Finnish, Swedish and southern Greenland records. Comparison of the new RPI records with global geomagnetic field models and cosmogenic isotope production rates highlights the global character of the geomagnetic variations from the northeastern Greenland Shelf. Virtual geomagnetic pole (VGP) paths for the last 8 kyr compared to geomagnetic field strength maps at the core‐mantle boundary over the same time interval illustrate that, at times of high intensity, geomagnetic flux lobes could have an effect on VGP migration. We suggest that High Arctic PSV is most likely driven by millennial‐scale hemispheric geomagnetic flux lobe geometry changes.

The physical, sedimentological, mineralogical, and elemental geochemical properties of sediment cores collected in Smith Bay (22BC and 01PC), and near the front of Mittie Glacier (23BC and 01GC), were used to reconstruct glacial sediment discharge and trace the long‐term composition and provenance of detrital sediments for the northeastern part of Manson Icefield, southeast Ellesmere Island, over the last 9.2 cal ka BP. The mineralogical and geochemical signatures of core 01GC support the hypothesis of the gradual opening of Nares Strait between 8.8 and 8.2 cal ka BP and suggest a progressive retreat of glaciers surrounding Smith Bay. Lower sedimentation rates (−1) recorded in core 01GC during the Middle Holocene, combined with the presence of coarse‐grained sediments, indicate a landward retreat of Mittie Glacier and sea‐ice free conditions during summer in Smith Bay from 5.4 to 3 cal ka BP. During the Late Holocene, the deposition of six coarse‐grained layers close to Mittie Glacier seems to have been triggered by subglacial lake discharges upstream of its western arm. The only known surge of Mittie Glacier, recorded between 1992 and 2007, is identified in the core 23BC by a marked increase in coarse‐grained sediment input.

Sediment transported to fjords is redistributed by turbidity currents and sometimes fails on steep sidewall slopes, forming marine geohazards that are known to impact infrastructure. Since marine geohazards are poorly understood in Arctic Fjords due to lack of data and monitoring, a comprehensive study of Southwind Fjord, Baffin Island, was undertaken to assess the modern processes leading to marine geohazards and their products on the seabed. Repeat measurements of bathymetric changes and flow measurements from moorings revealed that turbidity currents with measured speeds up to 1.75 ms⁻¹ lead to the migration of cyclic steps in the submarine channel of the prodelta. Fast and dense heads of turbidity currents transport sand kilometers away from the channel‐mouth during larger events and remain confined in the prodelta channel and on the fjord basin floor. Clayey silts are deposited on the sidewalls of the fjord as a result of both overflowing turbidity currents and settling of meltwater plumes. Since sand is confined to the fjord bottom, there is no regional weak layer on the sidewall that is responsible for the large number of submarine landslides observed on the slopes. Low factor of safety of sidewall sediment (1.7 at 2.5 m depth) indicates that limited environmental loading of the sediment can trigger shallow (≤3 m) failures. This is confirmed by repeat bathymetric and core data showing asynchronous failures caused by icebergs and subaerial debris flows. This study provides a comprehensive overview of modern seabed processes and provides new perspectives on the wide variability of causes of marine geohazards in glacierized fjords that will be useful to interpret other similar environments with limited seafloor data.