Le lundi 19 avril 2021,  Fanny VAN DER LINDEN présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Océanographie) sous la direction de Bruno DELILLE et Jean-Louis TISON (Co-tutelle avec l’Université Libre de Bruxelles) .

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Autumn to spring inorganic carbon processes in pack and landfast sea ice in the Ross Sea, Antarctica ».

Abstract

The Ross Sea, the southernmost sea on Earth, presents several iconic features of polar seas: sites of deep water formation, high summer primary production, floating ice shelves, the annual cycle of advance and retreat of sea ice cover, polynyas and katabatic winds. Furthermore, sea ice in McMurdo sound (Western Ross Sea) is one of the most productive marine environments. However, sea ice inorganic carbon dynamics and related air-ice CO₂ fluxes have never been documented in the Ross Sea.

Two surveys were carried out in the Western Ross Sea to bridge over a critical gap in the current understanding of sea ice: autumn and winter processes. The land-based land-based YROSIAE projectwas a temporal survey from late winter to summer within landfast sea ice. The ship-based PIPERS projectwas a unique opportunity to study the early stages of sea ice formation (in polynyas) and more common consolidated pack ice in autumn.

Based on these two consistent surveys, this work aims to (i) examine the bulk ice pCO₂ dynamics in landfast sea ice from late winter to summer (ii) investigate the seasonal pattern (net source vs net sink) and diurnal pattern of air-ice CO₂ fluxes (iii) analyse the depth-dependent physical and biogeochemical processes involved in inorganic carbon dynamics (iv) assess the precipitation of calcium carbonate in autumn and during a full bloom season.

CO₂ fluxes were measured using the chamber technique in autumn, late winter and spring, over open surface water, frazil ice patch, grey unconsolidated ice and consolidated first-year ice. These new autumn and winter data provide a first step to set up the budget of air-ice CO₂ fluxes over the year and evaluate the large-scale influence of these fluxes on the annual uptake of CO₂ by ice-covered oceans. Our results confirm that sea ice acts as a CO₂ source for the atmosphere during ice growth, with enhanced fluxes reported at the early stages of sea ice formation, and shifts to a sink in spring. In late spring, diel pattern superimposed upon this seasonal pattern and was potentially assigned to either ice skin freeze‐thaw cycles or diel changes in net community production. The snowpack plays a complex role in CO₂ exchanges and can no longer be considered as an inert reservoir lying at the sea ice surface.

The main features of the DIC₃₅ distribution through the ice column were: (i) a marked depletion at the surface from autumn to spring induced by the CO2 releases to the atmosphere (ii) bubble-driven gas enrichment below or within impermeable layers and (iii) an initial DIC₃₅ enrichment in the bottom layer disappearing in spring when the seasonal peak in biomass occurs.

At the bottom of landfast ice, in spring, a particular assemblage of microorganisms, the biofilm, led to a massive biomass build‐up counterintuitively associated with nutrients accumulation. This biofilm formation may also promote calcium carbonate precipitation. However, in young pack ice or in cold landfast ice in early spring, limited calcium carbonate precipitation was reported. This suggests that calcium carbonate precipitation is not an ubiquitous process, especially in winter and autumn Antarctic sea ice. Comparison of calcium carbonate precipitation and pCO₂ measurements advocates that the calcium carbonate precipitation is rather controlled by pCO₂ than temperature.

Le Jury sera composé de :

M. A. BORGES (Président), MM. S. ACKLEY (University of Texas at San Antonio), B. DELILLE (Promoteur), F. FRIPIAT (ULB) (Secrétaire), D. NOMURA (Hokkaido University), J.-L. TISON (ULB) (Promoteur).

En raison de l’épidémie du coronavirus, cette défense de thèse se fera en visioconférence via Lifesize.
Le lien public pour accéder à la défense est :

Adresse : https://call.lifesizecloud.com/5919884