Jonas Frère’s PhD

Evolution of carbon cycle in Dziani Dzaha Lake: from monitoring magmatic degassing to reconstructing paleoenvironments

Dziani Dzaha is a volcanic lake located on Petite Island in Mayotte archipelago. It is studied since 2010’s especially as an analog of Precambrian environments.

From 2018 to 2020, the birth of Fani Maore, an underwater volcano 50 km offshore Mayotte, provoked a sismovolcanic crisis. During the crisis, the island subsided of 20 cm and many earthquakes were felt by the population.

Regarding the lake, a new plume of water and gas of magmatic origin was observed in the middle of the lake by 2020, and changes in the physicochemistry (lowering of pH, δ¹³C_DIC…) were measured in 2021.

This thesis focuses on the characterization of the carbon cycle perturbation in lake Dziani Dzaha. The different objectives are: determine if the lake could be used to monitor the magmatic degassing ; study the evolution of a pCO₂ paleo-proxy (isotopic fractionation associated to photosynthesis) ; model the carbon cycle perturbation and evaluate the ecosystem response (resistance, resilience, etc.).

The methods used are : physicochemistry of the water column acquired on the field, chemical analyses of the water column (alkalinity, DIC, major ions…), isotopic analyses of main carbon reservoirs (DIC, POC, gaseous CO₂…), geochemical modelling (PhreeqC).