Abstract EN:

In this thesis, I investigate the nature, space-time variability, distribution and associated heat flux of hydrothermal activity at the Lucky Strike hydrothermal field (Mid-Atlantic Ridge, 37°17'N). As part of the MoMAR experiment to monitor hydrothermal activity, we used an ROV to acquired photomosaics of the seafloor and to deploy temperature sensors at black smoker, cracks, and diffuse hydrothermal outflows. Seafloor imagery provides detailed and accurate constrain on the distribution, geometry, and nature of hydrothermal outflow. Repeated surveys allow us to evaluate the temporal variability of these systems. Temperature records show two main temporal patterns: (1) episodic variability with rapid temperature changes, and (2) systematic variability at tidal periods. Cross-spectral methods applied to the temperature and bottom pressure records reveal robust phase relationships, particularly for the high-temperature records, as predicted by poroelastic theory. These results demonstrate the tidal pressures diffusely propagate through the porous matrix hosting sub-surface flow, which results in phase lags between the surface pressure and the fluid discharge temperature. We use th,is observation to constrain hydrologie parameters of the sub-surface circulation system at the Lucky Strike field. Finally, I also investigated in the laboratory the nature of flow associated with a heat source comparable to a crack. Temperature gradients between the heat source and the ambient seawater modulate the frequency and ascent velocity of thermals forming above the cracks, a dependency that may be applied in the future to calculate heat fluxes from video imagery acquired in the field.