A non-invasive method for estimating heat flux out of a hydrothermal crack derived from fluid dynamics analog modeling

Abstract:

Direct observations of sub-surface flow parameters and processes at mid-ocean ridge hydrothermal systems are difficult to perform, which makes it challenging to estimate the associated flow parameters such as velocity or heat flux. Analog modeling of hydrothermal processes can act as a means to estimate these important hydrothermal parameters, as well as to explore the sensitivity and response of these systems to natural perturbations. In this study, we conduct analog experiments that focus on low-temperature hydrothermal outflow from linear cracks, which can account for a sizeable proportion of the total hydrothermal flux (up to 90% at certain hydrothermal fields). We approximate these cracks in the laboratory as an ideal heat source with a linear geometry, which release a continuous flow of hot material. So far, fluid dynamics studies have documented the linear and weakly non-linear convective regimes in this configuration, namely the rise of a laminar sheet of fluid above the linear heat source, and the eventual onset of oscillations. However, direct observations above cracks suggest the existence of 3D instabilities such as plumes. We therefore have undertaken a laboratory study to investigate the conditions of existence and the characteristics (velocity and temperature) of these 3D instabilities. The latter are indeed observed for all the experimental conditions we tested, but at different depths in the water column. Scaling laws relating the frequency and velocity of these 3D perturbations in the water column to the thermal characteristics of the crack (heat flux, temperature difference with the ambient fluid) were derived. Hence it become possible to infer the heat flux out of a linear hydrothermal crack from the observation of the instable flow above it by standard video-imagery from deep-sea vehicles. Such a non-invasive method would be a valuable tool for hydrothermal fields studies.