Investigation of turbulent flows and near-bottom hydrothermal plumes at mid-ocean ridges

We investigate the characteristics of turbulent flows within near-bottom hydrothermal plumes at mid-ocean ridges through quantitative analysis of video images from manned submersibles using the Particle Image Velocimetry (PIV) method. High-quality video images of near-bottom hydrothermal vents were selected from the Data Library and Archives of the Woods Hole Oceanographic Institution (WHOI), consisting of multiple examples of vent fields in the Atlantic, Pacific, and Indian Oceans. Selected video segments of near-bottom hydrothermal plumes were decomposed into a series of still-image frames at a typical time interval of 1/30 second between consecutive frames. The PIV method was then used to track the motion of individual turbulent parcels, which were identified based on their relatively high concentration of optically visible particles than the surrounding water column. Finally, the velocity fields of the individual turbulent parcels, as well as the integrated fluxes of the composing hydrothermal plume, were calculated. Preliminary investigation of hydrothermal plumes at the TAG area of the Mid-Atlantic Ridge revealed significant spatial and temporal variations in the fluid dynamics of turbulent fluid parcels and near-bottom hydrothermal plumes: (1) Each rising hydrothermal plume is composed of a string of turbulent fluid parcels of variable sizes with a typical dimension of several cm. The calculated instantaneous velocities of individual turbulent parcels could reach tens of cm/s. (2) Turbulent fluid parcels within the hydrothermal plume were observed to grow rapidly through coalescing with adjacent parcels and interacting with ambient water column. (3) The cross-sectional dimensions of the near-bottom hydrothermal plumes were observed to increase several times within upwelling distance of tens of cm, indicating rapid entrainment of ambient fluids into the rising plumes. The overall vertical fluxes of the rising plumes are calculated to have changed significantly with time as the individual turbulent fluid parcels evolve rapidly. We are currently analyzing interaction of turbulent flows during the mergence of two adjacent hydrothermal plumes. Together, the above results reflect the significant roles of turbulent flows in fluid dynamics of near-bottom hydrothermal plumes.