Scientific Motivation

With such subjects in mind, I have sought to use laboratory explorations to enrich my understanding of how heat (and other fluid constituents) can be transferred by buoyant fluids. Through a number of simple experiments, I am beginning to gain new insights into fluid flow that is otherwise difficult to envision -- and each insight helps to interpret the diverse features present in hydrographic data from real volcanic systems.

In the summer of 1995, I helped to collect hydrographic data (CTD + light transmission) from the Northeast Pacific ocean above a volcanic region called the Endeavour Segment of the Juan de Fuca Ridge. The data reveal a variety of thermal anomalies, including prominent plumes that have risen about 180 meters above the bottom, and occasional warm patches within 50 meters of the bottom. Shown below are two portrayals of the temperature anomaly encountered as the instrument package was towed along the axis of the volcanic ridge, nearby four regions (white boxes) where fields of venting sulphide structures have been observed via submersible. The first map is a plan view of the tow track, along which warm colors depict positive potential temperature anomalies. The colors correspond to the anomaly magnitudes labeled in the second plot -- a contoured cross-section of the anomalies encountered in the water column during the transect. Note that the tow proceeded from north to south as time (top axis) proceeds.

The topographic contours above establish that the axis of volcanic activity (where thermal sources are presumably most intense) is located in a 100m-deep 1km-wide valley at the apex of a 10km-long ridge. This geometry suggests that small-scale fluid flow (in the valley) may be influenced by boundaries not unlike those of a long, thin tank; larger-scale flow (that extends above the ridge crests) may experience the effectively infinite environment of the Pacific Ocean, a situation more difficult to simulate in the close confines of a laboratory tank.

The anomalies depicted below are centered closely over the Main Endeavour and Mothra vent fields, which are located at different depths (the red line is the SEABEAM seafloor topography). The cores of the 2 (or 3?) thermal "blobs" are only 0.08oC above what they are expected to be far from the ridge and each as risen about 180m from its source elevation; note, however, that there is a significant thermal anomaly near the seafloor, of variable thickness, to the the north and south of the strong anomalies. Other tows show intermediate strength (0.03oC max) anomalies near the seafloor (less than 50m off the bottom); their significance is poorly understood.

Estimation of the heat flux from a vent field can be attempted using the temperature anomaly measurements and coincident, proximal current meter data. Yet, independent measurement of heat flux from individual vents suggests that plumes may derive heat not only from individual vents, but also through entrainment of warm fluid from surrounding diffuse sources ( Rona and Trivett 1992; Little et al. 1987). To what extent do plumes integrate the various seafloor sources of heat?

Motivated by tasks of mapping and heat flux estimation, I am interested in asking how buoyant fluid entrains as it rises to an equilibrium depth in a stratified fluid. While I think there are intriguing transitions to examine when buoyancy flux, momentum flux, and source geometry are varied, the hydrographic effect of turbulent flow similar (Ri=0.3) to a black smoker is a first priority. I'd like to be able to describe (at least qualitatively) the nature of plume entrainment given the ambient stratification and source characteristcs: temperature, salinity, particulate concentration, exit velocity and diameter -- or perhaps simply momentum and buoyancy flux, and area of venting. Towards such an ability, I will attempt to measure entrainment trajectories, velocities, and radii for turbulent plumes similar to those found in the Northeast Pacific.

Prediction of the influence of a buoyant fluid source on the local velocity field has application to other intriguing questions: the biogeography of vent organisms, and the role of hydrothermal circulation in altering deep ocean water character; and potential mechanism for the formation of "megaplumes" that rise (in association with seafloor seismicity) more than 700 meters above the bottom.