Interactions and entrainment:

The geometric scaling is not unreasonable if a typical sulfide structure (10m high; some 3m across at its top) and linear tube worm field (3x50m) are considered. The deflection of the potassium permanganate tracers in this animation reveal that the entrainment is powerful at the level of the orifice, decreases upwards, and reverses strongly in the outflow zone. Note how the tracers closest to the rising plume are immediately swept in above the orifice, but hardly move at all below its level.

The diffuse flow and effluent are similarly affected. While the bulk of the effluent is ultimately integrated into the focussed source's flow, and the rising diffuse fluid trajectories are clearly bent towards the focussed source, the lower plume from the diffuse source barely moves. This effect -- due to the different elevations of sources, and their entrainment dynamics in a stratified fluid -- may help to explain why anomalously warm fluid is sometimes found close to the bottom. While it may be advected away, this experiment indicates that may be little influenced by the circulation induced by nearby, steady-state venting from strong focussed sources.

My intuition has been bolstered by these simulations, but a more careful consideration of relative scales is prerequisite to the quantitative characterization of the velocity fields that result from real hydrothermal venting. Numerical models may help in the assessment of the extent (radius) to which various sources of buoyancy and momentum on the seafloor effect the local circulation, and the potential utility in the interpretation of field data is clear.

Trivett (1994) expects that "the presence of black smokers has a negligible effect on the diffuse flow since the vertical velocity of the black smokers was observed to be of the order 1m/s, resulting in an entrainment velocity of order 10cm/s, decaying inversely with the distance from the plume." McDuff (1995), on the other hand, compares the theoretical entrainment velocity of a black smoker plume to the vertical rise velocity of fluid from a tube worm field, and concludes that "the radius of entrainment is ~15-30m and dissuse flow in the vicinity of a black smoker plume should be entrained."

A closer look at the relevance of this experiment based on scale analyses and considerations of the role of turbulence is presented in the next, and final section.