Daniela Di Iorio and Guangyu Xu, University of Georgia

Deep sea hydrothermal plumes are driven by gravitational buoyancy forces and may coalesce with plumes from nearby sources and rise up to hundreds of meters above the orifice. Ambient ocean water is entrained into the plume during its ascent which makes the plume diluted and cooled.   Long term measurements of physical properties of hydrothermal plumes are limited and such properties are of crucial importance for understanding 1) how the plume interacts with the ambient oceanography, 2) how turbulence levels affect mixing within the plume and 3) how entrainment as a function of horizontal cross flows is an important component for modeling plume dynamics.

In Sept 2007 an acoustic scintillation system was deployed in the Main Endeavour vent Field to specifically quantify the long term temporal variability of the vertical velocity and turbulence of the hydrothermal plume of Dante at 20 m above the top of the sulfide structure.  The distance between the vertical array of transmitters and receivers was 91m and line-of-sight alignment was 19 ^oT along the Endeavour axis which was aligned with the direction of the dominant horizontal flows.  Six weeks of data was collected.  Using the space-time coherence of the acoustic amplitude signal, hourly vertical velocity measurements were obtained.  Theoretical developments comparing acoustic forward scattering from turbulence and from particles show that suspended particles having a measured density of 3.88 x 10⁴ particles/m³ produce negligible amplitude fluctuations compared to turbulence modeled by an isotropic and homogeneous Kolmogorov model for the temperature variability.

The vertical velocity and turbulence of the hydrothermal plume measured 20 m above Dante shows a significant correlation with the horizontal flow which is comprised of tidal and subtidal oscillations aligned with the axial valley.  The residual flow to the northeast is significant (~ 4 cm/s) and can increase the horizontal flow during the flooding tide and diminish the horizontal flow during the ebbing tide.  The hydrothermal plume of Dante and its interaction with the horizontal flow within the Main Endeavour Field can be generalized as follows: 1) the plume’s vertical velocity is maximum when the horizontal flow is weakest (during the ebbing tide) while it is a minimum when the horizontal flow is strongest (during the flooding tide); 2) the turbulent intensity within the plume reaches a maximum when the horizontal flow is weakest  (during the ebbing tide) while it reaches minimum when the horizontal flow is strongest (during the flooding tide).   When the horizontal flow is weak (during the ebbing tide), less ambient ocean water is entrained into the plume. In such a case, the plume is faster and hotter and the turbulent velocity and temperature fluctuations increase within the plume.  When the horizontal flow is strong (during the flooding tide), more ambient ocean water is entrained into the plume. In such a case, the plume is slower and cooler with reduced turbulent velocity and temperature fluctuations. 

Results from an integral plume model based on the conservation equations of mass, momentum, density deficit and dissolved tracers and taking into account ambient stratification and horizontal cross flows are compared with the acoustic scintillation results and several conclusions are reached and generalized as follows:  1) radius of the plume produced by Dante increases linearly along plume’s height above the orifice; 2) vertical velocity of the plume decreases exponentially along the plume’s height
and is enhanced during ebbing tide while suppressed during flooding tide; 3) The plume’s terminal height (where vertical velocity goes to zero) increases during ebbing tide and decreases during flooding tide; 4) the bending of the plume is enhanced during flooding tide and diminished during ebbing tide.

A poster outlining these ideas will also be presented.