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%                    P L U M E / D I S C U . T E X 
%                    doc: Wed Apr  8 09:18:03 1998
%                    dlm: Wed Jun  7 23:19:20 2000
%                    (c) 1998 A.M. Thurnherr
%                    uE-Info: 78 64 NIL 0 0 72 3 2 4 ofnI
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% - skew similarity with Nature/ledwell+00
% - Gaussian is brand new:
%	- confidence that plume has been sampled well
%	- layering does not exist

\begin{figure}[t]

\placeFig{HYD/flow_scheme}

\caption[1997 flow regime around \RR]{Sketch of the mean flow regime
around \RR during the 1997 survey; topographic shading is the same as
in \rfigNP{AMAR-bridge}; contour interval is \m{250}; black arrows
represent the flow below \m{2000}; the black dashed line indicates the
region where enhanced mixing is expected; white arrows represent the
flow above \m{2000}.}

\labelFig{flow-scheme}

\end{figure}

% - mixed overflow / boundary-current model with sketch

Using the hydrothermal particles as natural tracers allows the flow
downstream of the vent field to be determined in greater detail than is
possible from the hydrographic and direct flow observations alone
\rchaps{HYD}{CM}. The data sets suggest a circulation in the vicinity
of \RR during the 1997 survey as shown schematically in
\callout{\rfigNP{flow-scheme}}. The incident current below \m{2000}
flows along the western slope of \RR and across \RS which acts as a
hydraulic control point. Downstream of the sill the current descends
into the NE basin while continuing to flow clockwise around the ridge.
There is enhanced mixing in the downslope current which gradually
decreases in strength. Eventually, it detaches from the eastern slope
of the ridge, carrying particle-rich water into the interior of the
basin. Above \m{2000}, the mean incident current flows over the peak of
\RR. Because the density-averaged plume observed in 1997 extended above
the minimum depth of the blocking topography, a fraction of the
near-source plume may have been advected over the peak of \RR.

During the 1998 survey the situation was similar at depth but the flow
above \m{2000} had reversed. The boundary current along the western
slope of \RR was therefore strongly sheared, bisecting the plume. Only
its lower part followed the topography around \RR into the NE basin
whereas the upper part was carried into the interior of the SW basin.
The continuation of the SW-ward flow is not known but it appears
possible that at least some of the hydrothermal effluents could have
been advected across the rift-valley wall into the western North
Atlantic.

Upstream of \RS, close to the hydrothermal source, many nephelometry
profiles of both surveys are characterized by multiple vertical peaks.
Laboratory observations \citeEG{JFM/papantoniou+list89} and numerical
models \citeEG{JGR/lavelle97} show that even plumes rising from single
steady buoyancy sources into quiescent backgrounds are characterized by
highly inhomogeneous near-source effluent concentrations, indicating
that such variability is an intrinsic property of turbulent plumes. Our
1997 data show for the first time that a temporally and spatially
averaged hydrothermal particle plume can be Gaussian in depth. This
suggests that the particle distribution reflects turbulent plume
structure (as opposed to layering caused by multiple sources or
non-conservative particle behavior), consistent with the findings of
\rchapNP{NESPI}, where it is shown that the particle anomalies of the
\R hydrothermal plume are proportional to the hydrographic anomalies in
the water column.

The density-averaged plume is not exactly Gaussian in depth, however.
The slight but consistent skew with steeper gradients at the top of the
plume is reminiscent of the tracer distributions shown by
\citeN{Nature/ledwell+00}, indicating that similar processes may have
been responsible for the shape of the mean hydrothermal particle plume
observed in 1997.

