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%                    P L U M E / F L A M E . T E X 
%                    doc: Mon May 10 09:28:32 1999
%                    dlm: Wed Jun  7 23:16:08 2000
%                    (c) 1999 A.M. Thurnherr
%                    uE-Info: 178 0 NIL 0 0 72 3 2 4 ofnI
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\Subsection{plumeplan}{Horizontal Particle Distribution}

\begin{figure}[t]

\placeFig{plume/F_plume3d}

\caption[1997 particle plume (plan view)]{Depth-integrated
(\m{1500$--$2500}) nephelometry profiles of the 1997 \BGT data set
(arbitrary vertical scale); bathymetric contour interval is \m{250};
thin bars denote profiles without plume signals (from visual
inspection).}

\labelFig{F-plume3d}

\end{figure}

\begin{figure}[t]

\placeFig[0.6]{plume/F_PVD_G}

\caption[1997 flow near vent field (first week)]{Progressive vector
diagrams showing the first week of hourly flow measurements at mooring
G \protect\rfig{CM-stations}; the \m{2100} and \m{2300} curves are
horizontally offset by \km{5} and \km{10}, respectively; symbols are
drawn at midnight GMT.}

\labelFig{F-PVD-G}

\end{figure}

\callout{\rFigNP{F-plume3d}} shows the horizontal distribution of the
depth-integrated \BGT nephelometry profiles of the 1997 survey. (The
entire data set is shown, including the tows with the faulty CTD
instrument; see \rsecNP{F-methods}). In the SW basin the plume signals
are confined to the area north of the vent field, consistent with the
currents recorded on mooring G during the first week of deployment,
i.e.\ immediately after the end of the \F cruise \rfig{F-PVD-G}. A
representative width of the plume is difficult to estimate but the
value of \km{2} given by \citeN{EPSL/german+98} is most likely good at
least within a factor of two. Upstream of \RS the horizontal
variability between neighboring profiles is highest, whereas
downstream, on the eastern slope of \RR, the depth-integrated plume
signals are more uniform. The drop-off in signal strength away from the
ridge (east of \lonW{33}{50}) is consistent with horizontal mixing (the
drop-off coincides with the boundary between the ridge slope, where
hydrographic interleaving was observed \rsec{F-waterorigin}, and the
basin interior, where no such structures are apparent). Particle
settling and dissolution provide alternative mechanisms for the
decrease in plume nephels --- the small flow velocities in the NE basin
(e.g.\ \rfigNP{sig2200}) are consistent with increased sedimentation
rates. East of \RR the integrated plume signals decrease south of
\latN{\approx\!36}{15}, suggesting that the plume does not follow the
isobaths around the southern edge of the basin, consistent with the
methane data shown by \citeN{EPSL/german+98}.

A plot of the horizontal distribution of the nephelometry \emph{maxima}
is similar to \rfigNP{F-plume3d}, the main difference being a reduction
in signal strength of approximately 50\% across \RS (c.f.\
\rfigNP{F-bulkplumes}), consistent with vertical mixing.

%----------------------------------------------------------------------

\Subsection{plumeprofs}{Individual Nephelometry Profiles}

\begin{figure}[t]

\placeFig{plume/F_nephprofs}

\caption[1997 nephelometry profiles]{Selected \BGT nephelometry
profiles, including the one closest (\m{<200}) to the vent field \npa,
one from the eastern slope of \RR \npb, two adjacent ones from the same
tow-yo near the center of the NE basin \npc \& \npd, and the one
farthest away (\protect\km{\approx\!30}) from the vent field \npe;
successive profiles are horizontally offset by \V{0.5}.}

\labelFig{F-nephprofs}

\end{figure}

Based on the horizontal distribution of the nephelometry anomalies, the
\BGT plume data (thick bars in \rfigNP{F-plume3d}) were separated into
three regions: SW basin, eastern slope of \RR, and NE basin interior
(east of \lonW{33}{50}). \callout{\rFigNP{F-nephprofs}} shows
nephelometry profiles from each of these regions, selected to
illustrate the variability throughout the domain of observation.

In the near-source profiles (upstream of \RSp{}{)} the nephelometry
anomalies are confined between \m{2300} (the depth of the vent field)
and \m{1900}. They typically contain one to four peaks in the vertical
with the signal between peaks often dropping to background values. In
the \R data sets, the individual nephelometry peaks cannot be
correlated between profiles, suggesting that the horizontal scale of
the coherent plume fluid structures is small (typical \BGT tow-yo
spacing at plume depth is \km{1$--$2}). \REM{A subset of the
near-source nephelometry peaks coincides with layers of reduced
stratification, sometimes with thin (\m{<\!10} vertical extent) regions
of static instability.}

%----------------------------------------------------------------------

\Subsection{F-bulkplumes}{Mean Particle Plume}

\begin{figure}[t]

\placeFig{plume/F_bulkplumes}

\caption[1997 density-averaged particle plume]{Density-averaged \BGT
plume nephelometry profiles and depth-fitted Gaussian curves for the SW
basin (black), the eastern slope of \RR (gray, offset by \V{-0.03}) and
the interior of the NE basin (white, offset by \V{-0.06}); the
SW/slope/NE profiles are derived from 26/17/68 casts, respectively; the
SW basin profile is truncated at \m{2250} below which there are not
enough data points (less than four per bin) to calculate meaningful
averages.}

\labelFig{F-bulkplumes}

\end{figure}

For each of the three regions (SW basin, eastern slope of \RR, and NE
basin interior), a mean profile derived from the \BGT plume data set is
shown in \callout{\rfigNP{F-bulkplumes}.} Because of the variability of
the hydrography on tidal time scales which has a vertical scale
comparable to the thickness of individual near-source nephelometry
peaks \rsec{intwaves}, averaging of both nephels and depth was
performed in potential-density space with non-uniform bin sizes
depending on the stratification. All three mean profiles are
approximately Gaussian in depth, even the near-source one from the SW
basin which is derived from vertically structured individual profiles
such as profile \npa of \rfigNP{F-nephprofs}. The Gaussian curve fitted
to the SW basin profile is centered at \m{\mu=2120}, with a thickness
(one standard deviation) of \m{\sigma=96}, a peak value of \V{\Delta
n^*=0.25}\ls{Delta_n_star} and an RMS error of \V{0.018}; the
respective values for the profile from the eastern slope of \RR are
\m{\mu=2120}, \m{\sigma=102}, \V{\Delta n^*=0.18}, and RMS error
\V{0.009}; and those for the NE basin interior profile are
\m{\mu=2100}, \m{\sigma=137}, \V{\Delta n^*=0.06}, and RMS error
\V{0.004}, respectively. The peak of the mean plume exhibits no
vertical trend with distance from the source (neither in depth nor in
density space), indicating that the profiles of \rfigNP{F-bulkplumes}
represent the \emph{equilibrium plume} (neutrally buoyant without
vertical momentum). The rise height measured at the mean nephelometry
maximum is \m{200}. There is no evidence for vertical overshoot (plume
cap) near the source consistent with laboratory
\citeEG{JGR/helfrich+battisti91} and numerical \citeEG{JGR/lavelle97}
experiments which show that both rotation and cross flows tend to
suppress such an overshoot.

\begin{figure}[t]

\placeFig{plume/plumeskew}

\caption[1997 density-averaged particle plume skew]{Re-plot of the mean
nephelometry profile and depth-fitted Gaussian of the NE basin interior
\protect\rfig{F-bulkplumes} with added Gaussian fitted in
density-space.}

\labelFig{F-skew}

\end{figure}

The two mean profiles from the NE basin (and, to a lesser extent, the
SW basin profile as well) show consistent skews with respect to their
depth-fitted Gaussians, with steeper vertical gradients at the top of
the plume, indicating that it might be more appropriate to fit the
Gaussians in density space --- the curvature of density against depth
is greater than that implied by the plume skew however, so that the
mean plume lies between the two Gaussians as shown in  \rfigNP{F-skew}
for the NE basin interior profile.
