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%                    D I S C U / I M P L I C A T I O N S . T E X 
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%                    (c) 2000 A.M. Thurnherr
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\subsection{Geothermal Fluxes}

It is well known that heat-flux estimates derived from hydrothermal
equilibrium plume measurements are often associated with large
uncertainties \citeEG{review/mcduff95}. In case of the (preferred)
advective flux method the main causes are the spatial and temporal
variability of the hydrothermal plumes and of the background flow
fields. At the \R site both these problems are much less severe than it
is usually the case. Even though the variability within individual
profiles is high, a Gaussian mean plume (the first such observation in
a hydrothermal context) has been derived. Our data suggest that the
relationships between different hydrothermal tracers in individual
profiles can yield valuable information because typical tracers behave
conservatively over the short time scales required to sample single
profiles. The mutual consistency of the nephel vs.\ spice relationships
within the \R near-field plume allows the two tracers to be combined to
derive a Gaussian temperature-anomaly plume associated with
comparatively small uncertainties. The background flow advecting the
near-source plume away from the vent field observed during the 1997
survey was particularly simple, at least partially because of the
vicinity of the hydraulically controlled overflow across \RS. Taken
together, these observations significantly decrease the uncertainties of
the resulting heat-flux estimate.

On a regional scale, good hydrothermal flux estimates are important,
especially to constrain the sources of geochemical tracers. On a global
scale it appears unlikely that the fluxes can be quantified accurately
from plume measurements because of the large inherent uncertainties at
most sites. The problem is compounded by the uncertainty as to what
proportion of the low-temperature hydrothermal effluents are incorporated
into the high-temperature particle plumes.

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\subsection{Rift-Valley Hydrography and Flow}

Our observations indicate that the hypothesis of
\citeN{Progress/saunders+francis85} concerning the flushing of the \MAR
rift valley should be revised to allow for a quasi-steady state in
addition to (or instead of) periodic inflow events alternating with
periods of homogenization. Their hydrographic observations suggest that
there may be similar unidirectional along-valley flows in other (long)
\MAR segments. While geothermal processes do not appear to
significantly affect the heat budgets and dynamics within the rift
valley of the \A segments, our data indicate that strong diapycnal
mixing gives rise to hydrographic patterns similar to those observed in
canyons on the flank of the \MAR \citeEG{Nature/ledwell+00}. To what
extent mixing within the rift valleys of slow-spreading ridges is
important on a global scale cannot be determined without independent
confirmation of our diapycnal-diffusivity estimates nor without
observations from other segments.

\REM{The along-valley density gradient is consistent with the apparent
ubiquity of interleaving within the rift valley of the \MAR because it
allows inflow across multiple sills even in the absence of along-ridge
density gradients on the flanks. It will be noted that this implies the
possibility of interleaving of water with nearly identical $T/S$
properties. The resulting layers would be very difficult to detect
from hydrographic measurements alone.}

The transport budgets of the \A segments suggest that diapycnal mixing
may also provide an additional pathway (in addition to hydraulically
controlled outflows across sills) for the water to exit the rift valley
by ``entrainment'' into the overlying water column. It is difficult to
see how this hypothesis can be tested short of a tracer-release
experiment or by monitoring the input and dispersal of passive natural
tracers (such as $^3$He) over a large area. The apparent lack of
hydrothermal signatures in the vicinity of the \MAR (see below) is
consistent with the view that the hydrothermal tracers exit the rift
valley in a ``diffuse'' manner.

Without any diapycnal mixing and geothermal heating in the rift valley
the density drop between the hydraulically controlled inflows and
outflows would determine the amount of water transported along the
segments.  Diapycnal mixing complicates the system considerably. The
hydraulic-control assumption at the inflow appears to imply that it is
the upstream condition, i.e.\ the depth of the interface on the ridge
flank, which determines the inflow and, hence, the flux through the
system. In reality there is no such interface, however. The
``interface'' required to apply a \OneAndAHalfLayer hydraulic model is
taken from the depth below which there is an upstream/downstream
density difference across the sill, consistent with the view that the
corresponding pressure gradient drives a cross-sill flow below that
depth. The amount of inflow therefore depends on the processes which
control the level of that ``interface''. Hydraulic control implies that
if there is more inflow than the system can process the control is
flooded, stopping the inflow. Subsequent diapycnal mixing gradually
reduces the density of the stagnant water within the rift valley,
eventually ``un-flooding'' the control (essentially the mechanism
implied by \citeN{Progress/saunders+francis85}). It appears reasonable
to suggest that this process takes place before the control is fully
flooded, implying that  diapycnal mixing may also play a controlling
role by setting a limit for the density difference and, thus, for the
flux which is achievable at the inflow sill. Because ``entrainment''
provides an additional potential pathway for the water out of the rift
valley it is not clear if there has to be a limit beyond which the
amount of mixing does not influence the transport through the system
any more, as is the case in ``overmixed'' estuaries and marginal seas
where the maximal achievable exchange is purely hydraulically
controlled \cite{JMR/stommel+farmer53}. These questions merit further
investigation. It is suggested that laboratory experiments might be a
useful first step.

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\subsection{Dispersal of Hydrothermal Tracers and Biota}

It is not a new observation that the near-field dispersal of
hydrothermal tracers (and biota) released into the rift-valley water
column of the \MAR is primarily constrained by topographic effects.
Within the \A segments the unidirectional along-valley flow provides
an additional, dynamical, constraint. (The sheared flow observed in 1998
casts some doubt on that conclusion, however.) This appears to be the
first study where an attempt has been made to infer the fate of the
rift-valley water (and the associated hydrothermal tracers) beyond
segment boundaries. Passive tracers such as $^3$He must leave the
rift valley somewhere. Why are there apparently no basin-scale
hydrothermal signatures in the Atlantic? Does this rule out geothermal
heating as a driving process for the mid-depth westward flows as
suggested by \citeN{Science/talley+johnson94}? What are the
inter-segment colonization pathways for organisms associated with the
hydrothermal vent fields of the \MAR rift valley? These questions
cannot be answered in the context of this study. A more integrated
approach, which includes investigating the flow over the crests and on
the flanks of the ridges, is required. A proposal to that effect has
recently been submitted to NSF (proposal \#1051486).

