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%                    M I X I N G / I N T R O . T E X 
%                    doc: Tue Apr  4 11:00:58 2000
%                    dlm: Thu Jun  8 00:06:00 2000
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\begin{figure}[t]

\placeFig{mixing/sills}

\caption[\A sill locations]{\m{2250} bathymetric contours of the \A
rift valley and locations (and inferred flow directions where
available; c.f.\ \protect\rsecNP{mix-2000}) of the sills below
\m{2000}.}

\labelFig{mix-sills}

\end{figure}
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In this chapter mass (volume) and heat budgets of the \A segments are
calculated. The main goals are to investigate the pathways of the
rift-valley water, to infer the amount of mixing required to sustain
the observed along-segment hydrographic gradients, and to assess the
importance of geothermal heating within the rift valley. A steady-state
assumption is made, motivated by the hydrographic and flow observations
\rchaps{HYD}{CM}. To simplify the discussions the sills below \m{2000}
are labeled as shown in \rfigNP{mix-sills} (e.g.\ \RS is called sill
``R'').

In \rsecNP{mix-saunders} the heat budget beneath an isopycnal surface
grounding within the rift valley is calculated. While known geothermal
contributions (including the heat flux from the \R hydrothermal vent
field) do not have a significant impact, a bulk diapycnal diffusivity
($K_v$) of order \mmps*{5}{-3} is required to supply sufficient heat to
the water beneath the grounding isopycnal. Mass budgets for the rift
valley below \m{2000} are calculated next \rsec{mix-2000}. The pathways
of the rift-valley water are associated with considerable uncertainties
but the observations suggest that approximately half of the water
entering \sA below \m{2000} flows into the \FAM segment. The remainder
is most likely ``lost'' either by outflows across additional sills or
by ``entrainment'' into the overlying water column. Using the volume
budgets together with hydrographic data, heat budgets for the
along-segment flow below \m{2000} are calculated. Bulk
diapycnal-diffusivity estimates calculated from these heat budgets are
of order \mmps*{5}{-3} for both \A segments, i.e.\ consistent with the
value calculated in \rsecNP{mix-saunders}. The chapter concludes with a
discussion in \rsecNP{mix-discu}.