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%                    H Y D / F _ W A T E R O R I G I N . T E X 
%                    doc: Sat Feb 12 17:02:52 2000
%                    dlm: Mon Jun  5 18:44:09 2000
%                    (c) 2000 A.M. Thurnherr
%                    uE-Info: 70 13 NIL 0 0 72 3 2 4 ofnI
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% - T/S comparison

\begin{figure}[t]

\placeFig{HYD/F_sigcompare}

\caption[1997 valley/flank density comparison]{CTD potential density
profiles from the rift valley and from an eastern and a western
off-ridge background station; at each depth both the minimum and the
maximum densities of the rift-valley profiles are shown.}

\labelFig{F-sigcompare}

\end{figure}

\callout{\rFigNP{F-sigcompare}} shows the potential density range
(min/max) observed within the rift valley as well as density profiles
of an eastern and a western background station. The stratification
within the rift valley below \m{2200} is reduced with respect to the
ridge flanks, consistent with the observations of
\citeN{Progress/saunders+francis85}. The two background profiles
indicate a cross-ridge density gradient, bracketing the density range
observed in the rift-valley profiles above \m{2000}. The data of
\citeN{Progress/fukumori+91} show a similar cross-ridge gradient. This
density drop across the ridge axis implies that inflows into the rift
valley across sills from the east are energetically favored. 

% - T/S comparison

\begin{figure}[t]

\placeFig{HYD/F_tscompare}

\caption[1997 full-depth \ThetaS]{\ThetaS diagram showing the mean
water characteristics of the two deep basins on either side of \RR as
well as the characteristics of an eastern and a western off-ridge
background profile; potential density contours are referenced to \dbar{2000}
(\SigmaTwo), contour interval is $0.2$.}

\labelFig{F-tscompare}

\end{figure}

\callout{\rFigNP{F-tscompare}} shows the pressure-averaged \ThetaS
diagrams of the CTD stations of the NE and SW basins
\rfig{Rainbow-geography} as well as those of two off-ridge reference
stations. The rift-valley water characteristics are almost identical to
those of the eastern off-ridge profile for $\sigma_2>36.7$ (below
\m{\approx\!1300}) while there is a significant difference to those of
the western background profile. Therefore, the valley water most likely
originates east of the MAR as was previously asserted by
\citeN{EPSL/german+98}. The \ThetaS characteristics of the water in the
two basins separated by \RS are virtually identical, indicating that
the deep water of both basins most likely has the same origin,
consistent with the small range of potential densities observed in the
rift valley below \m{2500} \rfig{F-sigcompare}.

\label{sec:F-interleave}

\begin{figure}[t]

\placeFig{HYD/F_tsinterleave}

\caption[1997 rift-valley \ThetaS]{\ThetaS diagram showing interleaving
of cold/fresh water between \m{1720} and \m{1920} in a profile from the
SW basin; \SigmaTwo contour interval is $0.025$.}

\labelFig{F-tsinterleave}

\end{figure}

Individual CTD profiles indicate that the situation in the rift valley
is not as simple as the mean hydrographic properties suggest
\callout{\rfig{F-tsinterleave}}. Features such as the \ThetaS offset of
approximately \degC{-0.1} between \degC{4.1} (\m{1975}) and \degC{4.5}
(\m{1690}) in the rift-valley profile are common in the SW basin. In
the NE basin they are not observed away from the eastern slope of \RR
(i.e.\ east of \lonW{33}{50}). In profiles with hydrothermal plume
signatures \rsec{P-1997} such structures occur higher up in the water
column than the light-scattering peaks, indicating that these
hydrographic offsets are not caused by hydrothermal processes. Because
there is no evidence for additional water masses in this region the
non-hydrothermal \ThetaS offsets are assumed to be caused by
interleaving of western off-ridge water as noted by
\citeN{EPSL/german+98}. The depth and horizontal distribution of the
interleaving structures suggest a possible pathway for western water to
intrude into the rift valley across a sill near
\latN{36}{25}/\lonW{34}{10} \rfig{AMAR-SS97}.

