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%                    I N T R O / F L A M E . T E X 
%                    doc: Wed Apr  8 09:08:02 1998
%                    dlm: Wed Jun  7 21:54:05 2000
%                    (c) 1998 A.M. Thurnherr
%                    uE-Info: 80 69 NIL 0 0 72 3 2 4 ofnI
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\begin{figure}[t]

\placeFig[1]{intro/F_stations}

\caption[1997 stations \Fp{(}{)}]{\F stations; shading is the same as
in \rfigNP{AMAR-bridge}; contour interval is \m{250}; CTD stations are
marked with stars and \BGT tow-yos with circles. The \BGT tows marked
with black circles are the cross-sill tow of \rfigNP{sigBGT07} and the
cross-ridge tow of \rfigNP{sigBGT04}. The black star with a white
border near \RS indicates the position of the \h{10} CTD yoyo
\rfig{yoyo}; the small stars on the western slope of \RR show the CTD
tow-yo.}

\labelFig{F-stations}

\end{figure}

In May/June 1997 the first of two cruises to the \A segments (the \F
cruise) took place. Two different platforms were used to collect data
at the stations shown in \callout{\rfigNP{F-stations}}: \BGT, an
instrument developed for deep tow-yo lines through hydrothermal plumes
\cite{EOS/rudnicki+95}, and a standard CTD package. In addition to the
CTD stations shown, two eastern\REM{CTD001,014} (\km{\approx\!50} off
axis) and one western\REM{CTD031} (\km{\approx\!100} off axis)
background stations were occupied for reference. Total sampling time
was \d{24}.

%	- BRIDGET

The \BGT platform was used to collect data between \m{1700} and the sea
bed by tow-yoing it at speeds not exceeding 1.5 knots. It was equipped
with one and sometimes two self-contained \emph{FSI MicroCTD}
instruments, a \emph{Chelsea Instruments Aquatracka} Nephelometer
(measuring light-scattering at a \deg{90} angle), as well as a
12-bottle sampling rosette and auxiliary instruments such as attitude
sensors and an altimeter. The circles representing the \BGT tows in
\rfigNP{F-stations} show the locations of the intersections of the
tow-yo lines with the \m{2100} depth.

%	- CTD package

The CTD package was used for vertical profiles, a tow-yo, and a
ten-hour yoyo. It was equipped with a \emph{General Oceanics Neil Brown
Mk3c CTD,}  a \emph{Chelsea Instruments Mk2 Subaquatracka}
Nephelometer, a self-contained \emph{RDI Acoustic Doppler Current
Profiler} used as a lowered ADCP (LADCP), a 12-bottle sampling rosette,
as well as auxiliary instruments such as an altimeter.

%	- calibrations & accuracies

The temperature sensors of two of the three CTDs were pre-cruise
calibrated (the remaining one was taken as a spare). The \emph{Neil
Brown CTD} was re-calibrated 10 months after the cruise. Based on the
calibration information and direct instrument comparison on the \BGT
platform the temperature intercalibration error is estimated to be
\degC{\pm10^{-2}}. At the mean plume depth (\m{2100}; see
\rsecNP{P-1997}) this corresponds to a density uncertainty of
\kgpmmm*{\pm2}{-3}, the same order of magnitude as some of the
important hydrographic patterns (e.g.\ \rfigNP{F-sigprof}). Therefore,
data sets from different instruments were not combined. Because the CTD
data set is essentially a spatial subset of the \BGT data set
\rfig{F-stations}, this is not a problem --- the CTD data were used to
verify the patterns observed in the \BGT data.

The conductivity sensor of the \emph{Neil Brown CTD} was calibrated by
analyzing 207 bottle samples with an \emph{Ocean Scientific
International AUTOSAL Model 8400A} salinometer, resulting in a RMS
error of \psu*{2.7}{-3}. Because of hardware problems with one of the
two CTDs flown on \BGT not enough bottle samples were available to
reliably calibrate either conductivity sensor. The salinity calibration
of one of the CTDs that failed twice during the cruise was judged to be
too unreliable to be used at all. Therefore, the measurements of that
instrument were removed from the hydrographic data set. (The affected
tows are not shown in \rfigNP{F-stations}.) A trend of \psu*{5.8}{-4}
per day was removed from the data of the 2nd \BGT CTD. For calibration,
the resulting $T/S$ was fitted against the Neil Brown $T/S$. To assess
the quality of this intercalibration, the result was compared to the 37
available AUTOSAL samples for the \BGT instrument resulting in a RMS
error of \psu*{1.7}{-3} in the plume range (\psu{35$--$35.1}). This
corresponds to a density uncertainty of \kgpmmm*{\pm1.5}{-3}.

%	- Nephelometry

Nephelometry values are usually reported in arbitrary units called
``nephels'' \cite{EPSL/nelsen+87}; here, instrument voltage (V) is
used. To remove the offset between the CTD and the \BGT nephelometers
as well as the trends observed during long \BGT tow-yos (caused by
lens-fogging), an offset was applied to the individual profiles by
subtracting the mean value between \m{1500} (or the upper turning point
of the \BGT tow-yo profiles) and \m{1850} (above the highest particle
plume observations; \rsecNP{P-1997}). This results in some negative
nephel values. To compare the response characteristics of the two
nephelometers the mean values ($\pm1\sigma$) of the near-surface
light-scattering maximum were compared; the resulting agreement between
\V{0.15(\pm0.05)} derived from 19 \BGT profiles and \V{0.14(\pm0.03)}
derived from 25~CTD profiles indicates that the (assumed linear)
response characteristics of the two instruments are comparable.

%	- LADCP

The LADCP data presented here were derived from water-track data
averaged over \m{5} bins. In contrast to the data presented by
\citeN{EPSL/german+98} no averaging between the up- and the downcasts
was performed. Average layer velocities (e.g.\ \rfigNP{AC}) were
derived by separately calculating the mean of the northward and
eastward velocity components  --- this method may produce small values
in dynamically active layers where velocity components take on both
signs.

