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%                    N E S P I / F L A M E 2 . T E X 
%                    doc: Mon Mar 27 17:10:40 2000
%                    dlm: Wed Jun  7 23:42:34 2000
%                    (c) 2000 A.M. Thurnherr
%                    uE-Info: 66 19 NIL 0 0 72 3 2 4 ofnI
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\begin{figure}[t]

\placeFig{NESPI/RF2Coverlays}

\caption[1998 scaled nephelometry and spice profiles]{Overlays of
scaled nephelometry and spice profiles of the \FF data set; successive
profiles are horizontally offset by \protect\degC{0.03}; profiles \pa,
\pb and \pd correspond to profiles \pa, \pd and \pe of
\protect\rfigNP{F2-nephprofs}.}

\labelFig{F2-overlays}

\end{figure}

The bilinear method was applied to the data set from the 1998 survey as
well. Because different nephelometers were used during the two cruises
the magnitudes of the light-scattering peaks cannot be compared
directly (c.f.\ \rchapNP{plume}). Based on initial observations, 4 out
of the 8 profiles with plume signals (thick bars in
\rfigNP{F2-plume3d}) have detectable temperature anomalies associated
with the nephelometry peaks, defining the lower limit of detectability
in this data set as a nephelometer response of \V{0.2}. The
corresponding spice and scaled nephelometry anomalies of the 4 profiles
are shown in \rfigNP{F2-overlays}. To allow direct comparison with
\rfigNP{F-overlays} the same temperature scale is used. The maximal
temperature anomalies of the 1998 data set are of order \degC{-0.015},
i.e.\ approximately half of those observed in 1997. There is again a
general agreement between the vertical structures observed in the
light-scattering and in the corresponding spice profiles. As is the
case in the 1997 data sets, the bilinear method is biased towards the
hydrography below the particle plume.

Using the same criterion applied to the 1997 data for consistency
(i.e.\ spice vs.\ nephel $|r| \geq 0.4$) the bilinear method fails in
the case of profile \pd, most likely because of the weakness of the
plume signatures and because the particle layer coincides with a change
in the background $T/S$ slope. The remaining profiles yield spice vs.\
nephel regression slopes, correlation coefficients and RMS errors of
\degCpV*{-2.8(\pm0.3)}{-2}, $-0.83$ and \RMS{2.7} \pa;
\degCpV*{-2.8(\pm0.2)}{-2}, $-0.73$ and \RMS{2.7} \pb; and
\degCpV*{-2.7(\pm0.4)}{-2}, $-0.59$ and \RMS{2.6} \pc, respectively. As
is the case in the 1997 data sets, the regressions are mutually
consistent. The near-equality of the RMS errors of the CTD data sets
from the two surveys is consistent with the similarity of the two
instruments \rsecs{F-methods}{F2-methods}. 

\rExpr{mcd} indicates that differences in the equilibrium-plume
temperature anomalies can be caused by changes in the background
hydrography (\Rr) or in the hydrothermal effluent properties (\Ri). The
stability ratio of the water column \Rr is calculated from the
near-source CTD profiles in the depth range of plume rise
(\m{1900$--$2300} in 1997 and \m{1800$--$2300} in 1998), with
\pdegC*{\alpha=1.47}{-4} and \ppsu*{\beta=7.53}{-4} at \degC{4},
\psu{35} and \dbar{2000} taken from \citeN{books/gill82}. The
respective values are $\Rr=2.36(\pm0.11)$ for 1997 and
$\Rr=2.24(\pm0.04)$ for 1998, i.e.\ the difference is less than 5\%,
indicating that changes in the magnitude of the hydrographic anomalies
would primarily be caused by changes in the hydrothermal source fluid.
However, the small number of profiles taken during the 1998 survey does
not permit a mean plume to be derived so that no change in the effluent
properties can be inferred from our data.

