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%                    N E S P I / D I S C U . T E X 
%                    doc: Mon Aug 30 15:52:34 1999
%                    dlm: Wed Jun  7 23:51:17 2000
%                    (c) 1999 A.M. Thurnherr
%                    uE-Info: 89 59 NIL 0 0 72 3 2 4 ofnI
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% SIMILARITY OF TWO METHODS

The two new methods developed to calculate hydrographic anomalies
associated with hydrothermal particle plumes correspond to two new
anomaly definitions which are equivalent if the particle and spice
anomalies are linearly correlated and if the background $T/S$
relationship in the particle plume is linear, as is the case in the
data from the two quasi-synoptic surveys. The plume-bracketing method
is equivalent to the method of \citeN{Nature/lupton+85} if the $T/S$
background outside the particle plume is linear and both are equivalent
to the extrapolation method if the $T/S$ background above the plume is
linear, apparently implying that the extrapolation method is equivalent
to the method of \citeN{Nature/lupton+85}. Because extrapolation
requires much stricter adherence to linearity than interpolation,
methods based on the latter are preferable. If the particles within a
given peak of a single profile behave non-conservatively (implying
either reaction timescales which are shorter than the sampling
timescale or separate unmixed constituents making up the peak) the
bilinear method cannot be used. If, on the other hand, there are
background anomaly gradients coinciding with the particle peak, as was
argued for Pacific profile \pb, the plume-bracketing method cannot be
used. 

% SPICE/NEPHELS CORRELATIONS OF DIFFERENT METHODS / BACKGROUNDS
% IMPLICATIONS OF LINEAR CORRELATION

The near-source particle distribution of the \R hydrothermal plume
closely mirrors the corresponding hydrographic anomalies calculated
with the new methods. The linear correlations between the
light-scattering and the spice anomalies indicate that the nephelometry
anomalies of individual profiles behave conservatively. Similar
correlations were obtained by applying the bilinear method to three
Pacific profiles while the corresponding temperature anomalies derived
with the extrapolation method result in significantly lower
correlations in two of the three profiles. The linear relationships
resulting from the application of the new methods (and the
corresponding new anomaly definitions) imply that both temperature and
particle anomalies reflect the amount of dilution of the hydrothermal
effluents with background fluid entrained during plume rise. Mixing as
the dominant physical process governing the distribution of both types
of anomalies is consistent with the bias of the new methods towards the
background hydrographic properties and particle loads below the
equilibrium plumes, and with the Gaussian shape of the density-averaged
nephelometry profiles of the 1997 \BGT data set \rsec{P-1997}.

% CONSERVATIVE BEHAVIOUR ON LARGER SCALE

All three data sets of the \R hydrothermal plume yield mutually
consistent (within each data set) spice vs.\ nephel relationships,
indicating that the conservative behavior of the hydrothermal particles
is not limited to the short time scales associated with individual
profiles.  Linear correlations between particle, chemical and
hydrographic anomalies of hydrothermal plumes have been observed at
other sites, most notably in event plumes, while the corresponding
relationships in \emph{steady-state} (i.e.\ non-event) plumes are
usually reported to be more variable
\citeEG{EPSL/baker+massoth87,JGR/chin+94}\REM{1st: temp/particles, 2nd:
temp/chemical}. The results shown here indicate that this variability
may be caused partially by uncertainties in the estimates of the
background properties relative to which the hydrographic anomalies are
defined. This has important consequences for the interpretation of
anomaly ratios. The observed temporal and spatial variability of
``extrapolated'' hydrographic vs.\ particle \citeEG{JGR/baker94} and
vs.\ chemical anomalies \citeEG{EPSL/lupton+99} may be caused partially
by background variability, for example.

% RESIDUAL PROFILES => NEED PROCESSES

Comparing the results from the different methods applied to the Pacific
profiles indicates that heat anomalies derived with the extrapolation
method can be dominated by contributions from processes which are not
associated with the equilibrium particle plumes rising from
high-temperature hydrothermal vent fields. This may be one of the
reasons for the large difference between heat flux estimates calculated
from ``extrapolated'' temperature anomalies and those derived from
measurements at black-smoker vent orifices and in buoyant plumes on the
Juan de Fuca Ridge, discussed by \citeN{JGR/ginster+94}. Because of the
similarity of the residual temperature anomaly profiles with GBL
observations without associated particle plume signatures
\citeEG{JGR/thomson+95} and because of the lack of alternative process
candidates which could cause the observed isopycnal warming, it
appears reasonable to assume that the residual profiles are signatures
of geothermal heating. Not much can be concluded from only two profiles
but it is noteworthy that the residual temperature anomalies extend
above the equilibrium particle plumes in both cases. Therefore, it
appears unlikely that they are caused entirely by low-temperature (or
``diffuse'') hydrothermal circulation and heat conduction in the nearby
vent fields.

