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In this chapter the particle-plume observations of the two
quasi-synoptic surveys of the 1997 \F \rsec{F-methods} and the 1998 \FF
\rsec{F2-methods} cruises are presented. The main goal is to
investigate the horizontal and vertical distribution of the
hydrothermal particle anomalies in the water column in the vicinity of
the \R vent field \rfig{Rainbow-geography}. 

When the first hydrothermal plumes were discovered in the Galapagos
rift it was noted that some of the temperature anomalies were
associated with small transmissometry signals \cite{Nature/weiss+77}.
Subsequent discovery and investigation of high-temperature vent fields
revealed that metal-rich hydrothermal effluents give rise to particle
plumes, consisting primarily of precipitated metal sulfides and oxides
\citeEG{JGR/feely+87}. Because high particle loads in the water column
can readily be detected with transmissometers and nephelometers
(measuring light scattering), these instruments are used extensively to
track hydrothermal plume dispersal \citeEG{Nature/baker+85}, to
prospect for new vent fields \citeEG{EPSL/german+96}, and to
investigate the temporal variability of hydrothermal plumes
\citeEG{JGR/baker94,EPSL/rudnicki+94}. More quantitatively, optical
anomalies have been calibrated against total particle loads
\citeEG{Nature/baker+85} and against concentrations of hydrothermal
geochemical tracers \citeEG{EPSL/nelsen+87}.

There are a number of inherent problems associated with optical
particle measurements, however. Different instruments measure different
optical properties such as light transmission and scattering at
different angles and wavelengths. The optical properties of the water
column depend on the total particle loads, the particle size fractions,
and on the surface properties (shape, reflectance, etc.) of the
particles \citeEG{JGR/baker+lavelle84}. The surface properties of
hydrothermal particles depend on the chemical composition of the source
fluid. Therefore, separate calibrations are required for each study and
every instrument. Optical instruments furthermore tend to `fog'
resulting in calibration shifts with time; this is especially
problematic during long tow-yo casts \citeEG{EPSL/thomson+92}.

Plume-dispersal studies based on optical particle observations are
further hampered by variable particle behavior. Different studies have
shown near-constant particle loads on time scales of months in some
plumes \citeEG{Science/lupton+98}, rapidly (order of days) decreasing
particle concentrations because of sedimentation and dissolution in
others \citeEG{EPSL/baker+massoth87,EPSL/kadko+90}, and even increasing
particle loads, presumably because of in-situ precipitation continuing
for days after release of the effluents \citeEG{GRL/baker+95}.
Particles settling out of hydrothermal plumes have been inferred to be
recycled by re-entrainment
\citeEG{EPSL/german+sparks91,EPSL/lane-serff95,inbook/rudnicki95} and
sedimentation has been shown to be potentially important for the
dynamics of non-buoyant plumes \citeEG{JGR/hoyal+99}.

In the vicinity of hydrothermal vent fields particle distributions in
the water column are often highly inhomogeneous
\citeEG{EPSL/nelsen+87,misc/rudnicki+elderfield92}. Sometimes there are
significant differences between a profile's down- and upcasts
\citeEG{conf/radford-knoery+98}. Vertical structure in near-field
hydrothermal particle distributions has been attributed to layering
caused by multiple sources \citeEG{misc/rudnicki+elderfield92}, to
time-varying buoyancy fluxes and background flow fields
\citeEG{JGR/thomson+89}, and to turbulent plume structure
\citeEG{EPSL/nelsen+87}. Spatial variability in conjunction with
contouring can lead to misinterpretations concerning the number of
hydrothermal sources giving rise to observed particle distributions
\citeEG{EOS/rudnicki+95,misc/james+95}.

The background particle load of the water column is not constant, with
both biogenic inputs from the euphotic zone and sediment resuspension
in the bottom boundary layer influencing the distributions
\citeEG{DSR/eittreim+76}. Optical anomalies alone are therefore not
necessarily reliable indicators for hydrothermal processes, especially
in the presence of strong flows and in the vicinity of topography as is
the case in the \A segments.

Because of these observations the particle distributions observed
during the two quasi-synoptic surveys are analyzed qualitatively in
this chapter (in \rchapNP{NESPI} the optical measurements are
quantified by their associated hydrographic anomalies). In
\rsecNP{P-1997} the light-scattering observations of the 1997 survey
are presented. The hydrothermal particle plume rising into the boundary
current flowing along the western slope of \RR is advected northwards
away from the vent field, across \RS and into the NE basin where it
appears to detach from the eastern slope of \RR and to enter the NE
basin interior. Optical anomalies consistent with the hydrothermal
plume are detected up to the limit of the survey, \km{30} from the vent
field. Density-averaged nephelometry profiles show the particle anomaly
distribution to be Gaussian in depth, even in the near-field where
many of the individual profiles are characterized by multiple peaks.

In \rsecNP{P-1998} the light-scattering observations of the 1998 survey
are presented. During the \FF cruise the strongest particle anomalies
were observed SW of the vent field away from \RR and approximately
\m{100} higher in the water column than during the 1997 survey, whereas
the particles in the vicinity of \RR were observed further down in the
water column. This bipartition of the hydrothermal particle plume is
consistent with the current-meter observations which indicate strongly
sheared flow in the vicinity of the vent field. The chapter concludes
with a discussion in \rsecNP{P-discu}.



