Workshop Report - Ridge 2000 Program

most accurate estimates of heat and chemical flux from individual
vent fields. They also are required for optimal placement
of moorings designed to monitor variation in vent-field plumes
with time.
Vent-field monitoring is appropriate over time scales of
weeks to months or longer, in order to measure possible
variations in output resulting from subsurface processes. It
should include thermistors, current meters, transmissometers,
and chemical sensors. Sediment traps and novel remote sensors
such as electromagnetic and acoustic doppler current meters,
acoustic backscattering and tomography devices for measuring
plume structure, and various chemical sensors would also be
useful.
Particles represent a major problem, both in characterizing
their concentration and size distribution and in preventing
their reaction with ambient water after sampling. .!.!:!. situ
pumping and filtration as well as laser particle sizing should
be developed or adapted to solve these problems.
On the scale of one to several ridge segments, or about
200 km along the axis, measurements have traditionally been made
from surface ships. We recommend development of an apparatus
for efficiently detecting and characterizing hydrothermal .
plumes. The apparatus should be compatible with high-speed
("'8 kts) geophysical surveys as well as with slower and more
detailed vent-field surveys. It should at least measure
temperature, conductivity, and light transmittance every 30 m
over the lower 600 m of the water column and also higher, to
detect both normal plumes and larger transient plumes as
discovered in August 1986 on the S. Juan de Fuca Ridge. It
could consist of an ROV, or of a towed fiber optic sensor array,
with power supply and data processing equipment located on the
ship rather than on the cable to reduce drag. The apparatus
should be designed to accommodate additional chemical sensors as
they become available.
Monitoring from long-term moorings is also appropriate on
the scale of a ridge segment. primarily to detect large
intermittent, transient plumes such as that noted above, which
had a diameter of 20 km. These moorings could detect a similar
plume if spaced every 10 km. Once detected, it would be
important to track such a plume, possibly with an instrumented,
neutrally buoyant Lagrangrian drifter.
The largest scale with which we are concerned is that of an
ocean basin, equivalent to a 1000 km section of ridge axis. The
apparatus described above, for characterizing plumes, used
either alone or as part of a survey, should be capable of
rapidly determining the presence or absence of plumes along the
ridge axis over these distances. Off-axis advection and
dispersal on the scale of an ocean basin can be evaluated using
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