Workshop Report - Ridge 2000 Program

On the ridge segment scale, detailed plume mapping probably
by sensors towed from surface ships will inventory the vent
field distribution (both on and off axis ) for comparison to
axial morphology, petrographic heterogeneity, geophysical
variability, and faunal distributions. Techniques of flux
determination developed by the detailed vent field studies will
allow a reliable determination of the total segment - scale
flux. Variability on an annual to decadal scale must be
assessed by repeated surveys of selected segments, although the
contribution of large but infrequent hydrothermal events should
be continuously monitored by an appropriate distribution of
moored sensors. Identification of such events will pinpoint
fruitful locations for geological and geophysical studies.
Basin scale studies have equal along and across-axis
components and are central to the primary objective of
determining the global effect of hydrothermal venting on the
oceanic environment. Along-axis plume surveys, conducted
simultaneously with geophysical surveys and calibrated by a few
detailed segment-scale surveys, will provide a first-order
estimate of the global magnitude and distribution of the
hydrothermal flux. Across-axis surveys of the dispersing plume
will establish the hydrothermal link to oceanic chemistry,
mixing, and circulation. Complementary sediment studies provide
the geologic history of venting.
Data Collection and Measurement Capabilities
We have identified four different scales in space and time
over which data collection must occur.
On the scale of an individual vent orifice or chimney, we
require accurate measurements of the temperature and composition
of the exiting hydrothermal fluids, both high and low temperature,
including an estimate for the hydrothermal end-memberl s)
feeding a vent field. These data are basic for characterizing
the source of any hydrothermal plume. Measurements of mass and
heat flux from individual high-temperature vents are also
essential, along with video recordings of vent orifices to aid
in quantifying the proportion of the total flux which occurs via
hot, focused venting vs. cooler, diffuse venting. Technical
improvements such as acoustic or electromagnetic flowmeters
should be investigated for this application.
On the scale of a vent field, on the order of 100 m x 100 m,
we require detailed mapping of topography, geological and hydrothermal
features, and physical and chemical parameters in the
buoyant, rising plume( s) on a scale of a few meters to a few
hundred meters. Fine-scale measurements of temperature,
velocity. composition, and particle distribution are required
because the rising plume is characterized by large gradients in
space and time. Measurements on this scale should produce the
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