%======================================================================
%					 A B S T R A C T . T E X 
%                    doc: Thu Aug  5 15:52:56 1999
%                    dlm: Wed May 31 18:25:36 2000
%                    (c) 1999 A.M. Thurnherr
%                    uE-Info: 92 58 NIL 0 0 72 3 2 4 ofnI
%======================================================================

% Intro
%	- MORs, rift-valleys
%	- H/T circulation
% 	- relevance

Slow-spreading mid-ocean ridges such as the Mid-Atlantic Ridge are
characterized by deep axial rift valleys which are isolated from the
water on the ridge flanks. Topographic effects therefore have a
significant impact on the rift-valley hydrography and dynamics but
little is known about the details. Known processes of global importance
acting near the axes of mid-ocean ridges include high rates of
diapycnal mixing associated with the rough topography and
high-temperature hydrothermal circulation, a major source for a number
of chemical constituents of the ocean.

%	- site/goals
%	- methods

Physical data sets from the rift valley of two connected segments of
the Mid-Atlantic Ridge, which include the largest known hydrothermal
vent field of the Atlantic, were analyzed to investigate the
segment-scale hydrography, dynamics and geothermal fluxes. The data
include two quasi-synoptic hydrographic and particle plume surveys (one
year apart) and one-year-long records from an array of moored current
meters.

% Results
%	- rift-valley H/G & dynamics:
%		- uniform along-segment gradients
%		- hydraulic control
%		- persistent unidirectional flows (also BOF)
%		- lee waves
%		- internal waves
%		- semi-diurnal tides

The hydrographic properties of the rift-valley water were similar
during the two surveys, suggesting a stable state characterized by
inflow from the eastern ridge flank, unidirectional along-segment flow
(directly observed during an entire year), and monotonic along-valley
hydrographic gradients consistent with high rates of diapycnal mixing.
Geothermal processes do not appear to contribute significantly to these
patterns. The data contain signatures of a range of dynamical processes
consistent with high rates of diapycnal mixing, including hydraulically
controlled sill flows, topographic lee waves and high-energy tidal
flows.

%	- Gaussian plume
% 	- NESPI

The spatial distribution of the light-scattering anomalies associated
with the dispersing hydrothermal particle plume are consistent with the
dynamical observations. Close to the vent field the particle
distribution is highly inhomogeneous but density-averaged profiles
indicate that the mean plume is Gaussian in depth. To quantify the
fluxes associated with the hydrothermal plume the corresponding
hydrographic anomalies were determined. The complexity of the
hydrography within the rift valley precludes the application of
``standard'' methods so that a new method had to be developed resulting
in the first quantitative hydrographic anomaly measurements of an
Atlantic hydrothermal plume. The hydrographic and particle anomalies of
this plume are linearly correlated, indicating that the particles
behave conservatively in the near field.

%	- hydrothermal fluxes
%		- advective heat flux >> height-of-rise

Estimates for the heat flux associated with the hydrothermal plume were
derived using two established methods, one based on plume-rise modeling
and the other on the advection of heat anomalies away from the vent
field. Height-of-rise modeling yields values which are an order of
magnitude too low because the plume model relies on a point-source
assumption which is violated by the geometry of the vent field. The
uncertainties associated with the advection method are particularly
small at the site studied because of the uni-directionality of the flow
field, the small uncertainties of the hydrographic anomaly
measurements, and the Gaussian shape of the averaged near-field plume.
The resulting estimate for the heat flux associated with the particle
plume is \GW{2.5}. 

% 	- budgets
% 	- model

Mass and heat budgets of the rift valley indicate that high diapycnal
diffusivities are required to account for the hydrographic observations
and suggest that a portion of the water flowing along the rift valley
may be lost to the overlying water column. Inspired by the observations
a simple analytical and numerical model for the flow within the rift
valley was developed. The results indicate that the rift valley acts as
an efficient low-pass filter with characteristic time scales of weeks
to months, providing a plausible explanation for the persistence of the
along-segment flow.

