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%                    I N T R O / M O T I V A T I O N . T E X 
%                    doc: Thu Feb 10 12:17:38 2000
%                    dlm: Wed Jun  7 21:44:26 2000
%                    (c) 2000 A.M. Thurnherr
%                    uE-Info: 122 59 NIL 0 0 72 3 2 4 ofnI
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% MOR intro, types, segmentation

The mid-ocean ridge system is the largest mountain chain of the Earth,
spanning the entire globe with a total length of over \km*{60}{3} and
rising to a typical height of \km{3} above the abyssal basins of the
ocean. It marks the constructive boundaries between tectonic plates
where hot material rises from the asthenosphere forming new oceanic
lithosphere \citeEG{books/fowler90}. One of the principal parameters
characterizing mid-ocean ridges is the spreading rate of the underlying
plate boundaries. Together with the magma supply it determines the
cross-axial morphology \citeEG{Science/macdonald+91}. Most
slow-spreading ridges such as the \MAR are characterized by rough
topography and deep (\km{1$--$3}) median (or rift) valleys while
fast-spreading ridges are smoother and lack deep valleys. Mid-ocean
ridges are not continuous in the along-axis direction. At more or less
regular intervals the ridge axes are laterally offset by transform
faults and non-transform discontinuities separating the ridges into
individual segments. Transform faults are generally associated with
deep cross-axial valleys which connect the ridge axes to the open
ocean. Neighboring transform faults are typically a few hundreds of
kilometers apart. Non-transform discontinuities are characterized by
lateral offsets which are smaller than the width of the ridges
(\km{<\!30}). Because most such discontinuities occur at depth maxima
the rift valleys of slow-spreading ridges are often (partially) blocked
between segments. Typical distances between neighboring non-transform
discontinuities are a few tens of kilometers.

% Mixing

The water column overlying the rough topography of mid-ocean ridge
systems is associated with greatly enhanced levels of diapycnal mixing
\citeEG{Science/polzin+97}, which is required to balance the convective
deep-water formation, thereby closing the global circulation
\citeEG{DSR/munk66}. The processes giving rise to the enhanced mixing
are not well known but it has been inferred that approximately half of
the total energy required can be derived from astronomical tides
interacting with the topography \cite{DSR/munk+wunsch98}. Strong
vertical shear associated with hydraulically controlled overflows is
also known to greatly enhance diapycnal mixing
\citeEG{JGR/wesson+gregg94,Nature/polzin+96}. While it has been
hypothesized that canyons on the flanks of mid-ocean ridges are
locations where much of the diapycnal fluxes take place
\cite{Science/polzin+97,Nature/ledwell+00} the role of the rift valleys
of slow-spreading ridges has not been investigated in this context.

% Geothermal Fluxes

The young lithosphere near mid-ocean ridge axes gradually looses its
excess heat to the water column. Comparison of oceanic heat flow
measurements with thermal models indicate that, in addition to
conductive cooling, a large amount of the heat is transferred by
hydrothermal circulation (see \citeN{JGR/lowell+95} for a historical
review and \citeN{JGR/stein+stein94} for a recent evaluation of current
models and estimates). The most spectacular manifestations of
hydrothermal circulation are the high-temperature vent fields which
give rise to particle-rich plumes rising hundreds of meters above the
seafloor where they spread laterally into the surrounding water column.
In addition to the resulting hydrographic anomalies and the
corresponding dynamical responses, hydrothermal processes have profound
effects on ocean chemistry and biology. For some chemical species
hydrothermal circulation constitutes a source of similar magnitude to
riverine input, while particle precipitation in hydrothermal plumes
removes others \citeEG{review/elderfield+schultz96}. The input of heat
and chemicals also supports distinct and fascinating ecosystems
\citeEG{review/tunnicliffe91}. To assess the impact of hydrothermal
processes the fluxes associated with individual vent fields must be
quantified.

The dispersal and fluxes of hydrothermal plumes in the rift valley of
the \MAR have been the focus of a number of studies (see \rsecNP{BG-HT}
for a detailed review). Most of the attention has been directed at the
immediate vicinity of the vent fields, usually with the main emphasis
on plume-driven dispersal. Inter-segment dispersal has received little
attention and dispersal beyond the rift valley appears not to have been
investigated at all, possibly because of the apparent lack of
basin-scale hydrothermal signatures in the Atlantic
\citeEG{Science/talley+johnson94}. (Recently, hydrothermal helium
\cite{JGR/roether+98,DSR/ruth+00} and methane \cite{GRL/rehder+99}
anomalies have been reported in the vicinity of the \MAR crest,
however.) The dispersal of hydrothermal plumes is closely linked to the
biological colonization pathways between vent fields
\citeEG{DSR/kim+mullineaux98}.

The following main questions are addressed in this thesis:
\begin{enumerate}

\item What are the dominant processes determining the hydrography and
flow within the rift valley of the \MAR?

\item What role do conductive geothermal heating and hydrothermal
circulation play in this context?

\item Which processes (plume-driven, topographic, etc.) control the
segment-scale dispersal of hydrothermal plumes?

\item How are hydrothermal tracers and biota dispersed between segments
and how do they exit the rift valley?

\item What are the impacts of rift-valley processes on larger scales?

\end{enumerate} While some of these questions (especially 4 and 5)
require much more investigation than is possible in the context of a
thesis an attempt is made at providing answers by studying data from
the rift valley of two connected segments of the \MAR containing a
large hydrothermal vent field. It is expected that some of the insights
gained will be applicable elsewhere.

The data sets analyzed here were collected in the course of the \F
(Fluxes at \A Experiment) project \cite{misc/german+95}, the goal of
which is to perform a comprehensive study of the physical, geochemical
and biological processes associated with the dispersal of the effluents
rising from the largest (in terms of the lateral extent of the
non-buoyant plume) known hydrothermal site of the \MAR, located within
the rift valley near \latN{36}{15}. The project was set up in the
framework of the \emph{AMORES EC MAST III} program.

