%======================================================================
%                    I N T R O / B A C K G R O U N D . T E X 
%                    doc: Fri Feb 11 10:42:33 2000
%                    dlm: Wed Jun  7 21:48:58 2000
%                    (c) 2000 A.M. Thurnherr
%                    uE-Info: 283 49 NIL 0 0 72 3 2 4 ofnI
%======================================================================

\Subsection{BG-MAR}{Rift-Valley Hydrography and Flow}

% Saunders+Francis85:
%	- rift valley H/G received first special attention in context of H/T
%	- obs: 	- topographic blocking results in reduced strat below sills
%			  (also shown in JGR/rona+speer89, EPSL/murton+99 but with
%			  different explanation)
%			- deep temp near sills is consistent with sill depths
%			- temperature gradient within segments (0.02°C in short
%			  segs, 0.1°C in one long segment) with coldest temp near
%			  inflow sill (Fig.2)
%	- valley flushing:
%			- periodic arrival of dense water at lowest sill (similar to
%			  fjords) & hydraulically controlled (p.533) inflow
%			- periods of homogenization & upward density flux
%				- tidal stirring most likely
%				- geothermal heating possible but unlikely to be main
%				  cause because of episodicity (NB: only one MAR H/T src
%				  known then)

The first systematic study of the segment-scale hydrography within the
median valley of the MAR was carried out by
\citeN{Progress/saunders+francis85}. They reviewed 27 temperature
profiles taken in the rift valley of 7 segments between \degN{43}
and \degN{47} and report the following observations: \begin{itemize}

\item the density stratification within the rift valley is reduced in
comparison to profiles taken over the ridge flanks;

\item the near-bottom temperatures within short segments show
variations of order \degC{0.02};

\item the near-bottom temperature of a long (\km{\approx\!120}) segment
is characterized by a unidirectional gradient of order \degC{0.1};

\item differences between the near-bottom temperatures of adjacent
segments are as high as \degC{0.4};

\item the lowest temperature observed in each segment is similar to the
temperature in the off-ridge water column at the depth of the lowest
nearby sill which connects the rift valley to the ridge flank.

\end{itemize} From these observations
\citeN{Progress/saunders+francis85} infer that the water in the rift
valley is renewed by hydraulically controlled inflows across sills near
the locations where the coldest water is found. They investigate a
number of different processes which can potentially provide the density
flux within the segments to balance the inflows and conclude that the
most likely scenario is alternating inflow events and periods of
homogenization caused by diapycnal mixing (tidal stirring). They
discount geothermal heating as an unlikely primary driving mechanism
for the renewal of the rift-valley water because of the episodic nature
of volcanic eruptions and hydrothermal circulation. (At the time of
their investigation no hydrothermal vent fields had been discovered on
the \MAR, whereas subsequent studies have shown that hydrothermal
circulation is not uncommon in the rift valley; e.g.\
\citeNP{EPSL/german+parson98}.) Reduced rift-valley stratification has
been observed elsewhere on the MAR \citeEG{JGR/rona+speer89}. In one
case this was attributed to stirring and heating associated with a
hydrothermal vent field \cite{EPSL/murton+99}.

% + JGR/Wilson+95:
%	- rift-valley sampling approx 2 station per segment 33N--40N (NB:
%	  not high resolution, plots difficult to analyze for this
%	  interpretation because:
%		- lateral overlap of segments
%		- not clear how bathymetry is derived but not high-res enough to
%		  show sills between segments
%	- along-valley sigma section (fig.3c) shows gradient in AMAR
%	  segments below approx 1500m (difficult to evaluate because
%	  bathymetry shown does not show segment boundaries which are
%	  indicated by bathymetric highs in rift-valley by definition
% + other H/G rift valley observations
%	- single stations from cross-ridge sections
%	- near H/T sites, analyzed for specific signatures

\citeN{JGR/wilson+95} also investigated the hydrography within (and
overlying) the rift valley of the MAR. Because they contoured their
sections across segment boundaries their data, which covers the
segments between \degN{33} and \degN{40}, is difficult to evaluate in
the context of the findings of \citeN{Progress/saunders+francis85}.
Nevertheless, \MakeLowercase\figurename~3 of \citeN{JGR/wilson+95}
shows along-valley sloping isotherms and isopycnals in a number of
segments, most notably between \degN{35.6} and \degN{37.1} which
encompasses the segments investigated here. There are other studies
containing hydrographic data from the MAR rift valley, but these are
usually either single profiles from meridional sections across the
ridge \citeEG{DSR/roemmich+wunsch85} or profiles taken in the immediate
vicinity of hydrothermal vent fields, selected to show the hydrographic
signatures of the buoyant and non-buoyant plumes
\citeEG{JGR/rona+speer89,misc/rudnicki+elderfield92}.

% few dynamical / process studies

There are few investigations of physical processes within the MAR rift
valley which are not directly related to hydrothermal plumes.
\citeN{EPSL/rudnicki+94} analyzed the temporal variability of the
hydrography at the TAG hydrothermal site near \degN{26} over a
semi-diurnal tidal cycle. They show vertical excursions of the
isopycnal surfaces of the order of \m{100} consistent with the
semi-diurnal tidal frequency, and cite current meter observations
indicating that the semi-diurnal tidal currents of order \mps{0.1}
dominate the flow energy. Additional rift-valley current meter
observations were made at the \emph{Broken Spur} hydrothermal site near
\degN{29}. \citeN{EPSL/murton+99} report low-pass filtered along-valley
flow velocities peaking at \mps{0.08} and containing numerous flow
reversals during the 225 days of sampling, while 19 days' worth of data
from a different array of instruments deployed during another time in
the same segment show the flow to be dominated by the semi-diurnal
tidal currents peaking at \mps{0.05$--$0.1} \cite{misc/lukashin+95}.
Finally, observations from the \emph{Lucky Strike} hydrothermal site
near \degN{37} indicate that the flow was dominated by semi-diurnal
tidal currents of up to \mps{0.23}, and the residual of order
\mps{0.03} was predominantly parallel to the ridge axis and reversed
its direction twice during the first 3 months of data collection
\cite{EPSL/jean-baptiste+98}.

%======================================================================

\Subsection{BG-HT}{Hydrothermal Plume Dispersal and Fluxes}

% - Vent Fields & H/T Plumes

``Typical'' hydrothermal vent fields consist of a number of vent
chimneys, each with one or more orifices from which high-temperature
(\degC{\approx\!350}) effluents rise into the water column. At such
high temperatures the equation of state of sea water (the main
constituent of the hydrothermal effluents) is highly non-linear
\cite{misc/bischoff+rosenbauer85}. Sub-surface boiling and
phase-separation resulting in spatial heterogeneity of the effluents of
a single vent field may also occur, depending on the sub-surface
pressures and temperatures \citeEG{inbook/vondamm95}. Rapid initial
cooling results in precipitation of metal sulfides and oxides so that
high-temperature buoyant plumes take on the appearance of rising
``smoke'' \citeEG{EPSL/converse+84}. The (time-averaged) temperatures
in the plumes are reduced to near-ambient values close to the vent
orifices and linear equations of state can be used
\cite{EPSL/turner+campbell87}. Laboratory investigations of buoyant
plumes show high variability at any distance from the source
\citeEG{JFM/papantoniou+list89}, indicating that models of integrated
or ``bulk'' plumes \fullciteEG{RoySoc/morton+56} should not be compared
to individual profiles. The rising buoyant parcels of diluted
hydrothermal effluents mix with background water and eventually reach a
level where they are in equilibrium with the background density field
and start dispersing along isopycnal surfaces, possibly after some
vertical momentum-driven ``overshoot'' (where plume fluid is detrained,
c.f.\ \citeNP{misc/abraham+eysink69}). In the absence of other effects,
lateral plume spreading is restricted by the Earth's rotation limiting
the diameter of the equilibrium plumes to the order of the relevant
Rossby radius \cite{review/helfrich+speer95}. The rise height of
starting plumes in stagnant backgrounds is also affected by the Earth's
rotation if the buoyancy sources persist for longer than an inertial
period because of the cyclonic circulation set up around the buoyant
plumes \cite{JMR/speer+marshall95}. (The extensive literature on
turbulent jets and plumes
\citeEG{books/turner73,inbook/fischer+79,review/list82} and on observed
and modeled properties of hydrothermal plumes 
\citeEG{review/helfrich+speer95,review/speer+helfrich95,review/lupton95}
is not reviewed in detail here.)

% Diffuse

In addition to the focused high-temperature flows, lower-temperature
hydrothermal effluents not associated with strong particle anomalies
percolate through the walls of vent edifices and through porous
sediments, and rise as small plumes from cracks and fissures in the
vicinity of the ``smoker'' chimneys. There are observations 
\citeEG{EPSL/rona+trivett92,JGR/schultz+92} and at least one model
\cite{RoySoc/schultz+elderfield97} which indicate that the fluxes
associated with this ``diffuse'' hydrothermal constituent can be an
order of magnitude larger than the high-temperature fluxes in the same
vent fields but the range of ratios is unknown and the partition of the
total fluxes between ``focused'' and ``diffuse'' is an open problem
(see below).

% - H/T effluent anomalies
% - H/T flux quantification as major goal
% - direct measurement difficult because of distr of src

Hydrothermal effluents are characterized by a range of anomalous
hydrographic and chemical properties with respect to the background
water column \citeEG{inbook/vondamm95}. One of the principal goals of
hydrothermal research is to quantify the chemical and physical fluxes
associated with hydrothermal circulation. Direct measurement
\citeEG{EPSL/converse+84,JGR/schultz+92,JGR/ginster+94} is both costly
(involving manned submersibles) and difficult because of the
distributed nature of the sources. Particle anomalies, some chemical
anomalies, and sometimes even hydrographic anomalies associated with
the high-temperature hydrothermal plumes can be easy to detect in the
water column, on the other hand. Hydrothermal fluxes are therefore
often estimated from dispersal observations of the non-buoyant plumes.
Because it is not clear to what extent the effluents of the ``diffuse''
hydrothermal constituent are entrained into the high-temperature plumes
the calculated values may not represent the total fluxes. Heat-flux
estimates from the Endeavor segment of the Juan de Fuca Ridge, for
example, span two orders of magnitude \cite{review/mcduff95} with high
values derived from non-buoyant plume surveys
\cite{EPSL/baker+massoth87,EPSL/thomson+92} and low ones from direct
measurements at the vent orifices \cite{JGR/bemis+93}. This is
consistent with a large ``diffuse'' constituent entrained into the
high-temperature plumes but buoyant-plume measurements
\cite{JGR/ginster+94} do not support this view. Non-entrainment of
``diffuse'' effluents has also been inferred from observations at other
sites \citeEG{EPSL/rona+trivett92,JGR/trivett+williams94}.

A widely used method for estimating the fluxes associated with
non-buoyant hydrothermal plumes is based on the calculation of tracer
fluxes across vertical sections \cite{EPSL/baker+massoth87}. The large
uncertainties generally associated with this method reflect
uncertainties in the mean tracer and velocity distributions across the
sections \citeEG{review/mcduff95}. Determining mean tracer
distributions is complicated by spatial
\citeEG{EPSL/baker+massoth87,JGR/lavelle97} and temporal
\citeEG{EPSL/converse+84,JGR/schultz+92} variability of the
hydrothermal circulation, and by the inherent heterogeneity of
turbulent plumes. These problems are sometimes exacerbated by poor
sampling coverage \citeEG{GRL/baker+99}. In the case of heat flux
calculations from hydrographic anomaly fluxes the problem is compounded
by uncertainties as to how the anomalies should be defined in the first
place \citeEG{EPSL/thomson+92}. Uncertainties in the advection
velocities are caused by spatial and temporal variability
\citeEG{EPSL/thomson+92,EPSL/rudnicki+94}.

% - flux calculations:
%	- height-of-rise (Problems: strong dependence on N, cross-flow,
%	  uncertainty if bulk plume can be inferred from single profiles)

An alternative method for calculating hydrothermal fluxes from
non-buoyant plume observations consists in estimating the source
buoyancy fluxes by matching the height-of-rise of buoyant plume models
to the observations \citeEG{misc/rudnicki+elderfield92}. (The detailed
assumptions depend on the plume model used. Because there are currently
no numerical models which can resolve buoyant plumes rising from
multiple time-varying sources into turbulent, sheared backgrounds, the
most commonly used models are derived from that of
\fullciteN{RoySoc/morton+56}, extended to allow for separate
temperature and salinity buoyancy effects and sometimes also for
uniform crossflows \citeEG{misc/middleton+thomson86}.) Hydrothermal
fluxes can be calculated from the resulting buoyancy flux estimates and
effluent property measurements. This method has been applied primarily
to Atlantic hydrothermal plumes, possibly because the background
hydrography of the rift valley can be too complicated to allow the
quantification (or even detection) of the hydrographic anomalies
associated with the hydrothermal plumes \citeEG{EPSL/wilson+96}. The
simplifying assumptions underlying the height-of-rise method together
with its sensitivity to the observed parameters (height-of-rise,
background hydrography) give rise to the large uncertainties associated
with this method \citeEG{review/speer+helfrich95}.

% - chemical methods

Hydrothermal fluxes can also be estimated from geochemical anomaly
ratios in the plumes; this method is associated with large uncertainties
as well (see \citeN{review/elderfield+schultz96} for a discussion).

% - Atlantic flux calculations

Heat fluxes have been calculated for a number of Atlantic hydrothermal
vent fields. At \emph{TAG} near \degN{26} different applications of the
height-of-rise method resulted in estimates of \W*{500$--$940}{6}
\cite{misc/rudnicki+elderfield92} and \W*{70}{6}
\cite{EPSL/rudnicki+94} (a buoyant-plume method yielded \W*{10}{6} at
the same site; \citeNP{JGR/rona+speer89}). At \emph{Broken Spur} near
\degN{29} the height-of-rise method resulted in estimates of order
\W*{50}{6}\REM{layer method, depending on the assumed venting depth and
if the two fluxes are summed or not} \cite{misc/james+95} while a heat
budget calculated from current meter and temperature records yielded
\W*{275}{6} \cite{EPSL/murton+99}. The height-of-rise method applied to
the \emph{Lucky Strike} hydrothermal plume near \degN{37} resulted in
an estimate of \W*{120$--$400}{6} \cite{EPSL/wilson+96} while a heat
flux calculation based on the $^3$He flux in the non-buoyant plume and
the helium-heat ratio measured in the high-temperature effluents
yielded \W*{2.5$--$5}{9} \cite{EPSL/jean-baptiste+98}. Finally,
\citeN{GRL/german+96} applied the height-of-rise method to plume
observations from the \R hydrothermal vent field (the site investigated
here; \rsecNP{Rainbow}) resulting in an estimate of the order of
\W*{100}{6} (c.f.\ \rchapNP{HFLUX}).



