Transcript rtifinal - Econscience.org

A modeling approach for evaluating stability of subsurface habitats
Scott Veirs and Russ McDuff, School of Oceanography, University of Washington
[email protected] www2.ocean.washington.edu/~scottv/
How shall we model the extent of the subsurface biosphere?
Physical circulation models
Reactive transport models
Summary and background:
At most spreading centers, geophysical
data constrain the depth of circulation by
locating the axial heat source (hot rock or
magma). This figure juxtaposes
hydrothermal vent and micro-earthquake
locations at the Endeavour segment of the
volcanic ridge in the Northeast Pacific by
projecting them parallel to the axis onto a
cross-section of the ridge. Since
hyperthermophiles can’t live in the hot
rock or cold seawater, these data establish
the first-order extent of the axial
subsurface biosphere: approximately the
top 4 km of the oceanic crust.
Thorough characterization of microbial habitats within submarine
volcanoes demands that fluid flow models include the chemical
reactions that occur between hot rock and seawater. The subsurface
region where reduced chemicals in hydrothermal fluids mix with
oxygenated seawater may enable high microbial productivity
(McCollom and Shock, 1997). This poster explores the nature of
such mixing zones and suggests approaches to developing accurate
reactive transport models of specific subsurface environments.
Extant physical circulation models
(e.g. Steefel and Lasaga, 1994) map
the temperature and velocity fields in
the upper crust, further constraining
which portion of the subsurface is
inhabitable. Microbes might be
expected to thrive anywhere within
the volume bounded by the 50oC and
150oC isotherms. The models,
however, make important (possibly
inaccurate) assumptions about the
permeability structure of the upper
crust and its modification by
chemical precipitation and
dissolution.
Hydrothermal vents
UPFLOW
Earthquakes
HEAT SOURCE
Ridge Axis
Mixing and sulfate loss occurs at the
Galapagos Vent Field
Evidence of near surface processes:
Despite ~30 years of hydrothermal vent system study, we still know very
little about the processes which generate diffuse flow. Extant large-scale
models do not yet characterize the details of mixing, conduction, and
precipitation in the shallow subsurface, yet these processes are clearly
evident in the growing suite of vent fluid chemistry, mineralogy, and flux
observations from different hydrothermal fields. Identifying where and how
each process occurs and how they interact should lead to more accurate maps
(physical and chemical) of potential microbial habitat. As an example, we
focus on the sulfate ion (SO4) here because it can alter permeability in
hydrothermal systems (through anhydrite (Ca SO4 precipitation).
*
*
F=Volume Flux
Fseawater > ~15Fsource
Improved computing power and thermodynamic databases make possible
the addition of chemistry to physical circulation models. Depicted above
are the results for some minerals that may affect the availability of pore
space for microbes (images from www.cas.usf.edu/geology/faculty/steefel/steefel.html).
Even more subsurface mixing may occur
at the Main Endeavour Field
Separate regressions of Si and
Mg data against sample
temperature in Galapagos vent
fluids (Edmond, et al., 1979;
McDuff and Edmond, 1982) both
indicate that a high temperature
(~345oC) hydrothermal endmember (*) in equilibrium with
subsurface rocks was mixed
conservatively with >~15 parts
seawater. The sulfate data shown
here, however, extrapolate to a
lower end-member temperature
(~270oC, *), suggesting that
anhydrite precipitated in the
subsurface.
qconductive
qdiffuse
Schultz (1992) measured diffuse
and focussed vent temperatures and
volume fluxes (F) to calculate heat
fluxes (q) from a sulfide structure:
qfocussed
qseawater Upflow
Zone
qsource
Why doesn’t clogging occur?
Or does it?
qdiffuse ~ 10qfocussed
Assuming that only mixing is
involved in generating the low
temperature effluent (qconductive=0) a
balance of the volume and heat
fluxes dictates that
Fseawater ~ 30Fsource
Sulfate solubility diagram
(after McDuff and Edmond,
1982) shows that when a
high-temperature endmember (Ca-rich, Mg- and
SO4-poor) mixes
conservatively with seawater
(rich in Mg and SO4)
anhydrite precipitation is
expected at intermediate
temperatures. K is the
solubility product; Q is the
reaction quotient (actual
concentration ratio in the
hydrothermal mixture).
References:
A new approach to modeling:
Both mixing and conduction can affect chemical reactions
between rock and water in the subsurface. Construction of a
reactive transport model of the hydrothermal upflow zone
may help to disentangle the two processes. At the Main
Endeavour Field such a construction is aided by estimates of
key geophysical parameters and a thorough assessment of
end-member chemistry. Future measurements of
hydrothermal heat flux partitioning and of fluid chemistry
over a full range of venting temperatures will provide
opportunities for numerical model verification.
At the TAG hydrothermal site anhydrite is
precipitated through less mixing
Combinations of mixing,
conduction, and reaction...
…determine the rate of porosity
modification
A
AmSi=amorphous silica
Anh=anhydrite
Bn=bornite
Cp=chalcopyrite
Chrys=chrysotile
Hm=hematite
Py=pyrite
Tc=talc
A wide variety of mineral
assemblages are expected
to precipitate from
different mixtures of
hydrothermal fluid and
seawater at different
temperatures (Tivey, et al.,
1995). The mineralogy
observed in massive
anhydrite (ANH) samples
from the TAG
hydrothermal mound are
outlined.
B
C
An accurate reactive transport model of a hydrothermal upflow zone may
need to encompass any combination of mixing, reaction, and conductive
heat transfer. Many different trajectories through the T-mixing fractionsolubility space (depicted above) can generate diffuse fluids!
Similarly, the trajectory of a fluid will determine
how it modifies porosity in the upflow zone. The
solubility diagram (B) represents a situation in
which pure mixing may result in anhydrite
precipitation. Diagram (A) depicts schematically
how conductive heating and mixing together
result in more precipitation. Conversely, less
precipitation is expected when a hydrothermal
mixture experiences conductive cooling (diagram
C).
Thus, we expect that upflow zones that are
efficient at radiating heat will constitute more
stable physical and chemical environments for
microbes, while “hot” systems will tend to
quickly clog potential habitat.
Edmond, J., Measures, C., McDuff, R., Chan,
L., Collier, R., Grant, B., Gordon, L., Corliss,
J. (1979) Ridge crest hydrothermal activity
and the balances of the major and minor
elements in the ocean: The Galapagos data.
Earth a and Planetary Science Letters, 46:118.
McCollom, T. and Shock, E. (1997)
Geochemical constraints on
chemolithoautotrophic metabolism by
microorgansims in seafloor hydrothermal
systems. Geochimica et Cosmochimica Acta,
61, 20:4375-4391.
McDuff, R. and Edmond, J. (1982) On the fate
of sulfate during hydrothermal circulation at
mid-ocean ridges. Earth and Planetary
Science Letters, 57:117-132.
Schultz, A., Delaney, J., and McDuff, R.
(1992) On the partitioning of heat flux
between diffuse and point source seafloor
venting. Journal of Geophysical Research, 97,
B9:12299-12314.
Steefel. C. and Lasaga, A., (1994) A coupled
model for transport of multiple chemical
species and kinetic precipitation/dissolution
eactions with application to reactive flow in
single phase hydrothermal systems. American
Journal of Science, 294:529-592.
Tivey, M., Humphris, S., Thompson, G.,
Hannington, M., and Rona, P. (1995)
Deducing patterns of fluid flow and mixing
within the TAG active hydrothermal mound
using mineralogical and geochemical data,
Journal of Geophysical Research, 100,
B7:12527-12555.