Transcript rtipen - Econscience.org

Modeling chemical transport in the axial subsurface
Scott Veirs and Russ McDuff, School of Oceanography, University of Washington
[email protected] www2.ocean.washington.edu/~scottv/
Extant models:
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 region of the subsurface
where reduced chemicals in hydrothermal fluids mix with oxygenated
seawater holds the potential for maximum microbial productivity. This
poster explores the nature of such mixing zones and suggests approaches
to developing accurate reactive transport models of specific subsurface
environments.
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 microearthquake locations at the
Endeavour segment of the
volcanic ridge in the Northeast
Pacific by projecting them
parallel to the axis onto a crosssection 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: about the top 4 km of
the oceanic crust.
Extant physical circulation
models (e.g. Steefel and Lasaga,
at right) map the temperature and
velocity fields in the upper crust,
further constraining which
portion of the subsurface is
inhabitable. Thermophilic
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.
Increased computing power and improved thermodynamic data bases make
it possible to add chemistry to the physical circulation models. The results
(images from www.cas.usf.edu/geology/faculty/steefel/steefel.html) for
some minerals that may affect the availability of pore space for microbes
and are depicted above. At this scale, it would also be intriguing to map the
distribution of additional chemical species that are metabolic constraints on
microorganisms.
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 that are evident in a 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 (physical and chemical) maps of
potential microbial habitat. The focus on the sulfate ion (SO4) here because it
can alter permeability in hydrothermal systems (through anhydrite precipitation)
and is related to H2S, a reduced species of microbiological importance.
QseawaterTseawater ~ QsourceTsource
Main Endeavour Field
qconductive
Qseawater(<25) ~ Qsource(350)
Qseawater > ~15Qsource
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 end-member (*) in equilibrium with subsurface rocks was mixed
(conservatively) with ~15 parts sea water. The sulfate data, however, extrapolate to a lower
end-member temperature (~270oC, *), suggesting that anhydrite precipitated in the subsurface.
qdiffuse
qfocussed
Schultz (1992) measured diffuse and focussed
vent temperatures and volume fluxes (Q) to
calculate heat fluxes (q):
qdiffuse ~ 10qfocussed
qseawater
Assuming that only mixing is involved in
generating the low temperature effluent
(qconductive=0) a balancing of the volume and
heat fluxes dictates that
qsource
Qseawater ~ 30Qsource
Outstanding Questions:
New approaches 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 can help to disentangle the two
porcesses. At the Main Endeavour Field such a construction is aided by
estimates of key geophysical parameters and a thorough assessment of
end-member chemistry. More precise measurements of hydrothermal
heat flux partitioning and of fluid chemistry over a full range of venting
temperatures will provide opportunities for numerical model verification.
Sulfate solubility diagram
(after McDuff and
Edmond, 1982) shows that
when a high-temperature
end-member (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).
What is the rate of infilling in different subsurface environments?
What is the life expectancy of subsurface mixing zones?
Can habitats may eventually be defined by combining thermodynamic
databases (e.g. SUPCRT92) with accurate models of the axial subsurface
mixing zone?
How can McCollom’s (1997) temperature-based habitat descriptions be
made spatially explicit?.
References:
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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,
46:1-18.
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, B7:1252712555.