Extant models: Thorough characterization of microbial habitats within submarine volcanoes demands that physical flow models be combined with models of the chemical reactions that occur between hot rock and sea water. The region of the subsurface where reduced chemicals in hydrothermal fluids mix with oxygenated seawater is of special interest to aerobic microorganisms. 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 micro- earthquake locations by projecting them parallel to the axis onto a cross-section of the Endeavour Segment bathymetry. Since hyperthermophiles can’t live in the magma 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, at right) map the temperature, pressure, and velocity fields in the upper crust, further constraining which portion of the subsurface is inhabitable. Thermophiles might be expected to thrive anywhere within the volume bounded by the 50 o C and 150 o C 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 improvement of thermodynamic data bases make it possible to add chemistry to the physical circulation models. The results for some minerals that may affect the availability of pore space for microbes have been explored by Dr. Carl Steefel and are depicted above ( At this scale, it would also be interesting to map the distribution of those chemical species that are metabolic constraints for microorganisms. Evidence of mixing: Fluid circulation models can reproduce the exit temperatures measured at focussed hydrothermal vents (Wilcock). Flow models that integrate precipitation and dissolution reactions can match the patterns of mineralization observed in ophiolites and bore holes (eg. Sleep). Large-scale models, however, do not map mixing in the shallow subsurface, a process that is evident both in vent fluid chemistry and flux balances at a wide variety of hydrothermal fields. Identifying where and how mixing happens should lead to more accurate (physical and chemical) maps of potential microbial habitat. We begin here with the sulfate ion (SO 4 ) because it may clog hydrothermal systems (through anhydrite precipitation) and is related to H 2 S, a reduced species of microbiological importance. New approaches to modeling: Construction of a reactive transport models of the Main Endeavour hydrothermal upflow zone is now feasible. We have estimates of key geophysical parameters, a thorough assessment of high-temperature effluent chemistry, and evidence of subsurface processes like phase separation and segregation. More precise measurements of hydrothermal heat flux partitioning and measurement of fluid chemistry over a full range of exit temperatures will also provide opportunities for numerical model verification. Galapagos Vent FieldMain Endeavour Field Schultz (1992) measured exit temperatures and volume fluxes (Q) to calculate heat fluxes (q): q diffuse ~ 10q focussed Assuming that only mixing is involved in generating the low temperature effluent (q conductive =0), volume and heat conservation dictate that Q seawater ~ 30Q source q seawater q diffuse q conductive q source q focussed Elemental analyses of Galapagos vent fluids (1977; 1979) suggest that a high temperature (~345 o C) hydrothermal end member was equilibrated with subsurface rocks and subsequently mixed with sea water. The sulfate data, however, extrapolate to a lower temperature (~270 o C), suggesting that sulfate was lost in the subsurface during anhydrite precipitation. Q seawater ~ 15Q source Modeling chemical transport in the axial subsurface Scott Veirs and Russ McDuff, School of Oceanography, University of Washington www2.ocean.washington.edu/~scottv/ Fundamental geophysical and chemical observations from the Main Endeavour Field. ParameterValue(s)Source Bulk permeability Porosity Circulation depth~4 km maximumWilcock… Circulation geometryPrimarily fault bound? O(100m) upflow zones Delaney; Robigou; Others; Veirs, et al. Equilibrium temperature~ o C Equilibrium pressure~ bar (~m) Heat fluxesO(10 9 W)Baker; Bemis; Kadko; Thomson One important model result will be an estimate of the rate of infilling. Combined with a time series of vent field heat flux the rate of infilling will determine whether the Main Field system is clogging up or being maintained through tectonic deformation. Habitats may eventually be defined by combining the SUPCRT92 thermodynamic data base with an accurate model of the axial subsurface mixing zone, following the approach of McCollom ( ) who mapped habitats to temperature ranges in an idealized mixing zone. References: