1. 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 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: 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 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 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. 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 (SO 4 ) here because it can alter permeability in hydrothermal systems (through anhydrite precipitation) and is related to H 2 S, a reduced species of microbiological importance. 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. Main Endeavour Field Schultz (1992) measured diffuse and focussed vent 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) a balancing of the volume and heat fluxes dictates that Q seawater ~ 30Q source q seawater q diffuse q conductive q source q focussed Galapagos Vent 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 (~345 o C) 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 (~270 o C, *), suggesting that anhydrite precipitated in the subsurface. Q seawater T seawater ~ Q source T source Q seawater (<25) ~ Q source (350) Q seawater > ~15Q source Modeling chemical transport in the axial subsurface Scott Veirs and Russ McDuff, School of Oceanography, University of Washington scottv@ocean.washington.edu www2.ocean.washington.edu/~scottv/ Sulfate solubility diagram (after McDuff and Edmond, 1982) shows that when a high-temperature end-member (Ca-rich, Mg- and SO 4 -poor) mixes conservatively with seawater (rich in Mg and SO 4 ) anhydrite precipitation is expected at intermediate temperatures. K is the solubility product; Q is the reaction quotient (actual concentration ratio in the hydrothermal mixture). fg Outstanding Questions: 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: 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, B7:12527- 12555.