Geochemistry of Ocean Lithosphere Project

Discerning and explaining hydrothermal patterns
at the Endeavour Segment

Peggy Sullivan and Scott Veirs, 5/21/97

Objectives: Examine interactive processes; obtain an overview of the Endeavour Segment hydrothermal system; learn some geochemistry

Through hydrothermal cooling of magma and lava in the oceanic crust, sea water brings a family of dissolved chemicals into contact with crustal rocks. Pursuant feedbacks between crustal permeability and (bio)geochemical reactions must govern how subsurface hydrothermal circulation develops and evolves, and ultimately produce observed surface patterns of hydrothermal activity. Conversely, observed distributions of heat, salt, and chemical byproducts from the water-rock reactions, should help to constrain the nature of the subsurface system.

A general approach we are considering is to (1) review the observed distribution and chemical character of hydrothermal activity on the Endeavour segment, (2) through examination of theory (both published and presented in class) develop alternative hypotheses to explain the observed patterns, and (3) design a field program to test them.

The existence of hydrothermal activity on the Endeavour Segment of the Juan de Fuca Ridge was first established during a 19xx dredging transect of the axial valley, guided by 19xx SEABEAM bathymetry...

Further spatial constraints on the distribution of hydrothermal sources was obtained through conductivity-temperature-depth (CTD) and transmissivity (CTDT) measurements. (What chemistry was done early on?) While the nature of the deep sea current field over the ridge is apparently quite heterogeneous, its general character is sub-inertial and rotating (Allen and Thomson, 1993).

Regional hydrogeography (Baker et al; Lupton, 1995; Thomson et al; Kadko et al)

Ultimately, the CTDT transects specified targets for sea floor exploration via submersibles and long-term instrument deployments. Sea floor geological observations (Delaney et al).

Review basic characteristics of hydrothermal effluent (geo)chemistry (at the ES) (Butterfield, 1994; Von Damm, 1995; Lilley et al, 1995)

Summarize patterns of microbial abundance in various effluent types (Deming and Baross, 1993; Baross, personal communication)

  • Theoretical and conceptual bases for testable hypotheses
    1. Geology/Geophysics
      1. Geometry of the heat source (Archer et al, 1996; Rohr, 1994)
      2. Regional tectonic setting and geomorphology (Bhat, personal communication; Kappel and Ryan, 1986)
    2. Physics
      1. Theory of interacting water and hot rock (Lister, 1974, 1983)
      2. Properties of water at high P-T: phase separation and criticality (refs?)
      3. Formation of flow paths through dissolution and/or precipitation (akin to Kelemen, 1995)
      4. On-axis vs off-axis convection geometries (Travis, 1991)
    3. (Geo)Chemistry
      1. Examine what can be inferred about the subsurface geochemical interactions of sea water with specific crustal rocks (Thompson, 19xx; Von Damm, 1995; Lilley, 1995; Kelly and Gillis, 1993)
    4. Biology
      1. P-T tolerance ranges of hyperthermophiles (Pledger et al, 1994)
      2. Complications of subsurface organics/sediments? (Lilley; Bain)
      3. Subsurface biological activity may increase permeability through catalysis of dissolution reactions in glasses (Staudigel et al, 1995), or decrease permeability through growth in those regions of the subsurface in which the temperature is tolerable.

  • Design field program
    1. How to test the hypotheses?
    2. Alternative methodologies
      1. A standard procedure for exploring a new ridge segment
      2. Current considerations (Cannon; Kadko; Thomson; Mixing Zephyrs)
      3. Tomography? See alternative instrumentation below...
    3. Alternative instrumentation
      1. Mooring arrays
      2. Profilers
      3. AUV's
      4. Smart neutrally buoyant floats?

  • Budget

    References:

    Deming, J.W., and J.A. Baross. (1993) Deep-sea smokers: Windows to a subsurface biosphere? Geochimica et Cosmochimica Acta 57:3219-3230.

    Kappel, E.S. and W.B.F. Ryan (1986) Volcanic Episodicity and a Non-Steady State Rift Valley Along the Northeast Pacific Spreading Centers: Evidence from Sea MARC I Journal of Geophysical Research 91:B14:13,925-13,940.

    Lister, C. (1974) On the penetration of water into hot rock. Geophys. J. R. Astron. Soc. 39:465-509.

    Pledger, R.J, B.C. Crump, and J.A. Baross. (1994) A barophilic response by two hyperthermophilic, hydrothermal vent Archaea: An upward shift in the optimal temperature and acceleration of growth rate at supra-optimal temperatures by elevated pressure. FEMS Microbiology Ecology 14:233-242.

    Rohr, K. (1994) Increase of seismic velocities in upper oceanic crust and hydrothermal circulation in the Juan de Fuca plate. Geophysical Research Letters 21:2163-2166.

    Travis, B.J., D.R. Janecky, and N.D. Rosenberg. (1991) Three-dimensional simulation of hydrothermal circulation at mid- ocean ridges. Geophysical Research Letters 18:1441-1444.