Old Research > Mid-Ocean Ridge Hydrothermal Systems
A lot of my research involves mid-ocean hydrothermal systems.
Tim Crone, a 2007 Ph.D student worked with support from the National Science Foundation, the University of Washington's Royalty Research Fund, and the W. M. Keck Foundation to develop observational techniques to measure fluxes from black smoker systems and model the effects of ocean tides on subsurface flow in these systems. He recently moved to Lamont Doherty Earth Observatory and you can read about his research at his new web site and also check out his publications from his work at the UW on my publications page. He recently submitted a paper on which I am a coauthor documenting a link between an earthquake swarm on the Endeavour and change in black smoker hydrothermal outflow measured using an image correlation techique (Download the PDF).
A former postdoc, Fabrice Fontaine, who is now at the Institute de Physics de Paris de Globe (IPPG) was supported by the National Science Foundation to develop both conceptual and numerical models to understand brine storage and transport in high temperature systems. The following describes one of his projects.
Mid-ocean ridge hydrothermal systems are known to vent fluids with salinities substantially different from seawater. This is attributed to phase separation and the segregation of the resulting vapor and brine phases. Time series of vent temperature and salinity (chlorinity) show that some black-smoker vent fields have vented fluids with salinities well below seawater for over a decade. This raises important questions concerning chloride mass conservation and the fate of brines in these systems. One widely accepted model is that high-density brines formed during super-critical phase separation sink efficiently to the base of hydrothermal systems, leading to the development of a two-layer system in which a re-circulating brine layer underlies a single-pass seawater cell. However, there is no conclusive evidence for such a two-layer configuration or for the assumption that a brine layer will convect.
|Two-layer model for MOR hydrothermal systems. A convective brine layer with temperature > 500 degrees C settles below a superficial seawater layer (after Bischoff and Rosenbauer, 1989)|
From an analysis of brine properties in the two-phase area, we conclude that, if brines are stored in a layer at the base of high-temperature mid-ocean ridge hydrothermal systems they are unlikely to convect because phase separation will lead to a stable stratification.
|The stability of a conductive brine layer depends on its thickness (hbot) and the temperature contrast (DeltaT) in the layer. Layers with realistic thicknesses (100-500 m) and temperature contrasts (100-200 degrees C) are stable.|
In the abscence of convection, the brine layer beneath black smoker systems has to be thin (< 10m) to match the high heat fluxes. However, estimates of the rate at which brines are accumulating in the crust below the Main Field on the Endeavour segment of the Juan de Fuca Ridge and below vents near 9o50’N on the East Pacific Rise suggest that the brine layer is likely at least 100 meter thick.|| To resolve this apparent paradox we propose an alternative model which we support with inferences from single-phase numerical models. It is generally believed that the pressure gradients in mid-ocean ridge hydrothermal systems are close to cold hydrostatic. At the high temperatures and pressures characteristic of the deeper parts of these systems brines with salinities as high as 20-30 wt% NaCl have densities around 800-900 kg/m3 and will be buoyant in a cold-hydrostatic system.
|Evolution of the mean (middle lines), downwelling (top lines), and upwelling (bottom lines) neutral densities as a function of Nusselt number (Nu) and bottom temperature in two-dimensional numerical experiments of hydrothermal circulation at two bottom temperatures. Mean pressure gradients are closer to downwelling pressure gradients than upwelling pressure gradients.|
We argue that interfacial tensions between fluid and solid phases will likely favor the segregation of vapor into the main fractures and brine into the smaller fissures and backwaters. This allows the vapor to flow efficiently through the system and transport large heat fluxes while most of the porosity in the lower part of the system fills with brines that will rise only slowly because of their higher density and viscosity and the low permeability of brine filled fissures. Our numerical models suggest that brines that rise will reach a level of neutral buoyancy as they cool and enter high permeability regions in which the pressure gradients decrease.
|A new model for the dynamics and storage of brines in mid-ocean ridges (Full Sized Version)|
In 2009 Fabrice Fontaine was the lead author on a paper showing that chemical measurements of chlorinity and silica in hydrothermal fluids can be used to determine the temperature and depth of hydrothermal reaction zones (download the PDF).
In the future I hope to develop models of reactive chemical transport. Mid-ocean ridge hydrothermal systems support extensive biological communities that live on the the chemical energy that that can be obtained by catalyzing reactions that are kinetically limited and which occur when hot hydrothermal fluids are mixed with seawater. These reactions have been modeled extensively in static systems (i.e., what happens when you mix hot hydrothermal fluid with different amounts of seawater in a beaker) but they have not been combined with flow models that would predict where the chemical energy is available and by inference where in the subsurface different types of microbes might be found. The results of such modeling could be linked to the chemistry and microbiology of diffuse (low-T) fluids that are being increasingly sampled on the seafloor in mid-ocean ridge hydrothermal systems.