Subsurface Hydrology (courtesy of A. FIsher)
Thermally driven fluid flow through the oceanic crust profoundly influences the physical, chemical, and biological evolution of the crust and ocean. Although considerable research is focused on the spectacular black smoker systems at mid-ocean ridges, fluid flow in the ridge flanks accounts for a much higher proportion of global hydrothermal heat and mass fluxes across the seafloor and supports a potentially vast subseafloor biosphere. Fluid flow in the basement contributes to the heat loss from aging plates, profoundly alters basement rocks, influences the chemistry of the oceans, inputs volatiles at subduction zones, and cycles oxygen and nitrogen to considerable depths. Studies show that flow can extend over considerable distances but is highly heterogeneous and compartmentalized. There is still considerable uncertainty about how much of the crust is involved in active fluid circulation, whether circulation is continuous or episodic, and how ridge crest and ridge flank hydrological systems are connected. Resolving these issues requires long-term observations.
Borehole Observatories
The northern half of the Juan de Fuca plate is the best place to leverage existing data sets and existing borehole observatories to address first-order questions of crustal hydrology and evolution. This region is the site of one of the highest concentrations of Ocean Drilling Program boreholes and many of these are instrumented with CORKs (Circulation Obviation Retrofit Kits) that seal the borehole and facilitate monitoring of downhole temperatures and pressures and fluid sampling. A unique opportunity exists for the International Ocean Drilling Program (IODP) to use the cabled observatory infrastructure to conduct long-term four-dimensional (time and space) in arrays of boreholes that complement seafloor and water-column experiments in the region. The power and bandwidth of cabled observatories will facilitate a more extensive variety of long-term down-hole measurements and anthropogenic perturbation experiments. Recently IODP drilled several new holes on the flanks of the Juan de Fuca Ridge that will be available for hydrological monitoring, cross-borehole flow experiments, chemical and biological sampling, and microbial incubation experiments.
Strain Measurements
Over the past decade, results from the CORK observatories on the Juan de Fuca Ridge have demonstrated that fluid pressures in sediment-sealed basement formations are subject to transient perturbations following regional earthquakes. The characteristics of these perturbations can be used to infer the large-scale hydrological properties of the oceanic crust and estimate the regional patterns of strain that accompany large earthquakes. Thus, borehole monitoring represents an important component of multidisciplinary experiments envisioned for the cabled observatory that will seek to understand the coupling of tectonic and hydrological processes across the interior of the Juan de Fuca plate and between different plate boundaries.
Note: Parts of this page were extracted from RECONN: REgional Cabled Observatory Network (of Networks), Report of the Cabled Regional Observatory Workshop, October 7-10, San Francisco, CA, 2003.
