Research
Glenn Spinelli, Assistant Professor of Geophysics

Marine Hydrogeology
     In ocean crust, fluid flow between the basaltic basement aquifer and the ocean is an important control on heat transport, solute transport, and alteration of the crust. As sediments gradually accumulate on oceanic crust, they change the nature of fluid circulation within the basement aquifer. My interests in marine hydrogeology include fluid and heat circulation patterns in ocean crust, permeability evolution of ocean crust, and fluid and heat flow through seafloor sediments. In studies of the Juan de Fuca Ridge flank, I have modeled heat and fluid circulation patterns within the high permeability basaltic basement and determined the fluid seepage patterns through sediment around sites from Ocean Drilling Program Leg 168.

     As ocean crust subducts, fluid circulation within it may be an important control on subduction zone temperature. Subduction zone temperature has been suggested as a control on the distribution of seismicity. I am currently studying the evolution of hydrothermal circulation in subducting crust (with recently graduated M.S. student Troy Kummer). In this study, we are examining how far into a subduction zone hydrothermal circulation within ocean crust can persist, and the thermal consequences of that circulation.

     I have also been studying subduction zone hydrogeology, particularly fluid pressure and flow along the plate boundary fault. I have been examining sediment dewatering in the Costa Rica margin subduction zone. Large amounts of opal and smectite in the subducting sediment release water during diagenetic reactions and affect fluid pressures along the plate boundary fault. These diagenetic fluid sources may influence the pattern of seismicity or locking on the plate interface in the shallow subduction zone on the Costa Rica Margin.

Groundwater-surface water interactions/fluid flow through sediments
     Interactions between groundwater and surface water can affect fluid chemistry and chemical alteration of aquifer material. The groundwater seepage can be a source of fresh water to estuaries, lakes, rivers, and coastal environments; it can also transport nutrients and contaminants from aquifers or bottom sediments to surface waters. In San Francisco Bay, I have examined the potential importance of groundwater seepage transporting dissolved metals from sediments to overlying surface waters.

Above: Collecting and sampling a sediment core from San Francisco Bay to determine groundwater seepage rates. In both pictures, I'm on the left, Geoff Wheat (the geochemist working with us on the project) is on the right.

Sedimentology/sediment physical properties
     Variations in sediment type and porosity can lead to large differences in sediment permeability. My previous sediment oriented studies range from determining sedimentation patterns on the northern California margin from reflection seismic data to laboratory consolidation and permeability testing of sediments from San Francisco Bay, the Juan de Fuca Ridge flank, and the Congo Fan.

     My most recent work in this area examines the thermal, diagenetic, and consolidation history of sediment approaching the Nankai Trough subduction zone off Japan. I am currently working with M.S. student Russell White, examining opal cementation of hemipelagic sediment on the Nankai margin. I plan to sail on an IODP expedition to examine the cementation and deformation of sediment approaching and entering the Nankai Trough subduction zone.