Alan Williams, Ph.D. - US grants

The Pi's propose to continue investigations of the Salton Sea Geothermal system and particularly the interaction between deep hypersaline brines and Shallower, more dilute brines. These two reservoirs seem to be stratified within the sediment reservoir with a sharp density controlled interface paralleling the 250C isotherm. Isotopic data reveal that the two fluids may have different sources, and their distinctive chemical compositions imply different water-rock interaction histories along their respective flow paths. Both hydrothermal circulation and ore deposition are controlled by the salinity stratified nature of these fluids. Further work is intended to add to the existing data base using samples from commercial production wells. The ultimate goal is to understand the evolution of the two fluid reservoirs and their role in controlling metallogenesis.

The PI's will have access to extensive drilling technology in the newly developing Coso geothermal system. Water samples, cuttings, and cores will be studied to understand the nature of fluid/rock interaction in this system, which is relatively undisturbed and can provide a close approximator to natural circulation of fluids. The metamorphic/igneous basement rocks differ from the sediment hosted Salton Sea Geothermal field, and as such will have distinct effects on fluid compositions and evolution. Among the PI's objectives are determination of (1) fluid circulation paths and sources, (2) thermal evolution and age of the geothermal system, (3) veining mineralization, and (4) history of fracturing and cementation of the hydrologic system.

9316499 Elders This project involves the coordination of the participation of U.S. scientists in an international consortium from New Zealand, Japan and the USA which will carry out shallow research drilling on White Island, which is one of the world's most active, but accessible, andesite volcanoes. White Island is in the Bay of Plenty at the northern extension of the Taupo Volcanic Zone of New Zealand. Unlike other frequently active volcanoes, such as Kilauea and Etna, it is a "wet" system, i.e., its behavior is dominated by the interaction of shallow magma with meteoric water. As this system is highly compressed relative to those found in typical high-relief volcanoes, by drilling to depths of only hundreds of meters on White Island, it will be possible to study and directly sample the interface between an andesite magma and a geothermal system. ***

Sensory information is often acquired through active exploration. Knowledge of the world is gained by exploring a complex surface with hands or a visual scene with eyes. Yet relatively little is known about how neurons encode sensory stimuli in the context of natural patterns of sensing behavior, or about how sensory processing regions in the brain distinguish properties of the external world from the sensory consequences of the animal's own behavior.

A particularly clear example of active sensing is found in mormyrid electric fish. Electric fish use an electrical sense to navigate and find prey in the dark by probing the environment by emitting brief electric organ discharge (EOD) pulses. Nearby objects perturb the electric field around the fish, and these perturbations are detected by electroreceptors in the fish's skin. Each receptor encodes changes in local field strength as small shifts in the precise timing of individual action potentials following the EOD. The fish thus obtains a sequence of "snapshots" of the world, in which information about surrounding objects is encoded in the timing of action potentials.

In nature, the frequency and regularity of this sequence of snapshots varies depending on the behavioral context, whether the fish is probing objects, foraging, or quietly resting. Interestingly, the frequency chosen by the fish has a clear effect on the timing of electroreceptor action potentials within each snapshot: higher rates shift spikes later, and lower rates shift spikes earlier. The size of these effects is comparable to the effects of small invertebrate prey on which these fish feed. How does the fish detect and capture prey when its own sensing behavior has such a strong effect on the sensory input?

This study provides opportunity to explore how sensory processing regions of the fish's brain resolves the ambiguity, and whether a change in the input from electroreceptors is due to an external stimulus or to the animal's own sensing behavior. Neurons at the first stage of electrosensory processing integrate input from electroreceptors with signals from other areas of the fish's brain linked to the motor command that evokes the EOD. Such motor command signals could, in principal, "undo" the effects of EOD rate on electroreceptor input.

The research is expected to lead to a better understanding of how animals use internal knowledge of their actions to distinguish properties of the external world from the sensory consequences of their own behavior. At a more cellular level, the experiements are also expected to lead to a better understanding of how information contained in the precise timing of action potentials is decoded or interpreted by neural circuits.