Progress Report

**Note** This document and the above items are available and updated on the World Wide Web at URL: http://www2.ocean.washington.edu/~scottv/nasa/7progress.html

A year ago I was full of wonder after witnessing deep sea hydrothermal activity through the ALVIN submersible's starboard port. Now, showered by oceanographic observations presented both in class and in data I helped to collect, I'm brimming with questions! Learning simultaneously about specific ridge-crest processes, general oceanography, and mechanisms of global change has catalyzed my realization that the capacious and wonderful ocean is rich in unelucidated phenomena and possibilities.

Even as I am studying and researching hydrothermal systems, their geophysical genesis, physical dynamics, and biogeochemical evolution are becoming better understood. Indeed, alternative mechanisms by which ridge crest processes may affect global changes have frequently become apparent to me through both recent research results and my classes. A zooplankton survey in the Northeast Pacific waters overlying the ridge has revealed that surface and mid-water organisms migrate over 1000 meters to the top of the hydrothermal plumes, representing a possible flux of nutrients from the deep ocean to the photic zone, and I wonder if a resultant bloom might be detected by a satellite-borne color scanner. In physical oceanography, I learn that the heat flux from sea floor volcanism affects the global heat budget and can influence circulation. Chemical oceanographic mass budget models presented in class illuminate the potentially important and poorly-constrained role of water-rock reactions (during hydrothermal circulation) in the balance of magnesium and other elements. Thus, our observational understanding burgeons, and my integration of myriad, ongoing discoveries becomes an increasingly exciting and substantial challenge.

In my research efforts to interpret anomalous temperature, salinity, and particle concentration measurements made during transects of hydrothermal plumes above an active spreading center in the Northeast Pacific, I have encountered intriguing complexities (presented last autumn in a departmental seminar entitled, "What's Hot and What's Not Amongst the Mixing Zephyrs"). While I began with a goal of characterizing patterns of hydrothermal activity on the sea floor, and ultimately quantifying the heat flux from the fresh oceanic crust to the deep ocean water, my perspective on the physical dynamics of plumes was perturbed: I found that deep sea currents over the ridge are not uniform. Rather, they oscillate back and forth with tidal and inertial frequency, sometimes moving a plume far from its source! This complicates the task of calculating fluxes and mapping vent locations based on plume positions, but helps to explain some of the confusion we felt at sea when trying to better locate sources. Re-visiting the Endeavour Segment last summer during the REVEL research cruise was encouraging: the locations of plumes detected in 1995, refined after consideration of the current meter data, ultimately resulted in the discovery and mapping of a new field of sulfide structures during an exploratory dive by the ROPOS (remotely operated vehicle).

While the characteristics of focussed venting are clarified by the 1995 CTDT (Conductivity Temperature Depth Transmissivity) data, an unexpected anomalous signature is also apparent within 50 meters of the sea floor. I have hypothesized that the near-bottom temperature anomaly is spatially related to less-buoyant, diffuse flow sites (tubeworm beds), and am endeavoring to test this and alternative hypotheses. To map the extent and characteristics of the bottom layer and its relationship to the intense, known vent-fields will inform how heat and mass fluxes are partitioned between different hydrothermal phenomena, and help constrain the nature of the subsurface circulation.

At the meeting of the American Geophysical Union last December, I was gratified to receive constructive feedback from more-experienced plume researchers after presenting my poster, "CTDT Characterization of Ridge Crest Hydrothermal Activity." Spurred by those interactions, I have begun to ask how laboratory experiments and numerical simulations may aid my interpretation of the field data. It has been refreshing to approach ridge crest fluid dynamics as a student this quarter in the University of Washington Geophysical Fluid Dynamics Laboratory -- viewing the real phenomena I see in my data as specific examples of the way in which heat and mass can flow in porous media and stratified fluids. The field observations evidence what types of hydrothermal processes were operating during the summers of 1995 and 1996; the laboratory experience helps to constrain the range of fluid behaviors -- boundary layers, plumes, jets, convection cells -- that could create the observed signals in the deep sea volcanic environment.

This spring, I plan to further integrate my field data and laboratory results while completing the last classes of my curriculum: geochemistry, numerical methods, and thermodynamics. The integrative process will culminate in a public presentation of the physical characteristics of hydrothermal venting above the Endeavour volcanic segment. I will subsequently have a much improved understanding of hydrothermal processes -- how buoyant fluid arises, interacts with crustal geology, and ultimately propagates into the oceanic environment, transporting heat and mass -- and will be ideally positioned to further assess both the influence of sea floor volcanism on the Earth system, ramifications for inter-annual and long-term climate change, and the extent to which satellite sensors may detect surface expressions of ridge crest processes.

Last year I was awed by the ocean; now I am realizing how little is understood about ridge crest processes. They represent a facet of Earth's nature which is truly new to us. Only a few days ago, seismic activity was detected at the southern end of the Endeavour Segment -- a stretch of the ridge system along which tectonic extension, not volcanism, was thought to dominate. The character of the seismicity is very similar to that associated with two recent sea floor eruptions on the same ridge, each of which resulted in massive expulsions -- "megaplumes" -- of hydrothermal fluid. Such discoveries have been surprisingly common over the last 2 years (as monitoring has begun), and each catalyzes new ideas in me about how the vent systems work, how the Earth's crust is formed, and their potential influences on the planet. Establishing a basic description of the growing cast of hydrothermal phenomena is clearly going to be an ongoing process, and I look forward to contributing my insights during the next year.