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Project Progress

This cruise was prompted by the remote detection of earthquake activity about 34 km north-northeast of the Main Endeavour Vent Field on the Endeavour Segment of the Juan de Fuca Ridge. The earthquakes were detected in real time by an array of hydrophones that are part of the U.S. Navy’s Sound Surveilance System and monitored by the NOAA-PMEL VENTS Program’s T-Phase Monitory Program. The intensity of the earthquake activity (to >60 earthquakes per hour) and pattern indicate the seismic swarm may have been caused by a magmatic event with the possibility of a seafloor eruption. The purpose of this cruise was to study expression of this presumed magmatic event within the water column (significant 'event’ plumes or altered venting intensity or fluid chemistry of known vent sites) or at the seafloor (active venting or new lava flows). We were particularly interested in finding any large “event” plumes. The water column was studied for evidence of temperature and particle anomalies and elevated concentrations of metals, gases and microorganisms associated with any event plumes, as well as any chronic style (continuously venting) plumes found. The chemistry and microorganisms uniquely associated with such 'event’ plumes are thought to reflect geochemical and community conditions in the deep-subsurface of the hydrothermally active mid-ocean ridges. We were prepared to 'seed’ any event plume with neutrally buoyant RAFOS floats, intended to drift with the plume for several months before rising to the surface and telemetering its drift track and allowing a time series microbial geochemical study of the aging 'event’ plume.

Relevance to Astrobiology: A key aspect and justification for this project is the unique access to the subseafloor environment that seafloor eruptive events offer. Astrobiology has strong interests in extreme environments, especially those hosting chemolithotrophic consortia and hydrothermal systems. In particular, the spectacular, enormous volumes of hydrothermal fluids can be released nearly instantaneously, forming the so-called “event” plumes. We have found unusual chemical signatures in these event plumes as well as hyperthermophiles that have been found no where else. We currently believe that the formation of these event plumes during seafloor eruptive events provides access to parts of the mid-ocean ridge subseafloor biosphere that are not normally accessed via conventional sampling at chronic-style hydrothermal vents.

Summary of Findings and Preliminary Conclusions

1. 
   This February/March 2005 earthquake swarm did not induce a corresponding expression at the seafloor (e.g., eruption of a lava flow) or in the water column (e.g., formation of new hydrothermal venting, either chronic or event plumes).
2. 
   The most likely explanation for the absence of evidence of a significant event-related hydrothermal discharge is: The earthquake swarm was likely an intrusive, magmatic event that may not have 1) reached sufficiently shallow crustal depths to lead to extrusion (eruptive flows), 2) caused changes in hydrothermal circulation at the seafloor, and/or 3) did not stimulate new venting or changes to existing venting as discernable via surface ship sampling.
3. 
   Preliminary studies during this cruise indicate that electrochemical voltammetry) analysis will be sensitive enough to detect, for the first time, reactive reduced sulfur species within hydrothermal plumes. Further tests will be made in September 2005 with an in situ electrochemical analyzer suspended on a deep-sea hydrowire.
4. 
   This Feb/March event detection and response experience stimulated a constructive exchange of ideas and questions regarding the interpretation of available relevant seismological and geological data and the criteria used in evaluating the appropriate response or lack of response to this event. A paper has been submitted to the weekly journal EOS (Dziak at al., in review) that summarizes the geological, seismological and other geophysical data available at the time of all past SOSUS directed response cruises (1993 Co-Axial; 1996 Gorda Ridge; 1998 Axial Volcano; 2001 Jackon Segment, Gorda Ridge; 2001 Middle Valley; 2005 Endeavour) as well as events detected but not responded to (e.g., 1999 Endeavour). The emphasis of this paper is the correlation of data available during the brief period of time following initial detection of onset of seismic swarm and the decision whether or not to launch a rapid response.