The area between 9° north and 10° north latitude on the East Pacific Rise (often simply referred to as "9 north") has been a focus of mid-ocean ridge studies for almost 20 years.
Project Background
Studies of the hydrothermal systems at this site, and their associated biological communities, began in earnest in 1991. In March-April 1991 the first oceanographic expedition to use the submersible (DSV) Alvin at this site found evidence of a very recent volcanic eruption that had wiped out the biological communities and re-initiated the hydrothermal systems. This is often referred to as "time zero" and has provided an unparalleled opportunity to understand the time scales over which changes can occur on the seafloor. The rapidity of change observed here has truly revolutionized our ideas on processes in the deep sea.
In April 2006, evidence was discovered for another volcanic eruption in the same location as in 1991. This was very exciting as it marks the end of the "cycle" that began in 1991. The animal communities which had developed since 1991 were wiped out, and the hydrothermal system was reset, again to "time zero." Because we learned from the 1991 eruption how rapidly these systems evolve, especially in the time period right after the eruption, the National Science Foundation (NSF) funded an Atlantis/Alvin event response cruise to the site in June-July 2006 so we could set the baseline for future studies. We called this cruise "RESET06" for Recent Eruption Studies East (Pacific Rise) Time (zero) (20)06. Several types of results from this cruise now suggest this most recent eruption occurred sometime in 2005.
We know little about seafloor eruptions. We have no idea if the 1991 eruption was "typical." An important difference between the 1991 and 2005 eruptions is that we had instruments on the seafloor at this site that survived, and recorded through the 2005 eruption. For example, recording temperature probes like the one in the picture were in 15 of the vents. So far, we have recovered 4 of them. We therefore know much more about what happens leading up to, during, and immediately following an eruption than we have in the past.
Although the first cruise, "AdVenture 1," was funded as an Ocean Drilling Program (ODP) site survey in preparation for a drilling leg that occurred here in 1992, most other cruises to this site were funded by the National Science Foundation as part of the RIDGE Program. The goal of the RIDGE Program was: "To understand the geophysical, geochemical and geobiological causes and consequences of the energy transfer within the global rift system through time."
The successor program to RIDGE is RIDGE 2000 (R2K). R2K has identified three sites on the global ridge crest system where integrated studies will occur. The goal is to understand the links between the processes occurring at a single site: "from mantle to microbes" to better understand the ridge system as a whole including "cause and effect." One of the three integrated study sites (ISS) is the 9° north EPR area—the fast spreading ridge example. (The other two sites are the more slowly spreading Endeavour segment on the Juan de Fuca Ridge and the Lau Basin, a back arc spreading center in the western Pacific.) Therefore numerous cruises with various disciplinary objectives have been occurring at 9°.
This Expedition and Project
The current cruise is the last of four that have occurred since 1994 as part of a National Science Foundation proposal entitled "Temporal Variations in Hydrothermal Fluid Chemistry at 9-10° N East Pacific Rise: Elucidating Ties to Crustal and Biological Processes," for which Prof. Karen Von Damm is the principal investigator and chief scientist. The proposal initially called for three research cruises, but with the discovery of the eruption in 2006, a fourth "rapid response cruise" was added. An excerpt from the project summary for this NSF-funded proposal provides the context and goals of this research program.
We have observed pronounced temporal variability in the hydrothermal fluids at this site, most specifically around 9°50'N during the last decade. We interpret our chemical data to suggest that these changes in fluid composition are reflecting pronounced changes in the depth of hydrothermal circulation and hence of the heat source present at this site. We suggest that in 1991-2 the heat source was very shallow, essentially at or within 50m of the seafloor, that it gradually deepened until late 1995, began to shoal in early 1996, and as of 2002 is within 250m of the seafloor, shallower than it has been since 1993, based on our chemical data. Unfortunately, geophysical data are not available for most of this period to confirm our interpretation and to provide a rigorous link between hydrothermal fluid chemistry and magmatic processes within the crust. Also, we argue that the changes in conditions of hydrothermal circulation, primarily depth and temperature, result in changes in fluid compositions that affect the hydrothermal animal communities. While we have tied these results together loosely with the biologists, in this proposal we are proposing to link, through the hydrothermal fluid compositions, from active crustal processes to their manifestations in the hydrothermal communities, a specific example is the abundance of Fe and H2S in the fluids. We propose to address the following 3 hypotheses:
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