| Project
Background . . .
The area between 9° 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.
 |
|
| L
vent (Alvin dive 3753) will be
revisited for the first time
since 2002 on this cruise. |
The
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.
Although
this first cruise, “AdVenture 1" (for Alvin Diving
on the Venture Hydrothermal Field) was funded as an Ocean
Drilling Program (ODP) site survey, in preparation for a drilling
leg that occurred here in 1992, most of the other cruises at this
site were funded as part of the RIDGE Program, a National Science
Foundation
(NSF) initiative. 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 9N EPR area as the
fast spreading ridge example (the other 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 will occur
at 9N during the next few years.
This
Expedition and Project . . .
The current cruise is the first in a series of three that will occur
during the next five years, as part of an NSF 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. 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:
| 1.
|
Changes
in the depth of phase separation reflect changes in depth
to the heat source. Also, that the conditions of phase
separation indicate very closely the depth to the brittle/ductile
transition. |
| 2. |
Magma has migrated upward and will soon erupt and/or result
in another diking event. |
| 3. |
Changes in fluid compositions, reflecting changes in the
pressure-temperature conditions of reaction (and phase
separation) are the cause of changes in the biological
communities. |
| Our
overall goal is to link cause and effect in the mid-ocean
ridge hydrothermal systems, with vent fluids being the
medium of transfer of energy from the crust to the biological
communities, and a sensitive indicator of changes on relatively
short times scales (months) of conditions within the upper
oceanic crust. |
|
|
