Geotimes
Feature
The Complex World
along Mid-Ocean Ridges
Deborah R. Hassler, Liz Goehring and Charles Fisher
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In June of 1999, earthquakes rumbled near the Pacific Northwest coast
— not on land, but in the sea. It was not an unusual event. Along Earth’s
mid-ocean ridges, which stretch around the planet along the ocean floor,
tectonic activity is normal. But when these quakes struck, technology made
it possible for scientists to watch what happened.
Tectonic activity generated a swarm of earthquakes off the axis of the Endeavour
Segment of the Juan de Fuca Ridge, a portion of the mid-ocean ridge that
stretches along the ocean floor near the Washington coast. While the earthquakes
went unnoticed by people on land, they did affect a different type of community:
the ocean-bottom life forms thriving near hydrothermal vents along the ridge
axis. These vents are openings from which fluids heated from Earth’s
mantle enter the seawater, affecting ocean chemistry and supporting life
in this seafloor environment.
H. Paul Johnson et al., reporting in the Sept. 14, 2000, Nature,
and Johnson with Robert Dziak, reporting in the May 24, 2002, Science,
published observations showing that 4 to 11 days after the June 1999 earthquake
swarm, several vent sites 15 kilometers away and on the axis had increased
flow rates and temperature elevations of 2 to 12 degrees Celsius. Pore fluid
pressures increased instantaneously at drill holes 25 to 100 kilometers
away. After an eight-hour delay, temperatures and flow rates of fluids at
vents 220 kilometers away increased. These reactions are delayed compared
to the way a terrestrial aquifer would respond. The impact of submarine
earthquakes on oceanic hydrothermal systems is not well understood and appears
to be quite different from that on land.
Observations also showed that the earthquakes fueled an increase in biological
activity, including the formation of large bacterial mats and abundant suspended
bacterial floc. Microbial communities are particularly sensitive to changes
in temperature; a few degrees difference can change the dominant microbe
species present. Thus every earthquake or earthquake swarm has the potential
to significantly change the subsurface biosphere.
These observations show that seismicity significantly affects the environment
along the crests of mid-ocean ridges. This environment supports the biospheres
on and beneath the seafloor. The hydrothermal response to seismicity is
complex, and the impact is greater than what is expected given the amount
of seismic energy released. The processes that link crustal strain, earthquakes
and hydrothermal circulation through the ocean floor are the subjects of
active research.
These observations also demonstrate a direct relationship between geology
and biology along the planet’s mid-ocean ridge system. Understanding
this system requires research that combines traditional disciplines.
Much of the ridge science going on today is funded through Ridge 2000, a
National Science Foundation program that views spreading ridges as whole
systems and recognizes that the processes supporting the biospheres on and
beneath the deep ocean floor are interlinked with the geological processes
driving the systems. |
Earth’s
mid-ocean ridges stretch around the planet. Tectonically and volcanically active,
they are the places where new crust forms. The mid-ocean ridge system hosts
one of the most complex and hostile environments on our planet. Life here exists
without sunlight and is supported by vented hydrothermal fluids driven by heat
from magma chambers. Understanding the processes that occur on the ridges requires
the diverse expertise of physical and life scientists. Seafloor spreading and
life on ridge axes are a challenge to understand not only because of their remote
locations, but also because they operate as a complex interaction of geological,
geochemical and biological processes.
Since the exciting discoveries of these volcanically driven hydrothermal vent
systems in the mid 1970s, this underwater world has been actively researched.
By the mid-1980s, scientists recognized the need for long-term observations
to answer larger questions of whole system processes and interactions. As a
result, ridge-crest investigations were formally included as a major initiative
of the long-range plan of NSF, and a large-scale program began: RIDGE, the Ridge
Inter-Disciplinary Global Experiments. In its 12-year tenure, the RIDGE program
initiated more than 215 investigations, many involving four or more principal
investigators from a wide range of disciplines, to unlock the mysteries of this
deep-ocean cradle of planetary renewal.
Gatherings of minds
The idea of an interdisciplinary program to study ridges and associated hydrothermal
systems began in 1987 at a workshop convened by the National Academy of Sciences
at the Salishan Lodge in Oregon. More than 80 scientists from the United States
and abroad attended to discuss our understanding of oceanic lithosphere evolution,
and to identify primary objectives for ridge-crest science for the next decade.
Participants agreed that the global spreading-center network operated as a single,
dynamic system, one through which energy flows from Earth’s interior to
the lithosphere, hydrosphere and biosphere. They concluded that major strides
in understanding this system would demand an interdisciplinary approach to research.
The Salishan meeting also, for the first time, provided the opportunity for
broad community input into the planning process for such a large scientific
initiative.
Pictured to the right is a temporal
sequence of faunal community development within the 9 degrees and 50 minutes
north latitude BioTransect area at Biomarker #9 on the East Pacific Rise (from
Shank et al., Deep Sea Research II, 1998.). Pictures starting from the
top are numbered 1-5:
1) No vent megafaunal organisms were present when vigorous hydrothermal activity
began in April 1991. Profuse microbial material (floc) was expelled from a fissure
within 15 meters of the subsequent location of Biomarker #9.
2) In March 1992, the fissure was colonized by an extensive population of Tevnia
jerichonana with Riftia pachyptila noticeably absent.
3) By December 1993, numerous R. pachyptila had settled and formed a dense thicket,
engulfing the existing T. jerichonana.
4) In October 1994, the density, lateral extent, and height of the R. pachyptila
aggregation had dramatically increased.
5) In November 1995, the density and lateral extent of the R. pachyptila
aggregation had increased again and the rust-colored staining of worm tubes
was coincident with increased concentrations of iron in the diffuse vent fluids.
The arrow in 3, 4 and 5 indicates comparable corresponding location on the seafloor
in successive years.
By 1989, with financial support from NSF, RIDGE was established to understand
the geophysical, geochemical and geobiological causes and consequences of energy
transfer within the global rift system through time. From its inception, RIDGE
recognized that collaboration among scientists from different disciplines and
integration of data from different regions and on different spatial scales were
paramount to achieving the ambitious scientific objectives. To facilitate collaborations
within this diverse group, RIDGE sponsored (and co-sponsored with international
organizations) an average of five meetings or workshops each year, bringing
together scientists from all disciplines to share their findings and to plan
subsequent experiments and research programs.
One of the most successful and innovative approaches for fostering communication
and collaboration within the community has been the popular RIDGE Theoretical
Institutes (RTI). RTIs bring together field scientists, experimentalists and
theoreticians, and provide the required background for researchers and their
students to address complex, interdisciplinary problems.
The RTI concept has proven an important catalyst for new research as well as
an influential force on the direction of the mid-ocean research NSF funds. The
first RTI was held in 1990. Called “Mantle Flow and Melt Generation beneath
Mid-Ocean Ridges,” the institute produced an important collection of papers
published as an American Geophysical Union monograph. It also inspired the MELT
experiment, a large-scale, coordinated field experiment that collected seismic
and electromagnetic data to provide observational constraints on mantle flow
and melt migration beneath one of the fastest spreading portions of the mid-ocean
ridge system. Results of this major effort continue to define fundamental questions
of ridge research.
Since then, four more RTIs have been convened. Two in particular have focused
on understanding the linkages between life and geology at mid-ocean ridges:
Hydrothermal ecology.
The 1993 RTI, “Physical, Chemical, Biological and Geological Interactions
within Seafloor Hydrothermal Systems,” was intended to stimulate cross-disciplinary
inquiry and focus future investigations toward understanding the hydrothermal
systems as whole systems. Researchers were instructed to explain their science
and outstanding questions to a non-specialist audience. Scientists from diverse
disciplines grappled with elucidating the geological, geochemical and biological
processes of ridge systems and then identified potential areas for cross-disciplinary
research.
One area that proved fruitful was the study of larval dispersal and recruitment
of animals to new vents. This work spawned an important, interdisciplinary initiative,
the LARVE (Larvae At Ridge Vents) project. Understanding the process of larval
dispersal requires understanding the biological processes involved, such as
reproductive biology, genetics, larval physiology and recruitment ecology. Researchers
also need complementary information on geographic distribution and dynamics
of active vent sites, physical transport processes of benthic boundary-layer
currents on the spatial scale of vent habitats, buoyancy-driven mesoscale flow
features, and segment scale circulation.
Geology and biology.
The most recent RTI, “Subsurface Biosphere at Mid-Ocean Ridges,” explored
processes associated with the microbial habitats in porous oceanic crust. Physical
and chemical processes within young crust are found to provide fundamental controls
on the ecology and diversity of microbial communities living there. Likewise,
biological processes may play an important role in the chemistry of hydrothermal
fluids and crustal aging. This RTI stimulated interactions among marine geologists,
geophysicists and microbiologists to identify the key questions, observational
techniques and models necessary to further our understanding of the subsurface
biosphere. Elucidating the nature as well as the spatial and temporal extent
of the subsurface biosphere is now a fundamental goal of the Ridge 2000 program.
‘From mantle to microbes’
The Ridge
2000 Program is the new research initiative to understand Earth’s spreading-ridge
systems. Jointly sponsored by the Biological Oceanography and the Marine Geology
and Geophysics Programs of the Ocean Sciences Division of NSF, Ridge 2000 builds
directly on the scientific and technological successes of the original RIDGE
Program. Designed by five multidisciplinary community workshops over the past
several years, Ridge 2000 approaches the ridge system as an integrated whole:
from its inception in the mantle, to the seafloor biosphere, and to the overlying
water column.
Located at 9 degrees north latitude on
the East Pacific Rise, these hollow lava pillars are common remnants of drained
and collapsed sheet flows and lava lakes. They begin as gaps between lava-flow
lobes, eventually forming a pipe-like channel between the bottom and top of
a lava flow. Water trapped beneath the flow is heated and flows upward through
these channels. The cold water promotes the rapid growth of a lava crust around
the gaps.
Sometimes the pillars coalesce to form walls or can be attached to one another
by natural bridges, as shown here. After the eruption wanes, the molten interior
of the flow typically subsides, leaving a series of "bathtub rings,"
rather than a true lava flow stratigraphy. Photo couretsy of R. Lutz, Rutgers
University, Stephen Low Productions, and Woods Hole Oceanographic Institution
Interdisciplinary interactions are built into the Ridge 2000 science plan and
the scientific motivation is encapsulated by the phrase, “from mantle to
microbes.” This phrase expresses the inextricable linkages among the processes
of planetary renewal in the deep ocean and the origin, evolution and sustenance
of life in the absence of sunlight. Ridge 2000 aims for a comprehensive understanding
of the relationships among the geological processes of plate spreading and the
seafloor and sub-surface ecosystems that these processes support. The program
objective of Ridge 2000 is to develop whole-system models through coordinated
and interdisciplinary experiments. The program has two research themes: Integrated
Studies and Time-Critical Studies.
Integrated Studies support multidisciplinary research focused on specific ridge
locations. The observations of the diverse effects of the 1999 earthquake swarm
on the Endeavour Segment of the Juan de Fuca Ridge described earlier demonstrate
the need for this research strategy. Integrated Studies view oceanic spreading
centers as whole systems and recognize that the array of processes that support
the submarine and seafloor biospheres are interlinked. Organisms living in and
beneath the seafloor subsist on the energy and material that is transferred
from the mantle through volcanic and hydrothermal systems. The complex linkages
between life and planetary processes at spreading ridges can only be fully understood
through coordinated studies that span a broad range of disciplines.
Requiring researchers to focus their efforts at defined sites should lead to
better integration of data and understanding of ridge processes as a whole system.
Three sites have been chosen to begin the program: the segment of the East Pacific
Rise between 8 and 11 degrees north latitude; the Endeavour Segment of the Juan
de Fuca Ridge; and the East Lau Basin Spreading Center in the southern Pacific
Ocean.
Implementation plans for each of the Integrated Study sites were developed and
written this past April at an open community workshop. These plans identify
the geographic focus for each site and provide the guidelines for the set of
studies necessary at each site. These plans are available for download on the
Ridge 2000 Web site.
Rapid-response research
The Time-Critical theme of Ridge 2000 continues the RIDGE focus on observing
the immediate consequences of active processes on the seafloor. The mission
of this program element is to understand the geobiological impacts of magmatic
and tectonic events along the global mid-ocean ridge system. These events are
generally transient and include earthquakes that are associated with eruptions,
magma migration and faulting from seafloor spreading. Time-Critical Studies
are dedicated to facilitating rapid-response missions to observe the consequences
of these phenomena — on the seafloor and in the overlying ocean.
For example,
detecting earthquakes at the bottom of the ocean became possible at the end
of the Cold War, when previously classified military technology became available
for civilian oceanographic research. In 1993, the National Oceanic and Atmospheric
Administration (NOAA) began monitoring SOSUS (Sound Surveillance System), the
U.S. Navy’s cabled hydrophone system in the northeastern Pacific Ocean.
Within weeks of the monitoring program, researchers were able to detect earthquakes
from a volcanic eruption on the Juan de Fuca Ridge in real-time. The sensitivity
and locational accuracy of the SOSUS array were key to detecting this event,
which no land-based networks detected.
A black smoker vents through a polymetallic
sulfide chimney at 20 degrees 50 minutes north latitude and 2,615 meters depth
along the East Pacific Rise. Temperatures as high as 403 degrees Celsius have
been recorded at the orifice of such edifices, from which mineral-rich fluids
emanate. The "smoke" is made of sulfides that precipitate as the hot
hydrothermal fluids become oversaturated in sulfur and metals as they mix with
cold, ambient seawater.
In the following months, five nearby research cruises containing NOAA, Canadian
and RIDGE researchers were either diverted or sent to study this area now known
as the CoAxial Segment. Using conductivity, temperature and depth (CTD) profiles,
as well as multibeam sonar mapping, gravity and magnetic surveys, remotely operated
vehicles (ROVs), and the manned submersible Alvin, the scientists explored the
area where the acoustic signals were heard. These expeditions discovered new
hydrothermal plumes, new high-temperature vent sites, a fresh lava flow 2.5
kilometers long and new venting of bacterial floc to the overlying seawater.
The mission was not only a resounding success in the coordination of a rapid
response to an event, but it also brought the talents of more than 50 geologists,
geophysicists, physicists, chemists and biologists together to study one increment
in the seafloor spreading process.
Since 1993, four responses have been launched to study seismic events on the
Juan de Fuca Ridge and Gorda Ridge, both located in the northeastern Pacific
Ocean. The work done here has revolutionized our understanding of active processes
and has already provided fundamental new information about the linkages among
volcanic events at the seafloor, the development of hydrothermal plumes in the
ocean above the ridge crest, hydrothermal circulation and vent biota. Ridge
2000 will continue to monitor SOSUS and rapidly respond to events along the
ridge crest.
The next step: sharing the
data
A critical aspect of the success of the Ridge 2000 program (and its predecessor
RIDGE) is the open sharing of data. Early in the RIDGE program, researchers
saw that they would make significant scientific advances only if data were readily
available to everyone in a consistent form. RIDGE-sponsored data sharing began
with CD-ROM compilations of seafloor bathymetry. Now these types of data are
available on interactive, Web-hosted databases (for example, the Lamont Doherty
Earth Observatory global synthesis of high-resolution multibeam bathymetry,
and the petrology database, PetDB). These efforts became a basis for new ideas,
hypotheses and collaborations in ridge-crest research.
A component of
the Ridge 2000 program will be the creation of a data management system that
will catalog multiple levels of metadata with user-friendly, Web-based tools
for searching and accessing data. These tools include complex searches, relational
databases and effective visual display of all types of data. The greatest advances
in our understanding of the ocean basins will come from cross-disciplinary investigations
that are facilitated by the merging of different data types.
The vent fish Bythites hollisi emerges
from a high-temperature vent region known as Hole-to-Hell, at 9 degrees 50 minutes
north latitude along the East Pacific Rise. The species is common at various
vent fields along the Galapagos Rift and East Pacific Rise. A black smoker vents
in the background.
Ridge 2000 encourages the openness and sharing of data for the mutual benefit
of the scientific community. For Ridge 2000 to succeed as a community program,
all potential investigators must have ready access to data from the Integrated
Study sites to compete equally with previous investigators at a given site.
The program is committed to facilitating equitable access to all data that can
be used to develop hypotheses and research proposals.
To this end, the Ridge 2000 community has adopted a data policy. In accepting
NSF support within the Ridge 2000 program, each principal investigator is obliged
to meet the data policy requirements as an integral aspect of their participation
in the program. Rapid dissemination of metadata and data will maximize technology
transfer across the program and encourage scientific integration, coordination
of research and the construction and testing of hypotheses. Given that the program
anticipates funding for the next 10 to 12 years, the rapid sharing of all data
collected is central to its success.
Hassler is the Ridge 2000 program
coordinator (e-mail her at dhassler@psu.edu);
Goehring is the Ridge 2000 education and outreach coordinator (e-mail her at exg15@psu.edu);
and Fisher is a professor of biology (e-mail him at cfisher@psu.edu).
All are at Pennsylvania State University.
Visit the Ridge
2000 Program online.
