Why Are Scientists Exploring Submerged Sinkhole Ecosystems in Thunder Bay National Marine Sanctuary?
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on earth, and provide new knowledge about the forces that shape Earth’s surface.
At 5:12 am on April 18, 1906, Ernest Adams was thrown violently from his bed and watched in disbelief as the side of his San Francisco home crumbled to the ground. “I fell and crawled down the stairs amid flying glass and timber and plaster. When the dust cleared away I saw nothing but a ruin of a house and home that it had taken twenty years to build. I saw the fires from the city arising in great clouds and it was no time to mourn my loss so getting into what clothing I could find, I started on a run for Kearny St., five miles away. . .” (Adams, 1906).
In 1906, modern plate tectonic theory was several decades in the future, so no one who lived through the Great San Francisco Earthquake could know that their terrifying experience resulted from interaction between two large pieces of Earth’s crust now known as the Pacific and North America Plates. These tectonic plates are portions of the Earth’s outer crust (the lithosphere) about 5 km thick, as well as the upper 60 - 75 km of the underlying mantle. They move on a hot flowing mantle layer called the asthenosphere, which is several hundred kilometers thick. Heat within the asthenosphere creates convection currents (similar to the currents that can be seen if food coloring is added to a heated container of water). Movement of convection currents causes tectonic plates to move several centimeters per year relative to each other.
Where tectonic plates slide horizontally past each other, the boundary between the plates is known as a transform plate boundary. As the plates rub together, huge stresses are set up that can cause portions of the rock to break, resulting in earthquakes. Places where these breaks occur are called faults. The San Andreas fault exists along the transform plate boundary between the Pacific and North America Plates in California. The 1906 San Francisco Earthquake was caused by a 296 mile-long rupture along the San Andreas fault from the Mendocino Triple Junction to San Juan Bautista. A triple junction is a place where three of Earth’s tectonic plates intersect. At the Mendocino Triple Junction, the Pacific Plate and North American Plate intersect with the Juan de Fuca Plate. Other types of plate boundaries include convergent boundaries, which are formed when tectonic plates collide more or less head-on; and divergent boundaries, which occur where plates are moving apart. View animations of different types of plate boundaries at: http://www.seed.slb.com/ .
Understanding that the 1906 quake resulted from the movement of tectonic plates leads quickly to the realization that these plates are still in motion; in fact, the San Andreas fault is the fastest moving fault in western North America. This realization inevitably leads to the question, “When will a major earthquake like the 1906 quake strike again?”
To help answer this question, geologists study the history of past earthquakes along the San Andreas fault system. These studies, as well as thousands of years of historical records from China and Japan, tell us that giant earthquakes on faults like the San Andreas tend to occur every few hundred years. This interval is thought to be the time required for motion between tectonic plates to build stresses to levels that produce large quakes. In general, this evidence suggests that a 1906-size earthquake is not likely to strike Northern California for at least 100 years. Still, studies also show that stress has built up again along the San Andreas Fault system. For 70 years following the 1906 earthquake, there were only low levels of seismic activity in Northern California. Then, between 1979 and 1984, there were three quakes with magnitudes of about 6; and in 1989 a major (Loma Prieta) earthquake with a magnitude of 6.9. A similar pattern of earthquake activity took place during the 70 years prior to the 1906 quake.
The Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010 expedition will improve our understanding of the history of great earthquakes and how they are interrelated by investigating portions of the great plate boundary fault that lie offshore; areas that have virtually never been observed or explored. Many of these investigations will include observations that are fundamentally different from those that have been made on land. In addition, the expedition will use a state-of-the-art “green” research vessel that relies primarily upon sail power, and is constructed with a light composite hull. These features result in a vessel that is extremely fuel efficient and acoustically quiet, reducing interference with scientific sensors and producing almost no pollution.
The Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010 Expedition is directed toward three key questions:
- What is the history of past earthquakes associated with the San Andreas fault?
- What unique ecosystems are associated with active fluid and gas venting, topographic complexity, and continuous change associated with the fast-moving and active San Andreas fault system?
- How does tectonic history relate to biodiversity hotspots and how may these biologically-active areas respond to disturbances such as future earthquakes?
To answer these questions, the Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010 Expedition will:
Obtain high resolution multibeam and sidescan sonar images of the entire offshore fault, as well as subsurface images that provide information about its deep structure and evolution;
- Use an autonomous underwater vehicle (AUV) and remotely operated vehicle (ROV) to obtain higher resolution data from areas of special interest identified from larger maps, including information about unique and unexplored ecosystems associated with the continuous change resulting from earthquake and tectonic cycles;
- Collect samples of sediment and hard corals to obtain evidence of past earthquakes and when they occurred; and
- Use multifrequency water column sonars to study gases and fluids that are being released along the fault, as well as living organisms in the water column near the same areas.
Information about past earthquakes is often obtained by studying proxies. Proxies are natural records of biological and geological ecosystem features that are affected by certain events. Tree rings are a well-known proxy for year-to-year weather conditions, since the size of each ring is affected by growing conditions during the year in which each ring is formed. In fact, tree rings are sometimes used to obtain information about the history of earthquakes, since earthquakes cause stresses that can affect the growth of trees (for example, see http://www.ess.washington.edu/ .
A similar proxy for earthquake information is provided by deepwater corals. These corals build their skeletons from calcium and carbonate ions which they extract from seawater. During the Ocean Explorer 2002 Continuing The Legacy of Lewis And Clark Expedition, scientists found that skeletons of the bamboo coral (Isidella sp.) contained ring-like structures that probably are growth layers associated with changes in food supply. There were also gaps and other changes in the ring patterns that may correspond to specific disturbances.
Geological structures can also provide proxies. In many places along the continental shelf and continental slope, there are accumulations of sediments carried into the ocean by rivers. These accumulations continue to build until they become unstable and slide down the slope in a sort of avalanche called a turbidity current. This results in a layer of sediment on the seafloor called a turbidite. Since earthquakes may trigger turbidity currents, the presence of turbidites can provide a submarine record of past earthquakes. Turbidites and sediment layers are often studied using core samples collected from the ocean floor adjacent to the continental slope.
Earthquakes can also trigger changes in other geologic structures. One such structure of particular interest is hydrocarbon deposits within the seafloor. Hydrocarbons may exist as gases, liquids, or ice-like solids called hydrates. These hydrocarbons can provide an energy source for unique ecosystems called cold-seep communities (see “More About Cold Seep Communities and Methane Hydrates,” below). In addition, active gas and fluid venting can indicate areas where earthquake shaking has taken place.
Major technologies involved with the Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010 Expedition include:
Multibeam and Sidescan Sonar
Sonar (which is short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. In use, an acoustic signal or pulse of sound is transmitted into the water by a sort of underwater speaker known as a transducer. The transducer may be mounted on the hull of a ship, or may be towed in a container called a towfish. If the seafloor or other object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. The time elapsed between the emission of the sound pulse and the reception of the echo is used to calculate the distance of the object. Some sonar systems also measure the strength of the echo, and this information can be used to make inferences about some of the reflecting object’s characteristics. Hard objects, for example, produce stronger echoes that softer objects. This is a general description of “active sonar”. “Passive sonar” systems do not transmit sound pulses. Instead, they “listen” to sounds emitted from marine animals, ships, and other sources.
Side-scan sonar systems use transducers housed in a towfish, usually towed near the seafloor, to transmit sound pulses directed sideways, rather than straight down. Return echoes are continuously recorded and analyzed by a processing computer. These data are used to construct images of the seafloor made up of dark and light areas. These images can be used to locate seafloor features and possible obstructions to navigators, including shipwrecks.
Side-scan sonar systems do not provide bathymetric data. When this information is needed, multibeam sonar systems are used. A multibeam system uses multiple transducers pointing at different angles on either side of a ship to create a swath of signals. The time interval between signal transmission and return echo arrival is used to estimate depth over the area of the swath. In some systems, the intensity of the return echo is also used to infer bottom characteristics that can be used for habitat mapping. For more information about sonar systems, see http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html.
In addition to information about bottom topography, sonar can also be used to obtain information about the water column above the bottom. The multi-frequency water column sonar can detect fishes and other organisms, and can be used to construct a three-dimensional image of biomass in survey areas. When combined with information from other sensors, this can reveal relationships between the geology, substrate, gas and fluid vents, and water column biota.
Chirp Sub-Bottom Profiler
Sub-bottom profiling systems are used to study layers of sediment or rock under the seafloor. The basic principle is similar to sonar: a transducer sends a sound pulse vertically downwards towards the seafloor, and a receiver records the return of the pulse after it has been reflected off the seafloor. Parts of the sound pulse will penetrate the seafloor and be reflected by different sub-bottom layers. Sub-bottom profiling systems use a stronger sound signal than echolocation and lower sound frequencies. The signal from some “chirp” profilers are able to penetrate more than 100 meters into the seafloor. The sound pulse is often sent from an airgun towed behind the ship. Some sub-bottom profilers use several hydrophones towed behind the ship (called a towed array or streamer) which detect the reflected sound signal when it reaches the surface. The time it takes the sound to return to the ship can be used to calculate the thickness of the layers in the seafloor and their position (sloped, level, and other features). Sub-bottom profilers also give some information about the composition of the layers.
Autonomous Underwater Vehicles
Autonomous Underwater Vehicles (AUVs) are underwater robots that operate without a pilot or cable to a ship or submersible. This independence allows AUVs to cover large areas of the ocean floor, as well as to monitor a specific underwater area over a long period of time. Typical AUVs can follow the contours of underwater mountain ranges, fly around sheer pinnacles, dive into narrow trenches, take photographs, and collect data and samples.
Basic systems found on most AUVs include: propulsion, usually propellers or thrusters (water jets); power sources such as batteries or fuel cells; environmental sensors such as video and devices for measuring water chemistry; computer to control the robot’s movement and data gathering functions; and a navigation system. Until recently, once an AUV was launched it was completely isolated from its human operators until it returned from its mission. Because there was no effective means for communicating with a submerged AUV, everything depended upon instructions programmed into the AUV’s onboard computer. Today, it is possible for AUV operators to send instructions and receive data with acoustic communication systems that use sound waves with frequencies ranging roughly between 50 hz and 50 khz. These systems allow greater interaction between AUVs and their operators, but basic functions are still controlled by the computer and software onboard the AUV.
Navigation has been one of the biggest challenges for AUV engineers. Today, everyone from backpackers to ocean freighters use global positioning systems (GPS) to find their location on Earth’s surface. But GPS signals do not penetrate into the ocean. One way to overcome this problem is to estimate an AUV’s position from its compass course, speed through the water, and depth. This method of navigation is called “dead reckoning,” and was used for centuries before GPS was available. Dead reckoning positions are only estimates however, and are subject to a variety of errors that can become serious over long distances and extended time periods.
If an AUV is operating in a confined area, its position can be determined using acoustic transmitters that are set around the perimeter of the operating area. These transmitters may be moored to the seafloor, or installed in buoys. Some buoy systems also include GPS receivers, so the buoys’ positions are constantly updated. Signals from at least three appropriately positioned transmitters can be used to accurately calculate the AUV’s position. Although this approach can be very accurate, AUV operators must install the transmitters, and the AUV must remain within a rather small area.
A more sophisticated approach uses Inertial Navigation Systems (INS) that measure the AUV’s acceleration in all directions. These systems provide highly accurate position estimates, but require periodic position data from another source for greatest accuracy. On surface vessels and aircraft equipped with INS, additional position data are often obtained from GPS. On underwater vessels, the accuracy of INS position estimates is greatly improved by using a Doppler Velocity Logger (DVL) to measure velocity of the vessel’s speed. On some AUVs, several of these systems are combined to improve the overall accuracy of onboard navigation. For more information about INS and DVL systems, visit http://www.oceanexplorer.noaa.gov/explorations/08auvfest.
The Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010 Expedition will use a SeaBED AUV designed for high resolution imaging, bathymetric mapping and chemical sensing. This AUV is navigated using a DVL, and is rated to a working depth of 2000 meters. Mission durations are typically about ten hours, depending on the power required to operate instruments for a specific mission. The AUV is operated by a team of four people who attend to the mechanical systems, navigation, mission planning, data processing, deck handling and underway progress
monitoring. SeaBED has been used for a variety of missions including fisheries and coral reef habitat mapping, geological and geochemical mapping at turbidity current sites, archeological mapping of ancient shipwrecks off Greece, and coral reef habitat mapping off Puerto Rico and the US Virgin Islands.
Remotely operated vehicles (ROVs) are unoccupied robots linked to an operator by a group of cables. Underwater ROVs are usually controlled by an operator aboard a surface ship. Most are equipped with one or more video cameras and lights, and may also carry other equipment such as a manipulator or cutting arm, water samplers, and measuring instruments to expand the vehicle’s capabilities. Cradle of the Earthquake: Exploring the Underwater San Andreas Fault 2010 Expedition will use a ROV that carries a high quality video imaging system, high-resolution digital still camera and a manipulator for collecting biological samples from areas observed in the AUV imaging and mapping surveys. For more information about ROVs, visit http://oceanexplorer.noaa.gov/technology/subs/rov/rov.html.
Exploration Vessel: SRV Derek M. Baylis
For more than a century, most ocean exploration expeditions have been conducted from ships powered with petroleum fuels. The Derek M. Baylis (DMB) is 65-foot sailing vessel built specifically to provide a “green” alternative for modern ocean explorers. With a rear deck like a trawler and living quarters forward, the Baylis carries up to 24 passengers comfortably on day trips and 10 passengers for longer voyages. Because wind is its primary fuel, the ship can be operated quietly, economically and emit zero pollution. Low noise from the vessel will improve AUV tracking and data quality from the sonar systems, as well as reduce the likelihood of scaring fish away during mid-water surveys. Sails will add stability, and the reduced roll motion will improve the quality of multibeam data. The economical operation of the Baylis will also reduce the cost of ship operations during the 24 days that the expedition is at sea. The ship’s auxiliary engine cruises at 10 knots using only 2 gph of diesel. A removable transom, stern mounted titanium A-frame, and 22-foot long aft deck allow a wide range of gear to be easily deployed. The Seabed AUV and ROV will be deployed from the stern, while multibeam sonar, Chirp profiler, and AUV tracking transducers will be deployed from a pole mount to starboard. The vessel’s low freeboard (distance from the deck railing to the water surface) makes it easier to operate the AUV and ROV than on larger vessels. For more information about SRV Derek M. Baylis, see http://sealifeconservation.org/baylis.html
More About Cold Seep Communities and Methane Hydrates
The first chemosynthetic communities were discovered in 1977 near the Galapagos Islands in the vicinity of underwater volcanic hot springs called hydrothermal vents, which usually occur along ridges separating the Earth’s tectonic plates. Hydrogen sulfide is abundant in the water erupting from hydrothermal vents, and is used by chemosynthetic bacteria that are the base of the vent community food chain. These bacteria obtain energy by oxidizing hydrogen sulfide to sulfur:
CO2 + 4H2S + O2 > CH2O + 4S +3H2O
(carbon dioxide plus sulfur dioxide plus oxygen yields organic matter, sulfur, and water).
Visit http://www.pmel.noaa.gov/vents/ for more information and activities on hydrothermal vent communities.
Cold-seep communities are similar in that they are based upon energy produced by chemosynthesis; but while energy for hydrothermal vent communities is derived from underwater hot springs, cold-seep communities are found in areas where gases (such as methane and hydrogen sulfide) and oil seep out of sediments. These areas are commonly found along continental margins, and (like hydrothermal vents) are home to many species of organisms that have not been found anywhere else on Earth. Typical features of cold-seep communities that have been studied so far include mounds of frozen crystals of methane and water called methane hydrate ice, that are home to polychaete worms.
Methane is produced in many environments by a group of Archaea known as the methanogenic Archaeobacteria. These Archaeobacteria obtain energy by anaerobic metabolism through which they break down the organic material contained in once-living plants and animals. When this process takes place in deep ocean sediments, methane molecules are surrounded by water molecules, and conditions of low temperature and high pressure allow stable ice-like methane hydrates to form.
Methane hydrate is a type of clathrate, a chemical substance in which the molecules of one material (water, in this case) form an open lattice that encloses molecules of another material (methane) without actually forming chemical bonds between the two materials (visit http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/about-hydrates/about_hydrates.htm ).
Scientists are interested in methane hydrates for several reasons. A major interest is the possibility of methane hydrates as an energy source. The U.S. Geological Survey has estimated that on a global scale, methane hydrates may contain roughly twice the carbon contained in all reserves of coal, oil, and conventional natural gas combined. In addition to their potential importance as an energy source, scientists have found that methane hydrates are associated with unusual and possibly unique biological communities that may be sources of beneficial pharmaceutical materials.
While such potential benefits are exciting, methane hydrates may also cause big problems. Although methane hydrates remain stable in deep-sea sediments for long periods of time, as the sediments become deeper and deeper they are heated by the Earth’s core. Eventually, temperature within the sediments rises to a point at which the clathrates are no longer stable and free methane gas is released (at a water depth of 2 km, this point is reached at a sediment depth of about 500 m). The pressurized gas remains trapped beneath hundreds of meters of sediments that are cemented together by still-frozen methane hydrates. If the overlying sediments are disrupted by an earthquake or underwater landslide, the pressurized methane can escape suddenly, producing a violent underwater explosion that may result in disastrous tsunamis (“tidal waves”).
The release of large quantities of methane gas can have other consequences as well. Methane is one of a group of the so-called “greenhouse gases”. In the atmosphere, these gases allow solar radiation to pass through to the surface of the Earth, but absorb heat radiation that is reflected back from the Earth’s surface, thus warming the atmosphere. Many scientists have suggested that increased carbon dioxide in the atmosphere produced by burning fossil fuels is causing a “greenhouse effect” that is gradually warming the atmosphere and the Earth’s surface. A sudden release of methane from deep-sea sediments could have a similar effect, since methane has more than 30 times the heat-trapping ability of carbon dioxide. In 1995, Australian paleoceanographer Gerald Dickens suggested that a sudden release of methane from submarine sediments during the Paleocene epoch (at the end of the Tertiary Period, about 55 million years ago) caused a greenhouse effect that raised the temperatures in the deep ocean by about 6oC. The result was the extinction of many deep-sea organisms known as the Paleocene extinction event. More recently, other scientists have suggested that similar events could have contributed to mass extinctions during the Jurassic period (183 million years ago), as well as to the sudden appearance of many new animal phyla during the Cambrian period (the Cambrian explosion, about 520 million years ago).
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Director, Education Programs
NOAA Office of Ocean Exploration and Research