Why Are Scientists Exploring the Chile Triple Junction?
[NOTE: For additional information about plate tectonics and deep-sea chemosynthetic communities, please see the INSPIRE: Chile Margin 2010 Expedition Education Module.
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 the Earth’s surface.
Earthquakes and volcanoes are among Earth’s most spectacular and terrifying geological events, and both are linked to movements of Earth’s tectonic plates. Where underwater volcanic processes are active, underwater hot springs known as hydrothermal vents may form. The first hydrothermal vents were discovered near the Galapagos Islands in 1977. Here, researchers found large numbers of animals that had never been seen before clustered around plumes of hot fluid flowing from cracks in the lava seafloor. The presence of thriving biological communities in the deep ocean was a complete surprise, because it had been assumed that food energy resources would be scarce in an environment without sunlight to support photosynthesis. Researchers soon discovered that the organisms responsible for this biological abundance do not need photosynthesis, but instead are able to obtain energy from chemical reactions through processes known as chemosynthesis. Ocean explorers have also discovered other types of chemosynthetic communities that are distinctly different from those found near hydrothermal vents. Cold seeps are areas where hydrocarbon gases (such as methane) or oil seep out of sediments and are commonly found along continental margins. Like hydrothermal vents, cold seeps are home to many species of organisms that have not been found anywhere else on Earth.
In parts of the ocean where dissolved oxygen concentration is very low, hydrogen sulfide builds up and can support communities of chemosynthetic organisms. These communities are also found around dead whales (called whale falls) and large masses of dead kelp or trees. Although there are similarities between the chemosynthetic communities in these different habitats, hydrothermal vents, cold seeps, and large organic falls each support hundreds of endemic species (endemic means that these species aren’t found anywhere else). Even along the global ridge crest, which is almost continuous along its entire length, at least six different groups of endemic fauna have been recognized (Van Dover et al., 2002).
Figure 1. Along the western coast of Chile, three of Earth’s tectonic plates intersect in a way that does not occur anywhere else on the planet. Click image for larger view and image credit.
Along the western coast of Chile, three of Earth’s tectonic plates intersect in a way that does not occur anywhere else on the planet (see Figure 1). Chile, and the other countries of South America, lie on top of the South American tectonic plate. To the west of Chile, the Nazca Plate extends beneath the Pacific Ocean and meets the Pacific Plate along a divergent plate boundary called the East Pacific Rise. The southern edge of the Nazca plate adjoins the Antarctic Plate along another divergent plate boundary called the Chile Rise. The eastern edge of the Chile Rise is being subducted beneath the South American Plate at the Chile Triple Junction (CTJ), which is unique because it consists of a mid-oceanic ridge being subducted under a continental tectonic plate. The eastern portion of the Nazca Plate is also being subducted along the Peru-Chile Trench, and the Andes Mountains are one consequence of this process. Not surprisingly, complex movements of three tectonic plates at the CTJ result in numerous earthquakes. In fact, the largest earthquake ever recorded occurred along the Peru-Chile Trench in 1960. While earthquakes and volcanoes are often associated with massive destruction and loss of human life, the same processes that cause these events are also responsible for producing unique habitats for very different life forms.
The INSPIRE: Chile Margin 2010 Expedition was directed toward three key questions: about the CTJ:
- Does intense seismic activity just north of the CTJ cause increased flow of hydrothermal or cold seep fluids?
- Does the complex and unique interaction between ridge and trench tectonic processes at the CTJ produce different types of hydrothermal vent systems than have been found in other areas?
- Are similar organisms found in various chemosynthetic ecosystems near the CTJ, or do these ecosystems have species that are endemic to a particular type of habitat?
To answer these questions, the 2010 expedition was directed toward:
- Locating hydrothermal vent and cold-seep ecosystems near the CTJ;
- Mapping and photographing two new hydrothermal vent sites and two new cold-seep sites near the CTJ;
- Seismic monitoring near the CTJ; and
- Chemical analyses of hydrothermal and cold-seep fluids.
To locate hydrothermal vent and cold-seep ecosystems, expedition scientists used data recorders that can detect chemical and physical water characteristics that signal the presence of hydrothermal vents and cold seeps. Once plumes were located, scientists planned to use an autonomous underwater vehicle (AUV) to prepare high-resolution maps and collect overlapping photographs of vent and cold-seep sites. Unfortunately, less than halfway through the expedition, acoustic signals from AUV ABE (autonomous benthic explorer) suddenly stopped and the vehicle was never heard from again (the daily log for March 6, 2010). This meant that several objectives of the 2010 expedition could not be fulfilled. The INSPIRE: Chile Margin 2012 Expedition is intended to complete this unfinished work.
Hydrothermal vents and cold seeps cause changes to the chemistry and physical characteristics of the surrounding seawater. As this altered seawater diffuses away from vent and cold-seep sites, it produces plumes that are distinctly different from normal seawater. Scientists search for these plumes, since they provide clues about the location of hydrothermal vents and cold seeps. Redox potential is one of the key chemical characteristics used to locate cold seeps and hydrothermal vents. When an atom or molecule loses an electron it is said to be oxidized, and when an atom or molecule gains an electron it is said to be reduced. A reducing substance is one that reduces; in other words, it donates electrons. Similarly, an oxidizing substance is a substance that oxidizes; that is, it receives electrons. A reaction in which one or more electrons are transferred between two molecules is called a redox reaction. Moving electrons can produce electric currents, so electronic instruments can be used to measure these currents. The voltage produced by redox reactions is called redox potential. Chemosynthesis depends upon the availability of reducing substances such as hydrogen sulfide or methane that can donate electrons. Habitats in which these substances occur are called reducing habitats.
Another important change that can signal the presence of hydrothermal vents is the presence of unusually high concentrations of iron and manganese that dissolve in hydrothermal fluids while these fluids are still inside rocks below the seafloor. When these fluids enter seawater near vents, the metals precipitate and produce particles that can be detected with optical sensors. Results from the INSPIRE: Chile Margin 2010 Expedition suggest that hydrothermal vent systems near the CTJ may be different from other vent systems on mid-ocean ridges, and that a modified strategy may be needed to pinpoint their location. The big difference is that water samples collected near suspected location of CTJ vents did not have the expected increase in metal particles.
A possible explanation for this difference is that the tectonic plate margins of the CTJ are much closer to land than many other hydrothermal vent locations, and probably receive substantial amounts of sediment from the nearby South American continent (including the western watershed of the Andes Mountains). In other vent locations that receive substantial sediment input hydrothermal fluids must rise through sediment layers before they emerge from the seafloor, and the metals in these fluids precipitate while they are still within the sediments. This eliminates the metal particles that can signal the presence of vents, and also causes changes to the sediments as a result of the precipitated metals. To help detect vents that may be buried beneath sediments, the INSPIRE: Chile Margin 2012 Expedition will use an AUV named Sentry that carries instruments to measure redox potential, as well as sidescan sonar to provide images that show the topography of the ocean floor and reveal changes to sediments that may have been caused by precipitated metals from hydrothermal fluids.
In addition to AUV Sentry, the INSPIRE: Chile Margin 2012 Expedition will use other technologies including a towed camera system (TowCam), multicorer to collect sediment samples (and whatever may be living in the sediments), trawls to collect animals living on the seafloor surface, and instruments to measure temperature, conductivity, and depth (CTD). For more information on these technologies, please see the INSPIRE: Chile Margin 2010 Expedition Education Module; and the introductory essays for the INSPIRE: Chile Margin 2012 Expedition.