During an OASIS cruise to the Mid-Cayman Rise in January 2012, the manipulator arm of the remotely operated vehicle Jason placed the intake tube of an isobaric gas-tight sampler (IGTS) into the stream of fluid gushing out of a hydrothermal vent. The fluid contains gases that are in liquid form because of the high pressure of the deep ocean. In the past, bringing such samples to the surface resulted in loss of the gaseous portion. WHOI scientists and engineers developed the IGTS to keep samples of vent fluid at high pressure until they can be brought to a lab for analysis. WHOI geologist Chris German led the expedition, which visited the deepest known hydrothermal vents in the world. (Photo by Chris German and the Jason Group, Woods Hole Oceanographic Institution)
What are Hydrothermal Vents?
In 1977, scientists diving in the submersible Alvin made a stunning discovery on the bottom of the Pacific Ocean: vents pouring hot, mineral-rich fluids from beneath the seafloor. In addition, they also found the vents were inhabited by previously unknown organisms that thrived in the absence of sunlight. These discoveries forever changed our understanding of Earth and life on it.
Like hot springs and geysers on land, hydrothermal vents form in volcanically active areas—often on mid-ocean ridges, where Earth’s tectonic plates are spreading apart and where magma wells up to the surface or close beneath the seafloor. Ocean water percolates into the crust through cracks and porous rocks and is heated by underlying magma. The heat helps drive chemical reactions that remove oxygen, magnesium, sulfates and other chemicals from the water that entered the ocean through rain, rivers, and groundwater. In the process, the fluids also become hotter and more acidic, causing them to leach metals such as iron, zinc, copper, lead, and cobalt from the surrounding rocks. The heated fluids rise back to the surface through openings in the seafloor. Hydrothermal fluid temperatures can reach 400°C (750°F) or more, but they do not boil under the extreme pressure of the deep ocean.
As they pour out of a vent, the fluids encounter cold, oxygenated seawater, causing another, more rapid series of chemical reactions to occur. Sulfur and other materials precipitate, or come out of solution, to form metal-rich towers and deposits of minerals on the seafloor. The fluids also contain chemicals that feed microbes at the base of a unique food web that survives apart from the sun. Instead of relying on photosynthesis to convert carbon dioxide into organic carbon, the bacteria use chemicals such as hydrogen sulfide to provide the energy source that drives their metabolic processes and ultimately support a wide range of other organisms such as tubeworms, shrimp, and mussels.
Why Do They Matter?
Hydrothermal vents act as natural plumbing systems that transport heat and chemicals from the interior of the Earth and that help regulate global ocean chemistry. In the process, they accumulate vast amounts of potentially valuable minerals on the seafloor.
The mammoth copper mines of Cyprus, for example, were formed by hydrothermal activity millions of years ago before those rocks were uplifted from the seafloor to become dry land. Commercially valuable mineral deposits are believed to exist on the seafloor near hydrothermal vents, and a few companies have had plans in development for years to exploit some of these. The difficulty of mining in deep water near fragile ecosystems and the relatively small size of ocean bottom deposits compared to those on land have so far prevented seafloor mining from becoming commercially viable.
Vents also support complex ecosystems of exotic organisms that have developed unique biochemical adaptations to high temperatures and environmental conditions we would consider toxic. Learning about these organisms can teach us about the evolution of life on Earth and the possibility of life elsewhere in the solar system and the universe. Many previously unknown metabolic processes and compounds found in vent organisms could also have commercial uses one day.