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Background Information

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Mid-Cayman Rise study area.

Overview

This October a renowned team of oceanographers and astrobiologists will explore one of the deepest points in the Caribbean Sea, searching for life in extreme seafloor environments. Using the new hybrid underwater robotic vehicle Nereus, these scientists will extend their investigations beyond the reach of other research submersibles to the bottom of the Mid-Cayman Rise, whose maximum depth is just over 6,800 meters (4.2 miles) deep.

The study area, also known as the Mid-Cayman Spreading Center, is one of Earth’s deepest and slowest-spreading mid-ocean ridges—regions where two of Earth’s tectonic plates are ripped apart and new material wells up from the Earth’s interior. There, scientists will search for hydrothermal vent systems—natural, seafloor plumbing systems where cold seawater circulates down into the hot, freshly-formed oceanic crust releasing heat and mineral-rich fluids at the seafloor that support complex ecosystems of exotic organisms.

By exploring this extreme and previously uninvestigated section of the Earth’s deep seafloor, this research seeks to extend our understanding of the limits (in terms of extreme environments) to which life can exist on Earth and to help prepare for future efforts to explore for life on other planets.

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A detailed bathymetric base map of the Mid-Cayman Rise study area.  The 32 round targets show locations at which scientists plan to stop and sample the water column using the CTD—every 5 miles along and across axis, to complete a thorough survey for hydrothermal activity.

Where is the Expedition?

The Mid-Cayman Rise lies in the middle of the Cayman Trough (or Cayman Trench) which is located in the western Caribbean Sea, south of Cuba and Jamaica and close to the Cayman Islands. In the middle of the East-West trending Cayman Trough lies the Mid-Cayman Spreading Center—approximately 110 kilometers (68 miles) long and spreading apart at an ultra-slow rate of less than 20 millimeters (0.9 inches) per year.


Why Study this Area
Ultra-slow spreading ridges such as the Mid Cayman Spreading Center make up about 25 percent of the 60,000-kilometer (40,000-mile) long system of mid-ocean ridges that encircle the globe. Until recently, these ultra-slow ridge systems remained almost completely overlooked, because of an early hypothesis that their slow spreading rates would make hydrothermal activity rare or absent. This hypothesis has since been disproved and surveys along ultra-slow ridges in the Southwest Indian Ocean and the Arctic have shown that, on average, at least one vent site occurs approximately every 100 kilometers (60 miles) along the axis of these ridges.

While no hydrothermal exploration of the Mid-Cayman Spreading Center has previously been carried out, dredging and submersible studies along the walls of the rift valley have identified locations where rocks have been extensively altered hydrothermally. The discovery of these rocks, as well as the hydrothermal activity found on similar spreading centers, makes researchers optimistic that they will discover active hydrothermal vents in the Cayman Trough.

If the science team does find hydrothermal systems at such great depths, their discovery will likely extend the known limits to life on our planet in terms of pressure, temperature, and vent fluid chemical compositions. And because hydrothermal circulation can arise on any planet that currently has, or has experienced, liquid water and a source of heat, this new research along the Mid-Cayman Rise can provide new insights into the possible origins and evolution of Earth’s biosphere, as well as the conditions that might have given rise to comparable life-forming chemicals and/or life on other worlds.

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R/V Cape Hatteras

The 41-meter (135-foot ), steel-hulled research vessel Cape Hatteras is owned by the National Science Foundation and operated by the Duke/University of North Carolina Oceanographic Consortium. It has a cruising speed of 10 knots and can stay out at sea for up to 25 days.
» Learn more about the R/V Cape Hatteras

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» Take an Interactive Tour of Nereus to learn more.

HROV Nereus

HROV Nereus, built and operated by Woods Hole Oceanographic Institution engineers, is an unmanned vehicle that operates in two complementary modes.  It can swim freely as an autonomous underwater vehicle (AUV) to survey large areas of the depths, map the seafloor, and give scientists a broad overview.  When Nereus has located something interesting (for example, on this cruise we hope to find completely new and unexplored vent-sites) it can be brought back on board the ship and transformed into a remotely operated vehicle (ROV) tethered to the ship via a microthin, fiber-optic cable. Through this tether, Nereus can transmit high-quality, real-time video images back to skilled pilots on the ship, who can send commands to the vehicle to collect samples or conduct experiments with a manipulator arm.

For this expedition, Nereus will first operate in AUV mode.  After using the CTD to survey the area for hydrothermal plumes, researchers will deploy the AUV to locate, map, and photograph hydrothermal vents, at the seafloor. To do this, the Nereus operation team will first relocate each plume and home in to its source. The AUV will then descend close to the seafloor to obtain detailed bathymetry of the target site and intercept buoyant plumes from any high-temperature vents present.  Lastly, the vehicle will be programmed to photo-mosaic individual vent sites and any ecosystems they host.

After operations in AUV mode, the vehicle will be converted into ROV mode for the last phase of the expedition.  By this stage (Leg 2 of the cruise) we will also be ready to welcome a whole new set of specialist scientists on board.  In ROV mode, Nereus will have up to eight hours of sampling per deployment at any vent site we have found – or other areas of interest.  The vehicle will perform a combination of geochemical and biological sampling of vent fluids, minerals, hydrothermally altered rocks, large volume filtration (to sample chemicals or microbial life in the vent fluid) and extensive sampling of any megafauna.
» Learn more about Nereus

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Conductivity, Temperature, and Depth (CTD) Sensor

The first phase of the expedition will be to search for the plumes that rise up above hydrothermal vent sites using a Conductivity, Temperature, and Depth (CTD) sensor.  Buoyant, high-temperature plumes of fluid emanating from hydrothermal vents contain high concentrations of iron, manganese, and methane and rise hundreds of meters above the seafloor before being dispersed laterally by deep-ocean currents. It is the CTD’s job to detect temperature and other physical signals generated from the plumes and to collect water samples that will be analyzed on-board the ship as well as back in the lab after the cruise.

The CTD instrument package hangs from a conducting cable that is lowered vertically beneath the ship.  Arranged around this instrument pack in a rosette are a series of 24 sampling bottles that are open top and bottom with snap-shut caps.  As the instrument package is lowered to the seafloor, the various sensors send readings back, in real time, to a computer on the ship. Scientists monitor how the composition of the ocean changes at increasing depth from the surface to just above the seafloor and use the data sent back to the ship to map hydrothermal signals detected in the water column above the seafloor.  Then, as they haul the instrument package back up through the water column, they can choose where to take samples by using the top-side computer to send an electrical pulse down the same conducting cable that brings the data to the surface.  Each pulse triggers a sample bottle on the CTD frame, one bottle at a time, so that the end-caps snap shut and a water sample is collected at exactly the depth required.  When the CTD and water sample bottles are hauled back on the ship, the scientists transfer the water samples to the ship’s laboratory where they perform analyses to identify hydrothermal chemicals.
» Learn more about CTD's

Dates

October 7 - October 26: First leg of expedition.  CTD and Nereus AUV operations
October 27 - October 28: Port call, Georgetown, Cayman Islands.  Nereus switches to ROV mode
October 29 – November 6: Second leg of expedition.  Nereus ROV operations

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Last updated: October 1, 2009