Why Are Scientists Exploring Deep Underwater Caves Around the Island of Bermuda?
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, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
Anchialine caves are partially or totally submerged caves in coastal areas. Anchialine (pronounced “AN-key-ah-lin”) is a Greek term meaning “near the sea,” and anchialine caves often contain freshwater and/or brackish water in addition to seawater. These caves may be formed in karst landscapes as well as in rock tubes produced by volcanic activity. Karst landscapes are areas where limestone is the major rock underlying the land surface, and often contain caves and sinkholes formed when acidic rainwater dissolves portions of the limestone rock. Water in anchialine caves tends to stratify according to salinity, with the heavier seawater below the level of fresh and brackish water. This stratification produces distinctive habitats inhabited by a variety of species that are endemic to these habitats (endemic means that these species are not found anywhere else). Some of these species are “living fossils” known as relict species, which means that they have survived while other related species have become extinct.
Animals that live only in anchialine habitats are called stygofauna or stygobites. Investigations of these species have revealed some puzzling relationships, including:
- Some stygobite species appear to have been in existence longer than the caves they inhabit, which implies that these species must have arrived in the caves from somewhere else; but how could this happen if these species are only found in caves?
- Some stygobite species are found in caves that are widely separated, such as crustacean species found in caves on opposite sides of the Atlantic Ocean and species in Australian anchialine caves that are also found Atlantic and Caribbean caves.
- Geographic distribution of some species suggests a possible connection with mid-ocean ridges, such as shrimps belonging to the genus Procaris that are only known from anchialine habitats in the Hawaiian Islands, Ascension Island in the South Atlantic, and Bermuda in the North Atlantic.
- Some anchialine species are most closely related to organisms that live in the very deep ocean.
- Some anchialine species are most closely related to organisms that live in deep-sea hydrothermal vent habitats.
- An unusually large proportion of anchialine cave species in Bermuda are endemic to these caves, suggesting that these habitats have been stable for a long period of time.
Most investigations of anchialine caves have been confined to relatively shallow depths; yet, the observations described above suggest that connections with deeper habitats may also be important to understanding the distribution of stygobite species. Bermuda is a group of mid-ocean islands composed of limestone lying on top of a volcanic seamount. Because they are karst landscapes, Bermuda has one of the highest concentrations of cave systems in the world. Typical Bermuda caves have inland entrances, interior cave pools, underwater passages, and tidal spring outlets to the ocean. Bermuda’s underwater caves contain an exceptional variety of endemic species, most of which are crustaceans. Most of these organisms are relict species with distinctive morphological, physiological, and behavioral adaptations to the cave environment that suggest these species have been living in caves for many millions of years. Yet, all known anchialine caves in Bermuda were completely dry only 18,000 years ago when sea levels were at least 100 m lower than present because of water contained in glaciers. Such observations suggest the possibility of additional caves in deeper water that would have provided habitat for anchialine species when presently-known caves were dry.
The primary goal of the Bermuda Deep Water Caves 2011: Dive of Discovery Expedition is to explore the uppermost 200 meters of the Bermuda seamount and adjacent seamounts to confirm the existence of underwater caves at depths between 60 and 200 meters. A related goal is to document underwater features that indicate sea level during the last Ice Age, which was much lower than at present.
Key activities to achieve these goals are divided into three phases. The first two phases were completed in 2009. In Phase 1, high-resolution multibeam sonar was used to produce detailed maps that assist with locating deep-water caves and sea level benchmarks. During Phase 2, a remotely operated vehicle was used to examine and photograph sites of interest identified by the multibeam survey. In particular, expedition scientists were looking for signs of water movements around possible cave entrances, such as congregations of schooling fish, plumes of brackish water, sand ripples, or unusual abundance of filter-feeding organisms such as sponges. Phase 3 of the Expedition (2011) involves exploration of caves by technical divers to collect biological specimens and place or recover instrument packages.The 2009 multibeam sonar survey produced detailed maps of the entire shelf edge around the Bermuda Platform and 75% of the shelf edge around the Challenger Bank, in water depths greater than 150 m. Data from the mapping surveys were used to identify more than 100 points of interest. The mapping surveys also revealed what appeared to be several submarine landslides around the perimeter of the Bermuda Shelf. During the final week of the 2009 expedition, 33 video survey dives were completed at selected points of interest, and 25 cumulative hours of dive time were logged on these sites. For more information about results from the 2009 Expedition, see the Mission Summary.
Key questions for the Bermuda Deep Water Caves 2011: Dive of Discovery Expedition include:
- Do underwater caves exist around Bermuda between depths of 60 and 200 m?
- If such caves exist, how do organisms in these caves compare with their shallow water anchialine counterparts?
- If such caves exist, what do they reveal about the geologic history of the Bermuda Pedestal?
- If such caves exist, are they hydrologically-active and are they associated with aggregations of commercially-important fish species as is the case with shallow water marine caves?
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 system measures the strength of the signal and the time elapsed between the emission of the sound pulse and the reception of the echo. This information is used to calculate the distance of the object, and an experienced operator can use the strength of the echo to make inferences about some of the 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.
Remotely Operated Vehicle
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. The Bermuda Deep Water Caves 2011: Dives of Discovery Expedition used a SeaBotix LBV200L ROV capable of diving to 200 m. The ROV measures 53 mm x 245 mm x 254 mm, and weighs 11 kg in air. A 350 m fiber optic cable, 8 mm in diameter, connects a monitor aboard the surface ship to high resolution color and low-light black-and-white video cameras aboard the ROV. The ROV requires 1000 watts of power at 100 - 240 volts AC. Optional accessories include multibeam, scanning, and profiling sonars; grabbers; and precision positioning systems. Four thrusters are able to achieve a surface speed of three knots, and are mounted so that the ROV has forward/reverse, lateral, vertical, and rotational movements. To visualize these movements, imagine a man standing upright: if he steps forward or backward he has forward/reverse movement; if he steps to the left or right he has lateral movement; if he jumps up he has vertical movement; if he stands in one place and twists to the left or right he has rotational movement. Two other kinds of movement are pitch and roll: If he falls flat on his back or face he has pitch; if he falls to the left or right he has roll. For more information about ROVs, visit http://oceanexplorer.noaa.gov/technology/subs/rov/rov.html.
Doppler Current Meter
Currents can carry food, oxygen, and organisms in and out of underwater caves, so information about how water is exchanged between a cave’s interior and the surrounding ocean is essential to understanding the ecology of stygobite species. The Bermuda Deep Water Caves 2011: Dives of Discovery Expedition uses a Vector Doppler current meter that can be placed in a cave by divers and left for an extended period to obtain data on the direction, velocity, and cycles of underwater currents. This instrument measures current velocity using the Doppler effect. This effect is often observed when the sound of a train whistle seems to rise and fall in pitch as the train approaches, passes, then moves away. The Vector Doppler current meter has a transducer that sends pulses (pings) of sound at a constant frequency into the water. Particles in the water reflect the pings back to separate transducers. If the particles are moving toward the current meter, the reflected pings will have a slightly higher frequency than when they were transmitted. If the particles are moving away from the current meter, their frequency with be slightly lower. These frequency differences are called the Doppler shift. The particles are usually zooplankton, but may also be suspended sediment or small bubbles. Because these particles have been found to have the same average speed as the surrounding water, the Doppler shift can be used to estimate the velocity of water currents. For more information about Doppler current meters, see Gordon (1996) and http://oceanexplorer.noaa.gov/technology/tools/acoust_doppler/acoust_doppler.html.
Electronic Water Quality Analyzer
A YSI 6600 monitoring sonde may also be deployed in conjunction with the Doppler current meter to collect information about additional characteristics of cave environments. A sonde is a torpedo-shaped device that is used to gather water quality data. Sondes may have multiple probes, and each probe may have one or more sensors that measure specific characteristics of the surrounding water. Sondes used by the Bermuda Deep Water Caves 2011: Dives of Discovery Expedition have sensors that measure depth, salinity, temperature, pH, dissolved oxygen and chlorophyll concentration. Sondes may be connected to a computer or data display with a communications cable so that measurements can be made at intervals of a few seconds, and can be seen as soon as they are made. Some sondes also are capable of unattended operation for periods of days or weeks, and can be set to make measurements at longer intervals (minutes or hours). Data collected by an unattended sonde may be stored in a data logger built into the sonde, or may be transferred to an external data collection platform. For additional information, see http://oceanexplorer.noaa.gov/technology/tools/sonde_ctd/sondectd.html.
Mixed Gas SCUBA
Conventional SCUBA techniques using compressed air have several inherent problems. One of these is that as the pressure of a gas increases, the solubility of that gas in a liquid increases as well (Henry’s law). In water, pressure increases by one atmosphere for every 10 meters (33 feet) of depth. So, a diver at a depth of 20 meters is exposed to three atmospheres of pressure. If the diver breathes air from a demand regulator at 20 meters for a while, her blood will contain three times the amount of dissolved gases from the air than it did at the surface. If the diver rapidly ascends from 20 meters, the dissolved gases in her blood may form bubbles, creating a problem that may block critical blood vessels. This condition is called decompression sickness or “the bends,” and was first seen in miners working in pressurized coal mines (it was also a problem for workers constructing the Brooklyn Bridge, who spent hours working underwater in pressurized iron boxes called caissons, so yet another name for the condition is “caissons disease”). Since air is about 78% nitrogen, more nitrogen is dissolved in the blood than other gases so the bubbles formed during a case of decompression sickness are bubbles of nitrogen gas. Oxygen isn’t believed to be involved, since much of the oxygen dissolved in a diver’s blood is quickly bound by hemoglobin, and normal metabolism reduces blood oxygen concentration.
Another problem involves the effects of nitrogen and oxygen when they are breathed under pressure. The partial pressure of a gas in a mixture of several gases is equal to the percentage of the gas in the mixture multiplied by the total pressure of the gas mixture. So the partial pressure of nitrogen in air at one atmosphere (atm) pressure is about 0.78 atm. When the partial pressure of nitrogen in a diver’s blood rises above about 3 atmospheres (corresponding to a depth of about 30 m) a condition known as nitrogen narcosis may occur which is an effect similar to alcohol intoxication. The severity of the impairment depends upon individual susceptibility as well as environmental conditions (temperature, time of day, etc.). Oxygen may become toxic at partial pressures above 1.4 atmospheres (corresponding to a depth of about 180 m), causing convulsions. Individual thresholds vary widely and depend upon degree of exertion as well as environmental conditions.
Divers using conventional SCUBA techniques avoid these problems by closely monitoring their dive time and depth, since they both affect the amount of gas that dissolves in the blood. With mixed gas SCUBA, divers breathe special gas mixtures instead of air. Nitrox mixtures contain nitrogen and oxygen but with less nitrogen and more oxygen than ordinary air. Nitrox mixtures can be used at moderate depths without risking oxygen toxicity, and allow divers to greatly decrease the time needed for decompression. Trimix is a breathing gas mixture composed of helium, oxygen, and a third gas which is usually nitrogen. The advantage of trimix is that the concentrations of oxygen and nitrogen are reduced so that divers may descend to greater depths without risking oxygen toxicity or nitrogen narcosis. In addition, the density of the breathing mixture is reduced compared to air, which makes it easier to breathe at higher pressures.
Another advanced diving technique uses closed-circuit rebreather systems, which recapture oxygen in exhaled gases and allow a diver to carry much less breathing gas. In addition, modern closed-circuit rebreathers constantly monitor oxygen levels in the breathing mixture and are able to adjust the oxygen concentration to a level that is optimum for the divers’ depth. The result is much shorter decompression times and much less risk of oxygen toxicity. For more information on technical diving, visit the Cayman Islands Twilight Zone Expedition web page (http://oceanexplorer.noaa.gov/explorations/07twilightzone/background/techdive/techdive.html).
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Director, Education Programs
NOAA Office of Ocean Exploration and Research
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