Dean Blue Hole Long Island Bahamas 20110210

Watling's Blue Hole San Salvador Island, Bahamas

A blue hole is a large marine

cavern A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

sinkhole A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are locally also known as ''vrtače'' and shakeholes, and to openi ...

, which is open to the surface and has developed in a

bank A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets. Becau ...

or island composed of a

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonat ...

bedrock (

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. C ...

). Their existence was discovered in the late 20th century by fishermen and recreational divers. Blue holes typically contain tidally influenced water of fresh, marine, or mixed chemistry. They extend below sea level for most of their depth and may provide access to submerged cave passages. Well-known examples are the Dragon Hole (in the South China Sea) and, in the Caribbean, the

Great Blue Hole The Great Blue Hole is a giant marine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll from the mainland and Belize City. The hole is circular in shape, across and deep. It has a surface area of . ...

and Dean's Blue Hole. ''Blue holes'' are distinguished from ''

cenote A cenote ( or ; ) is a natural pit, or sinkhole, resulting from the collapse of limestone bedrock that exposes groundwater. The regional term is specifically associated with the Yucatán Peninsula of Mexico, where cenotes were commonly used for ...

s'' in that the latter are inland voids usually containing fresh

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...

rather than

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

Description

Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. Their water circulation is poor, and they are commonly anoxic below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

. The deep blue color is caused by the high transparency of water and bright white

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...

. Blue light is the most enduring part of the spectrum; other parts of the spectrum—red, yellow, and finally green—are absorbed during their path through water, but blue light manages to reach the white sand and return upon reflection. The deepest blue hole in the world at 300.89 meters (987 feet) deep is in the

South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Phil ...

and is named the Dragon Hole, or Longdong. The second deepest blue hole in the world with underwater entrance at is Dean's Blue Hole, located in a bay west of

Clarence Town Clarence Town is a town in the Bahamas. It is located on Long Island. Clarence Town is the capital of Long Island and has a population of 86 people as of 2010.Long Island,

Bahamas The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to 88% of the archi ...

. Other blue holes are about half that depth at around . The diameter of the top entrance ranges typically from (Dean's Blue Hole) to (

in Belize). The overall largest blue hole (taking into account depth and width) is located 100 kilometers from the coast of Belize. The Great Blue Hole is a massive 300 meters wide and 125 meters deep.

Formation

Blue holes formed during past ice ages, when the

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...

was as low as lower than at present. At those times, these formations were targets of the same erosion from rain and chemical weathering common in all

-rich terrains; this ended once they were submerged at the end of the ice age. Most blue holes contain freshwater and saltwater. The halocline is the point in these blue holes where the freshwater meets the saltwater and where a corrosive reaction takes place that eats away at the rock. Over time this can create side passages, or horizontal "arms", that extend from the vertical cave. These side passages can be quite long; e.g., over in the case of the Sawmill Sink in the Bahamas. The means in which blue holes are formed is through Karst processes. Karst processes need a specific type of topography. Rocks like limestone, gypsum and marble are soluble and so dissolution creates passages and cave systems underground. This paired with doline formation allows blue holes to be formed. Doline formation were once closed depressions formed by solution of superficial rock or subsidence collapse into an underground void. Most blue holes are formed through these processes, however, some show no signs of passages or cave systems which are requirements of karst and doline processes. Therefore, these blue holes must have other origins like possibly vertical reef development. Some blue holes however do not experience karst or doline processes during their formation. Some form through bedrock dissolution and collapse which in this case usually is controlled through tidal forcing, while others are controlled by carbonate dissolution, sea level fluctuations, and the presence of eogenetic carbonates.

Occurrence

Blue holes are typically found on shallow carbonate platforms, exemplified by the Bahama Banks, as well as on and around the

Yucatán Peninsula The Yucatán Peninsula (, also , ; es, Península de Yucatán ) is a large peninsula in southeastern Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north ...

, such as at the

at Lighthouse Reef Atoll,

Belize Belize (; bzj, Bileez) is a Caribbean and Central American country on the northeastern coast of Central America. It is bordered by Mexico to the north, the Caribbean Sea to the east, and Guatemala to the west and south. It also shares a wa ...

. Many deep spring basins formed by karst processes and located inland also are called blue holes, for example, Blue Hole in

Castalia Castalia ( grc, Κασταλία), in Greek mythology, was a naiad- nymph, a daughter of Achelous who inhabited the Castalian spring in Delphi. In older traditions, Castalian spring already existed by the time Apollo came to Delphi searching ...

Ohio Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...

Diversity

Many different fossils have been discovered that indicate the type of life forms that existed in blue holes. Other life forms such as marine life and marine fossils have also been noticed;

crocodile Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant ...

and

tortoise Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin: ''tortoise''). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like ot ...

fossils, for instance, have been found in blue holes. Important types of bacterial colonies have also been found in blue holes. Due to the conditions of a blue hole, they are forced to live off of sulfur compounds like hydrogen sulfide, which are toxic to most organisms. These special bacteria have produced many insights into the chemistry and biology of microbial life. Blue holes have a great diversity of microbes. They create biogeochemical pathways creating a unique and diverse environment within the blue holes. In the surface layer, oxygen, DOC, POC and chlorophyll need to be in low levels in order for cyanobacteria to respire. As depth increases, many branches and sub branches of microbes create specific niches based on the chemistry and nutrient availability of that depth. Microorganisms including

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

, meiobenthic, and nematodes also follow this pattern of organization, and inhabit the areas of the water column where the nutrients they rely on are most available. Nematodes, which are predominantly non-selective detrivores, are tolerable to the anoxic conditions at the base of blue holes, allowing them to survive where other species cannot. They thrive at the lowest depths of blue holes due to the abundance of organic matter that settles there. Similarly, foraminifera inhabit the lower depths, and even increase in diversity with depth. Meiobenthic organisms cannot survive the high sulfide found at depth, and remain in the surface layers of blue holes. Generally, the diversity of all forms of life is 2-3x greater in blue holes than other diverse areas of the ocean, including coastal and abyssal environments. When the diversity of microorganisms is larger, a proportional increase in larger organisms and their diversity is to be expected.

Sedimentation

Sediment accumulation is quite unique in blue holes. Sedimentation occurs at the center of holes rather than the edges. There are many different kinds of sediment that help preserve fossils and climate records. Main sediments that build up and create layers in blue holes are sapropel, detrital peat and

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

marls. Within these layers, microfossils can be found. Sediment cores taken from three blue holes in the Bahamas showed that with depth, more sapropel, detrital and freshwater peat, and lacustrine marls were found. At about 150 cm of sediment core, microfossils of wood, Charophytes and

Hydrobiidae Hydrobiidae, commonly known as mud snails, is a large cosmopolitan family of very small freshwater and brackish water snails with an operculum; they are in the order Littorinimorpha. Distribution Hydrobiidae are found in much of the wor ...

were found.

Chemistry

The chemistry of blue holes vary greatly depending on how they were formed. All blue holes have a layer of freshwater at the surface and more saline water as the depth increases. Many have pycnoclines and haloclines that show these zones, similar to the ocean around it. Many blue holes are great sediment traps and can preserve climate and fossil records dating back to the

last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eu ...

. The reason blue holes are able to preserve such records is due to the anoxic bottom water most blue holes contain. Chemical and Physical Characteristics of Blue holes

Chemical and Physical Characteristics of Blue holes

Stable Hydrogen and Oxygen isotopes can be used to help identify where the water within blue holes comes from. Scientists have discovered that many have

meteoric A meteoroid () is a small rocky or metallic body in outer space. Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...

or marine sources of saline water within them. Being able to identify where the water comes from in these columns allows scientists to see how tidally influenced they are. Most blue holes have a range in salinity from fresh water to hypersaline. Conduits and passageways allow for brackish water to enter as well. When the same isotopes of major ions are found in blue holes and in the surrounding ocean, it can be concluded that these blue holes are tidally influenced and have a marine water source, however, if the isotopes are similar to those found in meteoric lenses, then the source is meteoric.

Preservation

Due to water conditions at the bottom of blue holes, fossil preservation is very effective. Insufficient oxygen and light prevent decay; these conditions have resulted in preservation of fossils for thousands of years, allowing scientists to identify the skeletons of species extinct for years, as well as human skeletons. Sediments surrounding the fossils are rich in macro and

microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

s (twigs, leaves, pollen, spores, etc.), further improving their preservation. Fossils identified: Tortoise (undescribed species) Caracara (''Caracara creightoni'') Cuban crocodile (''Crocodylus gundlachii'') Cooper's Hawk (''Accipiter cooperii'') Birds (25 species) Snakes (3 species) Bats (4 species) Ancient Native Lucayans Plant fossils are also well preserved at the bottom of blue holes, and using sediment cores it is possible to determine what species of plants and trees existed around the area thousands of years ago. For example, The Bahamas today are dominated by tropical dry evergreens, and tropical pines, but sediment cores from the Abacos blue hole revealed preserved woody species such as ''Coccoloba, Exothea, Bursera,'' etc.

Expeditions

Exploring blue holes requires a level of competence and equipment appropriate to the depth and overhead penetration. In 2009 a team of scientists set out to study seven of these blue holes in the Bahamas. Through over 150 dives, the scientists, led by Keith Tinker, investigated bacteria able to live in anoxic environments. This allowed them to make connections to fields such as astrobiology where organisms thrive without oxygen or sunlight. In 2018, another group of scientists set out to explore the Great Blue Hole of Belize using two submarines of the latest technology. One of the major scientific contributions to result from this expedition was the first 3-dimensional map of its interior. The researchers captured features such as

stalactite A stalactite (, ; from the Greek 'stalaktos' ('dripping') via ''stalassein'' ('to drip') is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble an ...

s, the hydrogen sulfide layer, and other details that cannot usually be seen by the naked human eye. As part of a three-year study, a group of scientists set out in May and September 2019 to explore a blue hole nicknamed the " Amberjack Hole" located 30 miles off the coast of

Sarasota, Florida Sarasota () is a city in Sarasota County on the Gulf Coast of the U.S. state of Florida. The area is renowned for its cultural and environmental amenities, beaches, resorts, and the Sarasota School of Architecture. The city is located in the sou ...

. Individuals from Mote Marine Laboratory, Florida Atlantic University, Harbor Branch,

Georgia Institute of Technology The Georgia Institute of Technology, commonly referred to as Georgia Tech or, in the state of Georgia, as Tech or The Institute, is a public research university and institute of technology in Atlanta, Georgia. Established in 1885, it is part of ...

, the

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

, and the NOAA Office of Ocean Exploration participated in the expedition. The expedition gathered information about life around and within the hole, seawater composition, and the hole's bottom sediments. A follow-up expedition is planned in August 2020 to a deeper blue hole named the " Green Banana" off the coast of Florida. In contrast to the various successful expeditions completed, many explorers have perished in their attempts to reach the bottom of a blue hole. The Red Sea Blue Hole located in Egypt is nicknamed the "Divers' Cemetery" because at least 40 divers have died there.

Diving Risks

Despite the beauty and allure of blue holes, they are some of the most dangerous waters to dive.

Nitrogen Narcosis Narcosis while diving (also known as nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect) is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain ...

, begins to set in at depths below 70 feet, and causes disorientation and changes in consciousness. Divers experiencing Nitrogen Narcosis may become too confused to swim back into shallower waters, and some unfortunately never make it back to the surface. Water clarity and light both decrease significantly with depth, adding to the disorientation divers experience. If the risks weren't already great enough, divers must also remain aware of the animals they may encounter in these holes. Shark species including Bull sharks, reef sharks, and Hammerhead sharks have been seen making use of Blue holes, and while they rarely attack humans, it is important to keep an eye out and remain calm. In recent years, Dean's Blue Hole, and the Great Blue Hole have claimed over 200 lives each, many of which occurred due to divers pushing past the limits of their training. Diving in Blue Holes can be done safely, as long as you remain aware of your equipment, depth, and surroundings, and do not exceed your predetermined limits.

References

External links

World's deepest blue hole found in South China SeaBahamas Blue Holes GuideThe Blue Holes Foundation

Explanation at the Bahamas Caves Research Foundation
How Blue Holes Work
{{DEFAULTSORT:Blue Hole Cave geology Marine geology .Blue Underwater diving sites

Blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ...