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43-412: The Great Blue Hole is a large marine sinkhole off the coast of Belize . It lies near the center of Lighthouse Reef , a small atoll 70 km (43 mi) from the mainland and Belize City . The hole is circular in shape, 318 m (1,043 ft) across and 124 m (407 ft) deep. It has a surface area of 70,650 square metres (760,500 sq ft). It was formed during several phases of

86-508: A natural phenomenon , and have occurred throughout geological history. The Permian–Triassic extinction event , a mass extinction of species from the world's oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of carbon dioxide into Earth's atmosphere. Many lakes have a permanent or temporary anoxic layer created by respiration depleting oxygen at depth and thermal stratification preventing its resupply. Anoxic basins exist in

129-547: A blue hole nicknamed the " Amberjack Hole " located 30 miles off the coast of Sarasota, Florida . Individuals from Mote Marine Laboratory , Florida Atlantic University, Harbor Branch , Georgia Institute of Technology , the United States Geological Survey , and the NOAA Office of Ocean Exploration participated in the expedition. The expedition gathered information about life around and within

172-637: A byproduct of agricultural run-off and sewage discharge, can result in large but short-lived algae blooms. Upon a bloom's conclusion, the dead algae sink to the bottom and are broken down until all oxygen is expended. Such a case is the Gulf of Mexico where a seasonal dead zone occurs, which can be disturbed by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient concentrated "sludge", can be especially damaging to ecosystem diversity. Species sensitive to anoxic conditions are replaced by fewer hardier species, reducing

215-487: A consistent orientation, indicating that there had also been some past geological shift and tilting of the underlying plateau, followed by a long period in the current plane. The tilt indicates that this was movement of the land, rather than a rise in sea level alone. The initial measured depth of the Great Blue Hole was about 125 m (410 ft) which is the most often cited depth up to this day. An expedition

258-686: A marine water source, however, if the isotopes are similar to those found in meteoric lenses, then the source is meteoric. 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

301-529: A respiring organism is presented with little to no oxygen, chances of survival decrease. Therefore, eutrophication and anoxic conditions in water lead to a reduction in biodiversity. For example, the soft coral Xenia umbellata can resist some anoxic conditions for short periods. Still, after about three weeks, mean survival decreases to about 81%, and about 40% of surviving species experience size reductions, lessening in coloration, and compromised pinnate structures (Simancas-Giraldo et al., 2021). Another example of

344-422: A susceptible organism is observed with The Sydney Cockle, Anadara trapezia . Enriched sediments have lethal and sublethal effects on this Cockle and, as stated in [Vadillo Gonzalez et al., 2021], "movement of cockles was reduced in enriched sediments compared to natural treatments." A study collecting over 850 published experiments "reporting oxygen thresholds and/or lethal times for a total of 206 species spanning

387-529: Is a popular destination for recreational scuba divers making day trips from the coastal tourist communities in Belize. On-shore caves of similar formation, as large collapsed sinkholes, are well known in Belize and in the Yucatán Peninsula , where they are known as cenotes . Unlike the mainland cenotes which often link to underwater cave systems, there is little evidence of horizontal development in

430-625: Is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water, is nitrate . Denitrification occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing sulfate . This results in the byproduct of hydrogen sulfide (H 2 S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color: These sulfides will mostly be oxidized to either sulfates (~90%) in more oxygen-rich water or precipitated and converted into pyrite (~10%), according to

473-451: Is generally found in areas that have restricted water exchange. In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier, as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen . Anoxic waters are

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516-424: Is located 100 kilometers from the coast of Belize. The Great Blue Hole is a massive 300 meters wide and 125 meters deep. Blue holes formed during past ice ages , when the sea level was 100–120 metres (330–390 ft) lower than at present. During these times, the formations were exposed to the same erosion from rain and chemical weathering that is common to all limestone -rich terrains. The process ended once

559-735: Is oxygen free. Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount of oxygen used by marine organisms in the process of breaking down organic matter. BOD is influenced by the type of organisms present, the pH of the water, temperature, and the type of organic matter present in the area. BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams. As BOD increases, available oxygen decreases. This causes stress on larger organisms. BOD comes from natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff. Eutrophication , an influx of nutrients (phosphate/nitrate), often

602-762: Is the Dragon Hole , or Longdong, in the South China Sea at 300.89 metres (987 ft) deep, while the third deepest blue hole in the world is Dean's Blue Hole at 202 metres (663 ft) , located in a bay west of Clarence Town on Long Island , Bahamas . Other blue holes are about half that depth at around 100–120 metres (330–390 ft). The diameter of the top entrance ranges typically from 25–35 metres (82–115 ft) (Dean's Blue Hole) to 300 metres (980 ft) ( Great Blue Hole in Belize). The overall largest blue hole (taking into account depth and width)

645-535: 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 is the Taam Ja' Blue Hole in Chetumal Bay , which was found to have a depth of more than 420 metres (1,380 ft) in 2024. The second deepest

688-850: The Baltic Sea , the Black Sea , the Cariaco Trench , various fjord valleys, and elsewhere. Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, the Gulf of Mexico , and Hood Canal in Washington State. Anoxic conditions result from a combination of environmental conditions including density stratification , inputs of organic material or other reducing agents , and physical barriers to water circulation. In fjords, shallow sills at

731-499: The Quaternary glaciation when sea levels were much lower . Analysis of stalactites found in the Great Blue Hole shows that formation took place 153,000, 66,000, 60,000, and 15,000 years ago. As the ocean began to rise again, the cave was flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System , a UNESCO World Heritage Site . The site was made famous by Jacques Cousteau , who declared it one of

774-471: 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 were found. 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

817-616: The Blue Hole. In 2012, Discovery Channel ranked the Great Blue Hole as number one on its list of "The 10 Most Amazing Places on Earth". In 2018, they featured a two-hour special titled Discovery Live: Into the Blue Hole featuring Erika Bergman, Fabien Cousteau and Richard Branson . Cave dive sites: Blue hole Blue holes are distinguished from cenotes in that the latter are inland voids usually containing fresh groundwater rather than seawater . Blue holes are roughly circular, steep-walled depressions, and so named for

860-503: 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 stalactites , 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

903-419: The amount of dissolved oxygen it can hold. Following Henry's law , as water becomes warmer, oxygen becomes less soluble in it. This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on larger scales. Thus, bodies of water are more vulnerable to anoxic conditions during the warmest period of the day and during summer months. This problem can be further exacerbated in

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946-436: The beauty and allure of blue holes, they are some of the most dangerous waters to dive. Nitrogen narcosis begins to set in at depths below 20 m (70 ft) and causes disorientation and changes in consciousness. Divers experiencing nitrogen narcosis may become too confused to swim back into shallower waters, and some never make it back to the surface. Water clarity and light both decrease significantly with depth, adding to

989-1049: The case of the Sawmill Sink in the Bahamas. Blue holes are formed through Karst processes and require a specific type of topography . Rocks such as limestone, gypsum and marble are soluble and dissolution creates passages and cave systems underground. This process in combination with doline formation permits blue holes to be formed. Doline formations 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 although some do not show any sign of passages or cave systems as would be normally expected from karst and doline processes. This suggests that some blue holes may be caused by other processes such as vertical reef development. Some blue holes do not experience karst or doline processes during their formation. They form through bedrock dissolution and collapse, usually influenced by tidal forcing , carbonate dissolution, sea level fluctuations, or

1032-413: 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

1075-461: The chemistry and nutrient availability of that depth. Microorganisms including foraminifera , 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

1118-690: The disorientation divers experience. 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 occasionally attack humans. Cave dive sites: Anoxic sea water Anoxic waters are areas of sea water , fresh water , or groundwater that are depleted of dissolved oxygen . The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters , which are low (but not lacking) in dissolved oxygen. This condition

1161-430: 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 . The deep blue color is caused by the high transparency of water and bright white carbonate sand . Blue light

1204-411: The entrance may prevent circulation, while at continental boundaries, circulation may be especially low while organic material input from production at upper levels is exceptionally high. In wastewater treatment , the absence of oxygen alone is indicated anoxic while the term anaerobic is used to indicate the absence of any common electron acceptor such as nitrate , sulfate or oxygen. When oxygen

1247-443: The following chemical equations: Some chemolithotrophs can also facilitate the oxidation of hydrogen sulfide into elemental sulfur , according to the following chemical equation: Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and low levels of inflow of oxygenated water through the sediment. Below a few centimeters from the surface the interstitial water (water between sediment)

1290-465: The full taxonomic range of benthic metazoans." Individual species will have different adaptive responses to anoxic conditions based on their biological makeup and the condition of their habitat. While some can pump oxygen from higher water levels down into the sediment, other adaptations include specific hemoglobins for low-oxygen environments, slow movement to reduce the rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases,

1333-574: 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. Despite

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1376-405: 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

1419-508: 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 reason blue holes are able to preserve such records is due to the anoxic bottom water most blue holes contain. 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 or marine sources of saline water within them. Being able to identify where

1462-600: 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. Sediment accumulation is unique in blue holes. Sedimentation occurs at the center of holes rather than the edges. Many different kinds of sediment help preserve fossils and climate records. The main sediments that build up and create layers in blue holes are sapropel , detrital peat , and lacustrine marls . Within these layers, microfossils can be found. Sediment cores taken from three blue holes in

1505-424: The overall variability of the affected area. Gradual environmental changes through eutrophication or global warming can cause major oxic-anoxic regime shifts. Based on model studies this can occur abruptly, with a transition between an oxic state dominated by cyanobacteria , and an anoxic state with sulfate-reducing bacteria and phototrophic sulfur bacteria . The temperature of a body of water directly affects

1548-649: The presence of eogenetic carbonates . Blue holes are typically found on shallow carbonate platforms , exemplified by the Bahama Banks , as well as on and around the Yucatán Peninsula , such as at the Great Blue Hole at Lighthouse Reef Atoll, Belize . Many deep spring basins formed by karst processes and located inland also are called blue holes; for example, Blue Hole in Castalia , Ohio . Many different fossils have been discovered that indicate

1591-452: The sea level rose at the end of the ice age. Most blue holes contain both freshwater and saltwater. The halocline is the boundary surface between the freshwater and the saltwater in these blue holes 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 600 metres (2,000 ft) in

1634-606: The top five scuba diving sites in the world. In 1971 he brought his ship, the Calypso , to the hole to chart its depths. Investigations by this expedition confirmed the hole's origin as typical karst limestone formations, formed before rises in sea level in at least four stages, leaving ledges at depths of 21 m (69 ft), 49 m (161 ft), and 91 m (299 ft). Stalactites were retrieved from submerged caves, confirming their previous formation above sea level. Some of these stalactites were also off-vertical by 5˚ in

1677-486: 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 and tortoise 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 sulfur compounds like hydrogen sulfide , which are toxic to most organisms. These special bacteria have produced many insights into

1720-682: The vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released. Daily cycles are also influenced by the activity of photosynthetic organisms. The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise. Individual species’ reactions to eutrophication can vary widely. For example, some organisms, such as primary producers, can adapt quickly and even thrive under anoxic conditions. However, most organisms are highly susceptible to slight changes in aquatic oxygen levels. When

1763-405: 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

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1806-472: Was a layer of hydrogen sulfide at a depth of approximately 300 feet (91 m). The water at that depth and below becomes dark, anoxic and devoid of life. The submarine expedition also discovered the bodies of two divers at the bottom, out of three believed to have gone missing while diving there, and reported them to Belizean authorities. Out of respect, the crew declined to bring the bodies back to land, thus leaving them buried at sea . The Great Blue Hole

1849-458: Was conducted in the summer of 1997 to collect core samples from the Blue Hole's floor and document the cave system. To accomplish these tasks, all of the divers had to be certified in cave diving and mixed gases . In December 2018, two submarines descended into the Blue Hole in an attempt to map its interior. Using sonar scanning, the team was nearly able to complete a 3-D map of the 1,000-foot-wide (300 m) hole. One of their discoveries

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