Blue hole
A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock (limestone or coral reef).
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.
[2] The deep blue color is caused by the high transparency of water and bright white carbonate sand.
The overall largest blue hole (taking into account depth and width) is located 100 kilometers from the coast of Belize.
[5] Blue holes formed during past ice ages, when the sea level was 100–120 metres (330–390 ft) lower than at present.
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.
Doline formations were once closed depressions formed by solution of superficial rock or subsidence collapse into an underground void.
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.
[11] 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.
Meiobenthic organisms cannot survive the high sulfide found at depth, and remain in the surface layers of blue holes.
The main sediments that build up and create layers in blue holes are sapropel, detrital peat, and lacustrine marls.
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.
[13] Many blue holes are great sediment traps and can preserve climate and fossil records dating back to the last glacial maximum.
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.
A follow-up expedition is planned in August 2020 to a deeper blue hole named the "Green Banana" off the coast of Florida.
[15][16][17] In contrast to the various successful expeditions completed, many explorers have perished in their attempts to reach the bottom of a blue hole.
