The Bahama Banks – Little Bahama Bank
The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas.
Important Bank: Great Bahama Bank (95,798.12 km², has islands, area without islands)
Geologic history and structure
The limestone that comprises the Banks has been accumulating since at least the Cretaceous period, and perhaps as early as the Jurassic; today the total thickness under the Great Bahama Bank is over 4.5 kilometers (2.8 miles). As the limestone was deposited in shallow water, the only way to explain this massive column is to estimate that the entire platform has subsided under its own weight at a rate of roughly 3.6 centimeters (2 inches) per 1,000 years.
The waters of the Bahama Banks are very shallow; on the Great Bahama Bank they are generally no deeper than 25 meters (80 feet). The slopes around them however, such as the border of the Tongue of the Ocean (TOTO) in the Great Bahama Bank, are very steep. The Banks were dry land during past ice ages, when sea level was as much as 120 meters (390 feet) lower than at present; the area of the Bahamas today thus represents only a small fraction of their prehistoric extent. When they were exposed to the atmosphere, the limestone structure was subjected to chemical weathering that created the caves and sinkholes common to karst terrain, resulting in structures like blue holes.
Carbonate platform
A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonous calcareous deposits (Wilson, 1975). Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms cannot grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today’s reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.
Carbonate sedimentation
The mineralogic composition of carbonate platforms may be either calcitic or aragonitic. Seawater is oversaturated in carbonate, so under certain conditions CaCO3 precipitation is possible. Carbonate precipitation is thermodynamically favoured at high temperature and low pressure. Three types of carbonate precipitation are possible: bioticallycontrolled, biotically induced and abiotic. Carbonate precipitation is biotically controlled when organisms (such as corals) are present that exploit carbonate dissolved in seawater to build their calcitic or aragonitic skeletons. Thus they may develop hard reef structures. Biotically induced precipitation takes place outside the cell of the organism, thus carbonate is not directly produced by organisms, but precipitates because of their metabolism. Abiotic precipitation involves little or no biological influence.
Classification
The three types of precipitation described above determine different platform geometries. According to these characteristics a classification was created, consisting of three types of carbonate factories. A carbonate factory is the ensemble of the sedimentary environment, the intervening organisms and the precipitation processes that lead to the formation of a carbonate platform.
Tropical factory
In these platforms precipitation is biotically controlled, mostly by autotrophic organisms. Organisms that build this kind of platform are mostly corals, green algae, foraminifers and molluscs. These platforms are found only in warm (more than 20°C) and sunlit waters, high in oxygen and low in nutrients. This means that they are found between 30° north and 30° south of the equator. This type of factory is the most widespread today, and is often found fossilized.
Cool-water factory
As the name suggests, this type of platform extends its domain to cooler waters and higher latitudes than tropical factories. Precipitation is biotically controlled by heterotrophic organisms, sometimes in association with photo-autotrophic organisms such as red algae. The sea-waters of these platforms are characterized by a higher amount of nutrients than in tropical factories.
Mud-mound factory
These platforms are characterized by abiotic precipitation and biotically induced precipitation. They grow in waters high in nutrients and low in oxygen. Mud-mound factories are known only from the fossil record, especially Palaeozoic and Mesozoic.
1. You are here near the edge of the Little Bahama Bank. Look at this area. What kind of types of the carbonate factories do you think we have here at the Little Bahama Bank?
2. You can find here a special kind of sand. This is related to the Bahama Banks. What is the main component of the sand? Why do you think so?
3. Take a picture with you / your GPS with the beach and the sea in the background.
Send your answers for the questions by email or message to Fam.Weiss through our GC-account with your GC name and Earthcache name in the subject-line. We contact you if something is wrong.
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