The Rest Of The Story: Bahamas Banks

Marsh Harbor, Abacos, Bahamas
NoLL

Surprise! When we got to Manjack Cay we found the last thing in the world I expected, a WIFI Internet connection. Who would have thought? Anyhow, I did as I promised. I used the net to look up information about the geology of the Bahamas Banks. It turns out that my speculation that there must be some feedback mechanism that keeps the banks just below the surface of the water is correct, and also my speculation that they must have been high and dry when sea levels werer low was also correct. Read on. I'll paraphrase the stuff I found on Wikipedia.

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. 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 4500 meters.[1] As its limestones were 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 per 1,000 years. The weight of the platform has pushed down the earth's crust in the nearby region. Ergo the deep ocean trenches that surround the banks.

The waters of the Bahama Banks are very shallow; on the Great Bahama Bank they are generally no deeper than 25 meters.[2] The slopes around them however, such as the border of the Tongue of the Ocean 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 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, their limestones were subjected to chemical weathering that created the caves and sinkholes common to karst terrain, resulting in structures like blue holes.

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonous calcareous deposits. 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.

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.

The reef is that part of a carbonate platform created by essentially in-place, sessile organisms. Today’s reefs are built by hermatypic organisms. The reef is the rigid structure of carbonate platforms and is located between the internal lagoon and the slope. Survival of the platform depends on the existence of the reef, because only this part of the platform can build a rigid, wave-resistant structure.



p.s. We're in Marsh Harbor for a couple of days, riding out some rainy weather.