THE LIMESTONE ROCK

200 million years ago

PREHISTORIC BAHAMAS

As we walk along the shores and the beaches, as we go about our daily lives in the Bahamas, the land beneath our feet and the hard rock that comprises it largely goes unnoticed in our everyday life. But if you would pause for a moment to take a really good look at it, you would see that there is an unimaginably rich beauty to what rests beneath our feet. The rock encompasses a splendor more so than we can ever imagine, for it is this very thing we stand on, that begins the incredible tale of the story of the Bahamas.

Amazingly, the Islands of the Bahamas came from almost nothing. These land masses did not exist 200 million years ago and they were never part of the North American continent. The islands were formed over millions and millions of years. The Bahamas grew on a massive raised surface or platform that sat in the sea. This raised surface was created from the original movement of the giant land mass that produced all of the continents. The original giant land mass, the super continent, from where all seven continents started from, was named Pangea by scientists in 1927.

For most of the next 200 million years, the opening of the North Atlantic Ocean grew wider and wider as the land masses moved farther and farther apart. As Pangea separated, the continents gradually started taking shape. The Bahamas was slowly being exposed. Our prehistoric self was then only huge lengths or banks of rock under shallow sea water. The separation of Pangea and the continued separating and shaping of the continents also created the opening to the huge body of water we all know today as the North Atlantic Ocean.

In the PERMIAN PERIOD, there was the super continent PANGEA. Over the next 50 million years, it slowly began to separate through the process called CONTINETAL DRIFT. It continued to move until we get to present day. But continental drift continues. The continents are still moving ever slowly. In a million years, the map of world will undoubtedly look very different from the present day.

On this immense prehistoric shallow platform (scientists reckon that there were two that would later form the Bahama Islands), marine sediments started to accumulate. Marine sediments are the waste products and remains of sea creatures, shells and marine plants. Back then of course, creatures of the sea would have been prehistoric and bears little resemble to the marine creatures we know today. Millions of years ago, they would have been ginormous, fantastic and incredibly unusual looking. Over the next 120 million years, billions and billions of marine sediments accumulated and the first layers, that would later become land, was formed. Then the process kept happening over and over again. As one layer was formed, another layer would form on top of that. New layers would cause old layers to be pushed down.

Over a long, long, long, long, long time, the lower layers under the heavy weight of higher layers are pushed down, layered, squashed and compacted… to eventually become hard rock.

The rock that was formed from this process tells the 120 million year prehistory of the islands that make up the Bahamas. Billions and billions of marine sediments,[i] over millions and millions of years, formed the rock called LIMESTONE. Over the next 120,000,000 years, at least 20,000 feet of limestone sediment, accumulated on the two shallow platforms to form land.

The limestone carbonate platforms of the Bahamas, is one of the oldest in the world.[ii]

What is very important was that all of this was happening in relatively shallow water. It was the perfect positioning of this early structure that facilitated the formation of the land and the beautiful gradients in colour of the surrounding sea that we know today.

How do we know that the platform was under shallow sea?

Well, if the platform was under very deep water, land would not have developed above sea level and all those marine deposits that later formed the islands would have sunk down to probably form deep underwater coral banks and not the land we see today[iii].

More importantly, scientists have concluded that the carbon element in the limestone of the Bahama platform that was shaped in the late Jurassic period is in itself evidence of an absence of deep water at the time the limestone rock was formed. This is because carbonates that comprise the limestone are more likely to form only in shallow water[iv].

The astounding differences in the colour of the water highlights the shallow platform on which the islands grew. The light blue colour and the dark blue colour around the islands show the differences in depths of the water.

How science explains the rock…

Almost all living things need calcium. Animal shells are made mainly of calcium carbonate; the bones of mammals, like us humans, and other vertebrates are made mainly of calcium phosphate.

Calcium carbonate and calcium phosphate are not broken down by biological processes, so shells and bones do not decompose quickly like other parts of living things.

In the seas and oceans, when sea creatures die, the calcium carbonate in the shells of crustaceans and other animals will sink to the sea or ocean bottom and form layers of sediment, taking with them not only the calcium but also the carbonate.

Over time, we got enough layers of sediment to form land. Hence the reason why when scientists discovered that carbonates comprise the limestone rock that formed the Bahamas, we now know that the limestone is made up of wonderful, fantastic and curious prehistoric things that once lived millions and millions of years ago.

THE CONCH SHELL is 95% CALCIUM CARBONATE

A 2008 geological study of the limestone rock that makes up the centre of rum cay.

2008 Bahamas limestone rock and coral reef study by Geology and Environmental Sciences Department Hartwick College New York

What the limestone tells us…

The limestone tell us that at the very very bottom, if we dig down deep enough into the land, we get the continental mass, the base platform that the marine sediments accumulated on. As we move up, we then get multi layers of various types of marine deposits.

It is perhaps the most incredible thing to consider that once, the very rock that makes up the Bahamas, the very rock we stand on, was once a living thing.

In 1993, published research derived from 1985 drilling on the edge of the Blake Plateau of the Grand Banks in the Bahamas showed that the Mesozoic carbonate giga-bank had already existed in the late Jurassic and early Cretaceous periods, approximately 125 million years ago[v]. The Blake Plateau lies between the continental shelf and the deep ocean basin that runs along the southeastern coast of the United States all the way down to the Little Bahama Bank.

In 2000, another team of scientists studying the limestone rock in Eleuthera discovered something quite remarkable. They found ‘strawberry icing spread between layers of coconut cake’. Of course it was not real icing, what they found was thin bands of red fossilized soil separated between massive sheets of limestone rock. The formation of the limestone was like another time capsule for the geologists. The thick limestone layers on the high 20 to 30 meter cliffs in Eleuthera told the story of world’s ice ages[vi].

The thick limestone rock there was formed between breaks in the freezing of the world. The limestone on that part was formed during the warm breaks between the ice ages. During the warm breaks, red dust (as we shall read more about later) blew in and swept across the land. Scientists reckon that on those cliffs, there had to have been a beach that covered the area at different points over the past half million or so years. When the Earth was warmer sea levels rose, a layer or marine sediments formed, then the Earth froze, the water receded, red dust swept across the area, creating a future strawberry icing between the white rock, then the process kept happening over and over again.

Bahamas limestone helps rewrite science history

Underwater Stalagmites Force Reassessment of Carbon Dating

Charles Arthur Technology Editor, The Independent (London, England)

June 3, 2001

A GROUP of limestone stalagmites from a submerged cave beneath the Bahamas has thrown doubt on conclusions about human history drawn from carbon dating. The scientists who found them say there are also important implications for climate change.

The formations were recovered from a cavern that was created when sea levels were about 100m (330ft) lower than today. They show that, more than 20,000 years ago, there were dramatic shifts in the amount of radioactive carbon – often known as “carbon-14” – in the atmosphere.

Because the ratio of carbon-14 to its stable cousin, carbon-12, is used as the basis of carbon dating of fossils, any widespread variation in that balance would confuse the dating of items such as plants or animals that existed around those times.

“It means we have tended to underestimate the true age of objects from 20,000 to 40,000 years ago by up to 8,000 years,” said Dr David Richards, of the School of Geographical Sciences at the University of Bristol. However, he added, “this may change the timings, but it won’t change the order of events”.

80 million years ago

LAND BEGINS TO FORM

Around 80 million years ago other big changes were beginning to shape the islands. These changes were due to the continued shifts and movement of land called CONTINENTAL DRIFT.

Continental drift is also known by the very grand term of THEORY OF PLATE TECTONICS, which simple means the land moves and always has. Land moves by a little tiny bit at a time and incredibly still does today.

This tiny little bit averages about 2.5 centimetres or 1 inch each year.

This tiny 1 inch movement every year turns into 25 kilometres or 15 miles in one million years.

Over millions and millions of years, this little tiny bit becomes a great gigantic bit and this movement, separated and shaped the continents and islands we know today.

This continuous movement and shifting of land, millions of years ago, created deep troughs and channels in and between the LITTLE AND GREAT BAHAMA BANKS.

Deep troughs is a really fancy geology, geography and oceanography term than really means all of a sudden the water gets really deep when it was relatively shallow only a little bit before, then if you move on a bit further, becomes shallow again. The deep part in between relatively shallow parts is a trough.

The slow and continuous shifting and movement of the newly formed land caused the Bahamas to separate from Cuba and move farther from Florida.

The south-eastern Bahamas and the Turks and Caicos Islands broke into a series of small banks surrounded by deep troughs and basins.

Each of these newly separated banks became a free unit or island.

[i] “So if there was a megabank,” he says, “it would have been at least of late Jurassic age [about 150 million years], much older than originallyporposed, and that’s getting close to the time of rifting.”

[ii] Weisburd, S. (1985) Ocean Drilling: Banking on the Bahamas for a Leg to Stand On. Science News. V.127. Publication date: May 11, 1985. P. 294

[iii] Huggett, Richard J., (2004) Fundamentals of Biogeography 2nd ed. Routledge

[iv] Droxkerm Andrew W., (1993) Shallow Carbonates Drilled by DSDP and ODP, Oceanus, Winter 1993 Vol 36 Issue 4

[v] Droxkerm Andrew W., (1993) Shallow Carbonates Drilled by DSDP and ODP, Oceanus, Winter 1993 Vol 36 Issue 4

[vi] Monastersky, R., (2000) Climate’s Long-Last Twin, Science News Vol 157 Issue 9 February 26, 2000

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