What can Florida's reefs and limestone tell us about sea level change?

South Florida, particularly the Keys, are known for their coral reefs. Coral reefs support many different forms of life beside corals, including algae, invertebrates (sponges, snails, worms, and shrimps), fish, and other marine life that attach to the seabed or feed on organisms that live in or around the reef. As corals, algae, and other marine organisms grow, their bodies secrete calcium carbonate (CaCO3) that gets incorporated into their skeletons and shells. When these organisms die, this calcium carbonate becomes incorporated into sediments and is buried by younger reef growth. In this manner, slowly over time, coral reefs grow and accumulate on on the sea bed.

A Coral Environment


Coral reefs only grow in shallow, clear sea water (in the photic zone). Reefs will not form in areas where terrestrial sediments or murky storm waters might cover and smother reef organisms. For instance, the Gulf Stream moves sediments from the Mississippi River and other streams and rivers along the Gulf Coast. These sediments prevent reefs from developing along the northwestern coast of Florida. Now, Florida's coral reefs are now developing only in warm, clearwater conditions of South Florida and the Keys.

Many reef organisms are restricted to select environments. For instance, only strong corals and calcareous algae can tolerate the often rough wave energy on the seaside of the reef crest (see below). Whereas, other species are adapted only to the shallow, warmer, and more restricted regions in the inner reef or reef flats (between the reef and the shore). Other species are adapted to the cooler and deeper waters of the outer reef (or fore reef). Ocean currents erode and sweep some of the dead reef material into deeper water settings offshore.

Image source: U.S. Geological Survey (http://geopubs.wr.usgs.gov/fact-sheet/fs025-02/)

Reefs and Sea Level

When sea level is stationary, reefs will grow laterally in a seaward direction as reef sediments accumulate. Over long periods of time reef sediments will accumulate, and the growing pile will buid up in a seaward direction. Over time, this reef material will build broad shoals and platforms just below and near the ocean's surface. However, if sea level rises and reefs are submerged by deep water, tehy will die. The deep water isolates them form the necessary solar light and warm water conditions they require. Similarly, a drop in sea level could leve them exposed on land.

Image source: U.S. Geological Survey (http://geopubs.wr.usgs.gov/fact-sheet/fs025-02/)

Fossil Reefs and Limestone


Making Limestone

Over time, reefs accumulate on the seabed and become buried by other sediments. As more and more sediment is deposited on top, the weight of that sediment compacts the reef material. In addition, the orginal calcium carbonate from marine organisms acts as a cement between sedmimentary particles. The result of compaction and cementation converts reef sediments into the rock - limestone.

This is a cliff of limestone, now exposed in Walnut Canyon, Arizona. Fossils in the limestone indicate that the sediments were originally deposited in shallow, warm, clearwater marine water conditions.

Image source: U.S. Geological Survey
(http://3dparks.wr.usgs.gov/walnutcanyon/)

Fossils found in ancient coral reefs help geoscientists interpret the origin and evolution of reefs over time, and assist in interpreting changes in sea level.

Massive cliffs of fossiliferous limestone exposed in the mountaintops of the Guadalupe Mountains in West Texas suggest that a barrier reef existed in the region during Permian Period, about 280 to 250 million years ago. Today the reef rocks are exposed between 5,000 and 8,000 feet above modern sea level.

Image source: U.S. Geological Survey
(http://3dparks.wr.usgs.gov/guadalupe/)

This map shows the location of Walnut Canyon and Guadalupe Mountains. Sea level changes in the past are recorded in the rocks in these national parks. These limestones are hundreds of millions of years old. Tectonic forces have uplifted these ancient rocks to their present locations. In contrast, the limestone in Florida are relatively young and developed in a range of ony a few thousand to a few million years.


Activity: Can we predict where Florida's reefs might form in the future?

The maps below show locations where modern reefs are forming today, and the location of submerged reefs that are now beneath
the photic zone, and are now dead. The map also shows locations of modern and ancient limestone bedrock areas in Florida.

Click on images below for larger views.

Florida's limestone and reefs   Florida's limestone and reefs
Map A. Color for viewing & printing.   Map A. Black & white for printing.

Florida's changing sea levels... Past, Present, and Future?

Below are interpretations of sea levels in Florida interpreted from the fossil record. The image on the left
shows the location of Florida's shoreline as it probably existed about 18,000 years ago when sea level
was lowest during the last Ice Age. The image on the right shows the shoreline at sea level as it exists
in Florida. The two lower images are interpretations of what Florida's shorelines might look like if sea level
were to rise an additional 5 or 50 meters based on various stages of possible melting of continental
glaciers in Antarctica, Greenland and elsewhere in the world.

Map of Florida after about a 125 meter fall in sea level   Florida's shoreline today
Map B. Florida's shoreline about 18,000 years ago when sea level was about 125 meters lower than today.   Map C. Florida's shoreline today (in 2005)
Florida's shoreline after about a 5 meter rise in sea level.   Florida's shoreline after about a 50 meter rise in sea level
Map D. Florida's shoreline after about a 5 meter rise in sea level.   Map E. Florida's shoreline after about a 50 meter rise in sea level.


Step 1: Using the map (below) of the combined topography and bathymetry of Florida and the surrounding ocean.
Based on the information presented on maps A to E above:

Question 1: Can you predict where coral reefs might form if sea level were to rise 5 meters?
Question 2: Can you predict where coral reefs might form if sea level were to rise 50 meters?
Question 3: Where would you predict reefs to have formed when sea level had fallen 125 meters?

Use three different colored pencils to color in the location where you might expect reefs and reef sediments might
form and accumulate at the three different sea levels.

 
Map F. Florida's topography and bathymetry.
Color for viewing & printing.
  Map F. Florida's topography and bathymetry.
Black & white for printing.

Step 2: Using map E, locate the major cities in Florida, and the location of Disneyworld (Orlando), and Everglades National Park
in south Florida.

Question 4: Discuss and make notes about what may happen to Florida's cities, Disneyworld, and the Everglades ecosystem if
sea level were to rise 5 meters in Florida?
Question 5: Discuss and make notes about what may happen to Florida's cities, Disneyworld, and the Everglades ecosystem if
sea level were to rise 5 meters in Florida?
Question 6: Discuss and make notes about what may happen to Florida's cities, Disneyworld, and the Everglades ecosystem if
sea level were to fall 125 meters?

Extra Brain Teaser: read about Florida's sinkholes and what they tell of about sea level in the past!