Photo by M. Hamilton
(Click on the following link to lsten to an audio version of this blog … Florida keys part 1
Back in mid-December, my family took a trip to the Florida Keys. The purpose was to warm up after an already cold winter in Colorado and Seattle, to swim, to sit on a beach onto which waves only gently lapped, to sample as many run-based cocktails as possible, to eat good food and to enjoy each other’s company. We checked all of those boxes while we were there!
Map by NOAA. Public Domain
The Florida Keys are a chain of small, limestone islands that arc off to the southwest from the tip of Florida. From the road (The Overseas Highway of U.S. Route 1) that connects Key Largo down to Key West it is often possible, even when you are not on one of the 42 bridges that interconnect the land masses, to see both the Atlantic Ocean off to the east and the Gulf of Mexico off to the west. The Keys are narrow, low and flat. The average height of the islands is just six feet above sea level and the highest point in the Keys is only eighteen feet above sea level. Driving on the 113 mile Overseas Highway feels like a drive in the ocean! In many places, it feels like you are in a boat!
To understand how these little islands formed, you need to appreciate some events that have occurred over the past 2.6 million years.
The Pleistocene is the geological epoch that began about 2.6 million years ago and ended about 11 thousand years ago. The Pleistocene is sometimes called the Ice Age because during its time frame there were a large number of major, global cold periods that resulted in vast continental sheets of ice forming on the land masses of North America and Europe. In between these long periods of ice-bound glaciation there were shorter, more temperate interglacial periods during which most of these ice sheets melted. A huge amount of water was tied up in the ice masses and then released when they melted. So much water was tied up and then released that levels of the oceans fluctuated greatly over the span of the Pleistocene.
Map by USGS. Public Domain
The United States Geological Survey (USGS) estimates that at the peak of the last glacial period of the Pleistocene, just 14,000 years ago, ocean levels were 400 feet lower than they are today. The USGS also estimates that in the interglacial period just before that last glacial event, sea levels were 18 feet higher than they are today. These vertical changes in sea levels caused the coastlines of continents to drift dozens to hundreds of miles further off-coast or inland depending on the timing of the glacial and inter-glacial events. Islands all around the globe grew or shrunk or disappeared entirely with the fall and rise of the sea around them. The present day state of Florida at the last peak of Pleistocene glaciation was twice as large as it is today!
The base of what would become the Florida Keys was submerged under the warm, nutrient-poor, rising and falling ocean waters. A two mile thick mass of limestone laid down by the steady accumulation of calcium carbonate “tests” from countless generations of microscopic foraminifera (a component of the “zooplankton” of the first consumer-level of the marine trophic web) made a platform that grew closer and closer to the surface waters, and, eventually it was bathed in sunlight. When the light boundary was reached by the limestone base, coral, advantageously carrying in their own bodies photosynthetic algae that enabled them to thrive in the low-nutrient environment, began to grow.
Elkhorn coral. Photo by J. St. John. Wikimedia Commons
Elkhorn, staghorn, brain and star corals dominated the ancient coral reefs of the keys and slowly accumulated and contributed to more and more limestone. Soon the tops of the growing reef were high enough to be exposed by the falling ocean waters of the glacial periods. The Keys were born!
Once the peaks of the reef emerged, all sorts of things could happen. Plants could invade, animals could colonize, soil could form and the complex process of ecological succession could begin. Very notably, the shorelines could trap floating propagules of mangrove trees and start a vital, ecological sequence that would protect and stabilize the shorelines (I will write more about mangroves in next week’s blog!).
Some of the coral was ground down by wave action and weathered into sand particles, but this process was quite stunted in the Keys and remarkably little sand was actually made. Our kayaking guide told us that the waves were blunted by all of seagrass growing in the extensive, shallow waters offshore (I will write more about seagrasses in next week’s blog, too), but he was probably mistaken. The long coral reef (The Great Florida Reef) that sits a couple of miles east of the Keys and which runs on a similar arc as the island chain, breaks up incoming waves and shelters the ocean side of the Keys from incoming waves. The Great Florida Reef is four miles wide and 170 miles long. It formed after the last glacial period (i.e. after the “Wisconsinan Glaciation”) and is 5000 to 7000 years old. It is also the only living coral barrier reef within the continental United States!
Sandy beach near our beach house with Ari Drake. Photo by M. Hamilton
The net impact of this coral reef protection, though, is that there are very few natural, sandy beaches in the Keys! Since people coming to this ocean paradise want to be able to sit on a beach rather than simply lounge on the scratchy flotsam of a mangrove swamp, massive imports of sand from other locations (the Bahamas were mentioned as a primary sand source) had to be made. Most of the resorts and hotels in the Keys have built their beaches from imported sand!
So the coconut palm trees (an alien, imported species) sway in the ocean breezes and tiny, muted waves lap up on the, often, manufactured, sandy shores. But, there are also brown pelicans and magnificent frigate birds flying overhead (more on them in a later blog, too!), sea gulls calling out and floating in the wind out over the water. There are white ibises and great egrets stalking about on the really alien, grassy lawns looking for worms and insects, and bushes full of catbirds and eastern phoebes who are probably here to wait out the winter.
It isn’t exactly “real” in the sense of being a natural landscape, but it is an incredibly beautiful place!
(More next week!)