It's not every day snorkeling can be considered field research.
Three hundred seventy-five miles off the coast of Florida is a pristine tropical island where the past is an active part of the present.
The ruins of early 19th century cotton plantations are a frequent reminder of the British Loyalists who fled there during the American Revolution; many of the island's inhabitants today are descendants of their slaves. Older still is its name, San Salvador, which it was christened by Christopher Columbus upon his Oct. 12, 1492, landing. Because, by the way, it's the first land he found after crossing the Atlantic Ocean in search of the New World.
But even Columbus is too recent for a team of Texas Tech University researchers – and at the same time, much too old. Dustin Sweet, an assistant professor who studies sedimentology and stratigraphy in the Department of Geosciences, takes a class of graduate students there for one week every two years because it is a perfect environment to study how carbonate sediment forms into rocks and coral reefs: from the oldest rocks, about 150,000 years old, to the youngest, about 10,000 years old, and even the sediment waiting to become the rocks of the future.
"What I hope students learn is getting to take modern carbonate sediment and run it through their fingers, look at the different constituents – there's a snail, there's a clam and there's all this fine-grain sediment – and picture that mass of sediment in their hand being turned into a rock," Sweet said. "If that's turned into a rock, what type of rock would that make? That's what we study all semester but they never get to see the sediment being formed into that rock."
Carbonates, or what most people would consider limestone, are different than other sediments. For one thing, they come from living creatures.
"Carbonate sediments are most commonly either directly created from life as a sea shell or created through fish chomping on corals," Sweet said. "They are part of the living animal or they are created through biological processes. Other types of sediments are inorganic and created through physical or chemical erosion, then transported downhill out into the ocean."
Living creatures adapt to changing conditions in ways inorganic sediments cannot, so carbonate sediments – and the rocks they form into – can tell geologists about the environment at the time they were created.
"One of our tenets in geology is that the key to the past is the present," Sweet said. "To understand the past, we have to look at the present. Overall, we assume the processes that happen today – with some minor changes – are what happened in the past. There are some differences: there are different animals that lived than there are today, there are different types of coral, different types of algae, different things swimming in the ocean than there are today. But overall, the general processes of those living as a community in certain environments are very similar to what is today."
With that assumption, rocks can be used like puzzle pieces to construct an image of the past environment. Today, rocks on San Salvador generally show characteristics of being formed under high- or low-energy conditions – different rocks form in each condition. High-energy rocks are often found on the eastern side of the island, which faces the open Atlantic Ocean, because the repetition of large waves creates more energy. The low-energy rocks are often found on the western side of the island where the ocean is calmer. If geologists found a group of rocks that showed high energy characteristics, that would indicate the rocks' location probably faced open ocean in the past.
"We can start to paste together the old history from the rocks," Sweet said.
But the past is not the only thing Sweet is studying. One of the more enjoyable parts of the trip is the snorkeling expeditions to watch the sediment being formed that will someday become rock.
"Fish eat the corals and it's healthy because they're cleaning algae off the corals, but it's also a major sediment producer," Sweet said. "To actually see the fish bite the corals and hear it, and go around and do that, it's always fun and a perspective every student has mentioned."
While looking at current rocks can tell geologists about the conditions of the past, it's much harder to translate today's sediment and conditions into educated guesses about the future.
"Predictions on the future depend on a lot of processes," Sweet said. "Right now we're warming as an earth and sea level is rising; clearly that's going to affect that island and that depositional environment is going to change. But if we look at the geographical record of the island, it's had many ups and downs and changes of sea level, and the carbonate environment responds to that."
While in San Salvador, the group stayed at the Gerace Research Center, an arm of the College of the Bahamas that houses about 1,000 students a year from universities throughout the United States and Canada. It's one of the most modern places there.
"San Salvador itself is such a throwback island," Sweet said. "You get to go places, free reign of the island, to different beaches. Other spots in the Bahamas, it's either very inaccessible because it's too remote or it's very commercialized and not representative of a modern environment at all. It's a unique perspective there.
"It's getting a little more commercialized each time I go," he added. "I think 50 years ago it was mostly a trade-and-barter system. Now it's a cash economy, but it's not very far removed from the trade. It's a throwback to a small, different sort of life."
It seems the perfect place to study the intertwinement of the past and the present.
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