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30 feet deep, 6,000 years old

Scientists examining Tampa Bay's origins recover samples of early deposits in the bay. "We're really stretching back into deep time,'' says a USF professor.

By CRAIG PITTMAN, Times Staff Writer

© St. Petersburg Times, published July 19, 2002


Scientists examining Tampa Bay's origins recover samples of early deposits in the bay. "We're really stretching back into deep time," says a USF professor.

ON TAMPA BAY -- Geologist Terry Edgar peered through a small magnifying glass at the black lump in his hand. He started grinning, then handed it to Eckerd College marine science professor David Hastings, who began grinning too.

The lump was just a blob of mud flecked with bits of shell, but the two scientists could not have been more excited if it were a diamond.

"That's great!" Hastings shouted. "We're in business."

Ten minutes before, the lump of mud had been buried 30 feet beneath the bottom of Tampa Bay, a spot it had occupied for some 6,000 years. On Thursday morning, a team of scientists aboard a French research vessel dug it up as part of the most extensive effort ever made to examine the bay's prehistoric origins.

The scientists had gambled that boring deep into the bay bottom would turn up something worth studying. The mud proved their gamble had paid off.

"We couldn't ask for much better than this," said Edgar, a geologist with the U.S. Geological Survey's office in St. Petersburg. "It's part of a very early deposit in the bay, from when the sea first came in."

Tampa Bay, which at 400 square miles is the state's largest estuary, was not always the 12-foot deep bowl of brackish water it is now. At some point several millennia ago, it was a freshwater lake. No one knows when or how it changed, but one theory is that the bay's creation might have resulted from a massive sinkhole.

Sediment buried beneath the bay's bottom can tell geologists not only the bay's origin but also how it looked before man came along and changed it.

"If we can get a handle on the last few thousand years, then that will give us a baseline, an idea of how the area developed naturally," explained Greg Brooks of Eckerd College. "Then we can make a more accurate estimate of what the future development will be."

For instance, what made Hastings so ecstatic were the bits of shell. By analyzing the shell's chemical composition, he said, "I can deduce the temperature and salinity of Tampa Bay from when the animal was alive."

Such detective work would not have been possible without a stroke of luck: The French vessel, the 400-foot Marion Dufresne, happened to be ending an 18-day voyage across the Gulf of Mexico by docking at the Port of Tampa on Thursday. It will head to Cuba today.

When they learned of the ship's Tampa layover, officials from Eckerd, the University of South Florida and the U.S. Geological Survey quickly arranged to get some samples from the bay just off MacDill Air Force Base.

The Marion Dufresne is one of only a handful of vessels in the world equipped to bore into the sea bottom and retrieve deep samples of buried sediment. During its voyage across the gulf, the ship took samples from the deepest canyons, among other things finding a well-preserved fossil of sea grass. Despite being 5,000 years old, the fossil still had a few long-dead creatures clinging to its brittle stems and brown leaves.

"We're really stretching back into deep time," said Ben Flower, a University of South Florida marine science professor who, along with USF graduate student Heather Hill and Eckerd undergrad Jenna Lo Dico, was aboard the ship for the 18-day gulf trip.

The ship's crew collects samples by raising and then dropping a metal pipe topped by several tons of weights. The pipe plunges deep into the sea bottom, like a straw stuck into a thick milk shake. Fitted snugly inside the metal pipe is a PVC sleeve that collects the sample, called a core.

The crew then hauls the pipe back up on the deck and pulls out the sediment-packed PVC sleeve. Scientists then cut the PVC pipe into sections and slice each section in half lengthwise to look at the core from various depths.

The first pipe dropped into Tampa Bay with a loud "ker-splash!" about 8 a.m. When the crew hauled it back up, Edgar was quick to check the pipe's bottom, a piece called the core catcher. He had been afraid the pipe would pull up nothing but sand, which yields few secrets of the past.

Finding mud flecked with shell meant there would be plenty to study under the microscope and in the laboratory over the next few months.

The second attempt, however, ended in a disaster. The ship's crew put three times as much weight on the pipe, and when it plunged into the bottom it went too deep and broke.

When the crew tried to pull it back up, about half the pipe remained mired in the mucky bottom, with the top of the broken part visible just a few feet beneath the waves. Brooks volunteered to don a snorkel mask and dive down to wrap a chain around it so the crew could pull it back up. But the ship's captain nixed any retrieval effort because of liability concerns, Edgar said.

Instead the crew attempted to mark the spot with a bright red buoy -- which then drifted away. So Edgar said he will go back out and mark the location, and the scientists will try to figure out how to retrieve it later.

Before heading to port, the ship's crew built a new pipe section and tried one last time to get a sample from the bay. Once again, though, the pipe banged into the hard limestone beneath the soft sediment. Instead of breaking, it bent at a 30-degree angle.

Yet when Eckerd geologist Rebekka Larson checked the core catcher, she made the most intriguing find of the day. Stuck in the mud was a shell from a freshwater snail, evidence from the time when the bay was just a lake with no connection to the salty gulf.

"We did not expect even in our best hopes to find this," Edgar said. He was champing at the bit to get the entire PVC-encased sample back to the lab, explaining, "We're just anxious as hell to get these things opened up."

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