The allegation that "the Americans" left Panama at the end of 1999 is inaccurate in a personal sense because thousands, probably tens of thousands, of US citizens still call the isthmus home. It's also an exaggeration in the institutional sense --- while mangagement of the canal and the remaining US military bases were turned over to Panama, several US government agencies still operate here. One of these is the Smithsonian Institution, whose Smithsonian Tropical Research Institute (STRI) runs several scientific field research laboratories around Panama and maintains its headquarters in the Tupper Center in Ancon.

For visitors to Panama, STRI is probably best known for its small group tours of Barro Colorado Island in Gatun Lake, where it has been been conducting rainforest ecology studies for decades. Another of its activities, conducted in English at noon every Tuesday in the Tupper Center's auditorium, is a lecture series by various scientists who are doing field work under STRI's auspices. The lectures are open to the public and free of charge, and attract mostly other STRI scientists from diverse disciplines, along with a core of curious individuals who have no official ties with the Smithsonian. Because the audience members tend to be erudite but not of the speakers' specialties, the presentations are not so jargon-laden as to leave an intelligent
English-speaking person lost.

On July 2 the speaker was geologist Tony Coates, who described himself as "one of only a couple of geologically-oriented people at STRI." He has been researching Panama's geological history, using a number of techniques to construct a "temporal template of the rise and fall of the isthmus" over the past 20 million years or so.

As Coates explained, the isthmus came into place about four million years ago, as the result of a complicated set of movements among four distinct tectonic plates which find their confluence in Panama. He bases this conclusion on research at sites around the isthmus, in which layers of rocks are mapped and dated by their composition, their magnetic properties and the fossils they contain.

Paleontologists have compiled a good record of the time frames of when many of the plants, animals and single-celled creatures whose fossils are found in Panama's rocks existed. In the case of marine fossils, they also can discern deep water corals from those found in shallow seas, the remains of organisms that lived in certain temperatures and salinity levels from those that survived under different conditions, and creatures specific to the Pacific from those found in the Caribbean Sea and Atlantic Ocean. "In the fossil collection of the last 10 million years of isthmian history," Coates noted, "we have a better record than other disciplines do."

Originally from the pioneering ocean-bottom research that led to the tectonic plate theory, scientists know that the Earth's magnetic field reverses its polarity from time to time, and leaves the imprint of this change in volcanic rocks. Geologists can also easily distinguish the pillow-like lava formations created by underwater volcanism from the rocks left behind from lava flows on land.

Coates illustrated how this knowledge was combined in field work he did in Bocas del Toro, where he went around Popa Island and the Valiente Peninsula looking for weathered outcroppings of old rock formations and taking core samples to study. To date the geological strata the he studies, he said that people in his field first "get in the ball park by the fossils," and then use the polarity of the rocks and sometimes radiocarbon or the known decay rates of other isotopes encountered to get more specific dates. With enough core samples properly analyzed, it becomes possible to map the geological formations of an area. In this pursuit Coates and his colleagues can often rely on samples and measurements made not for the purpose of pure science, but by the oil companies' geologists in their search for valuable mineral resources.

Coates said that in the fossil record, microscopic plankton shells can be identified by species, and given what is known about the evolutionary changes in these organisms they are especially useful in establishing approximate dates. Fossils of marine molluscs, on the other hand, are very useful in that the growth lines in the shells will say a lot about the environmental conditions in which the animals lived. Large corals can only have existed in shallow seas, while many deep water coral species are identifiable as such. The otoliths --- tiny bones crucial to the ability of fish to hear underwater sounds --- that are found in a rock stratum's fossil record will tell a trained scientist whether they came from species found in deep or shallow waters.

What Coats found in Bocas was that the sea bottom that was to rise up and become part of the Isthmus of Panama rose to become a shallow sea, then fell to a depth of about 200 meters some six million years ago, then rose again to emerge as dry land. He dates the Valiente Peninsula as about 20 million years old, and Popa Island at about eight million years.

On the grander scale, Coates drew a picture of a gradually rising archipelago of volcanic islands which now form the spine of Panama from about El Valle to around Volcan Baru but long ago were to the northeast of their present location. This formation eventually became a larger island atop the Caribe plate, which ran into Central America. For a time, he theorized, there was a strait connecting the Caribbean Sea and the Pacific Ocean and dividing the Americas, with a swift current carrying Pacific species into the Caribbean but not vice-versa. The Azuero and Burica peninsulas were created by other tectonic plates crashing into Panama from the west, and the strait closed and created the isthmus when South America ran into it some four million years ago. Coates believes that, in the geological way of thinking, the Darien is probably the youngest part of Panama. He noted that, due to the ways that the four plates that find their confluence in this country are jamming together, we don't have the volcanic activity or severe earthquakes that our Central and South American neighbors do.

After the lecture, Coates explained that in more recent geological times, a cycle of ice ages and warming periods that is not connected to the tectonic forces that created Panama has caused sea levels to rise and fall. This has made the isthmus rise and fall relative to sea level, and widen and narrow correspondingly as shallow seas became land and were then inundated again, on a recurring cycle of about 130,000 years.

Panama's geological history in turn becomes important for other disciplines whose scholars come down here to study under STRI's auspices. For example, it explains to biologists how South America's wildlife evolved separately from Central America's until four million years ago, and to archaeologists looking for the traces of human migration from North America to South America how they may have difficulty finding traces on the present isthmus because the coastlines that the migrants probably followed are now underwater.

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