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Projects: NEO Earth Expeditions

1995 Belize Expedition

by Adriana Ocampo

Today, the Chicxulub crater lies between 300 and 1,000 meters (about 1,000 and 3,000 feet) below the surface, and even though the crater is extremely well preserved, the protective blanket of younger sediments makes detailed studies difficult. As a result, much is still not known, and in fact the crater's exact size, between 180 and 300 kilometers (110 and 190 miles) in diameter, remains controversial. More geophysical studies are under way, including the use of a space shuttle imaging radar (SIR-C), which reveals subtle surface structures, to get at the diameter question.

In search of more evidence and an understanding of how the end of the Cretaceous came about, in January 1995 an enthusiastic group of Planetary Society members and scientists traveled to the country of Belize, about 364 kilometers (226 miles), or about three crater radii, from Chicxulub. Belize is unique in that it contains an outcrop of Chicxulub ejecta that is more than 100 kilometers (60 miles) closer to the crater than any other.

The expedition included avid supporters of the impact-extinction hypothesis, and skeptics as well. To keep us on the right track, we had the help of one of the fathers of the theory, Walter Alvarez; microtektite specialist Philippe Claeys; paleontologist Francisco Vera Vega; field geologist and mapping expert Eugene Fritsche; and an expert in the geology of northern Belize, Kevin Pope. The richness of the expedition was enhanced by the multitalented Planetary Society group that completed the "dream team": Millie Alvarez, Sandi Atwood, Lu Coffing, Robert Cozzi, Gene Giberson, Kenneth Jones, Carmen Musgrave, John O'Brien, Joyce Stark, Richard Stark, Richard Weddle, Christina Wilder and Norman Wilson. The challenges were many, but there was certainly no lack of enthusiasm, and sense of inquiry, wonder and discovery.

Albion Island -- The Search Begins

Our search started on Albion Island in the Rio Hondo, near the city of Orange Walk (about a two-hour drive from Belize City) in northern Belize. The island is home to the town of San Antonio (population 500) and a dolomite quarry. We spent the first nine days working at the quarry.

The quarry presented us with a magnificent vista of the K/T boundary. Here we took stereo photographs of the boundary and mapped it in three dimensions. The stereo photography was an excellent tool for recording changes in this active quarry and would help us verify details of the stratigraphy back in the lab. Mapping the quarry gave us insight into the three-dimensional structure of the strata and the ejecta thickness.

The strata in which the quarry has been excavated are dome- shaped, and the quarry contains two sinkholes (or cenotes), which formed relatively recently. The dome may have been emergent (above water) when the material ejected from Chicxulub was deposited. This possibility has important implications for the depositional environment and geochemistry of the rocks at Albion Island.

We constructed detailed stratigraphic profiles and collected numerous samples, our activities mixed with bouts with mosquitoes and the dreaded "chechem" (a Belizean version of poison ivy that reaches tree size) and lots of camaraderie. The quarry provided us with a unique view of the last moments of the Cretaceous, complete with giant, 8-meter-diameter (26- foot) ejecta boulders that gave us but a glimpse of the tremendous power of the impact. The energy estimated to have been released by the Chicxulub collision is between 108 and 109 megatons, or equal to about twice all the world's present nuclear arsenal.

All these pieces of the puzzle are helping us understand and reconstruct an image of the end of the Cretaceous on Albion Island 65 million years ago, both before and after the impact. Impact events such as this one leave a complex array of deposits. During the expedition, we collected samples of carbonate from the ejecta that could give insight into our model predictions of target rock vaporization at Chicxulub. The relative amounts of heavy and light isotopes of carbon and oxygen in these carbonates are analyzed to determine if fractionation due to vaporization has occurred. Isotopes fractionate when they change states, such as going from a solid to a gas. For Chicxulub, when the target material was vaporized by the heat of the impact, the carbon dioxide vapors released should have contained higher concentrations of the lighter carbon and oxygen isotopes, with the heavier isotopes remaining behind in the solid ejecta. This signature may have been captured in the rocks at Albion Island if some of the gases produced by the impact condensed from the vapor plume.

Ferreting Out the Fossil Feast

During the last three days of our 1995 expedition, we concentrated on reconnaissance, searching for other K/T exposures farther south, near the Rio Bravo ecological reserve. This exploratory work took us deeper into the tropical forest, and although we have not confirmed any other K/T exposures more lab work and fossil identification are required. We did collect lots of samples and gained a greater respect for nature and the beauty of the lucky survivors of that dark episode 65 million years ago. By the end of the 12-day expedition, we had collected 200 kilograms (440 pounds) of rocks and fossils. Geologists use fossils as time markers in the geologic record. Among the discoveries made by Society members were fossil crabs and mollusks that proved to be critical in confirming the K/T age of the Albion deposits.

We left Belize with more questions than answers, as is often the case in the scientific process of discovery. Chicxulub and the K/T extinctions present the scientific community with a multidisciplinary set of problems that can be solved only by studying the intricate web of interactions that make up the Earth system. No single discipline holds the answers to this puzzle, and only a cooperative effort will bring about a complete understanding.

Adriana Ocampo is a Planetary Society advisor and a planetary geologist at NASA headquarters in Washington, DC. The research for the 1995 Belize expedition was supported by the Planetary Society and the NASA Exobiology Program.