【慢速听力: 196探索】LaBrea焦油坑：数千年前动物生活和死亡的地方
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A program about people, places and ideas.
La Brea Tar Pits: Where Animals Lived, and Died, Thousands of Years Ago
2007-10-2
VOICE ONE:
I'm Shirley Griffith.
VOICE TWO:
And I'm Steve Ember with EXPLORATIONS in VOA Special English. Today we tell about an unusual scientific research area in the United States.
It is filled with the remains of ancient animals. This unusual place is in the center of Los Angeles, California. Its name is Rancho La Brea. But most people know it as the La Brea Tar Pits.
(MUSIC)
VOICE ONE:
To understand why La Brea is an important scientific research center we must travel back through time almost forty thousand years. Picture an area that is almost desert land. The sun is hot. A pig-like creature searches for food. It uses its short, flat nose to dig near a small tree. It moves small amounts of sand with its nose. It finds nothing. The pig starts to walk away, but it cannot move its feet.
They are covered with a thick, black substance. The pig shakes one foot loose, but the others just sink deeper. The more it struggles against the black substance, the deeper it sinks. The pig attempts to free itself again and again. It now screams in fear and fights wildly to get loose. Less than a kilometer away, a huge cat-like creature with two long front teeth hears the screams. It, too, is hungry. Traveling across the ground at great speed, the cat nears the area where the pig is fighting for its life.
The cat jumps on the pig's back. It sinks its long teeth into the pig's neck. The pig dies quickly, and the cat begins to eat. Almost an hour passes before the cat is finished. When it attempts to leave, like the pig, it finds it cannot move. The more the big cat struggles, the deeper it sinks into the black substance.
Before morning, the cat is dead. Its body, and the bones of the pig, slowly sink into the sticky black hole.
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VOICE TWO:
Scientists say the story we have told you happened again and again over a period of many thousands of years. The black substance that trapped the animals came out of the Earth as oil.
The oil dried, leaving behind a partly solid substance called asphalt. In the heat of the sun, the asphalt softened. Whatever touched it would often become trapped forever.
In seventeen sixty-nine, a group of Spanish explorers visited the area. They were led by Gaspar de Portola, governor of Lower California.
The group stopped to examine the sticky black substance that covered the Earth. They called the area "La Brea" the Spanish words for "tar."
Many years later, settlers used the tar, or asphalt, on the tops of their houses to keep water out. They found animal bones in the asphalt, but threw them away. In nineteen-oh-six, scientists began to study the bones found in La Brea. Ten years later, the owner of the land, George Allan Hancock, gave it to the government of Los Angeles. His gift carried one condition. He said La Brea could only be used for scientific work.
VOICE ONE:
Today, the La Brea Tar Pits are known to scientists around the world. The area is considered one of the richest areas of fossil bones in the world. It is an extremely valuable place to study ancient animals. Scientists have recovered more than one million fossil bones from the La Brea Tar Pits. They have identified more than six hundred fifty different kinds of animals and plants.
The fossils are from creatures as small as insects to those that were bigger than a modern elephant. These creatures became trapped as long ago as forty thousand years. It is still happening today. Small birds and animals still become trapped in the La Brea Tar Pits.
(MUSIC)
VOICE TWO:
Rancho La Brea is the home of a modern research center and museum. Visitors can see the ancient fossil bones of creatures like the imperial mammoth and the American mastodon. Both look something like the modern day elephant, but bigger.
The museum has many fossil remains of the huge cats that once lived in the area. They are called saber-toothed cats because of their long, fierce teeth. Scientists have found more than two thousand examples of the huge cats. The museum also has many ground sloths and thousands of fossil remains of an ancient kind of wolf. Scientists believe large groups of wolves became stuck when they came to feed on animals already trapped in the asphalt.
VOICE ONE:
Since nineteen sixty-nine, scientists have been digging at one area of La Brea called Pit Ninety-One. They have found more than forty thousand fossils in Pit Ninety-One. More than ninety-five percent of the mammal bones are from just seven different animals. Three were plant-eaters. They were the western horse, the ancient bison and a two-meter tall animal called the Harlan's ground sloth.
Four of the animals were meat-eating hunters. These were the saber-tooth cat, the North American lion, the dire wolf and the coyote. All these animals, except the dog-like coyote, have disappeared from the Earth.
VOICE TWO:
Researchers say eighty percent of the fossils found are those of meat-eating animals. They say this is a surprise because there have always been more plant-eaters in the world. The researchers say each plant-eater that became trapped caused many meat-eaters to come to the place to feed. They, too, became trapped.
Rancho La Brea has also been a trap for many different kinds of insects. Scientists free these dead insects by washing the asphalt away with special chemicals. The La Brea insects give scientists a close look at the history of insects in southern California.
The La Brea Tar Pits have also provided science with interesting information about the plants that grew in the area. For many thousands of years, plant seeds landed in the sticky asphalt. The seeds have been saved for research. Scientists also have found pollen from many different kinds of plants.
The seeds and pollen, or the lack of them, can show severe weather changes over thousands of years. Scientists say these provide information that has helped them understand the history of the environment. The seeds and pollen have left a forty thousand year record of the environment and weather for this area of California.
VOICE ONE:
Many visitors to the tar pits wonder why they produce large gas bubbles. Now scientists from the University of California, Riverside, have the answer. Bacteria in the natural asphalt are eating away at the oil below the surface and producing methane gas. The scientists discovered more than two hundred kinds of bacteria. Most of them were species that were unknown. The bacteria were trapped in soil that was mixed with heavy oil almost twenty-eight thousand years ago.
The bacteria are able to survive in an extreme environment. The scientists say they live in the asphalt with no water, little or no oxygen and many poisonous chemicals. Scientists think the discovery of the bacteria might lead to new methods to clean oil spills and other uses.
(MUSIC)
VOICE TWO:
Thousands of visitors come each year to see the fossils that have been found at Rancho La Brea. They visit the George C. Page Museum. Mister Page was a wealthy man who became very interested in the scientific work being done at the tar pits. He gave the money to build the museum and research center.
At the museum, visitors can watch scientists dig bones from La Brea's Pit Ninety-One. The scientists dig very slowly, using small tools similar to those used by a doctor to examine teeth. They also use toothbrushes and cleaning fluids to help soften and clean away the asphalt.
VOICE ONE:
Visitors to the museum can also see the "fish bowl," a laboratory surrounded by glass. Here, they can watch scientists do their research. Visitors can watch the scientists clean, examine, repair and identify fossils that are still being discovered. Through this process, scientists are able to answer questions and solve puzzles about animals and their environment from thousands of years ago.
It is exciting to stand only a few meters away and watch scientists clean the asphalt off a fossil that is thousands of years old. Visitors quickly learn why researchers consider Rancho La Brea a very special place.
(MUSIC)
VOICE TWO:
This program was written by Paul Thompson. It was produced by Mario Ritter. You can learn more about the La Brea Tar Pits at our Web site, WWW.51VOA.COM. I'm Steve Ember.
VOICE ONE:
And I'm Shirley Griffith. Join us again next week for EXPLORATIONS in VOA Special English.