tar pit. An accumulation of natural tar or asphalt at the Earth’s surface, especially one that traps animals and preserves their hard parts. Tar pits form when petroleum in subterranean petroleum-bearing rocks oozes up to the surface.
Tar pits form when petroleum in subterranean petroleum-bearing rocks oozes up to the surface. As it rises, the petroleum loses its volatile components, forming a thick tar or asphalt deposit. The American Heritage® Science Dictionary Copyright © 2011.
An accumulation of natural tar or asphalt at the Earth’s surface, especially one that traps animals and preserves their hard parts.
It’s a great place to study how wildlife today act around this natural phenomenon. Asphaltic deposits or “tar pits” present a unique opportunity to study past ecosystems because they preserve many different kinds of fossils (and lots of them!).
How do tar pits form?
They form in the presence of oil, which is created when decayed organic matter is subjected to pressure underground. If this crude oil seeps upward via fractures, conduits, or porous sedimentary rock layers, it may pool up at the surface.
Tar pits are pools of asphalt. However, at the beginning of their formation, they were not always sticky and dense. The pools were composed of crude oil that originated below Earth’s surface. Crude oil is a mixture of heteroatom compounds, hydrocarbons, metals, and inorganic compounds. Heteroatom compounds are organic molecules that contain elements that are not carbon or hydrogen, while hydrocarbons contain only carbon and hydrogen. Crude oil is less viscous than asphalt because it contains a higher percentage of light hydrocarbons. Light hydrocarbons include the following alkanes: methane, ethane, propane, and butane. These molecules have very low molecular weights. Crude oils may also contain some inorganic impurities, such as CO 2, H 2 S, N 2, and O 2. At the surface, these light molecules may evaporate out of the crude oil, leaving behind the heavier, stickier molecules. Asphalt, or bitumen, usually contains hydrocarbon molecule chains with 50+ carbon atoms. The longer the hydrocarbon chain, the more viscous it becomes, and the boiling point increases.
For thousands of years, Native Americans used tar from the La Brea Tar Pits as an adhesive and binding agent. They would use it as waterproof caulking to line their boats and baskets. When Westerners arrived at the tar pits, they began mining and extracting the tar for roofing material in nearby towns.
In one study, the predominant bacteria found in the La Brea Tar Pits were of the Gammaproteobacteria class in the Chromatiales order, more simply referred to as purple sulfur bacteria. Purple sulfur bacteria do not use water as their reducing agent, so oxygen is not produced during respiration.
Tar pits are often excavated because they contain large fossil collections. Tar pits form above oil reserves, and these deposits are often found in anticlinal traps. In fact, about 80 percent of petroleum found on Earth has been found in anticlinal traps.
Other bacteria discovered in the tar pits were of the Rubrobacteraceae family. These bacteria are known for being some of the most radiation-resistant organisms on the planet. Pitch Lake, another asphalt pit in Trinidad and Tobago, is also a habitat for microbial communities of archaea and bacteria.
Once animals step into the tar, they become immobilized and begin sinking immediately if the asphalt is warm and sticky enough. Predators that see these helpless animals usually would advance into the tar pits with the hope of catching their next meal. As a result, prey are usually found beneath the predator during excavation projects.
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