La Brea Tar Pits
|La Brea Tar Pits|
Gas bubble slowly emerging at La Brea Tar Pits
|Location||Hancock Park, Los Angeles|
|Official name: Hancock Park La Brea|
The La Brea Tar Pits (or Rancho La Brea Tar Pits) are a group of tar pits around which Hancock Park was formed, in urban Los Angeles. Asphaltum or tar (brea in Spanish) has seeped up from the ground in this area for tens of thousands of years. The tar is often covered with dust, leaves, or water. Over many centuries, the bones of animals that were trapped in the tar were preserved. The George C. Page Museum is dedicated to researching the tar pits and displaying specimens from the animals that died there. The La Brea Tar Pits are a registered National Natural Landmark.
- 1 Location and formation
- 2 History
- 3 Scientific resource
- 4 Flora and fauna
- 5 Other tar pits
- 6 See also
- 7 References
- 8 External links
Location and formation
The La Brea Tar Pits and Hancock Park are situated within what was once the Mexican land grant of Rancho La Brea, now part of urban Los Angeles in the Miracle Mile district, adjacent to the Los Angeles County Museum of Art and the Craft and Folk Art Museum.
Tar pits are composed of heavy oil fractions called asphaltum, which seeped from the earth as oil. In Hancock Park, crude oil seeps up along the 6th Street Fault from the Salt Lake Oil Field, which underlies much of the Fairfax District north of the park. The oil reaches the surface and forms pools at several locations in the park, becoming asphalt as the lighter fractions of the petroleum biodegrade or evaporate.
This seepage has been happening for tens of thousands of years. From time to time, the asphalt would form a deposit thick enough to trap animals, and the surface would be covered with layers of water, dust, or leaves. Animals would wander in, become trapped, and eventually die. Predators would enter to eat the trapped animals and also become stuck.
The tar pits visible today are actually from human excavation. The lake pit was originally an asphalt mine. The other pits visible today were produced between 1913 and 1915, when over 100 pits were excavated in search of large mammal bones. Various combinations of asphaltum and water have since filled in these holes. Normally, the asphalt appears in vents, hardening as it oozes out, to form stubby mounds. These can be seen in several areas of the park.
As the bones of dead animals sink into the asphalt, it soaks into them, turning them a dark-brown or black color. Lighter fractions of petroleum evaporate from the asphalt, leaving a more solid substance, which encases the bones. Apart from the dramatic fossils of large mammals, the asphalt also preserves microfossils: wood and plant remnants, rodent bones, insects, mollusks, dust, seeds, leaves, and even pollen grains. Examples of some of these are on display in the museum.
Radiometric dating of preserved wood and bones has given an age of 38,000 years for the oldest known material from the La Brea seeps. The pits still ensnare organisms today, so most of the pits are fenced to protect humans and animals.
The Native American Chumash people living in the area distinguished themselves as the finest boat builders among the California Natives. Pulling fallen Northern California redwood trunks and pieces of driftwood from the Santa Barbara Bay, Chumash ancestors soon learned to seal the cracks between the boards of the large wooden plank canoes using the natural resource of tar. This unique and innovative form of transportation allowed access up and down the coastline and on the Channel Islands.
The Portolà expedition, a group of Spanish explorers led by Gaspar de Portolá, made the first written record of the tar pits in 1769. Father Juan Crespí wrote, "While crossing the basin the scouts reported having seen some geysers of tar issuing from the ground like springs; it boils up molten, and the water runs to one side and the tar to the other. The scouts reported that they had come across many of these springs and had seen large swamps of them, enough, they said, to caulk many vessels. We were not so lucky ourselves as to see these tar geysers, much though we wished it; as it was some distance out of the way we were to take, the Governor [Portolá] did not want us to go past them. We christened them Los Volcanes de Brea "the Tar Volcanoes".
For some years, tar-covered bones were found on the Rancho La Brea property but were not initially recognized as fossils, because the ranch had lost various animals, including horses, cattle, dogs, and even camels, whose bones closely resemble several of the fossil species. The original Rancho La Brea land grant stipulated that the tar pits be open to the public for the use of the local Pueblo. Initially, they mistook the bones in the pits for the remains of pronghorn antelope (Antilocapra americana) or cattle that had become mired.
Union Oil geologist W. W. Orcutt is credited with first recognizing that fossilized prehistoric animal bones were preserved in pools of asphalt on the Hancock Ranch in 1901. In commemoration of Orcutt's initial discovery, paleontologists named the La Brea coyote (Canis orcutti) in his honor.
Contemporary excavations of the bones started in 1913–1915. In the 1940s and 1950s, public excitement was generated by the preparation of previously recovered large mammal bones. Subsequent study demonstrated the fossil vertebrate material was well preserved, with little evidence of bacterial degradation of bone protein.
By the 1970s, research attention had shifted to smaller specimens, such as preserved insects and plant parts, including microfossils, such as pollen grains. These remains have contributed to an understanding of the Los Angeles basin during the glacial age, revealing a cooler and more moist climate.
Source of methane discovered
Methane gas escapes from the tar pits, causing bubbles that make the asphalt appear to boil. Asphalt and methane appear under surrounding buildings and require special operations for removal to prevent the weakening of building foundations.
In 2007, researchers from UC Riverside discovered that the bubbles were caused by hardy forms of bacteria embedded in the natural asphalt. After consuming petroleum, the bacteria release methane. Of the bacteria sampled, about 200 to 300 were previously unknown species.
George C. Page Museum
The George C. Page Museum, part of the Natural History Museum of Los Angeles County, was built next to the tar pits in Hancock Park on Wilshire Boulevard. Construction began in 1975, and the museum opened to the public in 1977. It tells the story of the tar pits and presents specimens from them. Visitors can walk around the park and see the tar pits. On the grounds of the park are life-size models of prehistoric animals in or near the tar pits. Of more than 100 pits, only Pit 91 is still regularly excavated by researchers and can be seen at the Pit 91 viewing station. Paleontologists supervise and direct the work of volunteers.
Excavation of newly uncovered pits
On February 18, 2009, George C. Page Museum formally announced the 2006 discovery of 16 fossil deposits which had been removed from the ground during the construction of an underground parking garage for the Los Angeles County Museum of Art next to the tar pits. Among the finds are remains of a saber-toothed cat, six dire wolves, bison, horses, a giant ground sloth, turtles, snails, clams, millipedes, fish, gophers, and an American lion. Also discovered is a nearly intact mammoth skeleton, nicknamed Zed; the only pieces missing are a rear leg, a vertebra and the top of its skull, which was sheared off by construction equipment in preparation to build the parking structure.
These fossils were packaged in boxes at the construction site and moved to a compound behind Pit 91, on Page museum property so that construction could continue. Twenty-three large accumulations of tar and specimens were taken to the Page Museum. These deposits are worked on under the name "Project 23". As work for the public transit Metro Purple Line is extended, museum researchers know more tar pits will be uncovered, for example near the intersection of Wilshire and Curson.
Flora and fauna
Among the prehistoric species associated with the La Brea Tar Pits are mammoths, dire wolves, short-faced bears, ground sloths, and the state fossil of California, the saber-toothed cat (Smilodon fatalis).
Only one human has been found, a partial skeleton of the La Brea Woman dated to approximately 10,000 calendar years (~9,000 radiocarbon years) BP, who was 17 to 25 years old at death and found associated with remains of a domestic dog, and so was interpreted to have been ceremonially interred. John C. Merriam of the University of California led much of the early work in identifying species in the early 20th century.
The park is known for producing myriad mammal fossils dating from the last glacial period. While mammal fossils generate significant interest, other fossils, including fossilized insects and plants, and even pollen grains, are also valued. These fossils help define a picture of what is thought to have been a cooler, moister climate in the Los Angeles basin during the glacial age. Among these fossils are microfossils, which are retrieved from a matrix of asphalt and sandy clay by washing with a solvent to remove the petroleum, then picking through the remains under a high-powered lens.
Tar pits around the world are unusual in accumulating more predators than prey. The reason for this is unknown, but one theory is that a large prey animal would die or become stuck in a tar pit, attracting predators across long distances. This predator trap would catch predators along with their prey. Another theory is that dire wolves and their prey may have been trapped during a hunt. Since modern wolves hunt in packs, each prey animal could take several wolves with it.
Below is a partial list of extinct and extant mammals with their scientific names included on the right.
- Columbian mammoth (Mammuthus columbi)
- American mastodon (Mammut americanum)
- Harlan's ground sloth (Paramylodon harlani)
- Jefferson's ground sloth (Megalonyx jeffersonii)
- Shasta ground sloth (Nothrotheriops shastensis)
- Giant bison (Bison latifrons)
- Ancient bison (Bison antiquus)
- American camel (Camelops hesterus)
- Stilt-legged llama (Hemiauchenia macrocephala)
- Western horse (Equus "occidentalis")
- Mexican horse (Equus conversidens)
- Peccary (Platygonus compressus)
- Pronghorn (Antilocapra americana)
- Dwarf pronghorn (Capromeryx minor)
- California tapir (Tapirus californicus)
- Elk (wapiti) (Cervus canadensis)
- Deer (Odocoileus sp.)
- Short-faced bear (Arctodus simus)
- Brown bear (Ursus arctos)
- Black bear (Ursus americanus)
- Scimitar cat (Homotherium serum)
- Sabre-toothed cat (Smilodon fatalis)
- American lion or Naegele’s giant jaguar (Panthera leo atrox)
- Jaguar (Panthera onca augusta)
- American cheetah (Miracinonyx inexpectatus)
- Cougar (Puma concolor)
- Bobcat (Lynx rufus)
- Dire wolf (Canis dirus)
- Gray wolf (Canis lupus)
- Coyote (Canis latrans)
A partial list of extinct and extant birds found as fossils at La Brea.
- California condor
- Daggett's Eagle
- Teratornis (Teratornis merriami)
- La Brea or northern caracara
- Sandhill crane
- Canada goose
- Mallard duck
- Night heron
- La Brea Stork
- Common raven
- Horned lark
- Common poor-will
- Great horned owl
- La Brea owl
- Greater roadrunner
- Mourning dove
- Band-tailed pigeon
- California quail
- Turkey (†Californian Turkey)
Reptiles, amphibians, and fish
Current living species
The petroleum fly (Helaeomyia petrolei) completes part of its life-cycle in the tar pits, feeding on trapped insects and other animal matter.
Other tar pits
Rancho La Brea is the most famous, but two other asphalt pits in southern California contain fossils: the Carpinteria Tar Pits in Carpinteria, Santa Barbara County and the McKittrick Tar Pits in McKittrick, in Kern County. Other asphalt deposits can be found in Texas, Peru, Trinidad, Iran, Russia, Venezuela, and Poland.
- "Hancock Park". Office of Historical Preservation, California State Parks. Retrieved 2012-10-07.
- Khilyuk, Leonid F.; Chilingar, George V. (2000). Gas migration: events preceding earthquakes. Gulf Professional Publishing. p. 389. ISBN 0-88415-430-0.
- Kielbasa, John R. (1998). "Rancho La Brea". Historic Adobes of Los Angeles County. Pittsburg: Dorrance Publishing Co. ISBN 0-8059-4172-X..
- Orcutt Ranch Horticultural Center Rancho Sombra del Roble. Los Angeles Historic-Cultural Monument # 31.
- "Animal Bones 50,000 Years Old Found In Tar". Los Angeles Times (Los Angeles). June 17, 1946. Retrieved January 27, 2010.
- McMenamin, M.A.S., et al. (1982). "Amino acid geochemistry of fossil bones from the Rancho La Brea Asphalt Deposit, California". Quaternary Research 18 (2): 174–183. doi:10.1016/0033-5894(82)90068-0.
- Jia-Rui Chong, "Researchers learn why tar pits are bubbly", Los Angeles Times, May 14, 2007.
- Page Museum. "Page Museum - La Brea Tar Pits". Page Museum web site. The Natural History Museum of Los Angeles County Foundation. Retrieved 2010-06-01.
- "Cache Of Ice Age Fossils Found Near Tar Pits". Los Angeles: KCBS-TV. Associated Press. February 18, 2009. Retrieved February 18, 2009.[dead link]
- Thomas H. Maugh II (February 18, 2009). "Major cache of fossils unearthed in L.A.". Los Angeles Times (Los Angeles). Retrieved February 18, 2009.
- "Workers Unearth Mammoth Discovery near La Brea Tar Pits". Los Angeles: KTLA. February 18, 2009. Retrieved February 18, 2009.[dead link]
- "Nearly intact mammoth found at L.A. construction site". USA Today. February 18, 2009. Retrieved 1 September 2010.
- J.C. Merriam (1914) Preliminary report on the discovery of human remains in an asphalt deposit at Rancho La Brea, Science 40: 197-203
- F.R. O'Keefe, E.V. Fet, and J.M. Harris (2009) Compilation, calibration, and synthesis of faunal and floral radiocarbon dates, Rancho La Brea, California, Contributions in Science 518: 1-16
- G.E. Kennedy (1989) A note on the ontogenetic age of the Rancho La Brea hominid, Los Angeles, California, Bulletin, Southern California Academy of Sciences 88(3): 123-126
- R.L. Reynolds (1985) Domestic dog associated with human remains at Rancho La Brea, Bulletin, Southern California Academy of Sciences 84(2): 76-85
- "Fauna and Flora of Rancho La Brea" (last visited April 13, 2013).
|Wikimedia Commons has media related to La Brea Tar Pits.|
- Page Museum - La Brea Tar Pits
- UCMP Berkeley website describes the geology and paleontology of the asphalt seeps.
- Setting the La Brea site in context.
- La Brea Tar Pits Visitor Guide
-  Pit 91