The La Brea Tar Pits, located in urban Los Angeles, California, contain one of the planet’s richest deposits of Ice Age fossils. The sticky pools of asphalt have perfectly preserved the remains of countless organisms, providing a detailed look at the ancient ecosystem of Southern California. The true age of the La Brea Tar Pits is not a single point in time, but a continuous process spanning tens of thousands of years, with most discovered fossils dating to a specific window in Earth’s history.
How the La Brea Traps Formed
The formation of the tar pits results from underlying geological activity. Crude oil from the deep Salt Lake Oil Field, roughly 1,000 feet underground, seeps upward through fissures in the earth’s crust, primarily along the 6th Street Fault. This crude oil reaches the surface, forming pools of petroleum in what is now Hancock Park. Once exposed to the atmosphere, the lighter components evaporate. This natural distillation leaves behind a dense, sticky residue known as asphalt, bitumen, or pitch—what Spanish settlers called “brea.” The resulting asphalt is a viscous substance that easily traps any animal or plant matter. New asphalt continuously rises to the surface, meaning the pits have been forming for tens of thousands of years.
Dating the Fossil Record
Determining the age of the La Brea fossils relies on radiocarbon dating (Carbon-14 dating). This specialized technique analyzes the decay rate of the carbon isotope C-14 found in organic materials like bone and wood, allowing scientists to assign a specific age to the remains.
The age range of significant fossil discoveries spans from over 50,000 years ago to around 10,000 years before the present. The vast majority of large mammal remains were entrapped during the Late Pleistocene Epoch. The oldest known fossil material recovered has been dated to approximately 38,000 years ago.
Extracting accurate dates is challenging because the asphalt saturates the porous bones. This requires extensive chemical pretreatment to remove petroleum contamination before dating the remaining bone collagen. Despite these difficulties, radiocarbon dating has established a precise chronology, showing that entrapment occurred in pulses over this 40,000-year window.
The precise dating reveals that Ice Age mammal populations were stable until around 13,250 years ago, followed by a sharp decline leading to extinction between 13,070 and 12,900 years ago. This focused timeline records a narrow, highly active period of environmental change.
What the Pleistocene Timeframe Reveals
The 50,000-year timeline covered by the La Brea fossils corresponds to the Late Pleistocene Epoch, or the last Ice Age. This dated window provides scientists with a detailed record of the environmental and climatic shifts in the Los Angeles Basin. The fossils allow researchers to study how species like the Columbian mammoth, the saber-toothed cat, and the dire wolf adapted to or perished from these changes.
The sheer volume of preserved megafauna, especially the high number of carnivores, offers unique insights into the ancient ecosystem. Scientists use this information to reconstruct past food webs and analyze how the ecosystem responded to a warming and drying climate.
Research suggests that the disappearance of large mammals around 13,000 years ago coincided with an increase in wildfires and a shift from a woodland environment to the drier chaparral system seen today. By examining the bones, scientists link the extinction of two-thirds of North America’s large mammals to specific environmental pressures. The data collected from the pits connects the ancient past to current climate challenges by revealing the ecological consequences of rapid environmental change.