What Is a Tar Pit? A Trap That Kept Filling Itself for Fifty Thousand Years
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Natural asphalt seeping to the surface forms a sticky pool that traps animals, and because a trapped animal attracts predators and scavengers who become trapped in turn, the process repeats indefinitely. The resulting deposits contain enormous numbers of bones in a ratio unlike anywhere else.
How the trap works
Crude oil migrating upward through rock reaches the surface at certain places, and the lighter fractions evaporate, leaving a heavy viscous asphalt. In warm weather it softens and becomes sticky, and shallow pools can be covered by water, dust and leaves so that the surface looks like solid ground or a drinking pool. An animal stepping into it becomes stuck, and the more it struggles the deeper it sinks, with even a few centimetres sufficient to hold a large mammal. Its distress calls and eventual death attract predators and scavengers, which enter the same trap and are caught in turn, and that is the mechanism that produces the deposits' most distinctive feature. Modelling suggests that a single large trapped herbivore could account for a considerable number of predators over the following days, and one entrapment event every decade or so would be enough to produce the observed accumulation over tens of thousands of years, so the pits were not constantly lethal but relentlessly so over time.
What the deposits contain
The best studied example is at Rancho La Brea in Los Angeles, and its contents are unusual in several respects:
- •A predator to prey ratio inverted relative to any living ecosystem, with dire wolves and sabre-toothed cats vastly outnumbering herbivores, because the trap selected for animals attracted to distress
- •Enormous numbers of individuals, with dire wolf remains from thousands of animals recovered, making it the largest sample of that species anywhere
- •Excellent preservation, since asphalt excludes oxygen and bacteria, so bones are intact rather than crushed and retain organic material
- •Microfossils including insects, plants, pollen, seeds, rodents and birds, which reconstruct the environment in detail and are scientifically as valuable as the large mammals
- •Evidence of injury and disease in the bones, which is abundant enough to support population-level study of how these animals lived and what they suffered
- •Very few human remains, with a single partial skeleton recovered, which is consistent with people being able to recognise and avoid the hazard
The problems the material poses
Asphalt is a superb preservative and a terrible medium to work in. The bones are saturated with it and must be cleaned with solvents, which is slow, hazardous and expensive, and enormous quantities of excavated material remain unprocessed decades after recovery. The deposits are jumbled rather than articulated, since animals sank, decayed and were disturbed by later arrivals and by the movement of the asphalt itself, so skeletons are not found intact and individuals must be reconstructed statistically. Early excavation in the twentieth century prioritised impressive large bones and discarded small material, losing information that later workers would have valued, which is why modern work processes everything and is correspondingly slower. Dating is complicated by contamination, since petroleum carbon is ancient and swamps radiocarbon measurements, requiring careful pretreatment. Recovery of ancient DNA has succeeded despite early pessimism, and genomic work on the dire wolf established that it was far more distantly related to grey wolves than its appearance suggested.
Where else they occur
Asphalt seeps are found wherever oil reaches the surface, and several have produced fossil assemblages. Venezuela, Trinidad, Peru, Azerbaijan, Poland and Russia all have deposits, and the Talara seeps in Peru have yielded a substantial ice age fauna including many birds. Pitch Lake in Trinidad is the largest natural asphalt deposit in the world and has been exploited commercially for centuries. Historically these seeps were valuable for exactly the property that makes them fossil traps, since asphalt waterproofs boats, roofs and baskets, seals joints, and was used as a construction and preservation material in the ancient Near East, where it was collected from seeps around the Dead Sea and along the Euphrates and traded widely. Indigenous peoples of southern California used the material extensively for waterproofing and adhesive, and the modern site sits within a city that grew up around the oil field the seeps announced.
The takeaway
Surface asphalt softens in heat and holds an animal in a few centimetres, and its distress draws predators into the same trap, which is why the deposits contain far more carnivores than any real ecosystem held. Asphalt excludes oxygen and bacteria, so bones are exceptionally preserved and jumbled rather than articulated. Petroleum carbon interferes with radiocarbon dating. Genomics showed dire wolves were only distant relatives of grey wolves.