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5.1: Deterioration and Decomposition

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    78460
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    DETERIORATION

    Most artifacts related to the earliest known humans are made of stone or clay. The oldest stone artifacts found are at least 2 million years old. In contrast, some of the oldest organic remains found are woven Fort-Rock-type sandals from Elephant Mountain Cave in southwestern Oregon that date to approximately 10,000 years ago. That is quite a difference! Why are some artifacts preserved so much longer than others? It depends on the material of the artifact and the environment in which the object was deposited when it first entered the archaeological record. If the conditions are right, an artifact's preservation may be extended such as in the case of human remains and artifacts found in peat bogs and ice patches.

    Whether the artifact is made of stone, metal, or an organic material such as wood, they are all subject to deterioration. The agents of deterioration can be grouped into three categories: chemical, physical, and biological (Fagan, 2006). While all three types of deterioration will cause damage to an artifact, it is the biological agents that can cause the most harm to organic materials. This is why there are very few organic artifacts that have been recovered from prehistory.

    Agents of Deterioration

    Chemical Physical Biological
    Water and Oxygen Corrosion on metals Water movement Fracture and wear of artifacts; erosion Bacteria Start the process of decay
    Acidic soils Dissolve bones Cycles of wet and dry or freezing and thawing Causes wood to crack and rot; causes stone and concrete to crack Insects Infestations of organic materials in the process of decomposition
    Sunlight Irradiation of materials Rainwater Erosion of artifacts and walls Fungi Destructive to wood and plant matter (especially in warm, damp climates)
    Saline soils Retards decay in organic material

    Severe reactions with copper, iron, and silver
    Natural events such as earthquakes and volcanoes Damage and move artifacts; changes the landscape of the site Animals such as dogs, hyenas, and rodents. Scavenge bones and other organic matter from abandoned sites

    CONDITIONS OF DECOMPOSITION

    Organic remains including skeletal, animal, plant,and textiles, all suffer from decomposition, and their survival depends largely on the material surrounding them, called the matrix, and on the climate. In terms of the matrix, chalk tends to preserve bones quite well because it acts as a desiccant (drying agent), removing all of the moisture from objects within it. Acidic soils, on the other hand, tend to destroy bones and wooden artifacts quite quickly. Other conditions of the matrix that can slow down decomposition and promote preservation include natural salt and oil pools that kill bacteria. A famous example in California is the La Brea Tar Pits located in central Los Angeles. This naturally occurring asphalt pool has preserved more than 35 million specimens of plants and animals from the last 50,000 years!

    USA_tar_bubble_la_brea_CA.jpg
    Figure \(\PageIndex{1}\):Tar Bubbles at the La Brea Tar Pits in Los Angeles, California
    512px-LA_BREA_TAR_PITS,_LOS_ANGELES.jpg
    Figure \(\PageIndex{2}\): Tar pits preserve animal and plant remains.

    Although stone can crack and weather, it is inherently more durable compared to organic materials used for basketry, textiles, and tools. Artifacts made of wood, papyrus, and bone, for example, are perishable and prone to rapid decomposition (rot and decay), making them difficult to recover. Decomposition occurs when organisms such as molds and bacteria inhabit and often consume organic material. Three conditions are required for microorganisms to survive: warmth, moisture, and oxygen. Without all of these conditions present, the environment is sterile and decomposition will not occur, greatly increasing the potential for organic artifacts to be preserved. Therefore, a sterile environment that is free from microorganisms provides the best conditions for the preservation of a wide variety of archaeological remains.

    sterile environment

    An environment free from microorganisms.

    La Brea Tar Pits

    Today, we use asphalt for things like paving roads and parking lots, but humans have been using asphalt for up to 40,000 years, which is supported by evidence such as bitumen-coated flint implements from the Mousterian period (Harris, 2015). However, asphalt is not only used for tools and highways, tar pits create the conditions for the preservation of the organic remains of prehistoric life. After the animals were trapped in the tar pits, the skin and flesh decomposed. However, the bones would quickly sink into the asphalt's oxygen-deprived environment where they began the fossilization process. The asphalt was absorbed into the bones and created the fossils we find today. The world's best-known deposit of asphaltic fossils is located in Los Angeles, California, at the La Brea Tar Pits. The tar pits are naturally occurring deposits of asphalt (or bitumen), which have been preserving the remains of trapped animals for thousands of years, including the bones of a young woman dating to about 9,000 BC.

    fossils

    The preserved remains or traces of remains of ancient humans, animals, and plants.

    The tar pits at La Brea were used by both Native Americans and European settlers. They were aware that animals would get trapped and die in the pits, but it was in 1875 that they discovered that the pits contained the fossilized remains of prehistoric times as well (Harris, 2015; La Brea Tarpits History). However, systematic recovery of the fossils did not occur until the early 20th century. At first, they focused on fossils of the megafauna (large mammals of the region), but by 1969 they began to study and collect the remains of smaller animals and flora (plants of the region) (Harris, 2015; La Brea Tarpits History). Since 1906, over a million bones have been recovered from the tar pits with 231 species of vertebrates, 159 species of plants, and 234 species of invertebrates (La Brea Tar Pits and Hancock Park).

    Interactive Element


    Visit their website to learn more about La Brea Tar Pits.


    REFERENCES

    Fagan, B. M. (2006). Archaeology: A brief introduction. Upper Saddle River, NJ: Pearson Prentice Hall.

    Harris, J. M. (2015). La Brea and Beyond: The Paleontology of Asphalt-Preserved Biotas. Los Angeles, Calif: Natural History Museum of Los Angeles County.

    La Brea Tar Pits and Hancock Park. La Brea Tar Pits. (n.d.). https://tarpits.org/experience-tar-pits/la-brea-tar-pits-and-hancock-park.

    La Brea Tar Pits History. La Brea Tar Pits. (n.d.). https://tarpits.org/la-brea-tar-pits-history.


    Images

    Figure 5.1.1 Tar Bubbles at the La Brea Tar Pits. (2004). By Daniel Schwen under CC BY-SA 2.5 via Wikimedia Commons.

    Figure 5.1.2 Lake Pit with Life-size models of mammoths with one entrapped in the tar. (2008). By Jerrye and Roy Klotz MD under CC BY-SA 3.0 via Wikimedia Commons.


    A derivative work from

    "Digging into Archaeology: A Brief OER Introduction to Archaeology with Activities" by Amanda Wolcott Paskey and AnnMarie Beasley Cisneros, Faculty (Anthropology) at Cosumnes River College & American River College, ASCCC Open Educational Resources Initiative (OERI), 2020, under CC BY-NC 4.0.


    5.1: Deterioration and Decomposition is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.