Environments that are conducive to saprophytic organisms are not conducive to biological preservation and vice versa (Table 1). Carbone and Keel in 1985 listed four environmental factors that influence preservation of biological assemblages: soil acidity, aeration, relative humidity, and temperature. Saprophytic organisms are intolerant of highly acidic soils and live almost exclusively in alkaline soils. Therefore, acidic soils will tend to preserve organic components of biological materials, whereas alkaline soils will tend to have poor biological preservation of organics due to increased saprophytic activity.
An example of this is pollen preservation in the southwestern United States. Because soils in the Southwest are highly alkaline, preservation of organic materials in open areas tends to be rare. Bryant and colleagues in 1994 analyzed 509 pollen samples from soil collected by a CRM firm along a proposed pipeline route in deposits ranging in age from 1000 to 5000 years old. The deposits all had a pH value above 6.0 with anhydrous carbonates as the most common compounds. Only 243 (48%) of the samples contained a significant amount of fossil pollen with a mean pollen concentration value of 6 545 grains/gram of soil. As a control, 90 modern surface soil samples were collected in west Texas, and area where the soil is less alkaline than in the Southwest. All of the samples contained a significant amount of pollen with an average concentration value of 21311 grains/gram of soil. Pollen has an outer covering (exine) made of sporopollenin, one of the strongest natural substances known. However, alkaline soils are conducive to fungi and bacteria which eat pollen, creating a biased array in which pollen with more sporopollenin in its exine is better preserved than pollen with less.
However, it is well known that preservation of biological remains is usually excellent in the southwestern United States due to the dry/arid conditions which do not allow saprophytes to thrive even though the soil is alkaline. Preservation is mainly found in enclosed areas (caves, rockshelters, pueblos, etc.) rather than in the open areas because the latter have increased exposure to weathering (wind, erosion, rain).
Conversely, alkaline soils tend to preserve mineral components better than acidic soils. Bone is made up of minerals (hydroxyapatite, calcium carbonate, trace elements) and organics (collagen, bone protein, fats, lipids) in an approximately 2:1 ratio. The organic components of bone will tend to be eaten by saprophytes in alkaline soils, leaving mineral bone components intact. Therefore, the mineral (structural) components of bone are preserved and can be recovered in alkaline conditions. Bone tends not to survive in acidic conditions because acids dissolve structural bases of minerals.
While alkaline soils tend to destroy organic assemblages (plant remains and organic components of bone) due to saprophytic activity, such soil types tend to preserve the mineral components of bone and shell better than acidic soils. For example, soils in Maine tend to be acidic, which limits the preservation potential of the mineral component of bone. When bone is preserved at interior sites it has been calcined. Although calcined bone is preserved in interior sites with acidic soils, bone (mainly uncalcined) is prevalent at archaeological shell midden sites along the coast. Bone preserves in these sites because weathering and degradation of the calcareous shell matrix produces an alkaline environment conducive to preservation of mineral components of bone. Therefore, bone preservation at archaeological sites in Maine depends upon site location and soil alkalinity.
Preservation of biological remains is also influenced by mechanical destruction through seasonal freeze/thaw cycles, increased biological preservation in dry/arid, or dry/frozen regions, and anaerobic environments which decrease saprophytic activity therefore increasing preservation. For example, although soils in Maine are acidic and tend not to preserve mineral components of bone, such acidic soils, because they inhibit saprophytic organisms, should preserve other organics like botanical remains. This is not the case, however, because of the seasonal freezing/thawing cycle which mechanically destroys chemical composition. Preservation of organics in this type of environment can be achieved when they have been carbonized or burned making them more structurally durable. Cold temperatures also limit the decay of biological remains because saprophytes do not live in such an extreme, cold environment. Biological assemblages in the Arctic can be as well preserved as in dry/arid regions.
Saprophytic organisms cannot live without oxygen, so anaerobic (lacking oxygen) environments will be conducive to biological preservation. Such environments include peat bogs, which are famous for preservation of ‘bog bodies’ and other biological remains, and waterlogged sites (see Sites: Waterlogged), where wood is commonly preserved, such as stakes from prehistoric fish weirs. Relatively anaerobic conditions also exist under thick layers of clay or silt deposits, a good environment for preservation of biological materials. Depth of deposit is an important component of preservation in such environmental conditions: the deeper material is buried, the more anaerobic the environment is, and the better the preservation potential.
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