Chemical Weathering of Bone in Archaeological Soils

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Abstract

Weathering of hydroxyapatite, Ca5(PO4)3(OH) in bone probably is initiated by organic and carbonic acids formed by the microbial decomposition of collagen. This weathering, independent of soil properties, is caused by protons replacing Ca from hydroxyapatite. As collagen is depleted, proton production decreases and weathering may either continue if protons are available from the soil or be arrested if Ca from the soil displaces the protons previously added to the hydroxyapatite. The theoretical Ca/P weight ratio of unweathered bones is 2.15. Weathered bones that have been stabilized by Ca may have this ratio or a higher one if extra Ca has been added. A group of weathered bones from one site with a slightly acid soil had an average ratio of 1.67, which probably promotes further weathering, while bone at the same site with an average ratio of 4.09 was less weathered and apparently stabilized.

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Taphonomic and ecologic information from bone weathering

Anna K. Behrensmeyer (1978)

Bones of recent mammals in the Amboseli Basin, southern Kenya, exhibit distinctive weathering characteristics that can be related to the time since death and to the local conditions of temperature, humidity and soil chemistry. A categorization of weathering characteristics into six stages, recognizable on descriptive criteria, provides a basis for investigation of weathering rates and processes. The time necessary to achieve each successive weathering stage has been calibrated using known-age carcasses. Most bones decompose beyond recognition in 10 to 15 yr. Bones of animals under 100 kg and juveniles appear to weather more rapidly than bones of large animals or adults. Small-scale rather than widespread environmental factors seem to have greatest influence on weathering characteristics and rates. Bone weathering is potentially valuable as evidence for the period of time represented in recent or fossil bone assemblages, including those on archeological sites, and may also be an important tool in censusing populations of animals in modern ecosystems.

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Apatite crystallites: effects of carbonate on morphology.

R Z LeGeros, O Trautz, J P LeGeros … (1967)

Carbonate is a substituent in the apatite structure; when present, it limits the size of the growing apatite crystals and so influences their shape that they grow more equiaxed than needle-like. The tendency for carbonate apatites to be equiaxed is related to the nature of the chemical bonds formed in the crystal. The interference of carbonate with the good crystallization of apatite, and its weakening effect on the bonds in the structure, increase the dissolution rate and the solubility, thereby presumably contributing to the susceptibility to caries of dental apatites containing carbonate.