Chapter 2: Radiocarbon Dating and its Problems

 

Which problems does one face, when trying to radiocarbon date a certain fossil correctly? How reliable are these datings?

 

Thomas W. Stafford, Jr., is at the Center of Geochronological Research INSTAAR, University of Colorado, at Boulder, Colorado. He reports about his findings in: Late Pleistocene Megafauna Extinctions and the Clovis Culture: Absolute Ages Based on Accelerator 14C Dating of Skeletal Remains, in: Megafauna & Man, 1990, L. D. Agenbroad et al. (eds.), Hot Springs, South Dakota:

 

“Bone 14C dates are often in error of 2,000 to 5,000 years... The difficulty in working with fossil bone is that it is a complex matrix that becomes increasingly more heterogeneous over geological time. The inorganic mineral phase of bone, carbonate hydroxyapatite (dahlite), contains natural amounts of CO3-2 that exchanges with groundwater, soil, and sediment-derived carbonates, thereby contaminating its 14C content. The remaining 20% (by weight) of bone is organic matter, predominantly the protein collagen, plus minor amounts of other proteins (e.g. osteonectin and osteocalcin), lipids, and peptides.

 

Over geologic time, these organic constituents lose chemical properties that make them easily separable, the proteins denature, hydrolyze, and are leached from the bone, and finally, substantial amounts of foreign humates and peptides enter the bone from the surrounding sediments and soils. The result is a complex and heterogenous organic chemistry.” (1990:118).

 

“Only when the XAD-purified phases are used is consistent accuracy attained. Fractions such as total collagen, base-leached collagen, and gelatin are the fractions that have been and continue to be used for 14C dating of bone. Dates on these fractions should be considered questionable because it is not possible to know, when their 14C dates may be valid or invalid in the examples from the Domebo and Dent mammoths, respectively.” - Stafford, T. W. (1990:119).

 

“Radiocarbon dating of individual amino acids from the Del Mar fossil bone and the Tepexpan human fossil corroborated the conclusion that poorly preserved fossils are contaminated at the molecular level. ... The results indicate why an age determination on a single chemical fraction is no absolute proof-of-age, even if the dated fraction is a specific amino acid.” (1991:54, 55).

 

“Collagenous bones yield accurate 14C dates because chemical pretreatment will eventually isolate a fraction that is free of contaminants. In contrast, noncollagenous bones should not be dated by 14C because foreign organic matter consists of a large proportion to 100% of the bone’s organic carbon content. No pretreatment step is presently known that will yield an absolute 14C date for poorly preserved bones.

 

“The preponderance of erroneous 14C dates on collagenous bone result from using a chemical fraction that retains foreign carbon, usually humic and fulvic acids. Heterogenous and chemically impure fractions such as acid-insoluble-collagen, gelatin, or base-leached collagen are very likely to contain humate contaminants. Dating results from the Domebo and Dent mammoths, and the Del Mar and Anzick human fossils are evidence that these poorly characterized chemical fractions can date accurately for some fossils and be thousands of years in error for others. Only by dating highly purified hydrolysates, individual amino acids, or both, can consistent accuracy be achieved for collagenous fossils.

 

“In contrast, non-collagenous bones have proven undatable by existing techniques. The reason is that they contain little if any endogenous protein. The only fraction that has any value from non-collagenous bones is the ‘humin’ fraction, which comprises high molecular weight residues that are resistent to acid hydrolysis. The ‘humin’ residues may be partly derived from endogenous amino acids and will provide a minimum age for the bone.” - Stafford, T. W. et al. (1991:65).

 

“The most important result from our experiments is that no one molecular fraction or pretreatment protocol will guarantee the accuracy of a 14C measurement. Sedimentary environments, geochemical cycles and diagenetic processes are too complex to dictate any one dating or pretreatment protocol. Molecular-level 14C dating must be combined with high-quality litho- and biostratigraphic data to ensure that as many errors as possible are removed.” - Stafford, T. W. et al. (1991:61).