For example, a kinetic model is used to estimate the age of an undated sample by extrapolating beyond a calibration point, or a calibration curve developed for one thermal regime is applied to another using a kinetic model to adjust the reaction rate for differences in site temperature. F., 1993, Applications of organic geochemistry for Quaternary research; Aminostratigraphy and aminochronology, in Engel, M. A., eds., Organic Geochemistry: New York, Plenum Press, 755-783.
Because the extent of racemization is dependent on both temperature and time, the average postdepositional temperature ("effective diagenetic temperature"; Wehmiller, 1977) can be determined from the D/L ratio if the sample age is known independently. Amino acid studies of the Del Mar, California, midden site—Apparent rate constants, ground temperature models, and chronological implications.
This relation, together with a model of racemization kinetics, is used to calculate the age of a sample if its D/L ratio and temperature history are known. Aminostratigraphic dating methods in Quaternary geology, in Noller, J.
The extent to which these reactions have progressed in a fossil is proportional to the length of time elapsed since the organism lived and to the ambient temperature of the reaction medium.
The most reliable of the complex network of diagenetic reactions is amino acid racemization, which involves the interconversion of L-amino acids to their D-isomeric configurations.
Most living organisms use amino acids exclusively of the L-configuration.
Upon death and removal of biologic constraints, the L-amino acids begin to racemize to their D-configuration.
The compound then alternates between each form while the ratio between the ( ) and () groups approaches 1:1, at which point it becomes optically inactive.
Since ( ) and () are experimental parameters, the absolute configuration of atoms in chiral molecules are commonly described using R and S, from the Latin rectus (right-handed) and sinistrus (left-handed).
Fossil remains of biogenic minerals contain trace quantities of indigenous organic matter, which is preserved for long periods of geologic time.
After synthesis by an organism, proteins and their constituent amino acids degrade through a complex series of biogeochemical reactions (Mitterer, 1993).
Chemical Whale Tales Since 1981, whale hunters in Alaska have found at least six harpoon heads in bowhead whales that they have killed in the Beaufort Sea, southwest of the Arctic Ocean. Anthropologists at the Smithsonian Institution (Washington, DC) estimate that they are between 130 and 200 years old and were probably used by Inupiat hunters in the late 1700s.
This circumstantial evidence led Craig George and fellow researchers at the Department of Wildlife Management (North Slope Borough, Barrow, AK) to believe that bowhead whales, named for their exaggerated jaw curve, could have a life span of more than 200 years.
For ion exchange analysis, the minimum sample size for each shell (or group of shells or shell fragments) is ~20 mg.