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ARCHAEOLOGICALArchaeoastronomyBIOLOGICALPackrat Middens
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Dating Techniques
Author: Darrell Kaufman
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Dr. Darrell Kaufman of the Northern Arizona University Geology Department loading samples onto a High Performance Liquid Chromatograph (HPLC) for amino acid racemization (see table below). |
Two equally important data sets are required to reconstruct land-cover changes over long time scales: (1) the biological or physical data used to infer past environments, and (2) the chronological data required to place the environmental changes into a temporal framework. Accurate age control for biological and geological evidence of land-cover change is needed to determine when the changes of interest took place and at what rate. Consequently, much effort has been devoted to developing dating techniques. The premier methods applicable to materials from the Holocene epoch (the last 10,000 years) are radiocarbon (14C) and dendrochronology. Where trees and other organic material are lacking, other techniques are employed, including: short-lived isotopes (210Pb, 137Cs); luminescence; amino acid racemization; obsidian hydration; tephrochronology; paleomagnetism; and lichenometry.
Summary of dating methods applicable to the Holocene (modified, in part, from Colman and Pierce, 1997):
| Method | Basis | Applicability | Notes |
| Lead-210 (210Pb) | Accumulation and decay rates of 210Pb produced in the atmosphere by the decay of 222Rn | Lacustrine and marine sediment less than ~100 years old | Dates time of sediment deposition; subject to uncertainties in flux, accumulation and bioturbation rates |
| Cesium-137 (137Cs) | 137Cs fallout produced during above-ground nuclear testing | Water-lain sediment deposited during the last few decades | Used to assess deposit age in relation to peak of 137Cs fallout in the early 1960's |
| Luminescence | Electrons generated by ionizing radiation accumulate in mineral crystals following heating or exposure to light; amount of light released during laboratory treatment is compared with that released after a know radiation dose | Fired ceramics; wind-deposited sediment; soils | Dates time of last exposure to light or heat; both optical and thermal techniques are in practice |
| Amino acid racemization | Chemical reactions that involve proteins within fossils; the primary reaction for geochronological proposes is the inversion of L-amino acids to D-amino acids, known as racemization | Carbonate shells are most reliable (snails, clams, ostracodes, foraminifera); application to Holocene requires use of rapidly racemizing amino acids, particularly aspartic acid | Rates of reactions depend on temperature; generally used for relative-age determinations; yields numerical ages when calibrated by other techniques; can also be used to estimate past temperature changes for samples whose ages are known |
| Obsidian hydration | Thickness of hydrated layer on the surface of obsidian increases with age | Obsidian | Dates time of surface formation; generally used for relative-age determinations; yields numerical ages when calibrated by other techniques |
| Tephrochronology | Volcanic ash identified by chemical or petrographic techniques provides marker horizon | Requires a trap for volcanic ash; generally proximal to volcanoes | Outstanding for correlation because ash erupts nearly instantaneously and because deposits can be widespread; age resolution depends on accuracy of independently dated eruptions |
| Paleomagnetism | Depends on correlation of the temporal changes in the magnetic vector with a known chronology of magnetic variation | Lacustrine, marine, or wind-blow deposits | Requires a reference for correlation; subject to errors due to magnetic overprinting and physical disturbance |
| Lichenometry | Lichen thallus grows systematically in proportion to age | Exposed, stable rock suitable for lichen growth | Dates time of surface stabilization; requires calibration for numerical ages; subject to errors due to factors that control growth rate: climate, taxonomy, and rock type |
See also:
Dendrochronology
Fire Scars
Radiocarbon
Dating
Resources:
Bradley, R. S. 1999. Paleoclimatology: Reconstructing climates of the Quaternary. Second edition. International Geophysics Series, volume 68. Academic Press, San Diego, CA, 613 pp.
Colman, S. M., and Pierce, K. L. 1977. Summary table of Quaternary dating techniques. Miscellaneous Field Studies Map, volume MF-904. U.S. Geological Survey.
Easterbrook, D. J., editor. 1988. Dating Quaternary sediments. Geological Society of America Special Paper 227, Boulder, CO, 165 pp.
Rutter, N. W. and Catto, N. R., editors. 1995. Dating methods for Quaternary deposits. Geological Association of Canada, Newfoundland, 308 pp.