Isotope analysis

Isotope analysis is the identification of isotopic signature, the distribution of certain stable isotopes and chemical elements within chemical compounds. This can be applied to a food web to make it possible to draw direct inferences regarding diet, trophic level, and subsistence.

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Oxygen isotopes

Oxygen Isotopes and their Relative Abundances:

¹⁶O = 99.763%

¹⁷O = 0.0375%

¹⁸O = 0.1995%

Present in the ratios above, oxygen atoms of all isotopes are incorporated in to molecules , including water. All isotopes of oxygen have similar properties, but water that incorporates ¹⁶O isotopic oxygen evaporates preferentially to water with the ¹⁸O isotope.

In isotopic analysis, the absolute abundances of isotopic oxygen are not considered. Rather, the ratio of ¹⁸O to ¹⁶O in the sample is compared to the ratio in a standard (VSMOW – Vienna Standard Mean Ocean Water) using the equation:

The values are reported in permil units (permil = per mille = per thousand) using the symbol ‰. While the differences between samples and the standard may appear small, a difference of even 1 permil is significant.

Variation by latitude

As moist air masses are carried away from the equator by the prevailing weather patterns they lose the heavier, more easily condensed, ¹⁸O water leading to lower and lower isotopic oxygen ratios toward the poles. Consequently, the amount of ¹⁶O relative to ¹⁸O in the water vapour becomes less and less as it approaches the poles, losing ¹⁶O water in the form of rain and snow.

Weighted Annual ¹⁸O in Global Precipitation [Source: GNIP-IAEA, 2002]

Variation occurring from the hydrological cycle

The ratios of isotopic oxygen are also differentially affected by global weather patterns and regional topography as moisture is transported. Areas of lower humidity cause the preferential loss of ¹⁸O water in the form of vapour and precipitation. Furthermore, evaporated ¹⁶O water returns preferentially to the atmospheric system as it evaporates and ¹⁸O remains in liquid form or is incorporated into the body water of plants and animals.

Schematic overview of the hydrological cycle showing approximate depletions in d¹⁸O [Source: GNIP, IAEA, 1996; Modified: Wade, 2006].

Tissues affected

Isotopic oxygen is incorporated into the body primarily through ingestion at which point it is used in the formation of, for archaeological purposes, bones and teeth. The oxygen is incorporated into the hydroxylcarbonic apatite of bone and tooth enamel.

Bone is continually remodelled throughout the lifetime of an individual. Although the rate of turnover of isotopic oxygen in hydroxyapatite is not fully known, it is assumed to be similar to that of collagen; approximately 10 years. Consequently, should an individual remain in a region for 10 years or longer, the isotopic oxygen ratios in the bone hydroxyapatite would reflect the oxygen ratios present in that region.

Teeth are not subject to continual remodelling and so their isotopic oxygen ratios remain constant from the time of formation. The isotopic oxygen ratios, then, of teeth represent the ratios of the region in which the individual was born and raised. Where deciduous teeth are present, it is also possible to determine the age at which a child was weaned. Breast milk production draws upon the body water of the mother, which has higher levels of ¹⁸O due to the preferential loss of ¹⁶O through sweat, urine, and expired water vapour.

While teeth are more resistant to chemical and physical changes over time, both are subject to post-depositional diagenesis. As such, isotopic analysis makes use of the more resistant phosphate groups, rather than the less abundant hydroxyl group or the more likely diagenetic carbonate groups present.

Applications

Anthropology

Bone recovered from archaeological sites can be analysed isotopically for information regarding diet and migration. Carbon and nitrogen isotope composition are used to reconstruct diet, and oxygen isotopes are used to determine geographic origin.

To obtain an accurate picture of palaeodiets, it is important to understand processes of diagenesis that may affect the original isotopic signal.

Archaeology/Anthropology

Bone recovered from archaeological sites can be analysed isotopically for information regarding diet and migration. Tooth enamel and soil surrounding or clinging to the remains may also be used in isotopic analysis. Carbon and nitrogen isotope composition are used to reconstruct diet, and oxygen isotopes are used to determine geographic origin. Strontium isotopes in teeth and bone can be used to determine migration and human movement.

The isotopes are imbued into the fauna during its lifetime through eating, drinking and particles inhaled. This process ends with the organism's death, from this point on isotopes no longer accumulate in the body, but do undergo degradation. For best result the researcher would need to know the original leaves, or an estimation there of, of isotopes in the organism at the time of its death.

To obtain an accurate picture of palaeodiets, it is important to understand processes of diagenesis that may affect the original isotopic signal. It is also important for the researcher to know the variations of isotopes with in individuals, between individuals, and over time.

Ecology

All biologically active elements exist in a number of different isotopic forms, of which two or more are stable. For example most carbon is present as ¹²C, with approximately 1% being ¹³C. The ratio of the two isotopes may be altered by biological and geophysical processes, and these differences can be utalised in a number of ways by ecologists. The main elements used in isotope ecology are Carbon, Nitrogen, Oxygen, Hydrogen and Sulphur.

Forensics

Isotope analysis can be used by forensic investigators to determine whether two or more samples of explosives are of a common origin. Most high explosives contain carbon, hydrogen, nitrogen and oxygen atoms and thus comparing their relative abundances of isotopes can reveal the existence of a common origin. Researchers have also shown that analysis of the ¹²C/¹³C ratios can locate the country of origin for a given explosive.

Geology

Main article: Isotope geochemistry

Hydrology

Paleoclimatology

Photosynthesis

References