Isotopic signature

An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic '

The atomic mass of different isotopes affect their chemical kinetic behavior, leading to natural isotope separation processes.

Carbon isotopes

For example, different sources and sinks of

${\displaystyle \delta {\ce {^{13}C}}_{\mathrm {sample} }=\left({\frac {{\ce {^{13}C/^{12}C}}_{{\ce {sample}}}}{{\ce {^{13}C/^{12}C}}_{\mathrm {standard} }}}-1\right)\cdot 1000}$ ‰

Similarly, carbon in inorganic

The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:^[11]

• C₄ plants: -16 to -10 ‰
• CAM plants: -20 to -10 ‰
• C₃ plants: -33 to -24 ‰

Nitrogen-15 is extensively used to trace

A number of other environmental and physiological factors can influence the nitrogen isotopic composition at the base of the food web (i.e. in plants) or at the level of individual animals. For example, in arid regions, the nitrogen cycle tends to be more 'open' and prone to the loss of ¹⁴N, increasing δ¹⁵N in soils and plants.^[19] This leads to relatively high δ¹⁵N values in plants and animals in hot and arid ecosystems relative to cooler and moister ecosystems.^[20] Furthermore, elevated δ¹⁵N have been linked to the preferential excretion of 14N and reutilization of already enriched 15N tissues in the body under prolonged water stress conditions or insufficient protein intake.^[21][22]

δ¹⁵N also provides a diagnostic tool in planetary science as the ratio exhibited in atmospheres and surface materials "is closely tied to the conditions under which materials form".^[23]

Oxygen isotopes

Oxygen comes in three variants, but the ¹⁷O is so rare that it is very difficult to detect (~0.04% abundant).^[24] The ratio of ¹⁸O/¹⁶O in water depends on the amount of evaporation the water experienced (as ¹⁸O is heavier and therefore less likely to vaporize). As the vapor tension depends on the concentration of dissolved salts, the ¹⁸O/¹⁶O ratio shows correlation on the salinity and temperature of water. As oxygen gets built into the shells of calcium carbonate secreting organisms, such sediments provide a chronological record of temperature and salinity of the water in the area.

Oxygen isotope ratio in atmosphere varies predictably with time of year and geographic location; e.g. there is a 2% difference between ¹⁸O-rich precipitation in Montana and ¹⁸O-depleted precipitation in Florida Keys. This variability can be used for approximate determination of geographic location of origin of a material; e.g. it is possible to determine where a shipment of uranium oxide was produced. The rate of exchange of surface isotopes with the environment has to be taken in account.^[25]

The oxygen isotopic signatures of solid samples (organic and inorganic) are usually measured with pyrolysis and mass spectrometry.^[26] Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.^[26]

Sulfur Isotopes

Sulfur has four stable isotopes, ³²S, ³³S, ³⁴S, and ³⁶S, of which ³²S is the most abundant by a large margin due to the fact it is created by the very common ¹²C in supernovas. Sulfur isotope ratios are almost always expressed as ratios relative to ³²S due to this major relative abundance (95.0%). Sulfur isotope fractionations are usually measured in terms of δ³⁴S due to its higher abundance (4.25%) compared to the other stable isotopes of sulfur, though δ³³S is also sometimes measured. Differences in sulfur isotope ratios are thought to exist primarily due to kinetic fractionation during reactions and transformations.

Sulfur isotopes are generally measured against standards; prior to 1993, the

As a very redox-active element, sulfur can be useful for recording major chemistry-altering events throughout Earth's history, such as marine evaporites which reflect the change in the atmosphere's redox state brought about by the Oxygen Crisis.^[31][32]

Radiogenic isotopes

Lead isotopes

Lead consists of four stable

In archaeological studies, stable isotope ratios have been used to track diet within the time span formation of analyzed tissues (10–15 years for bone collagen and intra-annual periods for tooth enamel bioapatite) from individuals; "recipes" of foodstuffs (ceramic vessel residues); locations of cultivation and types of plants grown (chemical extractions from sediments); and migration of individuals (dental material).^[citation needed]

Forensics

With the advent of stable

A study was published demonstrating the possibility of determination of the origin of a common brown

Nuclear explosions form ¹⁰Be by a reaction of fast neutrons with ¹³C in the carbon dioxide in air. This is one of the historical indicators of past activity at nuclear test sites.^[36]

Solar System origins

Isotopic fingerprints are used to study the origin of materials in the Solar System.

Sulfur isotope evidence has also been used to corroborate the timing of the Great Oxidation Event, during which the Earth's atmosphere experienced a measurable rise in oxygen (to about 9% of modern values^[45]) for the first time about 2.3-2.4 billion years ago. Mass-independent sulfur isotope fractionations are found to be widespread in the geologic record before about 2.45 billion years ago, and these isotopic signatures have since ceded to mass-dependent fractionation, providing strong evidence that the atmosphere shifted from anoxic to oxygenated at that threshold.^[46]

Modern

• Isoscapes
• Isotope electrochemistry
• Isotope geochemistry
• Radiometric dating

References