Isotopic signature
An isotopic signature is a ratio of non-radiogenic stable isotopes, stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample material are measured by isotope-ratio mass spectrometry against an isotopic reference material. This process is called isotope analysis.
Stable isotopes
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 methane have different affinity for the 12C and 13C isotopes, which allows distinguishing between different sources by the 13C/12C ratio in methane in the air. In geochemistry, paleoclimatology and paleoceanography this ratio is called δ13C. The ratio is calculated with respect to Pee Dee Belemnite standard:Similarly, carbon in inorganic carbonates shows little isotopic fractionation, while carbon in materials originated by photosynthesis is depleted of the heavier isotopes. In addition, there are two types of plants with different biochemical pathways; the C3 carbon fixation, where the isotope separation effect is more pronounced, C4 carbon fixation, where the heavier 13C is less depleted, and Crassulacean Acid Metabolism plants, where the effect is similar but less pronounced than with C4 plants. Isotopic fractionation in plants is caused by physical and biochemical factors. The different isotope ratios for the two kinds of plants propagate through the food chain, thus it is possible to determine if the principal diet of a human or an animal consists primarily of C3 plants or C4 plants by isotope analysis of their flesh and bone collagen.
Within C3 plants processes regulating changes in δ13C are well understood, particularly at the leaf level, but also during wood formation. Many recent studies combine leaf level isotopic fractionation with annual patterns of wood formation to quantify the impacts of climatic variations and atmospheric composition on physiological processes of individual trees and forest stands. The next phase of understanding, in terrestrial ecosystems at least, seems to be the combination of multiple isotopic proxies to decipher interactions between plants, soils and the atmosphere, and predict how changes in land use will affect climate change.
Similarly, marine fish contain more 13C than freshwater fish, with values approximating the C4 and C3 plants respectively.
The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:
- C4 plants:
- CAM plants:
- C3 plants:
Nitrogen isotopes
, or 15N, is often used in agricultural and medical research, for example in the Meselson–Stahl experiment to establish the nature of DNA replication. An extension of this research resulted in development of DNA-based stable-isotope probing, which allows examination of links between metabolic function and taxonomic identity of microorganisms in the environment, without the need for culture isolation. Proteins can be isotopically labelled by cultivating them in a medium containing 15N as the only source of nitrogen, e.g., in quantitative proteomics such as SILAC.Nitrogen-15 is extensively used to trace mineral nitrogen compounds in the environment. When combined with the use of other isotopic labels, 15N is also a very important tracer for describing the fate of nitrogenous organic pollutants. Nitrogen-15 tracing is an important method used in biogeochemistry.
The ratio of stable nitrogen isotopes, 15N/14N or δ15N, tends to increase with trophic level, such that herbivores have higher nitrogen isotope values than plants, and carnivores have higher nitrogen isotope values than herbivores. Depending on the tissue being examined, there tends to be an increase of 3-4 parts per thousand with each increase in trophic level. The tissues and hair of vegans therefore contain significantly lower δ15N than the bodies of people who eat mostly meat. Similarly, a terrestrial diet produces a different signature than a marine-based diet. Isotopic analysis of hair is an important source of information for archaeologists, providing clues about the ancient diets and differing cultural attitudes to food sources.
A number of other environmental and physiological factors can influence the nitrogen isotopic composition at the base of the food web 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. This leads to relatively high δ15N values in plants and animals in hot and arid ecosystems relative to cooler and moister ecosystems. 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.
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".
Oxygen isotopes
Oxygen occurs naturally in three variants, but 17O is so rare that it is very difficult to detect. The ratio of 18O/16O in water depends on the amount of evaporation the water experienced. 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 is incorporated into the shells of calcium carbonate-secreting organisms, such sediments provide a chronological record of temperature and salinity of the water in the area.The oxygen isotope ratio in the 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.
The oxygen isotopic signatures of solid samples are usually measured with pyrolysis and mass spectrometry. Improper or prolonged storage of samples can lead to inaccurate measurements.
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. Sulfur isotope fractionations are usually measured in terms of δ34S due to its higher abundance 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 Canyon Diablo troilite standard, which has a 32S:34S equal to 22.220, was used as both a reference material and the zero point for the isotopic scale. Since 1993, the Vienna-CDT standard has been used as a zero point, and there are several materials used as reference materials for sulfur isotope measurements. Sulfur fractionations by natural processes measured against these standards have been shown to exist between −72‰ and +147‰, as calculated by the following equation:
| Natural Source | δ34S range |
| Petroleum | |
| River water | |
| Lunar rocks | |
| Meteorites | |
| Ocean water |
| Isotope | Abundance | Half-life |
| 32S | 94.99% | Stable |
| 33S | 0.75% | Stable |
| 34S | 4.25% | Stable |
| 35S | <0.1% | 87.4 days |
| 36S | 0.01% | Stable |
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.