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 (PDB)
standard: :\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
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 (CAM) plants, where the effect is similar but less pronounced than with C4 plants. Isotopic fractionation in plants is caused by physical (slower diffusion of 13C in plant tissues due to increased atomic weight) and biochemical (preference of 12C by two enzymes:
RuBisCO and
phosphoenolpyruvate carboxylase) 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 (
rice,
wheat,
soybeans,
potatoes) or C4 plants (
corn, or
corn-fed beef) by
isotope analysis of their flesh and bone collagen (however, to obtain more accurate determinations, carbon isotopic fractionation must be also taken into account, since several studies have reported significant 13C discrimination during biodegradation of simple and complex substrates). 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 (i.e. tree ring δ13C) 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:
Limestones formed by precipitation in
seas from the atmospheric carbon dioxide contain normal proportion of 13C. Conversely,
calcite found in
salt domes originates from carbon dioxide formed by
oxidation of
petroleum, which due to its plant origin is 13C-depleted. The layer of limestone deposited at the Permian extinction 252 Mya can be identified by the 1% drop in 13C/12C.
Nitrogen isotopes Nitrogen-15, 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 (particularly
fertilizers) 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 (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. 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 (~0.04% abundant). 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 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 (organic and inorganic) are usually measured with
pyrolysis and
mass spectrometry. Improper or prolonged storage of samples can lead to inaccurate measurements. as calculated by the following equation: \delta \ce{^{34}S}_\mathrm{sample} = \left(\frac{\ce{^{34}S/^{32}S}_\ce{sample}}{\ce{^{34}S/^{32}S}_\mathrm{standard}} - 1\right) \cdot 1000 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. ==Radiogenic isotopes==