An isotopic signature (also isotopic fingerprint) 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
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
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
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
,
paleoclimatology
Paleoclimatology ( British spelling, palaeoclimatology) is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of ...
and
paleoceanography
Paleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation and biological productivity. Paleoceanographic studies using environment model ...
this ratio is called
δ13C. The ratio is calculated with respect to
Pee Dee Belemnite (PDB)
standard:
:
‰
Similarly, carbon in inorganic
carbonate
A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
s shows little isotopic fractionation, while carbon in materials originated by
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
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 C
4 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 C
3 plants (
rice
Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...
,
wheat
Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
,
soybeans
The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed.
Soy is a key source of f ...
,
potatoes) or C
4 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 δ
13C 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 δ
13C) 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 C
4 and C
3 plants respectively.
The ratio of carbon-13 and carbon-12 isotopes in these types of plants is as follows:
* C
4 plants:
* CAM plants:
* C
3 plants:
Limestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
s formed by precipitation in
seas from the atmospheric carbon dioxide contain normal proportion of
13C. Conversely,
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
found in
salt domes originates from carbon dioxide formed by
oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
of
petroleum
Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...
, 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
Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created f ...
and
medical
Medicine is the science and Praxis (process), practice of caring for patients, managing the Medical diagnosis, diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, ...
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
Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the ...
function and
taxonomic identity of
microorganisms in the environment, without the need for
culture
Culture ( ) is a concept that encompasses the social behavior, institutions, and Social norm, norms found in human societies, as well as the knowledge, beliefs, arts, laws, Social norm, customs, capabilities, Attitude (psychology), attitudes ...
isolation.
Proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...
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
Biogeochemistry is the Branches of science, scientific discipline that involves the study of the chemistry, chemical, physics, physical, geology, geological, and biology, biological processes and reactions that govern the composition of the natu ...
.
The ratio of stable nitrogen isotopes,
15N/
14N or
δ15N, tends to increase with
trophic level, such that
herbivore
A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...
s have higher nitrogen isotope values than
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, and
carnivore
A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...
s 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
Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.
The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and ...
of
vegan
Veganism is the practice of abstaining from the use of animal products and the consumption of animal source foods, and an associated philosophy that rejects the commodity status of animals. A person who practices veganism is known as a ve ...
s therefore contain significantly lower δ
15N 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
The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, atmospheric, terrestrial ecosystem, terrestrial, and marine ecosystems. The conversion of nitrogen can ...
tends to be more 'open' and prone to the loss of
14N, increasing δ
15N in soils and plants.
This leads to relatively high δ
15N values in plants and animals in hot and arid ecosystems relative to cooler and moister ecosystems.
Furthermore, elevated δ
15N 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.
δ
15N also provides a diagnostic tool in
planetary science
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of ...
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
Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
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
Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology
The word ''pyrolysis'' is coined from the Gree ...
and
mass spectrometry.
Improper or prolonged storage of samples can lead to inaccurate measurements.
Sulfur Isotopes
Sulfur has four stable isotopes,
32 S,
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 δ
33S 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 (abbreviated to
CDT), 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:
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
Lead isotopes
Lead consists of four stable
isotopes:
204Pb,
206Pb,
207Pb, and
208Pb. Local variations in
uranium
Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
/
thorium/
lead
Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
content cause a wide location-specific variation of isotopic ratios for lead from different localities. Lead emitted to the atmosphere by industrial processes has an isotopic composition different from lead in minerals. Combustion of
gasoline
Gasoline ( North American English) or petrol ( Commonwealth English) is a petrochemical product characterized as a transparent, yellowish, and flammable liquid normally used as a fuel for spark-ignited internal combustion engines. When for ...
with
tetraethyllead
Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula lead, Pb(ethyl group, C2H5)4. It was widely used as a fuel additive for much of the 20th century, first being mixed with gasoline begi ...
additive led to formation of ubiquitous micrometer-sized lead-rich
particulates in car exhaust
smoke; especially in urban areas the man-made lead particles are much more common than natural ones. The differences in isotopic content in particles found in objects can be used for approximate geolocation of the object's origin.
Radioactive isotopes
The
14C isotope is important in distinguishing between materials made from modern sources of carbon and ancient sources of carbon. Cosmic rays transform
14N to radioactive
14C, which subsequently decays over the course of tens of thousands of years. Shielded from cosmic radiation underground,
fossil fuels like
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal i ...
or
petroleum
Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...
exhibit
14C levels below detectable limits. However, carbon circulating in the surface biosphere maintains a measurable amount of
14C. Thus, materials synthesized from fossil fuel sources can be differentiated from those made with modern materials by the absence/presence of
14C. Federal regulators use this technique to encourage and monitor the incorporation of renewable biofeedstocks into transportation fuels.
Hot particles, radioactive particles of
nuclear fallout
Nuclear fallout is residual radioactive material that is created by the reactions producing a nuclear explosion. It is initially present in the mushroom cloud, radioactive cloud created by the explosion, and "falls out" of the cloud as it is ...
and
radioactive waste
Radioactive waste is a type of hazardous waste that contains radioactive material. It is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, nuclear decommissioning, rare-earth mining, and nuclear ...
, also exhibit distinct isotopic signatures. Their radionuclide composition (and thus their age and origin) can be determined by
mass spectrometry or by
gamma spectrometry. For example, particles generated by a nuclear blast contain detectable amounts of
60 Co and
152 Eu. The
Chernobyl accident did not release these particles but did release
125 Sb and
144 Ce. Particles from underwater bursts will consist mostly of irradiated sea salts. Ratios of
152 Eu/
155Eu,
154Eu/
155Eu, and
238 Pu/
239Pu are also different for fusion and fission
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
s, which allows identification of hot particles of unknown origin.
Uranium has a relatively constant isotope ratio in all natural samples with ~0.72% , some 55
ppm (in
secular equilibrium with its
parent nuclide ), and the balance made up by . Isotopic compositions that diverge significantly from those values are evidence for the uranium having been subject to depletion or
enrichment in some fashion or of (part of it) having participated in a nuclear fission reaction. While the latter is almost as universally due to human influence as the former two, the
natural nuclear fission reactor at
Oklo, Gabon was detected through a significant diversion of concentration in samples from Oklo compared to those of all other known deposits on earth. Given that is a
material of proliferation concern then as now every
IAEA-approved supplier of Uranium fuel keeps track of the isotopic composition of uranium to ensure none is diverted for nefarious purposes. It would thus become apparent quickly if another Uranium deposit besides Oklo proves to have once been a natural nuclear fission reactor.
Applications
Archaeological studies
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).
Forensics
With the advent of stable
isotope ratio mass spectrometry
Isotope-ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.
This technique has two different applications in the e ...
, isotopic signatures of materials find increasing use in
forensics, distinguishing the origin of otherwise similar materials and tracking the materials to their common source. For example, the isotope signatures of plants can be to a degree influenced by the growth conditions, including moisture and nutrient availability. In case of synthetic materials, the signature is influenced by the conditions during the chemical reaction. The isotopic signature profiling is useful in cases where other kinds of profiling, e.g. characterization of
impurities, are not optimal. Electronics coupled with scintillator detectors are routinely used to evaluate isotope signatures and identify unknown sources.
A study was published demonstrating the possibility of determination of the origin of a common brown
PSA packaging tape by using the carbon, oxygen, and hydrogen isotopic signature of the backing polymer, additives, and
adhesive.
Measurement of carbon isotopic ratios can be used for detection of
adulteration of
honey
Honey is a sweet and viscous substance made by several species of bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of pl ...
. Addition of sugars originated from corn or sugar cane (C4 plants) skews the isotopic ratio of sugars present in honey, but does not influence the isotopic ratio of proteins; in an unadulterated honey the carbon isotopic ratios of sugars and proteins should match. As low as 7% level of addition can be detected.
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.
Solar System origins
Isotopic fingerprints are used to study the origin of materials in the Solar System.
[ For example, the ]Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
's oxygen isotopic ratios seem to be essentially identical to Earth's. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. Different oxygen isotopic signatures can indicate the origin of material ejected into space. The Moon's titanium isotope ratio (50Ti/47Ti) appears close to the Earth's (within 4 ppm). In 2013, a study was released that indicated water in lunar magma was 'indistinguishable' from carbonaceous chondrites and nearly the same as Earth's, based on the composition of water isotopes.
Records of early life on Earth
Isotope biogeochemistry has been used to investigate the timeline surrounding life and its earliest iterations on Earth. Isotopic fingerprints typical of life, preserved in sediments, have been used to suggest, but do not necessarily prove, that life was already in existence on Earth by 3.85 billion years ago.
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) 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.
Modern sulfate-reducing bacteria
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...
are known to favorably reduce lighter 32S instead of 34S, and the presence of these microorganisms can measurably alter the sulfur isotope composition of the ocean. Because the δ34S values of sulfide mineral
The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide () as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenide mineral, selenides, the tell ...
s is primarily influenced by the presence of sulfate-reducing bacteria
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...
, the absence of sulfur isotope fractionations in sulfide minerals suggests the absence of these bacterial processes or the absence of freely available sulfate. Some have used this knowledge of microbial sulfur fractionation to suggest that minerals (namely pyrite) with large sulfur isotope fractionations relative to the inferred seawater composition may be evidence of life. This claim is not clear-cut, however, and is sometimes contested using geologic evidence from the ~3.49 Ga sulfide mineral
The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide () as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenide mineral, selenides, the tell ...
s found in the Dresser Formation of Western Australia, which are found to have δ34S values as negative as −22‰. Because it has not been proven that the sulfide and barite minerals formed in the absence of major hydrothermal input, it is not conclusive evidence of life or of the microbial sulfate reduction pathway in the Archean.
See also
* Isoscapes
* Isotope electrochemistry
* Isotope geochemistry
*Radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...
References
{{reflist, 30em
Further reading
Carbon isotopes: you are what you eat
Ayacucho Archaeo Isotope Project
The pursuit of isotopic and molecular fire tracers in the polar atmosphere and cryosphere
Radiometric dating
Bioindicators
Anthropology
Isotopes
Forensic evidence