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Isotope analysis
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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.
Contents
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1 Techniques
o 1.1 Oxygen isotopes
1.1.1 Variation by latitude
1.1.2 Variation occurring from the hydrological cycle
1.1.3 Tissues affected
2 Applications
o 2.1 Archaeology
o 2.2 Ecology
o 2.3 Forensics
o 2.4 Geology
o 2.5 Hydrology
o 2.6 Paleoclimatology
o 2.7 Photosynthesis
3 References
4 External links
[edit] Techniques
[edit] Oxygen isotopes
Oxygen Isotopes and their Relative Abundances:
16
O = 99.763%
O = 0.0375%
18
O = 0.1995%
17
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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Applications
[edit] Archaeology
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. To obtain an accurate picture of palaeodiets, it is important to
understand processes of diagenesis that may affect the original isotopic signal.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 levels, or an estimation thereof, 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 within individuals, between individuals, and over time.
Isotope analysis has been particularly useful in archaeology as a means of characterization.
Characterization of artefacts involves determining the isotopic composition of possible source
materials such as metal ore bodies and comparing these data to the isotopic composition of
analyzed artefacts. A wide range of archaeological materials such as metals, glass and lead-based
pigments have been sourced using isotopic charaterization. Particularly in the Bronze Age
Mediterranean Lead Isotope Analysis has been a useful tool for determining the sources of
metals and an important indicator of trade patterns. Interpretation of Lead Isotope Data is,
however, often contentious and faces numerous instrumental and methodological challenges.
Problems such as the mixing and re-using of metals form different sources, limited reliable data
and contamination of samples can be difficult problems in interpretation.
[edit] 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 sulfur.
[edit] Forensics
A recent development in forensic science is the isotopic analysis of hair strands. Hair has a
recognisable growth rate of 9-11mm[1] per month or 15cm per year[2]. Hair growth is primarily a
function of diet, especially drinking water intake. The stable isotopic ratios of drinking water are
a function of location, and the geology that the water percolates through. 87Sr, 88Sr and Oxygen
isotope variations are different all over the World. These differences in isotopic ratio are then
biologically 'set' in our hair as it grows and it has therefore become possible to identify recent
geographic histories by the analysis of hair strands. For example, it could be possible to identify
whether a terrorist suspect had recently been to the Middle-East from hair analysis. This hair
analysis is a non-invasive method which is becoming very popular in cases that DNA or other
traditional means are bringing no answers.
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.
Stable isotopic analysis has also been used in the identification of drug trafficking routes.
Isotopic abundances are different in morphine grown from poppies in South-east Asia versus
poppies grown in South-West Asia. The same is applied to cocaine that is derived from Bolivia
and that from Columbia.[3]
[edit] Geology
Main article: Isotope geochemistry
[edit] Hydrology
[edit] Paleoclimatology
[edit] Photosynthesis
[edit] References
1. ^ S. Black., Crime Scene Analysis, Reading University, 2008
2. ^ P. White., Crime Scene to Court: The Essentials of Forensic Science; Second Edition, Royal
Society of Chemistry, 2004
3. ^ J.R. Ehleringer, J. Casale, D.A. Cooper, M.J. Lott., Sourcing Drugs With Stable Isotopes - [1]
Isotope geochemistry
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Isotope geochemistry is an aspect of geology based upon study of the relative and absolute
concentrations of the elements and their isotopes in the Earth. Broadly, the field is divided into
two branches: stable and radiogenic isotope geochemistry.
Contents
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1 Lead-lead isotope geochemistry
2 Samarium-neodymium
3 Rhenium-osmium
4 Protactinium:Thorium Ratios - 231Pa / 230Th
5 Noble gas isotopes
o 5.1 Helium-3
6 Ground water isotopes
o 6.1 Tritium/helium-3
7 See also
8 General online stable isotope references
9 References
[edit] Lead-lead isotope geochemistry
Lead has four stable isotopes - 204Pb, 206Pb, 207Pb, 208Pb and one common radioactive isotope
202
Pb with a half-life of ~53,000 years.
Lead is created in the Earth via decay of transuranic elements, primarily uranium and thorium.
Lead isotope geochemistry is useful for providing isotopic dates on a variety of materials.
Because the lead isotopes are created by decay of different transuranic elements, the ratios of the
four lead isotopes to one another can be very useful in tracking the source of melts in igneous
rocks, the source of sediments and even the origin of people via isotopic fingerprinting of their
teeth, skin and bones.
It has been used to date ice cores from the Arctic shelf, and provides information on the source
of atmospheric lead pollution.
Lead-lead isotopes has been successfully used in forensic science to fingerprint bullets, because
each batch of ammunition has its own peculiar 204Pb/206Pb vs 207Pb/208Pb ratio.
[edit] Samarium-neodymium
Main article: Samarium-neodymium dating
Samarium-neodymium is an isotope system which can be utilised to provide a date as well as
isotopic fingerprints of geological materials, and various other materials including archaeological
finds (pots, ceramics).
147
Sm decays to produce 143Nd with a half life of 1.06x1011 years.
Dating is achieved usually by trying to produce an isochron of several minerals within a rock
specimen. The initial 143Nd/144Nd ratio is determined.
This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir - which is an
approximation of the chondritic material which formed the solar system. CHUR was determined
by analysing chondrite and achondrite meteorites.
The difference in the ratio of the sample relative to CHUR can give information on a model age
of extraction from the mantle (for which an assumed evolution has been calculated relative to
CHUR) and to whether this was extracted from a granitic source (depleted in radiogenic Nd), the
mantle, or an enriched source.
[edit] Rhenium-osmium
Rhenium and osmium are chalcophile elements which are present at very low abundances in the
crust. Rhenium undergoes radioactive decay to produce osmium. The ratio of non-radiogenic
osmium to radiogenic osmium throughout time varies.
Rhenium prefers to enter sulfides more readily than osmium. Hence, during melting of the
mantle, rhenium is stripped out, and prevents the osmium-osmium ratio from changing
appreciably. This locks in an initial osmium ratio of the sample at the time of the melting event.
Osmium-osmium initial ratios are used to determine the source characteristic and age of mantle
melting events.
[edit] Protactinium:Thorium Ratios - 231Pa / 230Th
Uranium is well mixed in the ocean, and its decay produces 231Pa and 230Th at a constant activity
ratio (0.093). The decay products are rapidly removed by adsorption on settling particles, but not
at equal rates. 231Pa has a residence equivalent to the residence time of deep water in the Atlantic
basin (around 1000 yrs) but 230Th is removed more rapidly (centuries). Thermohaline circulation
effectively exports 231Pa from the Atlantic into the Southern Ocean, while most of the 230Th
remains in Atlantic sediments. As a result, there is a relationship between 231Pa/230Th in Atlantic
sediments and the rate of overturning: faster overturning produces lower sediment 231Pa/230Th
ratio, while slower overturning increases this ratio. The combination of d13C and 231Pa/230Th can
therefore provide a more complete insight into past circulation changes.
[edit] Noble gas isotopes
[edit] Helium-3
Helium-3 was trapped in the planet when it was created. Some 3He is being added by meteoric
dust, primarily collecting on the bottom of oceans (although due to subduction, all oceanic
tectonic plates are younger than continental plates). However, 3He will be degassed from oceanic
sediment during subduction, so cosmogenic 3He is not affecting the concentration or noble gas
ratios of the mantle.
Helium-3 is created by cosmic ray bombardment, and by lithium spallation reactions which
generally occur in the crust. Lithium spallation is the process by which a high-energy neutron
bombards a lithium atom, creating a 3He and a 4He ion. This requires significant lithium to
adversely affect the 3He/4He ratio.
All degassed helium is lost to space eventually, due to the average speed of helium exceeding the
escape velocity for the Earth. Thus, it is assumed the helium content and ratios of Earth's
atmosphere have remained essentially stable.
It has been observed that 3He is present in volcano emissions and oceanic ridge samples. How
3
He is stored in the planet is under investigation, but it is associated with the mantle and is used
as a marker of material of deep origin.
Due to similarities in helium and carbon in magma chemistry, outgassing of helium requires the
loss of volatile components (water, carbon dioxide) from the mantle, which happens at depths of
less than 60 km. However, 3He is transported to the surface primarily trapped in the crystal
lattice of minerals within fluid inclusions.
Helium-4 is created by radiogenic production (by decay of uranium/thorium-series elements).
The continental crust has become enriched with those elements relative to the mantle and thus
more He4 is produced in the crust than in the mantle.
The ratio (R) of 3He to 4He is often used to represent 3He content. R usually is given as a
multiple of the present atmospheric ratio (Ra).
Common values for R/Ra:
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Old continental crust: less than 1
mid-ocean ridge basalt (MORB): 7 to 9
Spreading ridge rocks: 9.1 plus or minus 3.6
Hotspot rocks: 5 to 42
Ocean and terrestrial water: 1
Sedimentary formation water: less than 1
Thermal spring water: 3 to 11
3
He/4He isotope chemistry is being used to date groundwaters, estimate groundwater flow rates,
track water pollution, and provide insights into hydrothermal processes, igneous geology and ore
genesis.
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(U-Th)/He dating of apatite as a thermal history tool
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USGS: Helium Discharge at Mammoth Mountain Fumarole (MMF)
[edit] Ground water isotopes
[edit] Tritium/helium-3
Tritium was released to the atmosphere during atmospheric testing of nuclear bombs.
Radioactive decay of tritium produces the noble gas helium-3. Comparing the ratio of tritium to
helium-3 (3H/3He) allows estimation of the age of recent ground waters.
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USGS Tritium/Helium-3 Dating
Hydrologic Isotope Tracers - Helium
[edit] See also
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Cosmogenic isotopes
Environmental isotopes
Geochemistry
Isotopic signature
Radiometric dating
[edit] General online stable isotope references
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USGS: Stable Isotopes and Mineral Resource Investigations in the United States
USGS: Fundamentals of Stable Isotope Geochemistry
Environmental Isotopes
Fundamentals of Isotope Geochemistry
[edit] References
Faure G., 1986. Principles of Isotope Geology, John Wiley & Sons. ISBN 0-471-86412-9
3
He/4He
Burnard P.G., Farley K.A., & Turner G., 1998. "Multiple fluid pulses in a Samoan harzburgite",
Chemical Geology, 147: 99-114.
Kirstein L. & Timmerman M., 2000. "Evidence of the proto-Iceland lume in northwestern
Ireland at 42Ma from helium isotopes", Journal of the Geological Society, London, 157: 923927.
Porcelli D. & Halliday A.N., 2001. "The core as a possible source of mantle helium", Earth and
Planetary Science Letters, 192: 45-56.
Re-Os
Arne D., Bierlein F.P., Morgan J.W., & Stein H.J., 2001. "Re-Os dating of sulfides associated
with gold mineralisation in central Victoria, Australia", Economic Geology, 96: 1455-1459.
Martin C., 1991. "Osmium isotopic characteristics of mantle-derived rocks", Geochimica et
Cosmochimica Acta, 55: 1421-1434