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 th ...
,
paleoclimatology
Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...
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 models ...
''δ''
18O or delta-O-18 is a measure of the ratio of
stable isotopes
oxygen-18
Oxygen-18 (, Ω) is a natural, stable isotope of oxygen and one of the environmental isotopes.
is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). Generally, in the radiopharmaceu ...
(
18O) and
oxygen-16 (
16O).
It is commonly used as a measure of the temperature of
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, as a measure of groundwater/mineral interactions, and as an indicator of processes that show
isotopic fractionation, like
methanogenesis
Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...
.
In paleosciences,
18O:
16O data from
coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and se ...
s,
foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
and
ice cores
An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains i ...
are used as a
proxy
Proxy may refer to:
* Proxy or agent (law), a substitute authorized to act for another entity or a document which authorizes the agent so to act
* Proxy (climate), a measured variable used to infer the value of a variable of interest in climate re ...
for temperature.
The definition is, in "per mil" (‰, parts per thousand):
:
‰
where the standard has a known isotopic composition, such as
Vienna Standard Mean Ocean Water
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ( ...
(VSMOW).
[
] The fractionation can arise from
kinetic
Kinetic (Ancient Greek: κίνησις “kinesis”, movement or to move) may refer to:
* Kinetic theory, describing a gas as particles in random motion
* Kinetic energy, the energy of an object that it possesses due to its motion
Art and ent ...
,
equilibrium, or
mass-independent fractionation.
Mechanism
Foraminifera shells are composed of
calcium carbonate (CaCO
3) and are found in many common geological environments. The ratio of
18O to
16O in the shell is used to indirectly determine the temperature of the surrounding water at the time the shell was formed. The ratio varies slightly depending on the temperature of the surrounding water, as well as other factors such as the water's salinity, and the volume of water locked up in ice sheets.
also reflects local evaporation and freshwater input, as rainwater is
16O-enriched—a result of the preferential evaporation of the lighter
16O from seawater. Consequently, the surface ocean contains greater proportions of
18O around the subtropics and tropics where there is more evaporation, and lesser proportions of
18O in the mid-latitudes where it rains more.
Similarly, when water vapor condenses, heavier water molecules holding
18O atoms tend to condense and precipitate first. The water vapor gradient heading from the tropics to the poles gradually becomes more and more depleted of
18O.
Snow
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water throughou ...
falling in
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
has much less H
218O than rain in
Florida
Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and ...
; similarly, snow falling in the center of ice sheets has a lighter signature than that at its margins, since heavier
18O precipitates first.
Changes in climate that alter global patterns of evaporation and precipitation therefore change the background ratio.
Solid samples (organic and inorganic) for oxygen isotope analysis are usually stored in silver cups and measured with
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements '' ...
and
mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is u ...
.
Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.
Extrapolation of temperature
Based on the simplifying assumption that the signal can be attributed to temperature change alone, with the effects of salinity and ice volume change ignored, Epstein et al. (1953) estimated that
a increase of 0.22‰ is equivalent to a cooling of 1 °C (or 1.8 °F).
More precisely, Epstein et al. (1953) give a quadratic extrapolation for the temperature, as
:
where ''T'' is the temperature in °C (based on a
least-squares
The method of least squares is a standard approach in regression analysis to approximate the solution of overdetermined systems (sets of equations in which there are more equations than unknowns) by minimizing the sum of the squares of the res ...
fit for a range of temperature values between 9 °C and 29 °C, with a standard deviation of ±0.6 °C, and δ is δ
18O for a calcium carbonate sample).
Paleoclimatology
δ
18O can be used with
ice core
An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ...
s to determine the temperature from when the ice was formed.
Lisiecki and
Raymo (2005) used measurements of δ
18O in benthic foraminifera from 57 globally distributed deep sea sediment cores, taken as a proxy for the total global mass of glacial ice sheets, to reconstruct the climate for the past five million years.
data: .
The stacked record of the 57 cores was
orbitally tuned to an orbitally driven ice model, the
Milankovitch cycles of 41 ky (
obliquity
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbi ...
), 26 ky (
precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...
) and 100 ky (
eccentricity), which are all assumed to cause
orbital forcing
Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the Earth's orbit around the sun (see Milankovitch cycles). These orbital changes modify the total amount of sunlight reaching the Earth by up ...
of global ice volume. Over the past million years, there have been a number of very strong glacial maxima and minima, spaced by roughly 100 ky.
As the observed isotope variations are similar in shape to the temperature variations recorded for the past 420 ky at
Vostok Station
Vostok Station (russian: ста́нция Восто́к, translit=stántsiya Vostók, , meaning "Station East") is a Russian research station in inland Princess Elizabeth Land, Antarctica. Founded by the Soviet Union in 1957, the station ...
, the figure shown on the right aligns the values of δ
18O (right scale) with the reported temperature variations from the Vostok ice core (left scale), following Petit et al. (1999).
See also
*
*
*
*
Isotopic signature
*
Isotope analysis
Isotope analysis is the identification of isotopic signature, abundance of certain stable isotopes of chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food w ...
*
Isotope geochemistry
Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope ratio mass spectrometry, and can reveal ...
*
Paleothermometer
References
*
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{{DEFAULTSORT:Delta18O
Bioindicators
Isotopes of oxygen
Environmental isotopes
Geochemistry