Dole effect
Encyclopedia
The Dole effect describes an inequality in the ratio of the heavy isotope
18O
(a 'standard' oxygen
atom with two additional neutron
s) to the lighter 16O, measured in the atmosphere
and seawater. This ratio is usually denoted δ18O.
It was noticed in 1935 that air contained more 18O than seawater; this was quantified in 1975 to 23.5‰. The imbalance arises mainly as a result of respiration in plants and in animals
. Due to thermodynamics
, respiration removes the lighter — hence more reactive — 16O in preference to 18O, increasing the relative amount of 18O in the atmosphere.
The inequality is balanced by photosynthesis
. Photosynthesis emits oxygen with the same isotopic composition (i.e. the ratio between 18O and 16O) as the water (H2O) used in the reaction, which is independent of the atmospheric ratio. Thus when atmospheric 18O levels are high enough, photosynthesis will act as a reducing factor. However, as a complicating factor, the degree of fractionation (i.e. change in isotope ratio) occurring due to photosynthesis is not entirely dependent on the water drawn up by the plant, as fractionation can occur as a result of preferential evaporation of H216O - water bearing lighter oxygen isotopes, and other small but significant processes.
The stability (to within 0.5‰) of the atmospheric 18O to 16O ratio with respect to sea surface waters since the last interglacial
(the last 130 000 years), as derived from ice cores, suggests that terrestrial and marine productivity have varied together during this time period.
The Dole effect can also be applied as a tracer in sea water, with slight variations in chemistry being used to track a discrete 'parcel' of water and determine its age.
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
18O
Oxygen-18
Oxygen-18 is a natural, stable isotope of oxygen and one of the environmental isotopes.18O is an important precursor for the production of fluorodeoxyglucose used in positron emission tomography...
(a 'standard' oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atom with two additional neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s) to the lighter 16O, measured in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
and seawater. This ratio is usually denoted δ18O.
It was noticed in 1935 that air contained more 18O than seawater; this was quantified in 1975 to 23.5‰. The imbalance arises mainly as a result of respiration in plants and in animals
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
. Due to thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
, respiration removes the lighter — hence more reactive — 16O in preference to 18O, increasing the relative amount of 18O in the atmosphere.
The inequality is balanced by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. Photosynthesis emits oxygen with the same isotopic composition (i.e. the ratio between 18O and 16O) as the water (H2O) used in the reaction, which is independent of the atmospheric ratio. Thus when atmospheric 18O levels are high enough, photosynthesis will act as a reducing factor. However, as a complicating factor, the degree of fractionation (i.e. change in isotope ratio) occurring due to photosynthesis is not entirely dependent on the water drawn up by the plant, as fractionation can occur as a result of preferential evaporation of H216O - water bearing lighter oxygen isotopes, and other small but significant processes.
Use of the Dole effect
Since evaporation causes oceanic and terrestrial waters to have a different ratio of 18O to 16O, the Dole effect will reflect the relevant importances of land-based and marine photosynthesis. The complete removal of land-based productivity would result in a Dole effect shift of -2-3‰ from the current value of 23.5‰.The stability (to within 0.5‰) of the atmospheric 18O to 16O ratio with respect to sea surface waters since the last interglacial
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
(the last 130 000 years), as derived from ice cores, suggests that terrestrial and marine productivity have varied together during this time period.
The Dole effect can also be applied as a tracer in sea water, with slight variations in chemistry being used to track a discrete 'parcel' of water and determine its age.