Dole effect
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The Dole effect describes an inequality in the ratio of the heavy isotope 18O (a 'standard' oxygen atom with two additional neutrons) to the lighter 16O, measured in the atmosphere and seawater. This ratio is usually denoted δ18O. It was noticed in 1935[1][2] that air contained relatively more 18O than seawater; this was quantified in 1975 to 23.5‰[3]. 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 composision (i.e. the ratio between 18O and 16O) as the water (H2O) used in the reaction[4], 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.
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[edit] 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‰[5].
The stability (to within 0.5‰) of the atmospheric 18O to 16O ratio with respect to sea surface waters during the Palæocene (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.
[edit] References
- ^ Dole, M., The relative atomic weight of oxygen in water and air, J. Am. Chem. Soc., 57, 2731, 1935.
- ^ Morita, N., 1935, The increased density of air oxygen relative to water oxygen, J. Chem. Soc. Japan, 56, 1291
- ^ Kroopnick, P. and H. Craig, Atmospheric oxygen: isotopic composition and solubility fractionation, Science, 175, 54--55, 1972.
- ^ Guy, R. D., J. A. Berry, M. L. Fogel, and T. C. Hoering, Differential fractionation of oxygen isotopes by cyanide-resistant and cyanide-sensitive respiration in plants, Planta, 177, 483--491, 1989.
- ^ Bender, M., T. Sowers, and L. Labeyrie, The Dole effect and its variations during the last 130,000 years as measured in the Vostok ice core, Global Biogeochemical Cycles, 8, 363-376, 1994
