Dole effect

The Dole effect, named after Malcolm Dole, 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 more 18O than seawater; this was quantified in 1975 to 23.5‰,[3] but later refined as 23.88‰ in 2005.[4] The imbalance arises mainly as a result of respiration in plants and in animals. Due to thermodynamics of isotope reactions,[5] 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,[6] 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,[clarify] and other small but significant processes.

  1. ^ Dole, Malcolm (1936). "The Relative Atomic Weight of Oxygen in Water and in Air". Journal of Chemical Physics. 4 (4): 268–275. Bibcode:1936JChPh...4..268D. doi:10.1063/1.1749834.
  2. ^ Morita, N. (1935). "The increased density of air oxygen relative to water oxygen". J. Chem. Soc. Japan. 56: 1291.
  3. ^ Kroopnick, P.; Craig, H. (1972). "Atmospheric Oxygen: Isotopic Composition and Solubility Fractionation". Science. 175 (4017): 54–55. Bibcode:1972Sci...175...54K. doi:10.1126/science.175.4017.54. PMID 17833979. S2CID 24579820.
  4. ^ Barkan, E.; Luz, B. (2005). "High precision measurements of 17O/16O and 18O/16O ratios in H2O". Rapid Commun. Mass Spectrom. 19 (24): 3737–3742. Bibcode:2005RCMS...19.3737B. doi:10.1002/rcm.2250. PMID 16308852.
  5. ^ Urey, H.C. (1947). "The thermodynamic properties of isotopic substances". J. Chem. Soc.: 562–581. doi:10.1039/JR9470000562. PMID 20249764.
  6. ^ Guy, Robert D.; et al. (1989). "Differential fractionation of oxygen isotopes by cyanide-resistant and cyanide-sensitive respiration in plants". Planta. 177 (4): 483–491. Bibcode:1989Plant.177..483G. doi:10.1007/BF00392616. PMID 24212490. S2CID 22767005.