Dole effect

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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.

Use of the Dole effect[edit]

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‰[clarify].[7]

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.

Millennial variations of Dole effect are found to be related to abrupt climate change events in the North Atlantic region during the last 60 kyr.[8] High correlations of Dole effect to speleothem δ18O, an indicator for monsoon precipitation, suggest that it is subject to changes of low-latitude terrestrial productivity. Orbital scale variations of Dole effect, characterized with periods of 20-100 kyr, responds strongly to Earth's orbital eccentricity and precession, but not obliquity.[9]

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.

References[edit]

  1. ^ Dole, Malcolm (1936). "The Relative Atomic Weight of Oxygen in Water and in Air". Journal of Chemical Physics 4 (4): 268–275. 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. doi:10.1126/science.175.4017.54. PMID 17833979. 
  4. ^ Barkan, E.; Luz, B. (2005). "High precision measurements of 17O/16O and 18O/16O ratios in H2O". Rapid Commun. Mass Spectrom. 19: 3737–3742. doi:10.1002/rcm.2250. 
  5. ^ Urey, H.C. (1947). "The thermodynamic properties of isotopic substances". J. Chem. Soc: 562–581. doi:10.1039/JR9470000562. 
  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. doi:10.1007/BF00392616. 
  7. ^ Bender, M.; Sowers, T.; Labeyrie, L. (1994). "The Dole effect and its variations during the last 130,000 years as measured in the Vostok ice core". Global Biogeochemical Cycles 8 (3): 363–376. doi:10.1029/94GB00724. 
  8. ^ Severinghaus, J.P.; Beaudette, R.; Headly, M.A.; Taylor, K.; Brook, E.J. (2009). "Oxygen-18 of O2 records the impact of abrupt climate change on the terrestrial biosphere". Science 324 (5933): 1431–1434. doi:10.1126/science.1169473. 
  9. ^ Landais, A.; Dreyfus, G.; Capron, E.; Masson-Delmotte, V.; Sanchez-Goñi, M.F.; Desprat, S.; Hoffmann, G.; Jouzel, J.; Leuenberger, M.; Johnsen, S. (2010). "What drives the millennial and orbital variations of δ18Oatm". Quaternary Sci. Rev. 29: 235–246. doi:10.1016/j.quascirev.2009.07.005. 

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