Dole effect

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.

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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.

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.

References