Lesson plan
Dole effect
The Dole effect, named after Malcolm Dole, describes an inequality in the ratio of the heavy isotope 18 O (a "standard" oxygen atom with two additional neutrons) to the lighter 16 O, measured in the atmosphere and seawater. This ratio is usually denoted δ 18 O. It was noticed in 1935 that air contained more 18 O than seawater; this was quantified in 1975 to 23.5‰, but later refined as 23.88‰ in 2005. The imbalance arises mainly as a result of respiration in plants and in animals. Due to thermodynamics of isotope reactions, respiration removes the lighter—hence more reactive— 16 O in preference to 18 O, increasing the relative amount of 18 O in the atmosphere. The inequality is balanced by photosynthesis. Photosynthesis emits oxygen with the same isotopic composition (i.e. the ratio between 18 O and 16 O) as the water (H 2 O) used in the reaction, which is independent of the atmospheric ratio. Thus when atmospheric 18 O 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 and other small but significant processes.
Article summary
The Dole effect, named after Malcolm Dole, describes an inequality in the ratio of the heavy isotope 18 O (a "standard" oxygen atom with two additional neutrons) to the lighter 16 O, measured in the atmosphere and seawater. This ratio is usually denoted δ 18 O. It was noticed in 1935 that air contained more 18 O than seawater; this was quantified in 1975 to 23.5‰, but later refined as 23.88‰ in 2005. The imbalance arises mainly as a result of respiration in plants and in animals. Due to thermodynamics of isotope reactions, respiration removes the lighter—hence more reactive— 16 O in preference to 18 O, increasing the relative amount of 18 O in the atmosphere. The inequality is balanced by photosynthesis. Photosynthesis emits oxygen with the same isotopic composition (i.e. the ratio between 18 O and 16 O) as the water (H 2 O) used in the reaction, which is independent of the atmospheric ratio. Thus when atmospheric 18 O 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 and other small but significant processes.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
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Key claims and bibliography are generated from the same tracked claims and sources as the article page.