<P> T = A + B ⋅ ((δ 18 O) calcite − (δ 18 O) water) (\ displaystyle T = A + B \ cdot \ left (\ left (\ delta () ^ (18) (\ text (O)) \ right) (\ text (calcite)) - \ left (\ delta () ^ (18) (\ text (O)) \ right) (\ text (water)) \ right)) </P> <P> Where T is temperature in Celsius and A and B are constants . </P> <P> For determination of ocean temperatures over geologic time, multiple fossils of the same species in different stratigraphic layers would be measured, and the difference between them would indicate long term changes . </P> <P> In the study of plants' photorespiration, the labeling of atmosphere by oxygen - 18 allows us to measure the oxygen uptake by the photorespiration pathway . Labeling by 18 O 2 gives the unidirectional flux of O 2 uptake, while there is a net photosynthetic 16 O 2 evolution . It was demonstrated that, under preindustrial atmosphere, most plants reabsorb, by photorespiration, half of the oxygen produced by photosynthesis . Then, the yield of photosynthesis was halved by the presence of oxygen in atmosphere . </P>

How many electrons in an atom of oxygen-18