EM

First you say there is no trend. Now you seem to be saying that there is a trend, but it’s not linear. I never said that it was linear.

BTW, LJO shows zero, not 0.5.

JR

Not even a trend. Except at high latitudes it looks pretty uniform.

JR

There is an equilibrium between 18O in water in the ocean and 18O in atmospheric CO2.

When CO2 dissolves in water it forms carbonic acid. This is a reversible reaction

CO2 + H2O <=> H2CO3

O=C=O + H-O-H. <=> O=C-OH
I
OH

If the CO2 molecule carries an 18O, there is a 50% chance that it will become part of a hydroxyl group and a further 50% chance that when the carbonic acid decomposes the 180 will become part of a water molecule.

Thus there is a 25% chance that an 18O from a dissolved CO2 molecule will transfer to a water molecule.

If the water molecule carries an 18O it will become part of a hydroxyl group. There is then a 50% chance that it will become part of the CO2 molecule when the carbonic acid decomposes.
.

Damm. My carefully constructed carbonic acid molecule got disrupted. 😕

CO2 + H2O <=> H2CO3

O=C=O + H-O-H. <=> O=C-OH
.........................................I
...,.....................................OH

.

Biological systems swap Plants take up 18O in CO2, which becomes incorporated into carbohydrates during photosynthesis.

They also take up 18O in water. Photosynthesis breaks down the water into hydrogen which ends up in carbohydrate and any 18O is released into the atmosphere in oxygen molecules.

When the carbohydrate is broken down by respiration some of the 18O from the carbohydrate, and from oxygen molecules, ends up in water and some in CO2.