[mjaice]

hi,
I'm doing some research on oxygen isotopes in ice cores and how you can derive seasonal layers from the O16/O18 ratio.

I'm getting a little confused though.

Of course, the heavier isotopes will precipitate out more readily and will evaporate less easily. In colder temperatures, there is less of the heavy isotope than in warmer temperatures. With respect to ice cores, in summer layers you would expect there to be more dO18 than in winter layers.

I am focusing on the polar regions which are more negative than standard mean ocean water. My main question is this:

If dO18 condensates easier than dO16, then you would expect there to be more dO18 in snow formed at -30C than at -20C, correct? If that is the case, why is there LESS dO18 in winter layer than in summer layers in ice cores?

My answer to myself is due to atmospheric transport...in winter, the dO18 precipitates out of the air mass earlier than in summer when its on the way toward the poles?

Any help would be great,
Cheers.

[mjc123]

If dO18 condensates easier than dO16, then you would expect there to be more dO18 in snow formed at -30C than at -20C, correct?

No. The difference decreases as you lower the temperature.
Consider air containing two vapour components H and L. H is heavier and has a higher boiling point than L. As you gradually lower the temperature, H will start to condense before L does. The initial condensate will be rich in H, even if it is only a small fraction of the overall mixture. As the temperature falls, both H and L will condense, but the ratio of L to H condensing will increase, and as more L condenses the condensate will approach the composition of the initial mixture.
You probably know, for example, that liquid air contains a higher fraction of oxygen than the atmosphere, because oxygen is less volatile than nitrogen, so more condenses out at liquid air temperature. However, if the temperature was reduced enough - to liquid helium temp, in the extreme - then essentially all the nitrogen and oxygen would condense, and the condensate would have the same composition as air.
Your point about atmospheric transport appears to be right, see e.g. http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/

[mjaice]

thank you very much for the excellent explanation!

[mjc123]

Glad to be of help.