Oxygen Isotopes & Climate

I watched a program on the science channel a few nights ago about the extreme climate shifts that have occurred during the past several million years. The findings were based on ice cores taken arctic ice sheets.

The idea is that during a glacial period the percentage of the heavier isotopes of water (primarily O18) is less because O16 is lighter and evaporates more readily.

The graphs shown on the program indicated that climate shifts had been abrupt and extreme. Obviously, the findings are based on the assumption that the relationship of O16 to O18 remains constant. Suppose it doesn't?

Is there anything that might change the relationship?

I'm not putting this forth as a theory, just a question which I think has some logic to it. If more O16 is evaporated, then more O18 remains in the sea. If O18 is heavier, then it would tend to sink to the bottom layers of the ocean, and in subduction areas, a higher percent would be subducted. When an andesite type volcano erupts, the ejecta would contain a higher percentage of O18. There have been times when huge eruptions have occurred. If there is more O18 in the atmosphere, could it skew the findings?

In looking at those charts, it seemed to me that the supposed climate fluctuations and the number of them they represented were just too radical to be reasonable.