Hey - got a question about Isotopes. In the Ordovician, the Glacier growing on Gondwanaland at the time retained isotopically light Oxygen. Therefore isotopically heavy O18 accumulates in oceans causing certain types of fauna to secrete CaCO3 with a heavy O18. My question is why does this occur? Why do glaciers retain light O, while oceans retain heavy O? Thanks for your time.

My initial, simplistic response would be: it's because the heavier isotope ends up in the carbonate in marine organisms. Then it ends up at the ocean bottom, or on top of the oceanic crust, where it isn't recycled for, say a hundred mil years or so.

So it separates out naturally, it's heavier and ends up, with a little help from calcigenic lifeforms, at the lowest points on the earth's surface (the ocean basins), the lighter stuff stays in the atmosphere, as a "free" isotope.

Of course, what you actually get is a ratio of isotopes. The heavier isotope has a higher ratio to the lighter, in ocean cores, than in glacier core samples.

During evaporation more light 16O isotopes escape, leaving the heavier 18O isotopes behind. So if the evaporated isotopes do not return to the ocean because it accumulates on ice sheets then the concentration of 18O isotopes in the ocean increases. And hence this increase in concentration is reflected in the secreted carbonates.