How can ice core samples preserve air when it's hot outside?

BiggusGeekus · November 28, 2005, 11:03pm

Ice core data has certainly been consistient and reproduceable. I’m not wondering about that. But how can an ice sample trap air bubbles during warm spells when the ice is more prone to melt and not be ice in the first place?

Thanks,

-BG

Lemur866 · November 28, 2005, 11:25pm

Obviously ice core samples can only preserve air bubbles if the ice has remained frozen continuosly since the ice was first frozen.

So you have to get your ice cores from places like Greenland that have had a permanent ice cap since the Pleistocene. Mountain glaciers aren’t good enough, since even if a glacier has been on the mountain for thousands of years it isn’t the same ice, the ice flows down the mountain and is replaced by new ice.

BiggusGeekus · November 29, 2005, 5:48pm

OK, got it.

Follow up question:

How do we know that the ice has been accumulating for a half million years?

I assume it’s similar to how we know how stuff is underwater for the same period of time, but I’m still curious.

Fear_Itself · November 29, 2005, 6:29pm

Dust accumulates on the surface of the glacier in the summer, then is covered by new snow in the winter. Much like counting the rings on a tree, ice cores can be dated by counting the dust layers. Other techniques are also employed:

Many cores however come from regions where the yearly snowfall accumulation is too small for the annual layers to be distinguished, and other methods of dating must be used.

Even in cores where the yearly snowfall produces thick layers, the nature of glacier flow stretches and thins layers as they get buried deeper. This flow-thinning means that annual layer counting eventually becomes impossible in all deep cores.

Where layer-counting is not possible, dating generally relies upon mathematical models of ice-flow.

Another useful technique is to identify events which are seen in other types of past climate record. Other records, historical, tree-ring or sedimentary records all have independent methods of dating, and so the timing of a major climate shift or volcanic eruption can be used to synchronise the age-scales.