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Both 10Be and 36Cl are formed as charged ions in the ionosphere. Thus each annual layer starts 10Be and 36Cl poor, becomes 10Be and 36Cl rich, and then becomes poor again. Although what is said above is true, this is an exceedingly minor effect.

Both of these isotopes are produced by cosmic rays and solar irradiation impinging on the upper atmosphere, and both are quickly washed from the atmosphere by precipitation.

Of the irradiation dependent markers the two most important are 10Be and 36Cl.

The major disadvantage of this dating method is that isotopes tend to diffuse as time proceeds.

For similar reasons the ratio of deuterium to hydrogen acts the same way.

This process also depends on the relative temperatures of different years, which allows comparison with paleoclimatic data.

Thus, each annual layer starts 18O rich, becomes 18O poor, and ends up 18O rich.

This depletion is a temperature dependent process so in winter the precipitation is more enriched in H2(16O) than is the case in the summer.

As the water vapor travels towards the poles it becomes increasingly poorer in H2(18O) since the heavier molecules tend to precipitate out first.

Thus, water evaporating from the ocean it starts off H2(18O) poor.

The water molecules composed of H2(18O) evaporate less rapidly and condense more readily then water molecules composed of H2(16O).

Of the temperature dependent markers the most important is the ratio of 18O to 16O.

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