Measuring ancient CO2 levels

How can ancient CO₂ levels be measured?

E.g. from certain shells at the sea bottom.

These shells contain different types of boron.

The boron composition depended on the water acidity,

the acidity on the amount of dissolved CO₂ in the water

and dissolved CO₂ on CO₂ in the air.

Sources: 

- NATURE | VOL 406 | 17 AUGUST 2000

- Science 326, 1394 (2009)