The document discusses oxygen isotopes and how they can be used to study climate change during the Carboniferous Period. Oxygen isotopes are recorded in the calcite shells of microorganisms and preserved in limestone. Heavier oxygen isotopes are concentrated in the shells of microorganisms and limestone during glacial periods due to fractionation processes. Analyzing the ratio of oxygen isotopes over time in limestone reveals alternating warm and cold periods during the Carboniferous, dominated by the Carboniferous Ice Age when vast ice sheets covered the southern polar continents.