Terrace ages in relation to normalized oxygen isotope composition

In this figure, the lower graph shows oxygen isotope variations in foraminifera in deep-sea cores (Imbrie and others, 1984), which are excellent records of glacial-interglacial cycles. Oxygen isotope periods are called "stages," with odd-numbered stages corresponding to warm interglacial periods, such as the present (oxygen isotope stage 1). Cold glacial periods are given even-numbered stage names, such as the last major ice age (oxygen isotope stage 2). Because sea level is low during glacial periods (ocean water is taken up to form ice sheets) and high during interglacial periods (ice sheets melt and return water to the ocean), the oxygen isotope curve shown here for the past ~400,000 years is to a great extent a record of sea level fluctuations as well as a record of temperature fluctuations.