"by measuring the relative abundances of two stable isotopes of oxygen - 16o and 18o - in some fossil deposit, or in an ice core, or in a coal bed, or in some sedimentary layer, what can one determine about the conditions that pertained at the time those deposits were laid down

Stable isotopes refers to those isotopes that do not undergo radioactive decay. Oxygen atoms with atomic number of 16 and 18 are examples of stable isotopes. These isotopes of oxygen are usually found in substances such as polar ice and calcite. These substances are usaually measure with isotope fractionation and the ratios obtained are used to estimate water temperature of ancient oceans. From this, scientists can find out facts about ancient climates,