However, until relatively recently, these variations were not commonly investigated because of the great effort and expense involved in chemical analysis.
Most relevant to chemostratigraphy in general was the discovery by Harold Urey and Cesare Emiliani in the early 1950s that the oxygen isotope variability in the calcite shells of foraminifera could be used as a proxy for past ocean temperatures.
An extreme example of this type of investigation might be the discovery of strata rich in iridium near the boundary between the Cretaceous and Tertiary systems globally.
A more prosaic example of chemostratigraphic reconstruction of past conditions might be the use of the carbon-13/carbon-12 ratio over geologic time as a proxy for changes in carbon cycle processes at different stages of biological evolution.
However, many sedimentary rocks are much harder to date, because they lack minerals with high concentrations of radionuclides and cannot be correlated with nearly datable sequences.