Oxygen isotopes (-O) are widely used in correlation of Quaternary marine sediments.
Oxygen isotope concentrations in mollusk shell and calcareous algal material normalize with seawater while the organisms are alive.
In addition to the chemical and physical characteristics of volcanic ash, select igneous minerals in the ash can be used for absolute dating (discussed below).
For more information, contact Andrei Sarna-Wojcicki. Strontium Geochronology - With modern isotope separation equipment, the content of selected elemental isotopes can now be measured in concentrations to parts-per-million to parts-per-billion and beyond.
Tradition paleontological and biostratigraphic correlation methods are still perhaps the most common relative dating methods used by geologists.