Cartoon of oxygen isotope changes during production of glaciers via seawater extraction. The ratio of 18O to 16O in a sample is expressed by scientists as the deviation (designated by the Greek letter “δ”) from the ratio of isotopes in a standard, where d18O = [sample ratio]/[standard ratio]-1). Note how during low sealevel (when glaciers are expanding) the ocean becomes enriched in 18O, leading to a positive isotopic value (+1‰), while the glacier becomes “depleted” in 18O, giving it a negative isotopic value (-30‰). (See the glossary for an expanded overview of the “δ” notation under “Oxygen Isotopes.”)