Oxygen-18

Oxygen-18 (^18O) is a natural, stable isotope of oxygen and one of the environmental isotopes. Unlike its more abundant counterpart, oxygen-16 (^16O), which makes up the majority of oxygen on Earth, ^18O is rare, comprising only about 0.2% of the oxygen in the Earth's water bodies and atmosphere. Due to its unique properties and relative scarcity, ^18O is used in various scientific fields, including paleoclimatology, hydrology, and medicine, for tracing and dating water cycles and understanding climatic processes.

Properties

Oxygen-18 has an atomic mass of 18.998403163(6) u. It has eight protons and ten neutrons in its nucleus, compared to the eight neutrons found in ^16O. This difference in neutron count gives ^18O a slightly higher mass and different nuclear properties. Despite these differences, ^18O is chemically identical to ^16O, participating in the same chemical reactions but at slightly different rates due to the mass difference.

Abundance and Distribution

The natural abundance of ^18O varies across different Earth environments. It is more abundant in the oceans than in the atmosphere. The ratio of ^18O to ^16O in water molecules can be affected by temperature and other environmental factors, making it a useful tool for studying past climates and ocean temperatures. This variation is often expressed using the delta notation (δ^18O) to represent deviations from a standard mean ocean water ratio.

Applications

Paleoclimatology

In paleoclimatology, the δ^18O record from ice cores and marine sediments is a valuable proxy for past temperatures and ice volume. Higher δ^18O values in ice cores typically indicate colder periods, as ^16O is preferentially evaporated and then trapped in ice sheets, leaving behind water with a higher concentration of ^18O.

Hydrology

In hydrology, ^18O is used to trace water movement and to study the water cycle. The distinct ratio of ^18O to ^16O in precipitation versus groundwater can help identify sources of water and understand the processes of evaporation, condensation, and infiltration.

Medicine

In medicine, ^18O-enriched water is used as a tracer in positron emission tomography (PET) scans. This application takes advantage of the isotope's natural abundance and its ability to be incorporated into biological molecules, allowing for detailed imaging of physiological processes.

Isotope Fractionation

Isotope fractionation refers to the partitioning of isotopes between two substances or phases, leading to a concentration difference of isotopes. This process is temperature-dependent for ^18O, making it a useful tool for studying temperature changes in paleoclimate research.

See Also

• Isotopes of oxygen
• Paleoclimatology
• Hydrology
• Positron emission tomography