Applications of Stable Isotopes to Studies of Paleohydrology and Paleoclimatology

Abstract

The term isotope, derived from Greek, means occupying the same position in the periodic table. Isotopes are variant forms of a particular chemical element that have the same number of protons (atomic number) but differ in the number of neutrons in the atomic mass. Isotopes come in two basic types: radioactive (unstable) and stable. Radioactive isotopes are nuclides that are unstable and spontaneously decay into other new isotopes, whereas stable isotopes are nuclides that do not appear to decay radioactively. Hydrogen and oxygen have a number of isotopes, including radioactive and stable isotopes. The two stable isotopes of hydrogen, 1H and 2H (also called deuterium and denoted as D), have natural abundances of 99.9885% and 0.0115% in hydrosphere, respectively. The third isotope of hydrogen, 3H (tritium), is unstable with a half-life of 12.23 years. The stable isotopes of oxygen, including 16O, 17O, and 18O, have natural abundances of 99.762%, 0.0379%, and 0.200% in hydrosphere, respectively. Other isotopes of oxygen are radioactive and very short-lived. In isotope geochemistry, it is a convention to use the atomic abundance ratio of the rare isotope to the major isotope (e.g. 18O/16O, 2H/1H) relative to a standard of known isotopic composition to describe the isotopic composition of samples as: δ (in ‰) = ( Rx Rs − 1 ) × 1000