Isotope geochemistry / Геохимия изотопов

Isotope geochemistry has grown over the last 60 years to become one of the most important fields in the earth sciences. It has two broad subdivisions, namely, radiogenic isotope geochemistry and stable isotope geochemistry. These subdivisions reflect the two primary reasons why the relative abundances of isotopes of elements vary in nature, which are radioactive decay and chemical fractionation; in this context, “fractionation” is any process in which the isotopes of the same element behave differently. One might recognize a third subdivision, i.e., cosmogenic isotope geochemistry, where interactions with high-energy cosmic rays produce nuclear changes. The growth in the importance of isotope geochemistry reflects its remarkable success in attacking fundamental problems of earth science, as well as problems in astrophysics, physics, and biology (including medicine). Isotope geochemistry has played an important role in transforming geology from a qualitative, observational science into amodern quantitative one.To appreciate the point, consider the Ice Ages, a phenomenon that has fascinated the geologist and the layman alike for more than 150 years. The idea that much of the Northern Hemisphere was once covered by glaciers was first advanced by Swiss zoologist Louis Agassiz in 1837. His theory was based on observations of geomorphology and modern glaciers in the Alps. Over the next 100 years, this theory advanced verylittle, other than the discovery that there had beenmore than oneice advance and that it was a global phenomenon. No one knew exactly when these ice advances had occurred, how long they lasted, or why they occurred. Stable and radiogenic isotopic studies in the last 60 years have determined the exact times of these ice ages and the exact extent of temperature change (i.e., about 3 C or so in temperate latitudes, and more at the poles). Knowing the timing of these glaciations has allowed us to conclude that variations in the Earth’s orbital parameters (the Milankovitch parameters) and resulting changes in insolation have been the pacemaker of these ice ages.  <...>