Geochemical analysis is the process through which scientists determine the chemical compounds that constitute Earth, its atmosphere, and its seas. To a lesser degree, geochemical analysis can also be used to understand extraterrestrial materials such as moon rocks or Martian soil samples. The process or geochemical analysis requires a thorough grounding in chemistry and Earth sciences, as well as an understanding of the different ways elements can interact in a given geologic situation. Geochemical analysis can be used to help predict where petroleum, metals, water, and commercially valuable minerals may be located. It can also be used to predict or trace leaks from waste disposal sites and to track and understand fluctuations in Earth’s climate throughout its history.
Geochemical analysis became important in the nineteenth and twentieth centuries, when chemists first began investigating the compounds that formed naturally in the earth, air, and water. Much of this early work was credited to a chemist named V. M. Goldschmidt, who with his students created detailed charts of the chemical breakdown of common compounds, mainly igneous rocks. He also created a series of guidelines known collectively as Goldschmidt’s rules for understanding the different ways in which elements interact to form different types of rock. Scientists have expanded on Goldschmidt’s program, forming a series of disciplines that help them predict and interpret the chemical composition through time of this planet, other objects in the solar system (including planets), and their constituent ingredients. Goldschmidt based his analysis of chemical behavior on two separate items: size and electrical charge.
Later scientists have added radiation to the process of geochemical analysis, grouping elements by their radioactive and stable isotopes. Isotopic analysis can give clues to the place of origin of the compound, and the environment in which it was first put together. Isotopes are also used to determine the age of a compound, and the study of the process through which they decay from one form to another is known as geochronology. Astronomers have discovered certain isotopes in compounds located in celestial bodies— like supernovae—which have relatively short half-lives, and they use these substances to help date the formation of the universe.
One of the most commercially popular subfields of modern geochemistry is geochemical prospecting, which is employed to locate such metals as uranium and gold or oil and natural gas. The method of geochemical prospecting was pioneered in Europe and the Soviet Union during the 1930s. It was adopted by prospectors in the United States after World War II (1914–1918). Prospectors find that the most profitable way to search for valuable rock and mineral samples is often to look in areas that have undergone extensive weathering, especially the beds of streams. Using their knowledge of weathering and dispersion patterns, geochemical prospectors examine samples drawn from areas where streams intersect each other and from places where fault lines to detect the presence of valuable substances. They also can detect minerals that have undergone chemical decomposition by analyzing the surrounding water and sand and silt deposits for trace remnants, which form a characteristic spread known as a secondary dispersion halo.
By examining the characteristics of these elements and comparing the results to a series of known features in areas like valency, ionic size, and type of chemical bond, geochemists can discover if commercial valuable minerals are present in the area. Other elements, especially volatile ones such as chlorine, fluorine, sulfur, carbon dioxide, and water, also serve as indicators of elements that may be found in the area. Prospectors searching specifically for petroleum look for a polymer called kerogen, thought to be a substance falling between the original organic material that makes up petroleum or natural gas, and the final product, in soil and rock samples. The presence of radon, which can be detected relatively easily because of its characteristic alpha radioactivity, in the water of streams is often an indicator of uranium deposits.
Geochemists have also developed a variety of innovative and cost-saving ways of performing geochemical analysis without requiring to be in direct contact with the rocks and minerals they are examining. One relatively common way is to examine the surface flora and fauna for traces of chemicals or metals. Certain plants growing in contaminated areas develop characteristic diseases, such as chlorosis or nongenetic dwarfism. Contamination can also be detected from chemical residue collected in the internal organs of fish, molluscs, and insects. Some geochemists have used dogs to recognize and locate minerals that are found in combination with sulfur compounds by teaching them to sniff out the gasses released in the oxidation process. Prospectors also use aerial surveys, computer mapping and modeling, and atomic absorption spectrometry to gather clues as to where the minerals they are seeking can be found. Apparatuses that can record gamma radiation are mounted in airplanes and used to locate radioactive minerals.
By examining the chemical composition of sea water and polar ice, geochemists can draw conclusions and make predictions about the environment. Although natural weathering processes can take various trace elements into the sea or lock them into ice caps, scientists also find that by analyzing these compounds they can determine the impact which humans are having on Earth and detect evidence of climate change, either induced by humans or as the result of natural processes, such as ice ages.
Geochemists also make valuable contributions to understanding the history of Earth in general and human beings in particular. They perform isotopic analyses on cores drawn from rock strata or chemical breakdowns on ice cores to determine how the world’s climate has shifted in the past. Specific events, for instance the ash fallout of a large volcanic eruption like Mount St. Helens or Krakatoa, or records of the hydrocarbons released by factories in Europe and American during the Industrial Revolution leave chemical traces in the sediments of sea and lake beds, and in the unmelting ice of the polar regions.
Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall.
Walther, J.V. Essentials of Geochemistry. Sudbury, MA: Jones and Bartlett, 2005.
Kenneth R. Shepherd