Geochemistry

Definition: Geochemistry concerns itself with the distribution of chemical elements and their isotopes in the geospheres of earth. Geochemistry examines the geological processes and their cycles which led to this ongoing distribution from the formation of the solar system to the present and the future of Earth. Geochemistry also studies rocks, minerals, soil, water, the atmosphere, and the biosphere. Geochemistry contributes to humanity’s discovery and exploitation of natural resources (from ores to hydrocarbons), the determination of suitable areas for agricultural production, and the mitigation of humanity’s effect on the environment, including conventional and nuclear waste management. Geochemists also contribute to climate studies.

Basic Principles

In the nineteenth century, geochemistry was conceptualized as a science that used chemistry to examine issues in geology. Swiss German chemist Christian Friedrich Schönbein coined the term geochemistry in 1836 and, soon after, the first textbooks in the field were published in Germany. A pioneer of geochemistry was American scientist Frank Clarke. Leading a United States Geological Survey (USGS) group in the early twentieth century, Clarke published the final edition of his geochemical data in 1924. The 1950s saw the building of a solid foundation of the discipline through the influential works of three European geochemists: Finnish geochemists Kalervo Rankama and Thure Sahama, and Austrian Norwegian geochemist Victor Moritz Goldschmidt. Goldschmidt died in 1947 before his influential textbook came out, in 1954. In his honor, the American Geochemical Society hosts the annual international Goldschmidt Conference bringing together scientists in the field.

In the early twenty-first century there are two major directions in geochemistry. One effort examines the formation of the earth and the distribution of chemical elements throughout geological times. This should lead toward understanding of how geological features are formed and precisely determining the age of existing geological formations. A key goal is to develop theories leading toward an extrapolation of these processes into the future. This field links up with planetary sciences, seeking connections between the geochemistry of Earth and that of other bodies in the solar system.

The second emphasis in geochemistry analyzes the current distribution of chemical elements and their isotopes in the geospheres of Earth. The practical applications are in natural resource discovery and exploitation, agricultural optimization, environmental protection, waste management, and engineering solutions.

Core Concepts

Geochemistry is interested in the original distribution of chemical elements on Earth. For this reason, the field also studies the formation of the solar system. Once Earth formed, there began a constant, ongoing process of changing the chemical composition of the planet based on chemical and distribution processes. For geochemists, it is important to learn about the history of these processes and develop methods to determine the age of particular rock and soil samples to place them in geologic time. Finally, geochemistry is concerned with understanding where chemical and distribution processes may lead in the future, determining the fate of the planet.

Origin of the Universe and Solar System. To explain the distribution of the chemical elements on Earth, geochemists must study the past. They start their inquiry with the calculated origin of the universe—the big bang, some 14 billion years ago—when all of its matter and energy was created. As the first stars formed, nuclear fusion of their core elements of hydrogen and helium moved them along a path of development leading to the creation of the chemical elements through nucleosynthesis; these elements then spread through the cosmos. Once the solar system formed out of a contracting molecular cloud of interstellar dust, the sun and planets appeared. For this point, geochemists look at the initial distribution of elements on Earth. They are aided in this endeavor by the study of the chemical composition of meteorites that have crashed on Earth. Old objects from the early days of the solar system, these meteorites preserve a record of the distribution of elements during those times. To look back into time, geochemists have also analyzed samples of lunar rock brought back by the Apollo missions to the moon; they have also remotely analyzed Martian rocks. These extraterrestrial rocks have not weathered as their counterparts on Earth have and therefore have preserved their earlier composition. The field of these studies is also called cosmochemistry.

Geospheres. Geochemists distinguish between the different geospheres of Earth. There is the core, knowledge of which comes from geophysical calculations and comparison with suitable meteorites. Earth’s core is very dense, with a solid inner and liquid outer core. Earth’s mantle, the largest geosphere, is analyzed in its reaction with the planet’s crust, which geochemists can observe both on the bottom of the oceans (as oceanic crust) and on dry land (as continental crust). The crust and the top portion of the mantle are defined as the lithosphere. Analysis of the biosphere relies on biochemistry to understand the chemical processes leading to the creation of a habitable area for humanity and other life. The Earth’s hydrosphere—primarily made up of Earth’s oceans and seas—is subject to geochemical analysis as well. Finally, analysis of the atmosphere examines the composition of the outermost geosphere of Earth, its development, and its future changes.

Inorganic Chemistry. Geochemistry relies heavily on inorganic chemistry, including crystal chemistry, to determine the chemical composition of rocks and minerals found on earth. Inorganic chemistry provides geochemists with the theoretical and analytical tools to determine how the electronic structure of the atoms of a particular element influences its actual properties. The periodic table of elements is utilized to classify the chemical elements found on Earth and to group them according to their characteristics.

Thermodynamics. The three laws of thermodynamics are crucial in geochemistry, because they allow analysis of the chemical reactions taking place on Earth over geologic time. Thermodynamics provide geochemists with a theoretical foundation to model how the chemical composition of Earth changed over time as heat influenced the distribution of chemical elements and the formation of new rock compounds.

Geochemical Processes. The understanding and analysis of geochemical processes is a key concept in geochemistry. Of particular relevance are the reactions of acids and bases, salts and their ions, as well as oxidation-reduction reactions. Classification of clay minerals is done according to the processes leading to their creation. Key geochemical processes are weathering and diagenesis, the latter referring to the change of sedimentary material during the process of becoming a sedimentary rock formation. The analysis of geochemical processes yields an understanding of the transport of elements moving them from one site to another over time.

Isotope Chemistry. Geochemists study isotopes of the elements. Elements have isotopes if they exist as separate atoms that have the same number of protons, but a different number of neutrons. This gives each isotope a different mass number made up from its protons and neutrons. Elements can have both stable isotopes and radioactive isotopes, as is the case with carbon. The decay of radioactive isotopes generates heat, and its measurement by geochemists is used to date events during geological time. This is done to learn more about the origin of igneous rocks, formed from magma and lava, and the chemical composition of the lithosphere. Isotopic fractionation—or the separation of isotopes during chemical processes—is analyzed to date rocks and fossils and to provide a measurable record of past climate changes and ocean temperatures.

Biogeochemistry. This subdiscipline of geochemistry was developed by Russian geochemist Vladimir Vernadsky, who introduced the idea in his 1926 book The Biosphere (translated into English in 1986). Biogeochemistry works on the premise that, since its appearance, life has significantly shaped the geological features on Earth. Of particular contemporary interest are the study and modeling of chemical element cycles such as that of carbon and nitrogen that affect life on Earth. Biogeochemistry is highly interdisciplinary, combining scientific research in many fields to develop its models for life’s impact on the biosphere.

Applications Past and Present

Mining. The principles of geochemistry were applied first to mining in order to analyze the chemical composition of ores centuries before geochemistry developed in its modern form. Two German Renaissance scientists, Georgius Agricola and Lazarus Ercker, were the first to describe what can be recognized as being methods for chemical analysis of different ores. In 1556, one year after his death, Agricola’s De re metallica(On the Nature of Metals, 1912) appeared. Ercker published his Beschreibung der allerfürnemsten Mineralischen Erzt und Bergwerksarten (Treatise on Ores and Assaying, 1951) in 1580. However, it was not until American chemist Frank Clarke published The Data of Geochemistry in 1908 that the principles of geochemistry were used in their modern scientific form. Clarke used geochemistry to analyze ore deposits he encountered during his work as chief chemist of the USGS, begun in 1883 and ending in 1925. His successors at the USGS developed the basis of contemporary geochemical prospecting by 1947.

Contemporary geochemists work in two capacities for the mining industry. First, as mineral exploration geologists, they bring their knowledge to prospecting for new ore deposits. They survey and take soil and rock samples from a prospective field, also looking for chemical anomalies in their samples that hint at an ore deposit. Second, as mine production geologists, they are responsible for providing the scientific and technical information for mining and processing mineral deposits efficiently. This includes consideration of economic, safety, and environmental aspects.

Petroleum Geochemistry. Petroleum geochemistry has been used to discover new oil fields since some time after the beginning of the oil age in the mid-nineteenth century. Using some of the principles and techniques from mineral prospecting, petroleum geochemistry has specialized in the quest to find new oil and natural gas deposits. Because of the immense value of a successful oil deposit discovery, petroleum geochemists are among the best-paid geochemists.

When supporting prospecting for oil deposits, petroleum geochemists seek to identify source rocks where hydrocarbons like oil and natural gas have formed from organic matter. Next, as geochemistry is concerned with tracking the distribution of chemical elements on the earth, their techniques and methods can be used to analyze possible paths and distribution of hydrocarbons from the original source rock. Observed findings of hydrocarbons on the surface or in water—getting there through seepage or leaks, for example—are analyzed to be tracked back to locate new deposits. Petroleum geochemists rely on standard geochemical methods such as isotope chemistry, and use tools like gas chromatography and mass spectronomy to analyze source rocks and hydrocarbon traces.

Radiometric Dating. In 1905, British chemist and physicist Ernest Rutherford applied the principles of radioactivity—discovered in 1896 by French physicist Henri Becquerel and his PhD students Marie Curie and Pierre Curie—to find the age of rock samples. Because radioactive isotopes of an element decay at a steady rate to form isotopes of another element, measuring and comparing the amount of the so-called parent element with amount of the daughter element in a rock or mineral sample enables the determination of its geological age. This is possible because of the exceptional length of the half-life of some radioactive isotopes, lasting billions of years. For example, the decay of uranium-238 (U-283) into lead-206 (Pb-206) has a half-life of 4.5 billion years, almost that of the contemporary estimate of Earth’s overall age. Thus, comparison of the measured relationship between uranium-238 and lead-206 in a rock sample can date its origin accurately in geological time.

In 1913, British geologist Arthur Holmes was the first to use radiometric dating of rock samples to calculate the age of Earth. His publication The Age of the Earth(1913) arrived at an age of 1.6 billion years. This shocked those in the scientific community who believed that the planet was much younger, even though Holmes’s figure was still about 3 billion years short of Earth’s true age. By the early twenty-first century, radiometric dating had been firmly established as a geochemical method for calculating the exact periods of Earth’s geological time scale. Because of its relatively short half-life of 5,730 years, the decay of the carbon-14 isotope into nitrogen-14 isotopes is used in radiocarbon dating for determining the age of archeological objects and more recent rock and soil samples.

Geochemical Modeling. Understanding geochemical processes has led to geochemical modeling. This is done to analyze the effects of chemical reactions behind geochemical cycles, and to model the cycles that have affected geological development and life on planet. Of particular interest has been geochemical modeling of geochemical cycles that directly affect humanity, such as the cycle of surface, ground, and atmospheric water. Modeling the mixing and dilution of chemical elements during geological processes—work undertaken to account for the chemical composition of the resulting binary mixtures—and assessing the relative abundance of elements ultimately seeks to develop a better understanding of Earth’s future development.

American geochemists Robert Garrels and Charles Christ applied geochemical modeling to studies in aqueous geochemistry, related to the chemical study of the world’s waters. They published their results in Solutions, Minerals and Equilibria (1965). While their study relied on a physical equilibrium status, contemporary geochemical modeling also examines nonequilibrium states. Of particular concern is reactive transport modeling in porous media, which simulates how chemical reactions influence the transport of a fluid medium like magma through the planetary crust.

Climate Change Studies. Geochemical analysis of the chemical composition of the atmosphere and the chemical reactions that occur there has found a major application in geochemistry’s contribution to the study of climate change, and in particular human-caused, or anthropogenic, climate change. In the 1970s, geochemists and other scientists found that the ozone layer protecting the Earth’s surface from the sun’s ultraviolet radiation was diminishing. Normally, ultraviolet radiation breaks up the molecular bond of the oxygen molecule to form ozone in the stratosphere, ten to fifty kilometers (six to thirty miles) above Earth. There, equilibrium of ozone creation and destruction existed. However, when people began to release chlorofluorocarbons and bromofluorocarbons—used for aerosols and refrigeration—into the atmosphere, the materials traveled into the stratosphere. There, fluorocarbons triggered chemical reactions destroying ozone molecules.

In reaction to these scientific findings, the United States, Canada, and Norway became the first countries to ban production of fluorocarbon-based aerosols in 1978. After discovery of the ozone hole over Antarctica in 1985—a discovery to which geochemists contributed—the Montreal Protocol of 1987 phased out production of all fluorocarbon products by 1996. This international agreement was amended over time to phase out other ozone destroying compounds, by 2010 for some and 2030 for others. By 2006, there was clear scientific evidence that the ozone layer was recovering, a process expected to be completed in 2050.

The geochemical study of global warming contributes to another key current issue. Beginning with an article in the American journal Science on August 8, 1975, awareness has risen that the increased release of so-called greenhouse gases into the atmosphere causes global warming. The issue has been contentious, as most greenhouse gases, particularly carbon dioxide, are created by human industrial activity, and limiting the emission of greenhouse gases thus has significant economic cost. Nevertheless, there has arisen international concern that the increasing greenhouse gases in Earth’s atmosphere will lead to an increased atmospheric absorption of infrared radiation emitted by rock and soil on earth, leading to a rise in Earth’s temperature. This rise could cause droughts as well as floods, particularly if the ocean levels were to rise due to the melting of polar ice caps. In response, international agreements to limit greenhouse gases were concluded. The United Nations Framework Convention on Climate Change was signed on May 9, 1992, and became effective on March 21, 1994. Out of it, the Kyoto Protocol arose, an international agreement to lower greenhouse gas emissions. Even though the Kyoto Protocol was signed on December 11, 1997, and became effective on February 16, 2005, and ended in 2020. The Paris Agreement, which became effective in 2015, took its place.

Soil Studies. Geochemistry has contributed to soil studies, in particular the analysis of soil formation and the distribution and transport of chemical elements in soil. Geochemists have participated in soil surveys mapping and classifying the different soils of a region and their possible uses in agriculture. Russian geologist Vasily Dokuchaev has been commonly credited with establishing soil science through his influential 1883 publication on Russian soils. In the twentieth century, geochemists added to soil science through research in soil biochemistry and soil mineralogy. With humanity’s need for food growing, cultivating and managing soil with the goal of optimizing agricultural production has been a key concern, to which geochemical research has contributed. The issue of understanding and mitigating human-caused soil degradation through pollution and contamination has also incorporated geochemical research.

Management of Conventional and Nuclear Waste. Geochemists contribute to both conventional and nuclear waste management. Geochemical expertise is needed in designing new landfill deposits with mitigation of groundwater and soil pollution. For nuclear waste, geochemists help to identify the most stable geological formations for final storage. They contribute geochemical expertise to the study of the processes in the rocks around existing and planned nuclear waste repositories and model rock evolution over geological time, as nuclear waste decays very slowly.

Engineering. Geochemistry is applied to develop engineering solutions, particularly in geologically unstable regions. Knowledge of the geochemistry of a proposed engineering site supports design of customized solutions. Geochemists have increasingly worked in cooperation with engineers and architects.

Social Context and Future Prospects

Geochemistry supports humanity’s quest for knowledge about the origin of its planet and the solar system, the chemical composition of its habitat, and possible further developments of Earth. In its practical applications, geochemistry also contributes to exploration and the efficient use of natural resources—particularly oil, natural gas, and mineral ores—and a sustainable use of the planet’s water and soil. These varied fields should provide geochemists with ample opportunities for further research and industry employment.

For geoscientists, both the US Bureau of Labor Statistics (BLS) and the American Geosciences Institute indicate about average future employment growth. These employment prospects also came with good salary expectations. The best-paid jobs were in petroleum geochemistry.

Bibliography

Albarède, Francis. Geochemistry: An Introduction. 2nd ed. Cambridge: Cambridge UP, 2013. Print.

Faure, Gunter. Principles and Applications of Geochemistry. 2nd ed. Upper Saddle River: Prentice, 1998. Print.

"Geochemistry." American Chemical Society, www.acs.org/careers/chemical-sciences/fields/geochemistry.html. Accessed 28 Aug. 2024.

Krauskopf, Konrad B., Elizabeth S. Carter, and Arthur Beiser. The Physical Universe, 18th ed. McGraw Hill LLC, 2022. Print.

Li, Yuan-Hui. A Compendium of Geochemistry. Princeton: Princeton UP, 2000. Print.

Misra, Kula. Introduction to Geochemistry: Principles and Applications. Chichester: Wiley, 2012. Print.

United Nations Framework Convention on Climate Change. “Status of Ratification of the Kyoto Protocol.” United Nations Framework Convention on Climate Change. UNFCCC, 2014. Web. 29 Dec. 2015.

United States. Bureau of Labor Statistics. “Geoscientists.” Bureau of Labor Statistics Occupational Outlook Handbook. US BLS, Office of Occupational Statistics and Employment Projections, 17 Apr. 2024, www.bls.gov/ooh/life-physical-and-social-science/geoscientists.htm. Accessed 28 Aug. 2028.

Walther, John. Essentials of Geochemistry. 2nd ed. Sudbury: Jones, 2009. Print.