Metal Stable Isotopes: Signals in the Environment

Metal stable isotopes are now being used to trace metal contaminants in the environment and as indicators of human systemic function where metals play a role. Stable isotope abundance variations provide information about metal sources and the processes affecting metals in complex natural systems, complementing information gained from surrogate tracers, such as metal abundance ratios or biochemical markers of metal metabolism. The science is still in its infancy, but the results of initial studies confirm that metal stable isotopes can provide a powerful tool for forensic and biomedical investigations.

Higher plants induce chemical reactions in the rhizosphere, facilitating metal uptake by roots. Fractionation of the isotopes in nutrients such as calcium, iron, magnesium, and zinc produces a stable isotope composition in the plants that generally differs from that of the growth medium. Isotope fractionation also occurs during transport of the metals within most plants, but its extent depends on plant species and on the metal, in particular, on the metal’s redox state and what ligand it is bound to. The metal stable isotope variations observed in plants create an isotope signature of life at the Earth’s surface, contributing substantially to our understanding of metal cycling processes in the environment and in individual organisms.

Theoretical, experimental, and empirical methods for estimating isotope fractionations often complement one another in precision and ease of application. In metal isotope systems, a combined approach to calibrating stable isotope fractionation shows great promise, but it is sometimes necessary to resolve significant disagreements between theoretical models and empirical data. Here we introduce some of the principles and techniques used to estimate metal isotope signatures in low-temperature environments, and we highlight potential sources of uncertainty and error. We also discuss strategies for integrating theoretical calculations with data from laboratory experiments and natural sample suites.

Calcium and magnesium isotope fractionation, recorded in shells of marine organisms that biomineralize calcium carbonate, is helping scientists to understand the transport of trace elements from seawater to the site of calcification, as well as trends in seawater composition throughout time. This knowledge would be difficult to obtain otherwise, and is important, especially now, for assessing the threat of ocean acidification to shell-producing marine organisms.

Knowledge about paleoredox conditions is essential for reconstructing how the oxygenation of the Earth’s surface environment has changed through time and affected the evolution of life on our planet. Some metal stable isotope systems, such as Mo isotopes, record the extent of ocean oxygenation directly. Others, such as Fe isotopes, record redox conditions indirectly through their effects on biological processes that are sensitive to the presence of oxygen. Studies of modern analogs and experiments have improved our understanding of the processes responsible for the observed isotope trends and have helped to advance the use of these isotope tools for paleoredox reconstructions.

Mercury (Hg) is a redox-active trace metal that is viewed internationally as a priority pollutant. Research into Hg stable isotope biogeochemistry is rapidly providing new insight into the behavior of Hg. With the recent discovery that Hg can exhibit both mass-dependent (MDF) and mass-independent fractionation (MIF) (range of >6‰ for both), Hg isotopes are providing a valuable new tool for tracing this important toxin through the environment. MDF alone, which occurs during redox transformations, biological cycling, and volatilization of Hg, can be exploited to increase understanding of the processes that control Hg distribution and bioaccumulation. The addition of MIF signatures greatly increases the usefulness of Hg isotopes because MIF provides a unique fingerprint of specific chemical pathways, such as photochemical reduction.

Recent advances in mass spectrometry have allowed isotope scientists to precisely determine stable isotope variations in the metallic elements. Biologically influenced and truly inorganic isotope fractionation processes have been demonstrated over the mass range of metals. This Elements issue provides an overview of the application of metal stable isotopes to low-temperature systems, which extend across the borders of several science disciplines: geology, hydrology, biology, environmental science, and biomedicine. Information on instrumentation, fractionation processes, data-reporting terminology, and reference materials presented here will help the reader to better understand this rapidly evolving field.