Profile of Joel D. Blum

Joel D. Blum uses isotopes to solve an unusually wide range of research problems. Renowned for innovative approaches, Blum, a professor of earth and environmental sciences at the University of Michigan, was among the first scientists to apply methods developed largely for lunar and planetary studies to answer longstanding questions about ecology and the environment. His research group has also developed methods for making high-precision measurements of metallic element isotope ratios in terrestrial, aquatic, and atmospheric systems. Elected to the National Academy of Sciences in 2020, Blum used mercury isotopic signatures to investigate the world’s deepest ocean trenches, and the findings are reported in his Inaugural Article (1). Blum and his colleagues found that anthropogenic mercury reaches these remote areas primarily via sinking fish carcasses, an insight that could aid global mercury pollution reduction efforts.

Joel Blum. Image credit: Alec Tremaine (photographer).

Family of Outdoor Enthusiasts

Born in Cleveland, Ohio, Blum was influenced by his parents. His father, who was a professor of applied social science at Case Western Reserve University, inspired Blum’s interest in research. His mother, an avid outdoorswoman, shared her enjoyment of hiking, skiing, and other activities. Blum says, “As a family, we camped and hiked and traveled every summer. Not surprisingly, my brother Alex became a geologist.”

For his undergraduate studies, Blum attended Case Western Reserve University, where he initially majored in political science. He decided, however, that geology was more compatible with one of his favorite pastimes, mountain climbing, and added a geology double major. After earning a Bachelor’s degree in 1981, Blum went to the University of Alaska, Fairbanks, for a Master’s degree in geological science.

Isotope Geochemistry Off the Grid

Desiring to stay in Alaska, Blum took a job as an exploration geologist for a mining company in 1982. He then became a project supervisor for the Alaska Department of Natural Resources, mapping geology in Alaska’s remote mountains during the summers and running the state’s geochronology laboratory during the winters. Blum says, “I lived in a remote one-room cabin way off the grid with my wife, Cindy, at a time before that was fashionable.”

The California Institute of Technology has a long history in stable and radiogenic isotope geochemistry, so Blum elected to continue his education there and earned a doctorate in geochemistry in 1990. His doctoral advisors were Gerald Wasserburg and Edward Stolper. “Jerry taught me the importance of making very careful and precise measurements in the laboratory,” Blum says. “Ed taught me about thinking independently and about the effective communication of science.” At the California Institute of Technology, he also met geochemist Clair Patterson. Blum says, “Patterson’s work using lead isotopes to study lead contamination inspired my work using mercury isotopes to study mercury contamination.”

Cosmochemistry, Asteroid Impact Studies

Blum’s research at the California Institute of Technology largely focused on cosmochemistry. An article coauthored with Wasserburg, Stolper, and colleagues, for example, investigated the origin of micrometer-sized platinum-group element alloys in refractory meteorites (2). These assemblages were thought to have formed before other solid materials in the Solar System. Blum and his team, however, countered the theory and demonstrated through experimental studies that the metals had equilibrated during metamorphism on protoplanetary bodies later in the Solar System’s history.

In 1990, Blum moved to Dartmouth College, in Hanover, New Hampshire, and served as a professor of earth sciences until 1999. At Dartmouth, he continued to conduct planetary science research and expanded his research portfolio. Using neodymium and strontium isotopes, Blum and his team identified the Chicxulub crater in Mexico as the source of impact glass spherules found globally in sediment at the Cretaceous-Tertiary boundary (3). Blum says, “The finding helped link this crater to the extinction of the dinosaurs.”

Combining Planetary Science and Environmental Science

Blum’s knowledge of technologies developed to study Solar System processes led to a concerted effort to apply them to environmental sciences. He and his colleagues were among the first to use strontium isotopic measurements to investigate mineral weathering and calcium cycling in forest-soil ecological systems (4). The strontium isotopic composition was also used to differentiate between carbonate and silicate weathering in the Himalaya of northern Pakistan, given that Himalayan weathering has been implicated in global climate change on geological timescales (5).

The rate of chemical weathering of the continents is often factored into long-term climate change models. Strontium isotope ratios provide a proxy for weathering, so Blum and a colleague investigated chronosequences of these isotopes and how their release from silicate weathering is affected by glaciation and mountain uplift (6). The resulting calculations help explain the correlation between the rate of marine strontium isotope ratio change and the intensity of glaciation.

Forest and Mountain Geochemistry

In 1999 Blum accepted a position as Professor of Earth and Environmental Sciences at the University of Michigan, where he currently holds the Arthur F. Thurnau, John D. MacArthur, and Gerald J. Keeler professorships. Continuing his active research program, in 2002 Blum reported the findings of a study at the Hubbard Brook Experimental Forest in New Hampshire, identifying that the mineral apatite played an important role in supplying calcium to forests where nutrients had been depleted by acid rain deposition (7). The finding has since been considered in estimations of soil acidification impacts on calcium and phosphorous cycling.

Blum and a colleague later measured the geochemistry of stream catchments in the New Zealand Southern Alps (8). He says, “We used the measurements to clarify the relationships between mountain uplift, physical erosion, and chemical erosion.” A key determination was that models attempting to explain such relationships overestimated carbon dioxide consumption rates due to mountain uplift.

Understanding Mercury Biogeochemistry

Another longstanding subject of interest to Blum is the global biogeochemical cycling of mercury, including problems resulting from mercury contamination. He says, “Part of my career-long effort is to get the word out on how big of an issue mercury poisoning can be and how important it is to limit mercury emissions.” Mercury accumulation in fish is particularly concerning given its direct link to human health. Blum and his team demonstrated that algal blooms in aquatic food webs can dilute methylmercury, an organic form of mercury that is readily absorbed by animals (9).

A breakthrough in understanding mercury biochemistry occurred in 2007 when Blum and a colleague reported the occurrence of mass-independent mercury isotope fractionation in laboratory experiments and in the environment (10). Mass-independent fractionation processes are rare, happening primarily during photochemical reactions. They are therefore useful in estimating the loss of methylmercury via photoreduction in aquatic ecosystems. Blum says, “This research paved the way for hundreds of papers that have used mass-independent mercury isotope fractionation to better understand the biogeochemistry of mercury.”

Mercury Isotopes in Environmental Geochemistry

Since the identification of mass-independent mercury isotope fractionation, Blum’s research group has developed methods to make high-precision measurements of mercury isotope ratios and has applied these tools to a wide range of biogeochemical research problems. For example, Blum returned to Alaska to measure the mercury isotopic composition of snow samples collected during atmospheric mercury depletion events (11). The study used mercury isotopes to investigate photochemical reactions of mercury on snow surfaces.

Blum’s team also used mercury isotopes to demonstrate that fish obtain mercury directly from contaminated sediments in San Francisco Bay (12). His team was also among the first to apply stable mercury isotopes to human health. In a 2013 article, Blum and colleagues uncovered the degree to which methylmercury is degraded to inorganic mercury in the human body (13). The same year, Blum’s group applied stable mercury isotopes to marine chemistry (14). “We analyzed a range of fish species that feed at varying depths in the North Pacific Ocean,” he says. “This allowed us to determine the depths at which methylmercury was forming and being photo-degraded.”

For these and other achievements, Blum was awarded the 2013 Clair C. Patterson Award from the Geochemical Society. The honor is particularly meaningful to Blum, given that the award was named after one of his role models in the field of environmental geochemistry.

From the Surface to Deepest Trenches

Since Blum’s initial application of mercury isotopes to marine chemistry, his team has been conducting research to determine how oceanic mercury is transferred from shallow to deep regions. Blum and colleagues studied mercury stable isotopes in flying fish as a monitor of surface photochemical methylmercury degradation (15). The researchers found that methylmercury in these fish is residual from a common photochemical reduction mechanism.

For his Inaugural Article, Blum led mercury isotopic research at two of the deepest ocean trenches: The Kermadec and the Marianas (1). The results showed that mercury reaches these remote locations in the Pacific Ocean and is primarily derived from the atmosphere via rainfall. “We have found that this mercury is deposited from the atmosphere to the surface ocean and is then transported to the deep ocean in the sinking carcasses of fish and marine mammals as well as in small particles,” he says. “Some of this mercury is emitted by natural processes, but much of it comes from human activity.” Sources of anthropogenic mercury include coal combustion, mining, waste incineration, metal smelting, and other manufacturing.

Blum’s findings are informing models used to draft legislation limiting mercury emissions, such as the international Minamata Convention on Mercury and the US Mercury and Air Toxic Standards (MATS). Implementation of MATS, informed in part by Blum’s earlier research, began during the Obama administration but was recently rolled back.

Inspiring a New Generation of Scientists

Blum’s recent and ongoing projects reflect the breadth of his research interests. He recently worked with colleagues to create a strontium isotope ratio map of southern Africa to analyze ostrich eggshell beads from archaeological sites (16). This allowed the multidisciplinary research group to explore patterns of spatial networking in humans dating back 30,000 years. At present, Blum is working on the geochemical behavior of mercury in forests and streams, while continuing to study mercury in the marine environment.

Blum attributes much of his success to the talents of his many students, postdoctoral associates, and technical staff over the years. To his students he says, “Careers can only be planned so much. There is a lot of serendipity involved. Do the things you love to do, keep your eyes open for opportunities, and take advantage of them as they come along.”