Volume 6, Issue 45 Summer 2009
Atomic Tracers
Donald DePaolo is the founder and director of the Isotope Geochemistry Center, a joint research facility of UC Berkeley and Lawrence Berkeley National Laboratory, and director of the LBL Earth Sciences Division. Image credit: courtesy Donald DePaolo
As anyone who has watched "CSI" knows, police use blood, mud, fibers and other environmental clues to pin suspects to their crimes. Such tracers can help reconstruct past events as though it were captured on film.
Donald DePaolo, a Berkeley professor of geochemistry, and head of the Earth Sciences division at Lawrence Berkeley National Laboratory (LBNL), is an expert at interpreting environmental tracers of a far smaller kind. He uses isotopes—atoms of the same element with different numbers of neutrons—to piece together the history of rocks, water, pollution, and even air.
"Because these isotopes have different masses, they also have slightly different chemical properties," DePaolo says. "We can use these small differences to learn all kinds of things about the Earth, biological systems, and even the atmosphere."
Thanks to the existence of isotopes, a single cup of water might contain up to six different types of water molecules. Because lighter isotopes tend to evaporate faster, water vapor has a higher proportion of light than heavy molecules. For this reason, snow, polar ice, and even coastal versus inland precipitation each have distinct isotopic signatures.
The many projects DePaolo has conducted include an analysis of calcium isotopes in extreme environmental conditions such as those found in Death Valley's brine springs. Image credit: courtesy Donald DePaolo
A similar effect occurs with the calcium ions living creatures use to build shells and bones. The chemical differences between isotopes means that the proportions of each present in animal bones or shells are distinctive. DePaolo's understanding of these vital effects once got him evaluating possible evidence of life on Mars.
When scientists identified calcium minerals known to be produced by living organisms within a Martian meteorite, they asked DePaolo if the deposits had biological origins. Though DePaolo found that life isn't necessary to produce these minerals, he realized he could apply his newfound knowledge to a very different aspect of chemistry: crystal formation.
Because light calcium isotopes tend to move faster and react more rapidly, more of them are incorporated into crystals grown under higher heat and temperature conditions. In essence, DePaolo says, "the isotopes keep track of what's going on as the crystal was made" - information important to both geology and materials science.
Collecting samples of volcanic rocks from the Western slope of Mauna Kea volcano in Hawaii. The samples were used to test a new method of dating young lava flows using natural uranium and helium in the mineral olivine. Image credit: courtesy Donald DePaolo
Just this spring, the Isotope Geochemistry Center that DePaolo heads was awarded a $20 million U.S. Department of Energy (DOE) grant to find better ways to store carbon dioxide underground. The idea is to inject this greenhouse gas underground, removing it from the atmosphere and reducing human contributions to global warming. Researchers affiliated with this Center for Nanoscale Control of Geologic CO2 will study how carbon dioxide interacts with rocks, such as sandstone, to maximize storage capacity and demonstrate that the gas will remain underground indefinitely. The Center, housed at LBNL, is one of 46 new Energy Frontier Research Centers (EFRC) funded by the DOE with a total planned commitment of $777 million over five years.
But DePaolo's wildest project involved piercing the heart of a Hawaiian volcano. As leader of the Hawaiian Scientific Drilling Project, he sank a borehole more than two miles into the ground at a disused rock quarry near Hilo International Airport.
Because Hawaii's lavas were erupted from deep underground, "it's one of the few places where you can make measurements of rocks near Earth's surface, and claim that you're actually looking 3,000 kilometers down into the planet," says DePaolo.
Olivine crystals separated from a basalt lava flow. Small amounts of uranium, thorium, and helium in these crystals can be used to determine the ages of the eruptions. This method can be used to date rocks that cannot be dated by other means, and is an important new means to aid studies in volcanology and anthropology. Image credit: courtesy Donald DePaolo
The sediment cores extracted over the project's two decades have given scientists a new perspective on the great mantle plumes that formed the Hawaiian Islands. DePaolo's research has focused on the trace amounts of helium detected in the lava. He sees the presence of this lightweight gas as evidence that material is being transferred from the planet's core into the molten lava plume.
The core was likely the first portion of the Earth to come together. At that time, the sun was surrounded by a gas cloud rich in helium. "If some of the Earth formed while there was that much helium around, a fair amount would have ended up in the core," DePaolo says.
All in all, says DePaolo, isotope science has suited his wide-ranging interests to a T. "Everything about it is interconnected. You can look at a lava flow in Hawaii and start to think about how the stars and planets formed 4.5 billion years in the past. That's the attraction of the field."