Noble Gas Geochemistry

Noble gases are very rare elements on Earth, because they do not form compounds with other elements and hence were almost completely lost when the Earth was formed 4500 million years ago. Because they are so rare, even tiny amounts of noble gases produced later can be detected and allow us to study a multitude of processes.

Some projects:

Tiny amounts of noble gases are produced in rocks at the immediate surface of the Earth when fast elementary particles from the cosmic radiation penetrate a rock and hit a target atom, for example silicon. The concentrations of these "cosmogenic" noble gases are thus a measure how long a rock has been at the surface of the Earth. We study the "exposure ages" of boulders on glacial moraines to date advances and retreats of glaciers in the past several ten thousand to several million years (for example in Antarctica). We also use cosmogenic nuclides to measure how fast landscapes erode in wet and dry climate zones, respectively (for example in the Atacama desert).

Over geologic time, radioactive decay of uranium and thorium in minerals such as apatite (calcium phosphate) and zircon (zirconium silicate) produces measurable amounts of helium. An age can be calculated by separately measuring the parent and daughter nuclide abundances in mass spectrometers. Because helium is retained in minerals only up to a certain temperature (about 70 ?C in apatite), the (U-Th)/He clock tells us when a rock now at the Earth's surface cooled below this “closure temperature” in the subsurface. This allows us to study, for example, how quickly mountains are uplifted and eroded.

When water is in contact with the atmosphere, some of the noble gases from the atmosphere are dissolved in the water, and the amounts of dissolved gases depend on the temperature. The water may then enter a groundwater system, where it separates from the atmosphere. Some groundwater systems contain water which separated from the atmosphere at different times, and hence form a "climate archive". Noble gas analyses in groundwater allow us therefore to determine the mean temperature at the Earth's surface over the past several ten thousand years. We are also exploring the use of other climate archives, such as waters in lake sediments and in fluid inclusions in dripstones.