New X-Ray Fluorescence Core Scanner Reveals Clues to Earth's Past Climate and History

Article excerpt

Fans of the Star Trek science fiction series will remember the "tricorder," an all-purpose sensor that Kirk, Spock, and McCoy waved over objects, from rocks and spaceships to alien life forms, to determine what they were made of.

"We now have our own version of the tricorder," said Liviu Giosan, a coastal geologist who was instrumental in acquiring and setting up an X-ray fluorescence (XRF) core scanner at Woods Hole Oceanographic Institution. It is the first of its kind in the United States.

Like the tricorder (and some instruments on NASA's Mars rovers), the XRF reveals intimate details of the composition of ancient mud and rock, which can contain a variety of clues about past climate and environmental conditions on Earth. The $450,000 instrument simultaneously captures digital photographs and X-ray images of samples, while detecting measurable amounts of any of 80 chemical elements from aluminum (atomic number 13) to uranium (atomic number 92) without breaking the surface of the core. It gathers all of this information in a matter of hours.

Traditionally, scientists (or more likely, their students) spend months to years sifting through mud and sediment cores to measure carbon, trace metals, pollen, microscopic shells, and other materials that accumulate over time in marshes and dunes or on the seafloor. By analyzing the sequential layers of this preserved detritus, scientists can reconstruct past changes in ocean temperatures, rainfall, wind, and vegetation patterns. They can determine when droughts, hurricanes, or blooms of marine plankton occurred.

To study core samples, scientists must cut them down the middle and meticulously dissect them. It is a time- and labor-intensive process that gradually destroys unique--and not easily replaceable--cores. Sometimes cores must be shipped to other locations (in WHOI's case, a local hospital) if scientists need an X-ray view to see layers undetectable by the eye.

With the XRF scanner, researchers place a core into a motorized, computer-controlled carriage that slides the sediments past a camera and X-ray source. A charge-coupled device captures high-resolution images a millimeter at a time. Two X-ray beams are then fired into the core: One creates an X-ray image; the other stimulates the atoms in the sample, causing energy to be emitted in the form of electromagnetic radiation (fluorescent light). Each element releases radiation with a distinctive wavelength that can be detected by the XRF.

It does all this without taking a single bit of the core, meaning that other scientists can virtually dig into the same mud and pursue other chemical clues locked inside.

"We now have the ability to generate high-quality, high-resolution geochemical records very quickly," said Konrad Hughen, a WHOI geochemist who has been reconstructing ancient climate from sediments collected near Peru, Chile, and Venezuela. …