Analyzing Ancient Sediments at Warp Speed

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 11) to uranium (atomic number 92) without breaking the surface of the core. It gathers all of this data in a matter of hours.

Time is money…and better science

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 coastal 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 undetected 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. A sequential pair of X-ray beams is 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. “The XRF is incredibly efficient and incredibly precise, and it will completely revolutionize how we do our work.”

Diversifying scientific assets

MIT/WHOI Joint Program student Jon Woodruff and geologist Jeff Donnelly have already used the new tool to expand their studies of ancient hurricanes. The sensitivity of the machine allows them to see evidence of ancient storms and climate shifts that they could not detect before—and some that they would not have looked for. The speed of XRF analysis gives them the ability to examine and compare cores from many different continents in the time it formerly took to analyze just one spot on the globe.

“The XRF is my favorite new toy,” said Donnelly. “It has fundamentally changed the experience of studying sediments. We can ask bigger and broader questions.”

The XRF also allows what may be the biggest leap forward in sediment science: the ability to quickly analyze many different environmental markers at the same time. Information about individual elements can be valuable, but scientists can learn more—and be more confident in their analyses—when they combine and compare multiple records.

For example, Hughen and others had found through previous analyses that aluminum and titanium concentrations both decreased sharply in sediments from the Cariaco Basin (off Venezuela) during a period of abrupt climate change about 11,600 years ago (known as the “Younger Dryas”). The decreases suggested that the period was drier, with less rain causing less material to run off from land into the ocean.

But with the XRF, Hughen simultaneously measured the concentrations of each element, giving a precise ratio of one to the other. Those ratios indicated a surge of particles from African deserts blowing into the ocean off Venezuela—corroborating, but different, evidence of a drier climate during the Younger Dryas.

Sharing the wealth

The arrival of the XRF in Woods Hole opens doors for the entire oceanographic community. Giosan and Donnelly are figuring out how to share their oceanographic “tricorder” with the rest of the research world.

“Liviu took the initiative and made the case for the instrument, invested a lot of his own valuable research time, and then was open-minded enough to let others try things out,” said Lex van Geen, a chemist at the Lamont-Doherty Earth Observatory and a 1990 graduate of the MIT/WHOI Joint Program. Van Geen contacted Giosan to see if the XRF could detect trace metals in sediments he extracted from the Pacific coast of Mexico as part of an effort to reconstruct past ocean temperatures and oceanic life.

“It’s a credit to WHOI and the National Science Foundation for providing the support to make this possible,” van Geen said. “Access to the XRF could have been much more constricted, but there is no sense of a monopoly on this instrument.”