Lab reconstructs climate history

Peter Kauffner

Although researchers at the University’s Institute for Rock Magnetism believe in using magnetism for good, it’s possible that at least one of their machines, a model 750R cryogenic magnetometer made by the 2-G corporation, has been turned to the dark side.
“We call it the Darth Vader, because it’s big and black,” said geology professor Subir Banerjee, the director of the institute. “That was back when we had fewer machines and we named all of them.”
Although Darth Vader no longer represents state-of-the-art technology, it’s still the lab’s workhorse machine.
“We use it to measure the overall magnetism of a sample before deciding what further tests are needed,” Banerjee said.
The institute, founded in 1990, uses the magnetic properties of rock and soil to determine their age and origin, thereby allowing scientists to reconstruct climate history. The iron-based mineral magnetite is particularly useful for analysts
The lab’s equipment was used during recent research on the windblown soil of western China. The research was summarized in a paper by Banerjee in the current issue of GSA Today, a monthly serial published by the Geological Society of America.
“In China, it’s a unique situation because they have deposits of dust which were brought over by dust storms in the spring,” Banerjee said.
The study focuses on loess, a type of soil common in northern China. Loess, also called yellow earth, was lifted off the Gobi and other deserts as dust and then blown elsewhere. In China, the loess is carried south and east by the monsoon.
“A lot of people are trying to study how climate inside a continent has changed with time,” Banerjee said. Banerjee is especially interested in comparing continental climate changes with those over the oceans. “The ocean climate is very easy to measure because it averages out with the sediments in the water and the (drill core samples) are very easy to take in the ocean.”
Because loess is built up continuously, much like ocean sediment, deeper deposits can be correlated with earlier periods in geologic history. Light and dark bands in the loess correspond to glacial and interglacial periods.
Banerjee’s paper was based on earlier work in which trenches more than 200 feet deep were dug into the loess at Xifeng in Gansu province and other locations.
Because Earth’s magnetic field reverses polarity — with magnetic north flipping from near the north pole to the south pole and eventually back again — about once every 100,000 years, the alignment of the magnetite in the soil can be used to determine the age it was deposited.
The institute is funded by grants totaling $500,000 a year from the National Science Foundation, the W.M. Keck Foundation and the University.
Closer to home, lab researchers recently finished reconstructing Minnesota’s climate during the past 1,000 years based on samples taken at Long Lake near Minneapolis.
Rinita Dalan, then a University graduate student, used the lab’s magnetic analysis of a mound in Chaokiea, Ill., created by the mound-building American Indians. Magnetic techniques were used to help determine the source of the soil used to build the mound.
At Patzcuaro, Mexico, University of Wisconsin archeology graduate student Chris Fisher is using the institute’s equipment to compare Indian and Spanish colonial methods of agriculture.
“He is looking at the erosion from agriculture that has happened in the valley and tying it to Spanish and pre-Colombian times,” Banerjee said. “He is finding that the Indians cultivated smaller areas, but more intensively,” than later Spanish farmers did.