USU lab shows the lighter side of sediment dating

By Marci Monson

How old is your dirt?  One lab answers that question while bringing cutting-edge research techniques to USU.

When you first walk into the Luninescence Laboratory on USU’s Innovation Campus, it may seem like any other research lab. White, industrial-looking cabinets and countertops line the walls, holding an assortment of what appears. to be standard lab fare: beakers, microscopes, and other unnamed machines.

It’s just a normal lab. Normal, if you ignore the inescapable fact that instead of being lit by fluorescent lighting, every countertop, container and workspace is illuminated only by a dim, orange light. Entering the lab requires going through a special revolving door, and like entering a photographer’s darkroom, it takes your eyes a minute to adjust once you step inside.

So why the orange light? Inside the lab, researchers are working on a process called Optically Stimulated Luminescence (OSL) dating. Researchers collect sand-sized sediment from all over the world and send it to the USU Luminescence Laboratory to determine when it was last exposed to sunlight. If the sediment were to be exposed to any other light than red or orange, the process would be ruined, and the dating signal would be reset.

OSL is currently the only technique that uses light to date sediment. It is used to date minerals like quartz and feldspar in contrast to radiocarbon dating, which is used to date organic material. It can be used to address questions of climate change, date archeological sites, and tracking paleoseismic fault activity and movement.

The process of dating begins with the collection of a sample from a site—an archeological dig, geological formation, earthquake fault—without exposing it to any kind of sunlight. This is a delicate process, done by enclosing the sample in a tube and sealing both ends. After the sample is retrieved, researchers transfer it to the lab where only the very center sediment within the sample tube (where there is no possible way for sunlight to have reached) is processed down to pure quartz of a specific grain size—a process that takes about a month. The analysts then measure aliquots—small equal fractions—of the sample in the OSL instrument, which measures the luminescence the samples emit and estimates the date that the sediment was last exposed to the sun.

The Luminescence Laboratory officially opened in January 2007, marking a new outlook for research and collaboration at USU. Although plans were made in 2005, the idea had been growing in the back of geologist Joel Pederson’s mind years earlier. Pederson had been working at USU for several years when he realized that many of his graduate students had been using this technique in other labs across the country.

The idea came to fruition when Pederson started working with researcher Tammy Rittenour to understand the erosion and formation of the Grand Canyon. As they worked, they became frustrated, as each time they needed a sample dated, Rittenour had to travel to the University of Nebraska–Lincoln (where she was a PhD student) to work in the OSL lab there. As time wore on, they decided it would be much more cost- and time-efficient to build a lab at Utah State.

Pederson submitted a grant proposal to the National Science Foundation for continued work in the Grand Canyon and in hopes of getting money to bring Rittenour to USU. The proposal was accepted, and, through this grant and donations from the Val A. Browning Foundation, the College of Science, the Department of Geology, and USU’s Vice President for Research Office, the lab was built.

“This lab is incredibly useful, and without the contribution of the Browning Foundation, this never would have happened,” said Pederson.

The day the lab opened, Pederson and Rittenour held an official opening party, where they celebrated their efforts and ran the very first sample at the lab—sediment sampled from USU’s campus. They even had a raffle to guess the age (over nineteen thousand years old). They were both excited about the prospect this lab had for research at USU.

By the time the Luminescence Laboratory opened, Pederson and Rittenour already had plenty of work to do. The lab is one of maybe a dozen other labs in the country and one of the very few that does contract work. This means that people send in and pay to have their samples looked at and dated by the USU lab. The money from this contract work then goes toward the rent and maintenance on the lab, sample processing costs, and most importantly, funding student research in the lab.

Starting on day one, the lab had a year’s worth of contract work to do. This pattern has stayed consistent as Rittenour, now the director of the lab, gets weekly calls from people all over the world wanting to send in their samples. In fact, they have had to turn down some contract work because the wait list is so long.
“Our work ranges from the Indian tsunami deposits to faults in Alaska, Costa Rica, and even Cache Valley,” said Rittenour. “There is a huge demand for the relatively new technique of OSL dating.”

The work of the lab has been a great asset to research at USU, partly because researchers can be trained on it relatively easily and quickly. This helps to foster research among both undergraduate and graduate students at USU and has opened all kinds of doors for student experiences on campus.

“Our research at the lab helps to bolster involvement of students in research,” said Pederson. “We used to have to send graduate students to other places in the country to do this kind of research, but now we have students from other schools coming here.”

How to Take a Sample for Luminescence Dating

Step 1: Position

Target fine-medium sand (not soil). If possible, sample a thick unit, more than one meter below the landform to. Place sharpened end of metal tube, with styrofoam plug, at the target point.  Place pounding cap over other end.

Step 2: Hammer

Pound sample tube into outcrop until you feel and hear change indicating the tube is full.

Step 3: Moisture

Sample horizon for moisture content using an air-tight container, such as a film canister.

Step 4: Dose Rate

Collect a sample for the environmental dose rate (radiation absorbed over time) into a one-quart ziplock freezer bag.  Fill bag half full or more.

Step 5: Extract Tube

Before the tube falls out on its own, stop excavating around it and gently turn tube to loosen and extract.  With open end of tube carefully pointed up, pack any extra space with more sediment, tamping it down lightly.

Step 6: Cap and Tape

Place cap on end and tape both vertically and horizontally on both ends with at least two layers of tape.

Step 7: Label

Mark tube, moisture sample, and dose rate bag with a permanent marker.

Step 8: Tape

Place clear packing tape over labels to prevent smearing and fading.

Step 9: Package

Place all samples into a large ziplock bag.

Step 10: Measurements

Record latitude, longitude, and elevation of the sample location, as well as any other sedimentological observations