Researchers Use Prehistoric Packrat Piddle To Refine Hydrologic Dating Techniques

In a paper published today in the journal Science, Mitch Plummer, a researcher at New Mexico Institute of Mining and Technology, shows how he and colleagues from Los Alamos and other organizations used fossilized packrat urine from prehistoric dens to determine that deposition rates of chlorine-36 - a naturally occurring isotope of chlorine and an important chemical tag that can be used to determine the age of water - were substantially higher than previously thought for periods older than 11,000 years ago.

In addition to giving researchers better data for dating hydrologic processes, the work also has helped Los Alamos scientists determine how water moves through Yucca Mountain, Nev., a proposed repository for high-level nuclear waste.

Chlorine-36 is produced naturally in the stratosphere when atoms are bombarded by cosmic rays, high-energy particles that streak through space from beyond our solar system. This so-called cosmogenic production of chlorine-36 has varied over time due to fluctuations in the strength of Earth's magnetic field. When the field is weak, more cosmic rays can reach the upper atmosphere and more chlorine-36 is produced. Moreover, because Earth's magnetic field deflects cosmic rays toward the North and South Poles, chlorine-36 production rates decrease with distance away from the poles.

Researchers can determine the rate at which chlorine-36 was deposited in the past by determining the ratio of chlorine-36 to regular chlorine atoms in materials that contain chlorine and then comparing that ratio to the age of the material.

Plummer and his colleagues analyzed chlorine-36 to chlorine ratios in fossilized packrat dens from northwestern and southern Nevada. Packrat urine binds together the twigs, leaves, seeds, feces and other debris that the animals use to construct their dens, or middens. In desert areas, middens can be preserved for tens of thousands of years if protected from the elements by caves or rock overhangs.

Packrats get their water by ingesting desert plants. The plants take in atmospheric chlorine-36 through rainfall. Consequently, packrat urine is a good indicator of chlorine-36 levels and can be used to help determine deposition rates of the cosmogenic isotope.

To determine the age of the urine, Plummer and his colleagues were able to take packrat midden debris such as leaves and twigs and determine their age through carbon-14 dating - useful for dating organic materials up to 40,000 years old.

Plummer and his colleagues were able to determine that the rates at which chlorine-36 was deposited at mid-latitudes during periods older than 11,000 years ago were nearly twice as high as previously thought.

"One explanation for why chlorine-36 levels are higher during the period beyond 11,000 years ago is because of the position of the jet stream during the last glacial period," said Jake Turin, co-author of the paper and a researcher in Los Alamos' Environmental Science and Waste Technology Group. "In order for stratospheric chlorine-36 to be deposited on Earth's surface, it must enter the troposphere where it can be washed out of the atmosphere in rain. Mixing of the stratosphere and troposphere occurs most strongly at the jet stream. Apparently during this period, the jet stream had moved southward and this could account for the increased levels of chlorine-36 deposited at mid-latitude."

Global climate change, then, could explain the larger-than-previously-expected levels of chlorine-36, levels higher than can be explained by variations in Earth's magnetic field, he said.

June Fabryka-Martin, co-author and a researcher in Los Alamos' Environmental Science and Waste Technology Group, said the research has helped Los Alamos researchers working on the Yucca Mountain Site Characterization Project to better understand the hydrology of the mountain. Fabryka-Martin and her colleagues are using cosmogenic isotopes and computer models to understand how water moves through the rock layers that make up Yucca Mountain.

Los Alamos researchers are using naturally occurring chlorine-36 and man-made chlorine-36 - referred to as "bomb-pulse" chlorine-36 - to identify paths through which water can flow into Yucca Mountain. The worldwide concentration of chlorine-36 substantially increased during atmospheric nuclear weapons tests conducted in the Pacific about 40 years ago, creating a unique bomb-pulse signature that has elevated levels of chlorine-36. Hence, water with the bomb-pulse signature is less than 50 years old.

During Yucca Mountain site characterization work, Fabryka-Martin and her colleagues collected subsurface samples along a tunnel route that starts at the surface of Yucca Mountain and declines at a shallow grade to a depth of 200 to 300 meters below the surface. The 25-foot-diameter tunnel is part of the Exploratory Studies Facility in the mountain. They looked for samples with chlorine-36 levels that were higher than natural background levels. Elevated levels of chlorine-36 in samples could indicate the presence of bomb-pulse chlorine-36.

Some of the samples had chlorine-36 levels that were lower than the bomb-pulse signature levels, but were higher than previously estimated background levels. These samples puzzled the scientists.

However, since the packrat midden samples indicate that background levels of chlorine-36 for periods older than 11,000 years ago were more than twice as high as original background estimates, Fabryka-Martin now has a plausible explanation for the samples that didn't quite fit the bomb-pulse signatures, but didn't quite match the previously believed background-level signatures, either.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.