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. 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. 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. 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.