Packrat Middens Give Clues to Ancient Climate

SOCORRO -- Two New Mexico Tech geoscientists and their research colleagues are analyzing ancient packrat middens--globs of crystallized packrat urine containing twigs, leaves, seeds, and other debris which the scavenging rodents drag into their nests--to draw up more accurate production and deposition histories of naturally occurring radioisotopes over the past 40,000 years.

By measuring a radioactive isotope, chlorine-36, in middens found in packrat nests in Nevada, some which are older than 38,000 years, New Mexico Tech doctoral candidate Mitchell A. Plummer and Tech hydrology professor Fred M. Phillips have found that deposition rates of chlorine-36 were substantially higher than previously thought for periods older than 11,000 years.

The research findings are included in an article featured in the July 25 issue of the prestigious journal Science.

The key element in the geochronologic study is chlorine-36, a radioisotope which forms when highly energetic cosmic-ray particles strike argon atoms in the upper atmosphere. The chlorine-36 then falls to the ground in rainwater or other precipitation. Plants absorb it into their cell structures; and animals, like the pack rat, eat the plants and excrete the radioisotope, which shows up with the chloride salts in their urine.

Since packrats obtain all their water from the plants they eat, and also because the desert-dwelling rodents have a habit of defecating and urinating on material they collect and hoard in their nests, the urine eventually evaporates and crystallizes around the debris and forms a large, resinous clump, preserving for millennia to come Nature's own "time capsule" record of the amount of chlorine-36 present when the midden was formed.

According to radiocarbon dating, the particular fossil middens used for analyses in the Science article ranged in age from 38-thousand-years-old to near modern.

In their study, Plummer, Phillips and their collaborators found evidence--garnered from urine salts found in the packrat middens--that before 11 thousand years ago, or near the end of the Pleistocene period, ratios of chlorine-36 to stable chlorine were twice as high as they are today.

These findings raise the possibility that rates of the cosmic radiation which were striking the Earth before the close of the Pleistocene were up to 50 percent higher than what is suggested by previous data collected from carbon-14 analyses.

In the Science article, the researchers further explain that the discrepancy between the new chlorine-36 data and the older carbon-14 record could have been due to an increase of carbon dioxide containing only low levels of carbon-14 being released into the atmosphere during that period, which would have depressed atmospheric radiocarbon activity.

Certain comparisons made in the study between the cosmogenic production rates of chlorine-36 and the Earth's paleomagnetic history tend to support prior scientific theories which hold that our planet's protective geomagnetic field fluctuates and was once weaker and therefore allowed more cosmic rays to easily penetrate the atmosphere.

However, the paper points out, that over the range of the study, chlorine-36 production histories differ significantly and sometimes do not directly coincide with recorded increases in geomagnetic field intensity.

"There are several other possibilities which may have contributed to the large decrease in chlorine-36 deposition which is seen around the end of the Pleistocene," Plummer relates. "Around that period of time, severe global climatic change, such as decreased precipitation and hotter weather, dried up a lot of lakes in the Great Basin and created saline playas; and this situation, coupled with prevailing winds, could have increased the amount of salt in the atmosphere, thereby depressing the chlorine-36-to-chlorine ratios.

"Another, and more likely, explanation is that the jet stream could have shifted southward during the late Pleistocene, allowing more chlorine-36 to be deposited at the latitude of our sampling locations," Plummer adds.

"On the other hand, it is possible that our chlorine-36 record reflects the actual cosmogenic production history better than does the carbon-14 record," the geoscientists conclude in their Science article.

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(George Zamora)