By studying growth rings inside of rocks, researchers have devised a new technique for measuring long term changes to climate and precipitation — and this new technique is already revealing new insights into North America’s climate history.
When tracking geological changes over time, scientists typically source ice cores, lake and ocean sediments, and the interior of caves. The new method, which scans the layers of grains and pebbles, will likely be used alongside these established ones.
Magnified cross-section of a 3mm-thick pedothem soil deposit. (Photo by Erik Oerter)
Like a tree’s annual growth ring, the intricate layers embedded within rocks have a story to tell. But unlike the rings of trees, these layers — some of which are just three millimetres thick — represent hundreds of years, rather than a single year. Called pedothems, they’re formed by the steady accumulation of minerals within the soil. As the study’s senior author Ronald Amundson points out, these rock rings create “some of the most detailed information to date on the Earth’s past climates”. The Berkeley study now appears at the Proceedings of the National Academy of Sciences.
By studying rocks found in Wyoming’s Wind River Basin, Amundson’s team was able to extract information about changes to North American climate during the Pleistocene Epoch. The research was a kind of proof-of-concept, one showing that similar analyses can be performed by palaeoclimatologists in any arid and semi-arid region, which is typically where rich deposits of pedothems are found.
“We can now begin to develop records of how local and regional climate boundaries have shifted through time and in response to worldwide warming or cooling,” added study lead author Erik Oerter.
To study the oxygen and carbon isotopes embedded with the rocks, the researchers used laser ablation and a tiny drill called an ion microprobe. Analysis of the isotopes themselves painted a 120,000-year-old picture of North America’s climate past.
Map showing the predominant weather patterns in mid-latitude North America from 70,000 to 55,000 years ago. According to the new study, the large ice sheet covering the northeast caused a strong high pressure system to persist above it, which drew Gulf of Mexico-sourced precipitation (red arrows) into the mid-continent and Wyoming (white star). The result: rainy summers during this time, and possibly drier winters. (Caption and image by Erik Oerter)
Of note, the researchers discovered that during a minor ice age some 70,000 to 55,000 years ago, the pattern of precipitation on the continent shifted from a west-to-east flow to a south-to-north flow. So instead of having major weather systems form in the North Pacific, storms began flowing from the Gulf of Mexico, “a finding that highlights the influence of large continental ice sheets on atmospheric circulation,” write the authors. This change brought wetter summers and drier winters in central North America— a reverse of the usual pattern.
“This is a new insight from geologic sources of palaeoclimate data,” said Oerter. “The techniques that we developed can now be applied to similar soil deposits to fill in key gaps in the palaeoclimate record. The information will be useful to improve the accuracy of climate models by providing known conditions to calibrate them to.”
Read the entire study at PNAS: “Pedothem carbonates reveal anomalous North American atmospheric circulation 70,000–55,000 years ago”.
Top image: Didier Descouens/CC BY-SA 3.0.