By Nell Greenfieldboyce
An ancient, well-preserved tree that was alive the last time the Earth’s magnetic poles flipped has helped scientists pin down more precise timing of that event, which occurred about 42,000 years ago.
This new information has led them to link the flipping of the poles to key moments in the prehistoric record, like the sudden appearance of cave art and the mysterious extinction of large mammals and the Neanderthals. They argue that the weakening of the Earth’s magnetic field would have briefly transformed the world by altering its climate and allowing far more ultraviolet light to pour in.
Their provocative analysis, in the journal Science, is sure to get researchers talking. Until now, scientists have mostly assumed that magnetic field reversals didn’t matter much for life on Earth — although some geologists have noted that die-offs of large mammals seemed to occur in periods when the Earth’s magnetic field was weak.
The Earth is a giant magnet because its core is solid iron, and swirling around it is an ocean of molten metal. This churning creates a huge magnetic field, one that wraps around the planet and protects it from charged cosmic rays coming in from outer space.
Sometimes, for reasons scientists do not fully understand, the magnetic field becomes unstable and its north and south poles can flip. The last major reversal, though it was short-lived, happened around 42,000 years ago.
Giant kauri trees can live for thousands of years and can end up well preserved in bogs. “The trees themselves are quite unique,” says Cooper. “They’re a time capsule in a way that you don’t really get anywhere else in the world.”
Inside trees that lived during the last magnetic flip, the researchers and their colleagues looked for a form of carbon created when cosmic rays hit the upper atmosphere. More of these rays come in when the magnetic field is weak, so levels of this carbon go up.
The trees, with their calendar-like set of rings, took in this kind of carbon and laid it down as wood. That let the researchers see exactly when levels rose and peaked and then fell again. One tree in particular had a 1,700-year record that spanned the period of the greatest changes.
By creating a precise timeline, the research team was able to compare the magnetic field’s weakening to other well-established timelines in the archaeological and climate records.
By Krissy Waite
For the past nine years, Paula Turkon, assistant professor in the Department of Environmental Studies and Sciences at Ithaca College, has been working on creating a timeline that can be used to date environmental and cultural changes in Northwestern Mexico using trees.
“The work that we’re doing is still preliminary,” Turkon said. “We’re building what we call a master sequence, and that can then be used to interpret things, but the construction of it is slow work because we don’t have anything to compare it to.”
Turkon’s research, “Chronology, Climate, and Culture in Prehispanic Mesoamerica: Contribution of Tree-Ring Studies,” aims to understand climate events — like droughts and heavy rainfall periods — and cultural changes in Northwestern Mexico using dendrochronology, or the study of tree rings. She said she became interested in this region when she was invited by her graduate school adviser to work there. Once there, she said, she wondered how people lived and produced food in such a dry environment.
The research is funded through a grant from the National Science Foundation. Turkon also works through Cornell University’s Tree-Ring Laboratory. She also works closely with colleagues at the National Institute of Forestry, Agricultural and Livestock Research and the National Institute of Anthropology and History in Mexico.
Turkon said her original research focused on understanding the degree to which people in this region were dependent on agriculture, which correlates with the amount of rainfall in the desert. She said she had trouble answering questions related to how agricultural and cultural changes happened because the data she had to reference were food remains, which are hard to put a date on. She said she also wondered if the variability of rainfall in the arid area was a factor in the past. She said she thought she could answer both questions using dendrochronology.
Dendrochronology is a technique in which the annual growth rings in trees, commonly known as tree rings, are studied to date events and environmental changes. Turkon said rainfall is the biggest factor that generally affects tree growth in the areas in Mexico where she takes samples. This means that a bigger, thicker tree ring could indicate a good rainfall year and a thinner tree ring could indicate a bad rainfall year. In Northern Mexico, it rains between 9.2 and 26.2 inches per year. Other factors, like the tree’s species, can also affect tree ring growth. Turkon said this technique has not been applied in this region of Mexico before.
After a record low winter run-off, some water experts are now calling this Arizona’s worst mega-drought in recorded history, even when compared to tree-ring data that goes all the way back to the 1300s.