Researchers determined that the composition of two stalagmites from a northwestern Madagascar cave showed a sudden shift from a carbon isotope ratio that is typical of tree and shrubs to a ratio more consistent with grassland.
It is well known that humans have had a dramatic impact on planet Earth since the Industrial Revolution, but scientists at the Massachusetts Institute of Technology (MIT) and the University of Massachusetts at Amherst have now determined that human settlers in Madagascar 1,000 years ago set fire to the forests to make way for cattle pastures. The researchers made their determination from the composition of two stalagmites from a northwestern Madagascar cave that showed a sudden shift from a carbon isotope ratio that is typical of tree and shrubs to a ratio more consistent with grassland. The shift occurred within 100 years, which is too sudden to be a result of any naturally occurring phenomenon.
The team’s results suggest that the landscape transformation was not due to climate change. They found that oxygen isotopes, indicating rainfall rates and climate in general, remained relatively unchanged in both stalagmites during that period. MIT’s David McGee, Assistant Professor in the department of Earth, Atmospheric and Planetary Sciences, says, “We went in expecting to just tell a climate change story, and were surprised to see a huge carbon isotope change in both stalagmites.”
Stalagmites are formed in caves as water percolates through the soil from the surface. They can be preserved for thousands of years, allowing their composition to serve as a historical record of what was happening above ground. McGee said that because these stalagmites are essentially fossilized groundwater deposits that precipitates in distinctive layers, they are a fairly sensitive recorder of what was happening on the surface.
McGee went on to say that both the speed of the shift and the fact that there is no signal of climate change led them to believe the results were due to human involvement. “What we see in the record is that the change from carbon isotopes that look like forest, to isotopes that look like grassland, happens really rapidly, within a century, and it would be unusual for a forest to naturally completely turn into grassland that quickly.”
The findings also help to explain the loss of some large animal species from the same period, such as giant tortoises, pygmy hippos and giant lemurs.
Settlers first colonized Madagascar about 3,000 years ago and introduced cattle farming to their lifestyle after years of being hunters and gatherers.
Results of the study are being published this week in the journal Quaternary Science Reviews. Researchers hope to sample more caves across Madagascar in the future to try and gain a better understanding of how early humans transformed the island.
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