The mighty monsoon winds that periodically bring rains that drench India first billowed around 12.9 million years ago, new research shows. The work provides the best look yet at the conditions that fostered the modern monsoon.
By examining sediments piled up around Indian Ocean islands, researchers uncovered a geologic history of the South Asian monsoon stretching back tens of millions of years. The monsoon winds began abruptly, researchers report online July 20 in Scientific Reports. That speedy start-up suggests that factors such as global cooling were at play in addition to the rise of the Himalayan mountain range, which scientists typically blame for the monsoon’s inception.
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The monsoon “came on really fast, and it came on because the whole system went over a threshold, not just the Himalayas,” says study coauthor Gregor Eberli, a marine geoscientist at the University of Miami in Florida.
Rainfall during the summer monsoon season accounts for more than 70 percent of India’s annual precipitation. The temperature difference between the continent and the adjacent Indian Ocean drives the winds. During winter, warm air over the ocean rises and draws in cool air from the land to the north. In summer, the land becomes warmer and the winds flip direction.
The snow and high elevation of the Himalayas drive the temperature difference between the land and sea. But the mountains grew over tens of millions of years, making it difficult to determine exactly when conditions favorable to the monsoon began. Previous estimates ranged from around 28.7 million to 7 million years ago.
Eberli and colleagues traveled to a place where the monsoon leaves its mark: the bottom of the Indian Ocean. Monsoon winds drive currents in the ocean, which in turn carry ocean sediments across the sea. Sediment accumulates in mounds similar to snowdrifts when currents are strong. The strong currents also pull nutrients from the seafloor toward the surface, boosting biological activity that in turn draws oxygen from the water. That lower oxygen supply leaves a chemical trace in the sediments.
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At a depth of about 500 meters below the sea surface, the researchers drilled a kilometer into the seafloor and extracted sediments dating back roughly 25 million years. A weaker precursor to the modern monsoon existed roughly 25 million years ago, the sediment data suggest. Around 12.9 million years ago, however, the winds revved up to their modern strength over the course of about 300,000 years, a relatively short time compared with the formation of the Himalayas.
The strengthening of the monsoon lines up with a period of global cooling and the growth of the polar ice caps. That climate shift may have boosted the temperature difference between the land and sea, supercharging the winds, the researchers propose.
Just because the winds were blowing doesn’t mean India was getting soaked, though, says Peter Clift, a geologist at Louisiana State University in Baton Rouge. The winds and rains are associated now, but “that might not have always been the case,” he says.
Editor’s note: This story was updated August 17, 2016, to correct the depth the researchers drilled into the seafloor.