Sediments from the Atlantic Ocean indicate that the now lush Amazon basin was much drier during the last ice age.
Between 12,000 and 13,000 years ago, the Amazon River carried little more than half the water that it currently does, says Mark A. Maslin, a geographer at University College London. Maslin and his colleague Stephen J. Burns of the University of Berne in Switzerland presented their findings in the Dec. 22, 2000 Science.
They base their argument on two oxygen isotopes, known as oxygen-18 and oxygen-16, in sediments off the coast of Brazil. Several factors, including water temperature, global ice volume, and the amount of fresh water carried out to sea by the Amazon River, affect the ratio of these isotopes. By comparing the isotopic ratios recorded in a sediment core drilled from the ocean floor at a site about 500 miles north of the river mouth with those found in a core taken further from the mouth, Maslin and Burns were able to calculate the volume of fresh water that the river dumped annually into the ocean.
The cores also indicate a brief yet substantial spike in the river’s flow rate about 11,800 years ago, when the Amazon almost doubled its flow for a century or so. This period coincides with a major warming that partially melted an Andean ice sheet, the scientists note. The rise in temperatures probably also triggered more precipitation across the Amazon basin because the increased river flow during the warming spell is more than can be explained merely by the amount of ice that melted during that period.