Tropical rain forest thrived throughout last Ice Age

January 5, 2007

ANN ARBOR—Radiocarbon dating and pollen analysis of sediments from a small lake in Brazil indicate that the western Amazon River basin remained covered with lush, tropical rain forest 14,000 to 30,000 years ago?a time when glaciers covered the northern latitudes and most scientists believed the Amazon basin was a vast, dry grassland.

The research, published in the Oct. 4 issue of Science, is significant because it is the first direct, dated physical evidence from anywhere in the lowland rain forest of the Amazon basin indicating the type of climate in this region during the last Ice Age.

“These data will come as quite a shock to many paleoclimatologists,” said Paul A. Colinvaux, a research scientist at the University of Michigan Center for Great Lakes and Aquatic Sciences and senior scientist at the Smithsonian Tropical Research Institute in Panama. “They contradict the widespread belief that a drier climate during the last Ice Age turned the Amazon lowlands into a savanna with isolated pockets of rain forest.”

Co-authors on the Science paper include P.E. De Oliveira of the Chicago Field Museum, J.E. Moreno and M.C. Miller of the Smithsonian Tropical Research Institute and M.B. Bush of Duke University. Colinvaux and his colleagues based their conclusions on a seven-meter-long sediment core they drilled from the bottom of Lake Pata, which is located in the dense tropical Amazon rain forest of northwestern Brazil.

Radiocarbon dating of material in the undisturbed sediment layers confirmed that the top 1.6 meters were deposited on the bottom of Lake Pata between about 14,200 and 30,800 years ago?a time of maximum glacial coverage during the last Ice Age. When they identified the 50 to 100,000 pollen grains found in each cubic centimeter of sediment, Colinvaux and his associates discovered that 70 percent to 90 percent were tree pollens. Almost all of them were from trees commonly found in today’s rain forest. Only trace amounts of grasses were found in the sediment. “This is conclusive evidence that savannas or other grasslands were never present anywhere near Lake Pata during this time period,” Colinvaux said.

A steady increase in pollen from a tropical conifer called Podocarpus, which today grows mainly at altitudes 1,000 meters higher than the Amazon lowlands, indicates temperatures during the glacial maximum were five- to six-degrees C cooler than today, according to Colinvaux. Changes in the color and appearance of Lake Pata sediments also suggest lake levels were lower during the time of maximum glaciation?a result of decreased precipitation, a longer dry season or lowered water table. “Although precipitation was reduced in the colder glacial times, the reduction was not sufficient to displace or fragment the rain forest,” Colinvaux and his co-authors conclude in the Science article.

Widespread acceptance of an “arid Amazon grassland” during the Ice Age was based on the existence of ancient sand dunes and deposits of windblown dust found in parts of South America north and south of the Amazon River basin, according to Colinvaux. “Evidence of aridity in other areas was simply extrapolated to include the Amazon,” he said. “With no data to the contrary, there was no reason to question the assumption.” Ecologists looking for a way to explain the enormous number of plant and animal species found in the Amazon rain forest developed the “refuge hypothesis” based on the assumption that the rain forest dried up and virtually disappeared during glacial times. “To produce two distinct species, you must have isolated populations that cannot interbreed. If during successive Ice Ages, the forest periodically turned into grassland interspersed with small, isolated forest areas, ecologists thought that could explain the Amazon basin’s biodiversity,” Colinvaux said. “It’s a beautiful hypothesis, but it’s wrong.”

Since the Amazon basin appears to have maintained its rain forest environment in spite of a five- to six-degree C temperature fluctuation during the last Ice Age, Colinvaux believes it probably did so during all previous Ice Ages as well. “If this is true, then the Amazon rain forest has existed for two million years and is an example of an extremely resistant ecosystem,” he said. Is the rain forest resistant enough to adapt to the five- degree C average temperature increase scientists predict global warming could produce by 2050? “I have a hunch the rain forest will get away with it,” Colinvaux said. “But another five degrees in addition to the six- degree increase since the end of the last Ice Age would represent hideous warming. With such unprecedented conditions, no one knows what could happen.”

Colinvaux’s research was funded by the National Science Foundation, the National Geographic Society and the Mellon Foundation.