The University Record, November 13, 1995
Clues to mysterious global climate shift
buried in ocean floor
By Sally Pobojewski
News and Information Services
U-M research scientist Enriqueta Barrera is using evidence from some of the world’s tiniest marine creatures to learn more about what the world was like 70 million years ago when dinosaurs were still thriving on Earth.
Until recently, most scientists believed this was a time of long-term global environmental stability with temperatures much warmer and more uniform than they are today. Many experts thought the only major global change during this period took place five million years later when the Earth apparently was struck by a large asteroid, and many species, including dinosaurs, suddenly became extinct.
By analyzing isotopes in the shells of microscopic sea creatures called foraminifera buried in ocean sediments, however, Barrera discovered signs of multiple, drastic changes in Earth’s global climate and oceanic circulation patterns that began about 71 million years ago—six million years before the asteroid collision.
“Something happened during this time to trigger global cooling, a major change in ocean circulation patterns, a 150-foot drop in sea level and mass extinctions of many tropical marine organisms,” says Barrera, associate research scientist in geological sciences. She presented the results of her research at the Geological Society of America meeting held in New Orleans Nov. 6-9.
As they grow, foraminifera incorporate varying ratios of oxygen isotopes from sea water in their shells. When they die, their shells pile up in layers on the ocean floor. Barrera analyzed changes in oxygen isotope ratios in foraminifera in ocean sediments deposited during a several-million-year interval at six locations in the South Atlantic, Indian and tropical Pacific Oceans. She found evidence for long-term, gradual, high-latitude cooling and a rapid and sharp decrease in deep ocean temperatures—possibly as much as 4 degrees C—lasting over a million-year time span beginning about 71 million years ago.
Associated with this decrease in deep ocean temperature, Barrera also found carbon and strontium isotope ratios in foraminifera that indicated a major change in the production and deposition of organic matter in the oceans and increased continental weathering. “These oceanic and continental changes occurred in conjunction with a suggested 150-foot drop in sea level. After about one million years, sea level seems to have gone back up.”
Barrera speculates that the drop in sea level may have been enough to dry up shallow shallow tropical seas, greatly reducing or stopping the production of warm and salty water flowing into the Pacific and Indian Oceans. This may have allowed colder water from polar latitudes to spread into equatorial areas.
