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 still walked the Earth. Until recently, most scientists believed that the dinosaur's era 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 about 65 million years ago, and that it was caused by Earth's collision with a large asteroid. The extinction of many species, including dinosaurs, is commonly traced to the collision.
But Barrera, an associate research scientist in geological sciences, thinks that five million or so years before any catastrophic asteroid collision, something else happened to "trigger global cooling, a major change in ocean circulation patterns, a large drop in sea level, and mass extinction of many tropical marine organisms."
She is not sure what happened, but she says the sharp climatic changes are recorded in the shells of microscopic sea creatures called foraminifera buried in the sediment of a stretch of water that scientists have dubbed the Tethys Sea after the wife of the mythological Greek titan Oceanus.
Seventy-one million years ago, Earth's continents were clustered together and sea level was much higher (see map). The Atlantic Ocean was very small, but the Pacific was enormous, covering about half the Earth.
The Tethys Sea---a shallow salty body of warm water teeming with corals and bi-valves that formed large reefs and with the tiny one-celled foraminifera---separated continents in the northern and southern hemispheres.
As they grow, foraminifera incorporate varying ratios of oxygen isotopes from sea water in their shells. As 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. (The Mediterranean Sea is the only remnant of the Tethys Sea.)
Barrera found evidence for long-term gradual high-latitude cooling and a rapid and sharp decrease in deep ocean temperatures---possibly as much as four degrees lasting over a million-year time span beginning about 71 million years ago. She concludes that "both 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 dried shallow areas of the Tethys region, 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. When sea levels went back up, this circulation pattern reversed again.
"Sedimentary analysis indicates the sudden extinction of many tropical shallow water reef-forming organisms, as well as other deep water organisms," she said. But it is still impossible to say whether it was the colder water, shifts in sea levels or something else that killed the aquatic life and whether their disappearance is linked to the extinction of the dinosaurs.
Barrera presented her findings at the Geological Society of America meeting in New Orleans in November. Her research has been funded by the National Science Foundation. Collaborating with her were U-M research scientist Charles Jones, Samuel M. Savin of Case Western Reserve University and Ellen Thomas of Wesleyan University.
Sally Pobojewski ( NIS )
. . . December 1995