The University Record, June 5, 1995

Geochemists find evidence of biogenic 'bloom' in North Pacific

By Sally Pobojewski
News and Information Services

A U-M research team has identified the first strong geochemical evidence indicating that a large section of the North Pacific--which today is a subarctic, barren water wasteland--supported a large population of plant and animal life 5 million years ago.

Fed by nutrients in currents welling up from the ocean depths, this area in the North Pacific once teemed with one-celled plants called phytoplankton and the animals that ate them, according to Gerald R. Dickens, graduate student in marine geochemistry.

Dickens believes this zone of concentrated biological activity, called a biogenic bloom, was part of a global phenomenon that occurred from 7.5 million to 3.5 million years ago during the early Pliocene when the Earth's high-latitude regions were much warmer than they are today.

Dickens and Robert M. Owen, professor of marine geochemistry, presented their research results at the American Geophysical Union meeting held in Baltimore May 30-June 2.

Evidence for biogenic blooms during this same time period has been suggested for the Indian Ocean, along the Pacific coasts of North and South America, and in the equatorial Pacific. Until now, however, scientists could only locate ancient blooms by counting the shells of microscopic animals buried in ocean sediment. Lots of shells indicated a high-productivity zone filled with ocean life. No shells indicated a "dead" zone.

By analyzing Ocean Drilling Program sediment cores from two North Pacific sites and several sites in the Indian Ocean, Dickens and Owen demonstrated that sediment deposited beneath high-productivity zones contains a distinctive geochemical "fingerprint" with low concentrations of manganese and iron oxide and high concentrations of barium, which can be used to track the age and location of biogenic blooms millions of years after their existence.

Dickens believes the "fingerprinting" technique will help scientists understand long-term changes in the Earth's ecosystem and climate. "It gives us an independent, more reliable tool for documenting the location and extent of other blooms that may have existed during Earth's geological history," Dickens said.

Dickens and Owens' work also may help resolve scientific controversies about what causes global warming to occur periodically in Earth's history. For example, geological evidence shows temperatures were much warmer at mid-latitude and sub-polar regions during the early Pliocene than they are today. The similar age of the Pliocene warming and the biogenic blooms suggests they are related, but scientists are still not sure about what caused the blooms, according to Dickens.

"There are several current theories about why these high-productivity zones were so much more extensive five million years ago than they are today," Dickens explained. "One possibility is that changing ocean circulation patterns during the Pliocene carried nutrients like phosphorus and nitrogen, the phytoplankton's basic food source, into bloom areas. An alternative idea is that an overall increase in mineral nutrients from river run-off and flooded continents provided more food for phytoplankton."

An analysis of Indian Ocean sediments by Dickens and Owen was published in the February 1994 issue of Paleoceanography. The work is supported by the U.S. Department of Energy through its Graduate Fellowships for Global Change program.