These abrupt changes were caused either by Oceanic Anoxic Events during the middle Jurassic to late Cretaceous when the oceans became uniformly depleted of oxygen or by a warming event in the early Paleocene around 55 million years ago.

Marine sediment cores contain calcareous plankton � single-celled organisms with a coating or shell of calcium carbonate � as fossils. These tiny photosynthesizing plants float in the ocean and move with the currents. They are around 10 micrometers in size, about half the width of a human hair. Anything bigger than phytoplankton eat them. Eventually, their calcium carbonate shell falls to the ocean floor to become part of the sediment.

The factors that were altered in the upper marine environment during the abrupt climate change events included increases in temperature and changes in thermal structure, changes in salinity and alkalinity, and changes in nutrient patterns and trace elements.

"In every case, changes in surface habitats resulted in transient plankton communities," Bralower told attendees at the 2006 annual meeting of the American Association for the Advancement of Science. "Although we have a poor understanding of ancient plankton ecology, it appears that extinctions were selective and targeted more specialized and often deeper-dwelling species."

For example, about 55 million years ago there was a warming event that geologists call the Paleocene/Eocene thermal maximum. During that time, there were mass extinctions of organisms living on the ocean floor, but surface phytoplankton populations dipped and for the most part came back. During this event one genus of phytoplankton - Fasciculithus � which had about five species went extinct.

"We do not have anything like Fasciculitus in the oceans today," says Bralower. "But, these organisms were probably highly specialized and existed in a very narrow ecological niche. The other thing is that, as soon as some group disappears, another species comes in to occupy that niche." About 120 million years ago, during an episode of oxygen depletion another genus inhabiting surface waters � Nannococus � which also had about five species, went extinct. Otherwise only a few species here and there were unable to survive these abrupt changes. However, on the ocean floor during these same times, mass extinctions occurred.

Other extinctions, such as that at the Cetaceous Tertiary boundary (K/T) that caused the demise of the dinosaurs, are thought to be caused by other than abrupt climate changes. The K/T event had mass extinctions on land and in the upper portions of the oceans, but not on the ocean floors.

During the abrupt climate changes that Bralower investigated, the temperature of the oceans changed about 11 degrees Fahrenheit over the course of 1,000 years.

"This rate of change in ocean temperature is probably slower than what is happening today in the oceans," the Penn State researcher adds. "We are not yet seeing the same effect in today's phytoplankton."

Besides being a major food source, phytoplankton are also important in the balance of carbon dioxide in the atmosphere as opposed to the carbon that is sequestered in the ocean sediment.

Photosynthesizing organisms use carbon dioxide to create energy and so remove carbon dioxide from the atmosphere. Some of the carbon that phytoplankton take out of the air as carbon dioxide is used to make their calcium carbonate coatings. Because these coatings eventually make it into the sediment, they do not immediately return to the atmosphere. It is not until chalk or limestone beds are exposed to the elements that weathering returns the carbon to the atmosphere.

"Today, we are sort of in the middle of a mass experiment," says Bralower. "With the oceans warming, we do not really know what the end result will be, but we can look to the fossil record to see how they were affected in the past. It appears that abrupt climate change affects plankton with selectivity and most of the organisms bounce right back after the change."

The National Science Foundation's Integrated Ocean Drilling Program supported this research.

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