Decline of carbon-dioxide-gobbling plankton coincided with ancient global cooling

Jan 08, 2009 By Hugh Powell
Diatoms are abundant oceanic plankton that remove billions of tons of carbon dioxide from the air each year. Their evolutionary history needs to be rewritten, according to a new Cornell study. Image: NOAA/Gordon Taylor

(PhysOrg.com) -- The evolutionary history of diatoms -- abundant oceanic plankton that remove billions of tons of carbon dioxide from the air each year -- needs to be rewritten, according to a new Cornell study. The findings suggest that after a sudden rise in species numbers, diatoms abruptly declined about 33 million years ago -- trends that coincided with severe global cooling.

The study is published in the Jan. 8 issue of the journal Nature.

The research casts doubt on the long-held theory that diatoms' success was tied to an influx of nutrients into the oceans from the rise of grasslands about 18 million years ago. New evidence from a study led by graduate student Dan Rabosky of the Department of Ecology and Evolutionary Biology takes into account a widespread problem in paleontology: that younger fossils are easier to find than older ones.

"We just tried to address the simple fact that the number of available fossils is colossally greater from recent time periods than from earlier time periods," Rabosky said. "It's a pretty standard correction in some fields, but it hasn't been applied to planktonic paleontology up till now."

More than 90 percent of known diatom fossils are younger than 18 million years. So an unadjusted survey of diatom fossils suggests that more diatom species were alive in the recent past than 18 million years ago.

The dearth of early fossils is understandable. Sampling for diatom fossils requires immense drill ships to bore into seafloor sediment. To find an ancient fossil, scientists first have to find ancient sediment -- and that's no easy task because plate tectonics constantly shift the ocean floor, fossils and all. Much of the seafloor is simply too young to sample.

So Rabosky and co-author Ulf Sorhannus of Edinboro University of Pennsylvania controlled for how many samples had been taken from each million-year period of the Earth's history, going back 40 million years. After reanalysis, the long-accepted boom in diatoms over the last 18 million years disappeared. In its place was a slow recent rise, with a much more dramatic increase and decline at the end of the Eocene epoch, about 33 million years ago.

With the new timeline, diatoms achieved their peak diversity at least 10 million years before grasslands became commonplace.

"If there was a truly significant change in diatom diversity at all, it happened 30 million years ago," Rabosky said. "The shallow, gradual increase we see is totally different from the kind of exponential increase you would expect if grasslands were the cause."

As an example of that kind of increase, Rabosky turned to another fossil record: horse teeth. Before grasslands, horses had small teeth suited for chewing soft leaves. But as grasslands appeared, much hardier teeth appeared adapted to a lifetime of chewing tough, silica-studded grass leaves. Diatoms ought to show a similar evolutionary response to the sudden availability of silica, Rabosky said, but they don't.

Although the new results don't explain the current prevalence of diatoms in the ocean, Rabosky said that whatever led to diatoms' rise at the end of the Eocene, the tiny organisms may have contributed to the global cooling that followed.

"Why diatom diversity peaked for 4 to 5 million years and then dropped is a big mystery," Rabosky said. "But it corresponds with a period when the global climate swung from hothouse to icehouse. It's tempting to speculate that these tiny plankton, by taking carbon dioxide out of the air, might have helped trigger the most severe global cooling event in the past 100 million years."

Provided by Cornell University

Explore further: Researchers devise new method to identify disease markers

add to favorites email to friend print save as pdf

Related Stories

World's wildlife critical to the economies of nations

10 hours ago

Wildlife is critical to the economies of nations. New Zealand's wildlife – whales, dolphins, red deer, thar, albatross, kiwi, tuatara, fish and kauri – attract tourists. And the tourists who come to see ...

We must defend science if we want a prosperous future

9 hours ago

Today's Australians are, by far, the best educated cohort in our history –- on paper, anyway -– but this is not reflected in the quality of our political discourse. We appear to be lacking in courage, ...

Recommended for you

Hundreds of starving koalas killed in Australia

50 minutes ago

Close to 700 koalas have been killed off by authorities in southeastern Australia because overpopulation led to the animals starving, an official said Wednesday, sparking claims of mismanagement.

Bridge jumper says sea lion saved him

1 hour ago

A man who jumped off San Francisco's Golden Gate Bridge to try to take his own life and was kept afloat by a sea lion said Wednesday suicide prevention was now his life's work.

Brazil receives macaw pair from Germany

1 hour ago

A pair of endangered blue macaws of the kind made famous by the hit animated "Rio" movies arrived in Brazil from Germany on Tuesday as part of a drive to ensure the bird's survival.

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

GrayMouser
1 / 5 (1) Jan 08, 2009
Which is the horse and which is the cart?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.