New Fossil Study May Resolve Prehistoric Climate Paradox

Bijal P. Trivedi
for National Geographic Today

October 9, 2001

Seventy million years ago carbon dioxide levels in the atmosphere were
probably at least twice current levels and the Earth, scientists
believe, was generally a much warmer place. Analysis of microscopic
fossils suggests however, that the tropics remained relatively cool.
Until now this discrepancy has shed doubt on researchers' ability to
predict future global warming. New evidence, however, indicates that
scientists were misled by the fossils.

The "cool tropics
paradox"—the leading enigma of past climate research according to
many scientists—involves a clash of geological data with climate
predictions based on computer simulations of ancient climates.

Climate models consistently predict that high carbon dioxide levels 70 million years ago, during the Late Cretaceous and Eocene epochs, caused a greenhouse effect that led to universally higher temperatures on Earth—when the poles were ice-free and heat-loving animals such as crocodiles and turtles roamed as far north as the Arctic Circle.

These predictions conflict with analysis of microscopic fossils from deep-sea ocean cores that suggest the warming was more pronounced at higher latitudes and considerably cooler at the tropics—cooler, in fact, than current temperatures.

New research led by Paul Pearson of the University of Bristol, United Kingdom, confirms a long-held suspicion that the ancient fossils were not all that they seemed.

The fossils come from microscopic shell-dwelling animals called planktonic foraminifera that live in the surface waters. When they die they sink to the ocean floor.

Scientists analyze the oxygen content of the foraminifera shells to determine the temperature of the water in which the animals lived—foraminifera living in warm surface waters have a high concentration of the isotope oxygen 16 compared with those that live among the cold currents of the ocean floor.

But Pearson's study reveals that as these shells sit on the ocean floor and get buried under millions of years of sediments, their chemical character is altered.

The result is that the fossilized shells contain oxygen profiles from both warm surface waters and much colder deep ocean currents. These oxygen isotope signatures are difficult to separate and skew the results.

"What had not been previously recognized was that these fossils were about 50 percent original and 50 percent re-crystallized material from surrounding deep-sea sediments," said Pearson. The report is published in the current issue of the journal Nature.

The flaw was recognized when Pearson's team analyzed foraminifera collected from clay deposits in Tanzania and compared these with samples collected from deep-sea ocean cores.

"What's different about the Tanzanian sediments is that the foraminifera were virtually encapsulated in clay," said geochemist Lee Kump of Pennsylvania State University in University Park, the author of an accompanying article in Nature.

The clay protects foraminifera from surrounding water and the sediments that might otherwise infiltrate and erode the shell.

Pearson's team discovered Tanzanian shells from the Late Cretaceous epoch had much higher concentrations of the oxygen 16 isotope than shells of similar age collected from deep-sea cores. The high oxygen 16 concentrations are a signature of shells that originally grew in warm waters.

The oxygen profile of the Tanzanian shells indicated that tropical sea surface temperatures were between 28 and 32 degrees Celsius (82 and 89 degrees Fahrenheit), which are consistent with climate predictions for the Late Cretaceous period. Analysis of deep-sea foraminifera suggests much colder temperatures: 15 to 23 degrees Celsius.

"This paradox has been a serious impediment to progress," said Kump. Considerable effort has been spent to resolve this paradox by reshaping climate models to account for the cool-tropics phenomenon.

"Researchers tried to make models fit the data by taking into consideration cloud cover and ocean currents that may have led to cooler tropical waters, but these creative ideas are hard to substantiate," said Kump.

"The new data rids us of this paradox so we can move on," says Kump.

The new study also improves confidence in the ability of climate models to predict greenhouse-induced changes as carbon dioxide levels rise.

Kump anticipates that this new study will be met with considerable resistance. "Decades of climate research are based on deep-sea planktonic foraminifera, now much of this research is of dubious value and may be thrown away," said Kump.

Although further studies are needed to verify these results, "researchers will undoubtedly need to choose their sample more carefully, looking for those that are unaffected by local conditions," he said.