for National Geographic Today
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.
