Falkowskiand his colleagues have measured the abundance of carbon 13, abyproduct of photosynthesis, in deep-sea core samples that go back 205million years. Because photosynthesis produces oxygen and leaves carbon13 behind, the presence of carbon 13 in the fossil samples allowsscientists to estimate precisely how much oxygen was in the atmosphereat any given time, Falkowski says.
From a steady 10 percent – thelevel at which dinosaurs flourished – the oxygen percentage rose to 17percent 50 million years ago and then to 23 percent by 40 million yearsago.
"In the fossil record, we see that see that this rise inoxygen content corresponds exactly to a really rapid rise of large,placental mammals," Falkowski says. "The more oxygen, the bigger themammals. We argue that the rise in oxygen content allowed mammals tobecome very, very large – mammals like 12-foot-tall sloths and hugesaber-toothed cats. They paved the way for all subsequent largemammals, including ourselves."
The results described inFalkowski's article, "The Rise of Oxygen Over the Past 205 MillionYears and the Evolution of Large Placental Mammals," stem from years ofanalysis of organic and inorganic core samples. Scientists have beenusing deep-sea core samples for years, but Falkowski and his colleagueshave achieved greater precision in their measurements, thanks to twohigh-precision, isotope ratio mass spectrometers housed in thegeological sciences department at Rutgers.
There were placentalmammals on Earth at the time of the great extinction of dinosaurs about65 million years ago, Falkowski says. They were, however, tiny, limitedcreatures; the extinction event itself, while eliminating thedinosaurs, did little to further the mammalian domination of theplanet. It was the subsequent spreading of shallow seas, the increasein plant life – and photosynthesis – in addition to the consequentincrease in oxygen content that gave the mammals the boost they needed,according to Falkowski.
In the last 10 million years, thepercentage of oxygen in the earth's atmosphere has decreased to 21percent. Falkowski says many scientists believed that great firesburned over the earth about 10 million years ago, reducing the numberof trees and, therefore, the amount of photosynthesis and oxygen.
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