Compositional History of Earth's Atmosphere
Ages of ice samples found on the Earth cover a span approaching 200,000 years. Gas bubbles trapped in
that ice can be used to learn about the composition of Earth's atmosphere at the time they were trapped
in the ice. But how can we tell what the Earth's atmosphere was like before that?
Recently, USGS scientists
have used a gas QMS to determine the oxygen level of ancient samples of Earth's atmosphere from a most
unlikely place - amber. The fossilized resin of conifer trees, amber is interesting to scientists as a
medium that traps insects, small animals, and plants, preserving them through geologic time for future study.
The recent extraction by scientists, of ancient DNA from organisms entombed in amber much like in the
science-fiction novel and movie, Jurassic Park is an example of why scientists are intensely interested
in amber. Minute bubbles of ancient air trapped by successive flows of tree resin during the life of the
tree are preserved in the amber.
The Oxygen-Rich Cretaceous Atmosphere
Analyses of the gases in these bubbles show that the Earth's atmosphere,
67 million years ago, contained nearly 35 percent oxygen compared to present levels of 21 percent. Results
are based upon more than 300 analyses by USGS scientists of Cretaceous, Tertiary, and recent-age amber from
16 world sites.* The oldest amber in this study is about 130 million years old.
Significance of Cretaceous Oxygen Levels?
The consequences of an elevated oxygen level during Cretaceous time are speculative. Did the higher oxygen
support the now extinct dinosaurs? Their demise was gradual in the transition from late Cretaceous to early
Tertiary times, as was the decrease in oxygen content of the atmosphere.
|This 84-million-year-old air bubble lies trapped in amber (fossilized tree sap). Using a quadrupole mass
spectrometer, scientists can learn what the atmosphere was like when the dinosaurs roamed the earth. USGS image.
Landis, G.P., Rigby, J.K.,Jr., Sloan, R.E., Hengst, R.A., and Snee, L.W., 1996, Pelé Hypothesis: Ancient
atmospheres and geologic-geochemical controls on evolution, survival, and extinction, in G. Keller and N.
MacLeod, eds., Cretaceous-Tertiary Mass Extinctions. Biotic and Environmental Changes, W.W. Norton, pp. 519-556.
Landis, G.P., and Snee, L.W., 1991, 40Ar/39Ar Systematics and argon diffusion in Amber; implications for
ancient earth atmospheres: in Kump, L.R., Kasting, J.F., Robinson, J.M., Atmospheric oxygen variation through
geologic time. Global and Planetary Change. v. 5, p.63-67.
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|Amber - the fossilized resin of conifer trees - provides a unique means of protecting intricate samples of the past. This mosquito, lying trapped for 45 million years in a piece of amber, is almost perfectly preserved. USGS image.
|This chart shows a major decrease in oxygen content in the atmosphere from 35 percent to the present day level of 21 percent. This decrease occured about the same time that the dinosaurs disappeared - 65 million years ago. USGS image.