The Carbon Cycle - Aaron B. Berdanier - Colorado State University

The Carbon Cycle

Test vials from the
Miller-Urey experiment
Scripps Institution of
Oceanography, UCSD

he origin and history of life on Earth is a difficult process to study. Scientists have taken many diverse approaches to understand when and how life developed. For example, in the early 1950s Harold Urey and Stanley Miller mixed elements to form a "primitive" ocean and atmosphere in test tubes at the University of Chicago to simulate a lifeless earth. They introduced electric shocks into the tubes and created amino acids - organic materials that are the basis of life. In a very different setting Stanley Awramik, a geologist at UC Santa Barbara, examined 3.5 billion year old rocks from western Australia in the 1980s and found microfossils that are some of the earliest evidence of life that we have.

These studies form the foundation for our understanding of life on Earth, but scientists have only recently begun to examine how life and the environment around it (atmosphere and lithosphere) interact. The Earth's early atmosphere had a strong influence on what forms of life existed - conditions were just right in the primitive earth for life to develop. But, those early forms of life were also integral in changing the atmosphere from one dominated by nitrogen gas, carbon dioxide, and water vapor to one characterized by oxygen. This shift allowed life to expand dramatically on the Earth and it was heavily mediated by photosynthesis and the early carbon cycle.

Some of the first photosynthesis likely happened in the ocean with sulfur, not oxygen. Scientists think that both sulfur- and oxygen-producing photosynthesis were found in ancient seas. But through time and because of the abundance of water on the early Earth, oxygen-producing photosynthesis evolved as the dominant pathway for life. Researchers have found evidence for oxygen-production photosynthesis in rocks that are at least 3.5 billion years old, in "banded iron formations." These sedimentary rocks are composed of Fe₂O₃, which is created when iron (Fe) reacts with oxygen in the air. So, banded iron formations are an indication of when oxygen became present in the atmosphere.

Even though oxygen-producing photosynthesis likely began around 3.5 billion years ago, the Earth's atmosphere was primarily anoxic (without oxygen) until around 2 billion years ago. Scientists think that this occurred because the first oxygen to reach the atmosphere would react quickly with other atmospheric gases and with exposed minerals on land. These reactions continued to happen until the oxygen had exhausted most of the reaction material. Oxygen began to accumulate in the atmosphere when the rate of O₂ production was greater than the rate of the oxidation reactions.

Composition of Earth's atmosphere

Oxygen levels in the atmosphere may have reached their present-day level of 21% around 430 million years ago. They have stayed near that (between 15 and 35%) ever since. The atmosphere's stability could be due to a basic balancing act between oxygen release and organic matter burial in sedimentary rocks - if one decreases so does the other one, maintaining the Earth's atmosphere in a pleasant range for life and driving the modern carbon cycle - but the topic of "homeostasis" between the atmosphere and the biosphere is a contentious topic of research. There is no doubt, though, that the oxygenation of the atmosphere was important in Earth's history. Bill Schlesinger, a biogeochemist at Duke University, has called the release of oxygen by photosynthesis "the single most significant effect of life on the geochemistry of the Earth's surface" because of its dramatic effect on the atmosphere and its influence on life today.

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| GDPE - CSU | Natl. Res. 220 | 605 415 3240 | Page last updated June 2008.