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تاريخ التسجيل : 11/05/2015
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The Rise of Oxygen in the Atmosphere

في الأحد مايو 17, 2015 5:38 pm
The Rise of Oxygen in the Atmosphere

Where did the oxygen to form the banded iron formations come
from? This puzzled scientists for many years. Then stromatolites were
discovered. (You looked at stromatolites in the previous section.)
Scientists thought that the oxygen must have come from ancient
cyanobacteria. Recall that cyanobacteria are phototrophs. They use
photosynthesis to convert energy from the Sun into food. Oxygen is a
by-product of photosynthesis. The population of cyanobacteria on Earth
increased. As a result, 3.5 to 2.5 billion years ago, more and more oxygen
was released into Earth’s atmosphere.
The atmosphere and oceans readily exchange gases. As the oxygen
content of the atmosphere increased, the dissolved oxygen content of
the surface waters in the oceans also increased. Oxygen reacted with the
dissolved iron and silica in the ocean. This resulted in the layers of the
banded iron formations. Until 2.3 billion years ago, oxygen was drawn
down into the oceans. It replaced oxygen used up in forming the banded
iron formations. This kept the concentration of oxygen in the atmosphere
to low levels. The levels were 1–2 percent of what they are today.
After about 2 billion years ago, the deposition of banded iron formations
began to slow down. There was less and less dissolved iron in the oceans.
The rate at which iron was resupplied to the oceans had decreased. By
1.8 billion years ago, there was very little dissolved iron in ocean water.
The banded iron formations stopped forming altogether.
The cyanobacteria flourished. They continued to produce oxygen.
However, the iron was no longer there to act as a sink for oxygen in the
oceans. Less oxygen was drawn from the atmosphere into the oceans.
As a result, oxygen began to build up in the atmosphere.
The change from an oxygen-poor atmosphere to an oxygen-rich
atmosphere took place slowly. Scientists think that it took over a billion
years. They have looked at soils in South Africa dated at 1.9 billion years.
The soils indicate that atmospheric oxygen at the time was 15 percent
that of the modern level. Current levels of oxygen were reached probably
about 600 million years ago.
Along with a buildup of oxygen, came an increase in ozone levels (O3). The
increase in oxygen and ozone had important implications for life on Earth.
Ozone absorbs much of the UV radiation that strikes the atmosphere. It
formed a protective layer around Earth. This made the continents a more
hospitable area for organisms to live.
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الجنسانثى مســآهمآتي : 7
تاريخ التسجيل : 20/05/2015
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رد: The Rise of Oxygen in the Atmosphere

في الأربعاء مايو 20, 2015 9:48 pm
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