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Changes
in the Atmosphere
Earth’s
early atmosphere contained only small amounts of free oxygen, probably
produced entirely by the reaction of sunlight with water vapor from
volcanoes. The oxygen-rich atmosphere that evolved later, and upon
which oxygen-breathing life now depends, was a result of the origin
of photosynthesis. During the Precambrian, vast numbers
of single-celled algae and cyanobacteria living in
the seas eventually released enough oxygen to transform the environment.
The oldest evidence of cyanobacteria dates to 2.7
billion years ago, although oxygen did not begin to build up in the
environment until about 2.3 billion years ago. During the transition
from oxygen-poor to oxygen-rich atmosphere, the first banded iron
formations may have formed.
Banded iron formations are silica-rich rocks that show alternating
thin layers of dark and red iron-rich rock. They are the most economically
important deposits of iron ore. The silica probably was dissolved
from volcanic ash and rock, and the iron came from sea floor vents
or the weathering of iron-rich volcanic rocks. In the absence of free
oxygen, iron dissolves in water. This must have occurred throughout
the Archean, resulting in ocean waters that contained a great deal
of dissolved iron. In the Proterozoic, however, the dissolved iron
bonded with oxygen released into ocean water by photosynthesizing
cyanobacteria to form magnetite (Fe3O4). This magnetite was then deposited
on the ocean floor. The alternating layers in banded iron formations
are thought to reflect the alternation of oxygen-rich and oxygen-poor
conditions on the sea floor.
A vast amount of iron dissolved in the oceans was available to react
chemically with oxygen, which kept oxygen from accumulating in the
ocean and atmosphere. Once all of the dissolved iron was used up,
the oxygen released by photosynthetic organisms could escape directly
into the atmosphere. As gaseous oxygen built up, the atmosphere began
to change from one that was chemically reducing to one that was oxidizing
(i.e., rust-forming), like today’s. Iron weathered from basaltic volcanoes was oxidized on land before it reached the oceans.
This resulted in the formation of red beds. The red
color of these rocks comes from the particular variety of iron mineral
precipitated on land, mostly hematite (Fe2O3).
Thus, the history of Earth’s early crust also tells the story
of its early atmosphere. Banded iron formations were precipitated
from about 3.1 to about 2 billion years ago—most (92%) during
the Proterozoic between 2.5 and 2 billion years ago. Until all the
available iron had been deposited in banded iron formations, oxygen
could not build up in the atmosphere. Red beds appeared only after
free oxygen was released into the atmosphere, beginning about 2.0
to 1.8 billion years ago. They are still being formed today. |
Eon Overview |
Earth's Crust as a Platform for Prokaryotic Life |
Eukaryotes and the First Multicellular Life Forms |
Changes in the Atmosphere |
Proterozoic Mountains and Glaciers
Department of Paleobiology Home |
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