Historical Geology Understanding Our Planet\'s Past

In effect, biological activity was responsible for the iron deposits since

photosynthetic organisms produced the oxygen. When plants began generat-

ing oxygen (Fig. 37), it combined with iron, keeping oxygen levels in the

ocean within the limits tolerated by the early prokaryotes. Throughout the

Archean, the amount of oxygen probably remained less than 1 percent due to

this regulating mechanism.Then, between 2.5 and 2 billion years ago, photo-

synthesis generated enough oxygen to react with iron on a large scale.

BIF deposits, composed of alternating layers of iron and silica, formed

around 2 billion years ago at the height of the early Proterozoic ice age. For

unknown reasons, major episodes of iron deposition coincided with periods

of glaciation.The average ocean temperature was probably warmer than today.

When warm ocean currents rich in iron and silica flowed toward the glaciated

polar regions, the suddenly cooled waters could no longer hold minerals in

solution. As they precipitated out of seawater, they formed alternating layers

on the ocean floor due to the difference in settling rates between silica and

iron, the heavier of the two minerals. After most of the dissolved iron was

Figure 37 The evolution
of life and oxygen in the
atmosphere.

Historical Geology

4

Billions of years ago

Evolution of shelled invertebrates

Evolution of higher organisms

Beginning of ordinary respiration

Beginning of photosynthesis

Beginning of first cells

Evolution of shelled invertebrates

Evolution of higher organisms

Beginning of ordinary respiration

Beginning of photosynthesis

Beginning of first cells

.0001

Fraction of present oxygen.001

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