Geology Reference
In-Depth Information
Figure 19.16 Paleoproterozoic-Age Banded Iron Formation (BIF)
a At this outcrop in Ishpeming, Michigan, the rocks are brillantly
colored alternating layers of red chert and silver iron minerals.
Ultraviolet radiation
Oxygen produced by photochemical dissociation
Oxygen produced by planktonic
photosynthetic cyanobacteria
Oxidation and photo-oxidation of Fe ++
Ferric hydroxide and silica precipitate
Ferric hydroxide
reduced,
dissolved
10 3 km
b Depositional model for the origin of banded iron formations.
early episode of intense meteorite and comet bombardment
accounting for a rapid rate of surface water accumulation.
Following Earth's early episode of meteorite and comet
bombardment, which ended about 3.8 billion years ago, these
extraterrestrial bodies have added little to the accumulating
surface waters. However, volcanoes continue to erupt and expel
water vapor (much of it recycled surface water), so is the volume
of the oceans increasing? Probably it is, but at a considerably
reduced rate. The reason is that much of Earth's residual heat from
when it formed has dissipated and the amount of radioactive
decay to generate internal heat has diminished, so volcanism is
not nearly as commonplace. Accordingly, the amount of water
added to the oceans now is trivial compared to their volumes.
Recall from Chapter 17 that one early attempt to
determine Earth's age was to calculate how long it took for
the oceans to reach their current salinity level—assuming,
of course, that the oceans formed soon after Earth did, that
they were freshwater to begin with, and that their salinity
increased at a uniform rate. None of these assumptions is
correct, so the ages determined were vastly different. We now
know that the very early oceans were salty, probably about as
salty as they are now. That is, very early in their history, the
continental red beds (Figure 19.13c) provide compelling
evidence that Earth's Proterozoic atmosphere was an
oxidizing one.
The Hydrosphere
We have outlined the history of the evolving atmosphere,
and now we turn our attention to the hydrosphere, another
of Earth's major systems. All water on Earth is part of the
hydrosphere, but most of it—more than 97%—is in the
oceans. Where did it come from, and how has it changed?
Certainly, outgassing released water vapor from Earth's
interior, and once the planet cooled suffi ciently, water vapor
condensed and surface waters began to accumulate, prob-
ably at some time during the Eoarchean. Another source
of water vapor, and eventually liquid water, was meteorites
and especially icy comets (Figure 19.3a). It is not known
which of these—outgassing or meteorites and comets—was
most important, but we do know that oceans were present
by the Eoarchean, although their volumes and geographic
extent cannot be determined. Nevertheless, we can envision
an early Earth with numerous erupting volcanoes and an
 
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