Geoscience Reference
In-Depth Information
calculated that an iron-based biosphere could have been, perhaps, 10%
as active as today's marine biosphere. Not too shabby.
Let's now summarize. It seems likely that before the evolution of
oxygen-producing organisms, numerous interesting and dynamic eco-
systems existed on Earth in environments ranging from the deep sea, to
the surface ocean, and to land-based hydrothermal systems, as well as in
lakes and in soils. In some instances, these ecosystems would have been
rather diminutive, and in other instances, quite obvious to the naked
eye. The most active ecosystem may have been based on the oxidation
of dissolved ferrous iron in the oceans. It seems most likely, however,
that the ancient biosphere would have been much less active than the
present one, which is driven by oxygen-producing organisms.
Can we find any evidence of such ancient ecosystems in the geologic
record? This is far from certain. One difficulty, as we shall explore in
more detail in
chapter 7,
is that we're really not quite sure when oxygen-
producing cyanobacteria first evolved. Therefore, we are not certain how
far back in time we must go to be confident that the rocks we explore
were deposited in a world free from oxygen-producing organisms. And
even so, there aren't many of these ancient rocks around, and those that
we can find are not in good shape. As we will explore in
chapter 6,
the
same plate tectonics that make the world such a habitable place, leave a
fragmented and compromised geologic record. The tossing, turning,
and rumblings of the planet promote the erosion and weathering of an-
cient rocks and also encourage their burial, heating, and deformation.
In short, most of the rocks once found at Earth's surface are gone, and
many of those that remain have been heated and badly deformed, and
this problem grows worse the further back in time we look.
But still, despite the insults of age, there are some clues to the types
of microbial life that occupied very ancient ecosystems. Indeed, a most
revealing place to look is the nearly 3.5-billion-year-old rocks from
“North Pole,” Australia.
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These rocks are in remarkably good shape
for their age, and they have been studied intensively by my Australian
colleague Roger Buick, now of the University of Washington. Roger
paints a picture of an active volcanic terrain at the ocean's edge. Imagine
semi-isolated marine lagoons washing over basaltic outpourings, and
accumulating sediment in an environment with locally high concentra-
