Geoscience Reference
In-Depth Information
Wetlands through time
9
9.1 Introduction
(Nature of Shark Bay 2010). The stromatolites
of Shark Bay became established there about
2000 to 3000 years ago when sea level and
water salinity reached modern conditions. The
story of stromatolites illustrates the deep time
dimension that is necessary to understand the
Earth's environmental system and how it came
into being.
Geologists apply a principle known as “uni-
formitarianism,” which states that the present
is the key to the past. In other words, under-
standing modern earth processes, such as vol-
canoes, glaciers, earthquakes and so on, gives
necessary analogies for interpreting the past.
This approach may be turned around; the
present is the consequence of the past. Indeed,
in order to interpret modern environmental
conditions, it is often necessary to know about
past developments that led to current circum-
stances. It would be impossible, for instance, to
understand why the southern portion of Lake
Michigan is sinking today while the northern
part is rising without knowing about the impact
of the last glaciation more than 10,000 years
ago (see chapter 8.3.2).
Given their situations in low-lying landscapes
that contain anaerobic water and soil, wetlands
tend to accumulate organic-rich sediment
through time. Both macro- and microfossils of
l ora and fauna are well preserved and abundant
in such sediments. Many of the most important
fossil assemblages have been found in strata of
ancient wetland deposits. Well-known American
Wetlands have existed since the earliest records
of fossil life on Earth. The most common fossils
from the Archean Eon (
2.5 billion years ago)
are stromatolites, which were essentially reefs
built by cyanobacteria (blue-green algae) that
proliferated in shallow marine environments.
In fact, the oldest known fossils dating from 3.5
billion years ago are stromatolites (Virtual Fossil
Museum 2010). Stromatolites pumped their
waste product, namely oxygen, into the early
anaerobic ocean of the world, which led to
deposition of massive banded-iron formation
and later of terrestrial red beds (see Color Plate
2-10), as atmospheric oxygen began to increase.
It is safe to say that stromatolites living in
shallow marine and coastal wetlands utterly
transformed the global environment prior to the
appearance of multicellular life (
c
. 700 million
years ago). Then, as now, prokaryotic life forms
the bulk of the biosphere, maintains biogeo-
chemical cycles, and renders the Earth habitable
for all other life as we know it today, and wet-
lands are essential ingredients in this environ-
mental scheme.
Stromatolites continue to survive in the
modern world in the Bahamas and at Shark
Bay, a World Heritage Area in western Aus-
tralia, where they live in hypersaline tidal pools
(Fig. 9-1). High salinity excludes most preda-
tors and competitors, which allows the stroma-
tolites to thrive as they did in ancient times
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