Geoscience Reference
In-Depth Information
formed during this time. Comparable modern day environments would
include the Everglades in Florida and peat bogs found in many places
around the world, especially in high latitudes.
The drop in oxygen concentrations that followed was probably due
to a change in paleogeography and the types of sediments deposited.
Beginning in the Permian Period, but continuing into the Triassic, a
supercontinent called Pangea became fully assembled. It seems that as
a result of this assembly, far fewer low-lying swampy areas were avail-
able. Also, because of its vastness, less of the rain that fell onto the
Pangean continent ran out to the sea, and more of it ran into the conti-
nent itself, forming vast expanses of well-drained sandy red bed depos-
its (we met red beds in
chapter 8)
. These sandy continental sediments
are virtually free of organic matter and thus their formation provides no
input of oxygen to the atmosphere. Therefore, in Bob's model, the shift
to major red bed deposition during the later Permian and Triassic Peri-
ods reduced the supply of oxygen to the atmosphere and this resulted
in a precipitous drop in oxygen concentration, perhaps to levels signifi-
cantly lower than in the present atmosphere.
Thus, changes in the types of sediments deposited, as controlled by
a combination of factors like plant evolution, paleogeography, and cli-
mate, seem to have controlled the input rate of oxygen to the atmo-
sphere, providing a major influence on atmospheric oxygen levels. There
may even be some evidence to back at least parts of this story up. It is
well known that gigantic insects were part of the Carboniferous and
early Permian biosphere; some of them were almost nightmarish in size.
Imagine, millipedes over a meter in length, giant spiders with leg spans
as broad as an office chair (50 cm or so), and dragonflies with wing-
spans up to 70 cm. Indeed, these giant dragonflies prompted the French
paleontologists Harlé and Harlé over a century ago to propose that
atmospheric oxygen levels in the Carboniferous Period must have been
higher than today's. Part of their reasoning was that higher atmospheric
pressures, enabled by higher oxygen levels, would help to keep these
massive fliers aloft. Also, and perhaps more importantly, they reasoned
that greater oxygen availability would allow for the higher respiration
rates required for such large organisms to fly, especially since they ob-
tain their oxygen by diffusion through their tracheal systems.
