Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 245

206 ATMOSPHERE EVOLUTION

of oxygen through their exoskeletons: to be big they
need to live in conditions of high atmospheric oxygen,
so that sufficient oxygen passively diffuses into their
blood to power muscles for flight.
Reduced greenhouse forcing and glaciation at the
end of the Palaeozoic with the assembly of Pangaea
reduced organic-matter burial, and there was a slow
rise in carbon dioxide levels to around six times PAL
in the Permian and Triassic. Levels of carbon dioxide
gradually decreased, and stabilized near present-day
levels in the Mesozoic. Oxygen tends to track carbon
dioxide inversely; geological evidence and palaeoclimate models suggest a maximum of near 35% oxygen
in the atmosphere at the beginning of the Permian.
During the Permian, the oceans were highly stratified,
with carbonate-rich water at depth that was depleted
in oxygen. This system was unstable: ocean hypoxia
could occur if ocean circulation intensified enough
to mix deep-water carbon dioxide and hydrogen
sulphide into surface waters. A protracted (20 Ma)
whole-ocean hypoxia event is considered to be a
major mechanism in the Permian–Triassic extinction
event, which wiped out 90%–95% of all marine
species (see Palaeozoic: End Permian Extinctions).

Figure 9 Phanerozoic carbon dioxide and oxygen concentra
tions.

Carbon Dioxide and Climate Changes

High-resolution information about changes in atmospheric chemistry over the past 160 000 years can be
obtained by studying the record of trapped gases in ice
cores from the Greenland and Antarctic ice-caps. Furthermore, oxygen isotopic values from marine sediments, marine planktonic and benthonic fossils, cave
deposits, and other sources can be used to estimate
marine palaeotemperatures. Direct measurements of
carbon dioxide and methane concentrations in ice
cores permit assessment of past atmospheric levels of
these gases, providing factors to incorporate into
models of past air temperatures and sea-levels.
Data from deep ice cores taken in polar regions, coupled with complex palaeoclimate models
(Figure 9), show large fluctuations in atmospheric
carbon dioxide, oxygen and methane levels, leading
to long-term temperature changes of the order of
Ỉ6 C or more. There is a strong correlation between
levels of atmospheric greenhouse gases and palaeotemperature. The periodicities in these data provide
clear evidence of the role of Milankovitch forcing by
changes in the Earth’s orbital parameters. The two
strongest Milankovitch cycles observed correspond
to the 26 000 year precession of the equinoxes and
the 100 000 year period of rotation of the Earth’s
orbital axis (see Earth: Orbital Variation (Including
Milankovitch Cycles)). Changes in greenhouse-gas
concentrations appear to follow rather than guide
long-term climate, suggesting that Milankovitch

Figure 10 Changes in the concentration of atmospheric carbon
dioxide over the last 60 years as measured at Mauna Loa, Hawaii.
Data provided by D. Keeling and T. Whorf.

cycles are the prime mechanism bringing the Earth

into a greenhouse or icehouse condition. Greenhouse
gases do provide positive feedback at the beginning of
temperature changes by boosting insolation. Anthropogenic emissions of greenhouse gases are not
governed by Milankovitch cycles and represent a separate and increasingly important climate-forcing
mechanism. Figure 9 shows that carbon dioxide concentrations changed by almost 100 ppm(vol.) towards
the end of the last glaciation. Modern levels of carbon
dioxide are near 370 ppm(vol.) and rising. Almost all
of this change has occurred since the Industrial Revolution, and high-resolution monitoring at the Mauna
Loa observatory shows clear diurnal and annual
cycles in carbon dioxide levels (Figure 10), with a
mean annual increase of 1.16 ppm(vol.) year 1. This
is over one hundred times the rate of increase of
carbon dioxide levels inferred from all available