Page 186 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 173
Studies of sedimentology across the PT
boundary in China and elsewhere have shown
a dramatic change in depositional conditions.
In marine sections, the end-Permian sediments
are often bioclastic limestones (limestones
made up from abundant fossil debris), indi-
cating optimal conditions for life. Other latest
Permian sediments are intensely bioturbated,
indicating richly-oxygenated bottom condi-
tions for burrowers. In contrast, sediments
deposited immediately after the extinction
event, in the earliest Triassic, are dark-colored,
often black and full of pyrite. They largely
lack burrows, and those that do occur are
very small. Fossils of marine benthic inverte- seabed and become buried. Over millions of
brates are extremely rare. These observations,
in association with geochemical evidence,
suggest a dramatic change in oceanic condi-
tions from well-oxygenated bottom waters
to widespread benthic anoxia (Wignall &
Twitchett 1996; Twitchett 2006). Before the
catastrophe, the ocean fauna was differenti-
ated into recognizably distinct biogeographic
provinces. After the event, a cosmopolitan,
opportunistic fauna of thin-shelled bivalves,
such as the “paper pecten” Claraia, and
the inarticulated brachiopod Lingula spread
around the world (see Box 7.2).
Geochemistry gave additional clues. At the
PT boundary there is a dramatic shift in
oxygen isotope values: a decrease in the value
18
of the δ O ratio of about six parts per thou-
sand, corresponding to a global temperature
rise of around 6°C. Climate modelers have
shown how global warming can reduce ocean
circulation, and the amount of dissolved
oxygen, to create anoxia on the seabed. A
dramatic global rise in temperature is also
reflected in the types of sediments and ancient
soils deposited on land, and in the plants and
reptiles they contain. In many places it seems
that soils were washed off the land wholesale.
After the event, the few surviving plants were
those that could cope with diffi cult habitats,
and virtually the only reptile was the plant-
eating dicynodont Lystrosaurus (see p. 450).
Life was tough in the “post-apocalyptic green- systems that normally reduce greenhouse gas
house”, as it has been called.
So what was the killing model? The key
comes from a study of carbon isotopes in
marine rocks. They show a sharp negative
excursion (see Fig. 7.7a), dropping from a
value of +2 to +4 parts per thousand to −2
parts per thousand at the mass extinction
level. This drop in the ratio implies a dramatic
12
increase in the light carbon isotope ( C), and
geologists and atmospheric modelers have
tussled over trying to identify a source. Neither
the instantaneous destruction of all life on
12
Earth, and subsequent fl ushing of the C into
12
the oceans, nor the amount of C estimated
to have reached the atmosphere from the CO 2
released by the Siberian Trap eruptions are
enough to explain the observed shift. Some-
thing else is required.
That something else might be gas hydrates.
Gas hydrates are generally formed from the
remains of marine plankton that sink to the
years, huge amounts of carbon are transported
to the deep oceans around continental margins
and the carbon may be trapped as methane in
a frozen ice lattice. If the deposits are dis-
turbed by an earthquake, or if the seawater
above warms slightly, the gas hydrates may be
dislodged and methane is released and rushes
to the surface. Because the gas hydrates reside
at depth, they are at high pressure, and in the
rush to the surface the pressure reduces and
they expand sometimes as much as 160 times.
The key points are that gas hydrates contain
12
carbon largely in the organic C isotopic
form, and they may release huge quantities
into the atmosphere rapidly.
The assumption is that initial global warm-
ing at the end of the Permian, triggered by the
huge Siberian eruptions, melted frozen cir-
cumpolar gas hydrate bodies, and massive
12
volumes of methane (rich in C) rose to the
surface of the oceans in huge bubbles. This
huge input of methane into the atmosphere
caused more warming and this could have
melted further gas hydrate reservoirs. So the
process continued in a positive feedback spiral
that has been termed a “runaway greenhouse”
effect. The term “greenhouse” refers to the
fact that methane is a well-known greenhouse
gas, causing global warming. Perhaps, at the
end of the Permian, some sort of threshold
was reached, beyond which the natural
levels could not operate. The system spiraled
out of control, leading to the biggest crash in
the history of life.
The current model tracks all the environ-
mental changes back to the eruption of the
Siberian Traps (Fig. 7.8). An immediate effect
was acid rain, as the volcanic gases combined