Past climate change - Climate Change: Biological and Human Aspects - page 77

the Cambrian and now was not a steady one, as is evident from times of speciation
and times of extinction, both beyond and within background rates. It is not possible
to review even a summary of the extensive palaeobiological literature here (for which
readers should seek out other texts such as Briggs and Crowther, 2001), but it is worth
citing a few of the major climate-related biological events. Then we can put recent
changes of the past million years or so, not to mention likely future changes over the
next thousand years (see Chapter 8), into a broader context.
3.3 Majorbio-climaticeventsofthepre-QuaternaryPhanerozoic
(542-2mya)
3.3.1 Late-Ordovicianextinction(455-435mya)
In the Ordovician period life was mainly marine. During most of this period life was
flourishing in the seas and oceans, and terrestrial life was largely restricted to algae
in splash zones at the edge of the sea and possibly in brackish environments. But by
the late Ordovician life was beginning to get an enhanced foothold on land. Yet at this
time life was threatened when globally about 22% of all families of species became
extinct, which makes the late Ordovician the time of one of the largest episodes of
mass extinction in the Earth's history. The extinction is thought to have happened in
two principal phases about 0.5-2 million years apart that wiped out nearly all major
benthic and planktic groups. Among the molluscs, 125 genera of nautiloids were
reduced to 25, the 88 gastropod genera went down to 55 and bivalve genera were
down from 84 to 32. The two phases of the late Ordovician were associated with
the growth and decay of a large Gondwanan ice cap when the southern continents
drifted over the South Pole. Sea-water temperatures fell by as much as 8 ◦ C, and
a similar degree of cooling even took place in the tropics, which caused the loss
of many tropical species. The sea level fell by between 50 and 100 m depending
on regional eustatic conditions. Changes in 13 C of about 7‰ compared to 12 Cin
sediments indicate a major change in carbon cycling (see section 1.3 and the text box
in section 3.2.2). The rapid global cooling and growth of the ice caps contrasted with
the greenhouse conditions prevalent before and after the event. The two extinction
phases are thought to have related first to the cooling (caused by the weathering of
recently formed mountains ranges: see the silicate carbon dioxide equation in section
3.3.3) and then second to the warming: species surviving the first change were then
faced with the second. What appears to be characteristic is that both events were
sudden, in geological terms. It is perhaps worth reflecting, as we shall see, that recent
(Quaternary) glacials exhibit periods of rapid, rather than gradual, climatic change.
(We will return to this when discussing climate thresholds in section 6.6.8.) As we
shall see with some future major extinctions, there was a fungal spike associated
with each event, suggesting a world of rotting biomass. With the late-Ordovician
extinction there was another spike: a spike of microbialites (including stromatolites,
thrombolites and dendrolites).