Past climate change - Climate Change: Biological and Human Aspects - page 103

was the end-Cretaceous event that provided more of a spur to the diversification of
mammals and the rise of modern mammals, even if the IETM/PETM facilitated the
further diversification of modern mammals.
Meanwhile, leaving the IETM/PETM behind, another major extinction was to
follow. But before looking at that it is worth reminding ourselves that there were
other CIEs than this initial Eocene event: remember the Toarcian extinction (183
mya) in the Jurassic. The reason why more is known about the Eocene event is
that there are more geological strata (and hence biological palaeorecords) to access,
with more recent deposits (in this case 55 million years old) than for older events.
Consequently we know more about the Eocene event. Nonetheless, other CIEs have
taken place and, as we shall see, today's anthropogenic warming is itself a CIE.

3.3.10 Eocene-Oligoceneextinction(approximately35mya;or33.9mya?)

The Eocene-Oligocene boundary has recently been re-dated (hence the two dates
given in this subsection's title, with the latter, younger date being the one appearing
in some recent literature). It was a cooler period, with palaeotemperatures at a latitude
of 45 ◦ dipping to within a few degrees of today. Antarctica, which was moving to
cover the South Pole, also saw the beginnings of an ice cap on some of its mountain
ranges, including the Gamberstev mountains that are currently under the centre of
East Antarctica's Dome A ice sheet (Bo et al., 2009). It was a time noted for one
of the biggest extinctions since the Cretaceous-Tertiary extinction. In Europe this
extinction was known as the Grand Coupure, although extinctions have been noted
on the other continents and the full explanation is not entirely clear. Nonetheless,
approximately half the mammal genera became extinct within about a million years
(Hallam, 2004).
But, was this extinction caused by cooling due to indigenous planetary climate
forcing (be it greenhouse or atmosphere/ocean circulation changes) or some other
factor? Evidence emerged in the mid-1990s of an asteroid impact aged 35.5-35.2
mya at Chesapeake Bay off the shore of Virginia, USA. This is a structure of 85 km
in diameter and the probable source of the huge field of tektites (characteristic glassy
nodules) covering mainly Georgia some 1100 km away and microtektites in Barbados
2250 km away. This was clearly a significant impact, albeit one that was smaller than
that associated with the end-Cretaceous event.
If this were not enough, soon after the Chesapeake discovery there was the dating
of a second crater, the 100 km-diameter structure in Popigai, northern Siberia, which
is therefore possibly the fifth largest impact crater ever found. (The Cretaceous-
Tertiary structure at Chicxulub, Yucatan, is 180-310 km in diameter.) The Popigai
structure contains some 2000 km 3 of impact melt rocks, and an iridium anomaly
has been detected in Italy (over 3200 km away) exactly matching the Popigai dates.
The structure had been known for many years but its date of origin was ambiguous,
with estimates ranging between 5 and 65 mya. But in 1997 Canadian and Russian
researchers announced that they had managed to refine the dating to 35.7 0.2 mya

(Bottomley et al., 1997). This suggests two possibilities. Either the Earth was subject
to two major hits within a few hundred thousand years, or it was subject to two major
ones simultaneously. Both are possible. Given that major hits are rare events that

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