K-T extinctions: the end of dinosaurs?

Since the early decades of the 19th century, it had been known that
different groups of organisms dominate different periods of Earth
history. One of the more notable groups was the dinosaurs, and
there was a steady reinforcement from palaeontological surveys of
the idea that none were to be found in rocks younger than the end of
the Cretaceous period (approximately 65 Ma). In fact, it came to be
recognized that the very end of the Cretaceous Period, leading into
the Tertiary Period (now universally referred to as the K-T
boundary) marked a major time of change. Many species became
extinct and were replaced in the Early Tertiary by a diversity of new
forms: the K-T boundary therefore seemed to represent a major
punctuation in life and consequently a mass-extinction event. The
types of species that became extinct at this time included the fabled
dinosaurs on land, of which there were many different varieties by
Late Cretaceous times; a multiplicity of sea creatures, ranging from
giant marine reptiles (mosasaurs, plesiosaurs, and ichthyosaurs), to
the hugely abundant ammonites, as well as a great range of chalky
planktonic organisms; while in the air the flying reptiles
(pterosaurs) and enantiornithine birds disappeared forever.

Clearly it was necessary to try to understand what might have
caused such a dramatic loss of life. The flip side of this general
question was just as important: why did some creatures survive?
After all, modern birds survived, so did mammals, and so did lizards
and snakes, crocodiles and tortoises, fish and a whole host of other
sea creatures. Was it just luck? Up until 1980, most of the theories
that had been put forward to explain the K-T extinctions and
survivals ranged from the sublime to the ridiculous.

One of the more persistent of the pre-1980 theories revolved around
detailed studies of the ecological make-up of the time zones closest
to the K-T boundary. The consensus suggested that there was a shift
to progressively more seasonal/variable climatic conditions at the
end of the Cretaceous Period. This was mirrored in the decline of
those animals and plants less able to cope with more stressful
climatic conditions. This was linked, rather inconclusively, to
tectonic changes towards the close of the Cretaceous Period; these
included marked sea-level rises and greatly increased continental
provinciality. The general impression was that the world was slowly
changing in character, and this eventually culminated in a dramatic
faunal and floral turnover. Clearly such explanations require a
longer timescale for the extinction event to take place, but the
Achilles heel was that this did not adequately account for the
simultaneous changes seen in marine communities. In the absence
of better-quality data, arguments waxed and waned with no obvious
resolution.

In 1980, this field of investigation was completely revolutionized
by, of all people, an astronomer, Luis Alvarez. His son Walter, a
palaeobiologist, had been studying changes in plankton diversity at
the K-T boundary. It seemed logical to assume that the interval
between the Late Cretaceous and Early Tertiary might simply
represent a longish period of ‘missing’ time – a genuine gap in
the continuity of the fossil record. To assist Walter in his studies
concerning the changes in planktonic communities at this critical
time in Earth history, Luis suggested that he could measure the
amount of cosmic dust that was accumulating in boundary
sediments in order to be able to provide an estimate of the extent
of this presumed geological gap. Their results shocked the
palaeontological and geological world. They found that the
boundary layer, which was represented by a thin band of clay,
contained enormous quantities of cosmic debris that could only be
explained by the impact and subsequent vaporization of a gigantic
meteorite. They calculated that this meteorite would have needed to
be at least 10 kilometres in diameter. Considering the effect of the
impact of such a giant meteorite, they further proposed that the
huge debris cloud generated (containing water vapour and dust
particles) after the impact would have shrouded the Earth
completely for a significant period of time, perhaps several
months or even a year or two. Shrouding the Earth in this way
would have shut down photosynthesis of land plants and planktonic
organisms, and triggered the simultaneous collapse of terrestrial
and aquatic ecosystems. At a stroke, the Alvarezes and their
colleagues seemed to have found a unifying explanation for
the K-T event.

As with all good theories, the impact hypothesis generated an
impressive volume of research. Throughout the 1980s, more and
more teams of researchers were able to identify cosmic debris and
violent impact-related signals in K-T boundary sediments from
the four corners of the globe. By the late 1980s, the attention of a
number of workers was drawn to the Caribbean area. Reports
showed that on some of the Caribbean islands, such as Haiti,
deposits of sediments at the K-T boundary not only showed the
impact signal, but immediately above this an enormous thickness
of breccia (broken masses of rock that had been thrown together).
This, as well as the greater thicknesses of the meteorite debris
layer and its chemical signature, prompted the suggestion that
the meteorite had impacted somewhere in the shallow sea in this
area. In 1991, the announcement was made that researchers had
identified a large subterranean meteorite impact crater, which they
called Chicxulub, on the Yucatán Peninsula of Mexico. The crater
itself had been covered by 65 million years of sediment, and had
only been visualized by studying seismic echoes of the Earth’s crust
(rather like the principle of underground radar). The crater
appeared to be approximately 200 kilometres across and coincided
with the K-T boundary layer, so Alvarez’s theory was vindicated in a
most remarkable way.

From the early 1990s onwards, study of the K-T event shifted
away from the causes, which then seemed to have been established,
to attempting to link the extinctions at this time to a single
catastrophic event. The parallels to the nuclear winter debate are
fairly clear. Advances in computer modelling, combined with
knowledge of the likely chemical composition of the ‘target’ rocks
(shallow sea deposits) and their behaviour under high-pressure
shock, have shed light on the early phases of the impact and its
environmental effects. At Yucatán, the meteorite would have
impacted on a sea floor that was naturally rich in water, carbonate,
and sulphate; this would have propelled as much as 200 gigatons
each of sulphur dioxide and water vapour into the stratosphere.
Impact models based on the geometry of the crater itself suggest
that the impact was oblique and from the south-east. This trajectory
would have concentrated the expelled gases towards North
America. The fossil record certainly suggests that floral extinctions
were particularly severe in this area, but more work elsewhere is
needed before this pattern can be verified. Alvarez and others’ work
on the effects of the impact suggested that dust and clouds would
have plunged the world into a freezing blackout. However,
computer modelling of atmospheric conditions now suggests that
within a few months light levels and temperatures would have
begun to rebound because of the thermal inertia of the oceans, and
the steady fall-out of particulate matter from the atmosphere.
Unfortunately, however, things would have become no better for
some considerable time because the sulphur dioxide and water in
the atmosphere would have combined to produce sulphuric acid
aerosols, and these would have severely reduced the amount of
sunlight reaching the Earth’s surface for between 5 and 10 years.
These aerosols would have had the combined effects of cooling the
Earth to near freezing and drenching the surface in acid rain.

Clearly these estimates are based only on computer models,
which may be subject to error. However, even if only partly true,
the general scope of the combination of environmental effects
following the impact would have been genuinely devastating,
and may well account for many aspects of the terrestrial and
marine extinctions that mark the end of the Cretaceous Period.
In a sense, the wonder is that anything survived these apocalyptic
conditions at all.