A New History of Life (45 page)

Sometimes it is the great science fiction writers who can best encapsulate the past. Here is one of the best descriptions of the most famous of all mass extinctions, the K-T event, and as stated in our introduction, we use this older term instead. We were delighted to discover this wonderful description from a book by the iconic writers William Gibson and Bruce Sterling, in their book
The Difference Engine
:

Storms of cataclysm lashed the Cretaceous Earth, vast fires raged, and cometary grit sifted through the roiling atmosphere, to blight and kill the wilting foliage, till the mighty dinosaurs, adapted to a world now shattered, fell in massed extinction, and the leaping machineries of Evolution were loosed in a chaos, to re-populate the stricken Earth with strange new orders of being.

As we well know, those “strange new orders of being” included the many kinds of mammals populating the Earth today. Yet how is it that we came to know with such certainty that it was indeed an asteroid that did in the dinosaurs? This “fact” has held sway since around 1990, or ten years after the Alvarez group of Berkeley published their bombshell findings that have utterly changed our understanding not only of mass extinctions but also of geological processes in general.

The study of mass extinctions was intricately interwoven into the very fabric of the nascent developing field of geology in its most fruitful early period, from about 1800 to 1860. For decades there had been a dispute about the most basic of principles that allowed explanation of how geological material and formations, as well as the kind of animals and plants present on the Earth, came into being. The fight was between those promoting the principle of uniformitarianism, with the
mantra that the present is the key to understanding the past on one side, and a group pushing a principle of catastrophism on the other. The latter was championed by the scholar who first recognized the reality of extinction, Baron Georges Cuvier of pre- and immediately postrevolutionary France, and by later disciples, the most important being Alcide d’Orbigny, whose lasting contribution to geology was the development and then modernization of geological time units. But for all the science they contributed, both Cuvier and d’Orbigny resorted to the supernatural to explain the startling evidence for mass extinction that they first discovered in their early studies of the fossil record. Both believed that a supreme being brought about occasional world-covering floods that wiped out most life and then repopulated the postflood lands and oceans.

There was ebb and flow in terms of the confidence that new generations of geologists and naturalists had in uniformitarianism and the contrasting catastrophism. Uniformitarianism eventually won out, as more and more sophisticated interpretation of rocks, their features, and their ages showed no evidence of a single world-covering flood, let alone the succession of them that would be needed to explain the ever-growing list of mass extinctions known to have occurred. From oldest to youngest, these were the Ordovician, Devonian, Permian, Triassic, and Cretaceous-Tertiary mass extinctions, now referred to as the big five. By the twentieth century, catastrophism was no longer accepted by anyone, save the occasional crackpot writers trying to cash in on the many humans hopeful that the past was more exciting than the grain-by-boring-grain accumulation of the thick stratigraphic record. But in one facet—the explanation of the mass extinctions—the uniformitarianists (including Charles Darwin) remained uneasy.

Geology concluded that the mass extinctions were very slow, stately events, and that given enough time, even observable changes in climate and even the level of the oceans could explain the extinction of so many species at each of the big five mass extinctions accepted by all geologists in the latter half of the twentieth century.

There were a few (but only a few) cries of dissent. One of the best was from Otto Schindewolf, professor of paleontology at the University
of Tübingen in southern Germany. Schindewolf objected to slow and stately as the reasons for mass extinctions. Instead (but only after a career of long and careful study of the fossil record and its changes), he speculated that perhaps far more catastrophic and rapid events caused the mass extinctions, and suggested that the effects of a nearby star going supernova might have been sufficient to cause one or more of the known mass extinctions. He even gave a name to this throwback to the past—he called it neocatastrophism, and he meant it to be a very nonuniformitarianistic way of explaining the past.

Schindewolf’s speculations fell on deaf ears in his profession. Slow climate change, slow sea level change were the “facts”—and presumed causes—of the great mass extinctions. For thirty years, from 1950 on, the field of geology slumbered, smug in its understanding that all was explainable by earthly (and slow) causes. That, then, was the state of play concerning mass extinctions from the time of Schindewolf’s speculations in the 1950s until 1980. And then it all changed. On June 6, 1980, the thirty-sixth anniversary of the D-Day invasion of Europe, the Alvarez paper on the asteroid causing the Cretaceous-Tertiary mass extinction invaded the old, stately, yet already tottering edifice of uniformitarianism in general, and the accepted causes of mass extinctions in particular.
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It was a shot that started a scientific war that in some sense continues to this day. The work of the Alvarez group answered that question.

The presence of numerous impact craters on every planet or satellite of the solar system with a solid surface gives stark and impressive evidence of the frequency and importance of these events, at least early in the history of our solar system. It is probable that impact is a hazard in most, or perhaps all, extra-solar planetary systems as well. Impacts are probably the most frequent and important of all planetary catastrophes. They can completely alter the biological history of a planet by removing previously dominant groups of organisms and thus opening the way for either entirely new groups or previously minor
groups. Thus the 1980 Alvarez work took on special significance on many fronts.

Two of the most important lines of evidence used to convince most workers in the field that the K-T extinction was indeed caused by large body impact were the discovery of both elevated iridium values within the boundary clays and abundant “shocked quartz” intermingled with the iridium. By 1997, high iridium concentrations had been detected at over fifty K-T boundary sites worldwide. Iridium is seen as an indicator of impact because it is quite rare on the Earth’s surface, but at much higher than Earth concentrations in most asteroids and comets. Shocked quartz grains are seen as indicators of impact because creating the multiple lamellae on sand-sized quartz found at most K-T boundary sites can be produced only by high pressure events, such as would occur when a large asteroid hits rock containing quartz at high velocity. No “earthly” conditions naturally create such quartz grains with multiple shock lamellae.

In addition to iridium and shocked quartz grains, K-T boundary sites have also yielded evidence of fiery conflagrations that must have occurred soon after the impact.
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Fine particles of soot were found in the same K-T boundary clays from many parts of the globe. This type of soot comes only from burning vegetation, and its quantity suggested that much of the Earth’s surface was consumed by forest and brush fires.

Although originally controversial, the mineralogical, chemical, and paleontological data gathered during the 1980s persuaded most experts that a large (~10–15 km diameter) comet or asteroid impacted the Earth ~65 MA, and that, at the same time, more than half of the species then on Earth became extinct rather suddenly in the K-T. The discovery of a large impact crater of precisely the right age in the Yucatán region of Mexico (the Chicxulub crater) largely swept away remaining opposition to the impact hypothesis.

The ultimate killer, according to Alvarez et al., was a several-month period of darkness, or blackout as they called it, following the impact. The blackout was due to the great quantities of meteoric and Earth material thrown into the atmosphere after the blast, and lasted
long enough to kill off much of the plant life then living on Earth, including the plankton. With the death of the plants, disaster and starvation rippled upward through the food chains.

Several groups have calculated models of lethality caused by such atmospheric change. Apparently a great deal of sulfur was tossed into the atmosphere as well. A small portion of this was reconverted into H
₂
SO
₄
, which fell back to Earth as acid rain; this may have been a killing mechanism, but was probably more important as an agent of cooling than direct killing through acidification. However, more deleterious to the biosphere may have been the reduction (by as much as 20 percent for eight to thirteen years) of solar energy transmission to the Earth’s surface through absorption by atmospheric dust particles (aerosols). This would have been sufficient to produce a decade of freezing or near freezing temperatures on a world that at the time of impact had been largely tropical, and confirmed the earlier Alvarez statement that the blackout played a part in the mass extinction. The prolonged winter was brought about by vastly increasing aerosol content in the atmosphere over a short period of time.

After the impact there was also dust, lots of it.
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The global climatic effects of atmospheric dust produced by the impact of a large (10 km diameter) asteroid or comet would include long-term (on the order of months) blackout producing light levels below that necessary for photosynthesis, accompanied by rapid cooling of land areas. But perhaps the most ominous prediction in this model is the formerly unappreciated effect that the giant volume of atmospheric dust generated by the impact has on the hydrological cycle. Globally averaged precipitation decreased by more than 90 percent for several months and was still only about half normal by the end of the year. In other words, it got cold, dark, and dry. This is an excellent recipe for mass extinction, especially for plants, as well as for the creatures feeding on plants.

Finally, it was realized that within hours after the impact, bits of rock would rain out of the sky at high speed, and coming through the atmosphere from the near outer space heights that they had been blasted up to, they came in hot enough to set the Earth’s
vegetation on fire. What may have been the largest forest fires in history, on all continents, ensued, and this alone may have killed off the dinosaurs on land.

It is now known that as many as 75 percent of all species went extinct in the K-T mass extinction. On land, it was marked by the loss of dinosaurs on one side, and the appearance of mammals on the other. In the sea it was the disappearance of ammonites on the Cretaceous side of the event, and the appearance of marine biotas dominated by clams and snails on the Paleogene side. But as age dating improves, we are seeing that the “mass extinction” at the Cretaceous-Paleogene boundary is far more complicated than the original single-strike theory can account for. We now know of at least two “pre-K-T” pulses of mass extinction prior to the final blow, which remains fully supported. But the last few years of research show that once again the effects of flood basalt volcanism were part of the killing mechanisms.

There are very few dinosaur fossils, and thus using them to try to learn the rate at which they went extinct is nearly impossible. The fossil record is full of microfossils, and it has been the study of these that lend great support to the claim of sudden extinction caused by the asteroid or comet’s fall. Yet we needed to know the fate of larger fossils both on land and sea, and the most studied of these latter have been the ammonite cephalopods, described in the last chapter.

The best place to study the extinction of the last ammonites that lived near the equator of the Late Cretaceous world is at the thick stratal outcrops lining the Bay of Biscay, a large region extending from southwestern France to northeastern Spain, and best of all is at the rocky coastline near the old Basque village named Zumaya.
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There, hundreds of meters of stacked strata ranging in age from 72 to about 50 million years in age are laid out like the pages of an open book. The mass extinction boundary lies in a bay defined by the rocks, and there is even a change of lithology and color marking this unmistakable boundary.

The oldest of the rocks along the Zumaya coastline dates to around 71 million years ago. The strata are composed of individual stratal beds each about six to twelve inches thick, and each bed is a pair: a thicker limestone, and a thinner, more lime-free rock called a marl. Thousands and thousands of these pairs are stacked one upon another and long ago lithified into the rocky coastline. From the kind of rock and from the fossils, we know that the strata were deposited in fairly deep water, in the deepest part of the continental shelf, or even over its edge, at depths between two hundred and perhaps four hundred meters deep.

The strata are aligned perpendicular to most of the coastline and tilted at a steep angle to the north. Walking along the coastline from south to north takes one ever younger in time. But the very steep tilt of the rocks and the high range of daily tidal change make a visit to a sizable portion of these two outcrops low-tide dependent and very difficult walking.

At the start of the walk, well down the beach (and thus down in time as well) from the long stairway that is the only entrance to this rocky site, fossils are everywhere. Most are large clams, the inoceramids discussed in the last chapter, but there are many ammonites as well, and not a few sea urchins, looking like large stuffed hearts. There are no vertebrate bones, no shark’s teeth—and certainly no dinosaurs—but these marine strata are the same age as nonmarine beds found around the world holding one of the great dinosaur assemblages known.