Sometimes ‘the facts’ discovered by science answer questions by themselves, but much of the time facts are used for competing hypotheses. Sometimes more facts settle the issue of which hypothesis better fits the facts, but like as not, the ‘better fit’ is a matter of interpretation. If the issue at hand is important enough, scientists will sometimes round up a group of specialists (a panel of experts) and try to assess the evidence (facts) for a consensus conclusion. This is the case for global warming. It is also the case for Chicxulub and a mass extinction of half the life on Earth.

Some of the facts were known a long time ago. Paleontologists studying the fossil record in the early 20th century noticed that suddenly (geologically speaking) there were far more animal fossils, especially dinosaurs, before 65 million years ago than after it. Eventually this point in geologic history was identified as a ‘mass extinction,’ in which about 50% of all species – plant, animal, marine and terrestrial – disappeared from the fossil record. This became a ‘boundary’ time that signaled the end of the Cretaceous Period and the end of the dinosaur era. It used to be called the K-T boundary, for Kreidezeit (German for Cretaceous) – Tertiary (the following period). Officially this has been changed to reflect a new geologic time division into Paleogene and Neogene periods. So now it’s the K-Pg boundary, except for those who still write K-T. (Scientific nomenclature, just like most other aspects of science, is subject to revision – selectively.)

The facts in the fossil history showed that the extinction happened, what they did not show was how it happened. Of course, this is science, so there were hypotheses, lots of them. Here’s a few: Mammals became the dominant animal form during this time, so maybe the mammals ate all the dinosaur eggs. Perhaps the climate changed because of dust from volcanic activity. It is known that the sea levels dropped considerably during this period (the Maastrichtian sea-level regression), which may have killed off life in the continental shelf area – causing a marine mass extinction and global climate changes. It was also suggested that the Earth may have been struck by one or more giant meteorites, spewing dust into the atmosphere with ensuing climate change. These hypotheses and many more like them were supported by some evidence, although none were considered conclusive. In overview, the various hypotheses tended to two main types: Gradualist (or intrinsic gradualism), which believes that the actual K-Pg period was several million years long and encompassed a number of causes, some of which played out slowly (gradually). The other type is called catastrophist (or extrinsic catastrophism), extinction caused by extraterrestrial objects hitting the Earth.

That’s the thing about scientific hypotheses – they can remain plausible but not conclusive for a long time because there isn’t enough evidence for a consensus on any of them.

Note: “Consensus” General agreement among those familiar with the issues. Not unanimous agreement. Not unconditional. New evidence may appear. In this case, there was no consensus because the evidence was too weak for any one or combination of hypotheses. You can make up almost anything if the evidence is sparse – and people do, just ask a lawyer.

In 1980 there was a new variation on the ‘Earth struck by extraterrestrial object’ hypothesis. Scientists including Luis Alvarez (Nobelist, physics) noticed that the element iridium, though quite rare, was found in thin layers in many locations on Earth. This layer occurred in sediment about 65 million years old. They theorized that the iridium layer was laid-down at the time of an impact from a massive, vaporized meteor about 100 kilometers in diameter. However, at that time, no crater of this size was known. So this hypothesis went on the shelf with the others.

It came off the shelf in 1990 when scientists discovered the massive Chicxulub crater. Chicxulub (pronounced chick-shoo-loob) was an asteroid. It crashed into Earth 65 million years ago on the Yucatan Peninsula in Mexico. Chicxulub was a rather big asteroid, about 12 kilometers (7.5 miles) in diameter travelling at 20 kilometers per second (44,640 mph). The impact was roughly equivalent to exploding an atomic bomb a billion times larger than the one used at Hiroshima. It produced a crater 100 kilometers (60 miles) across and blew into the atmosphere billions of tons of pulverized earth, rock, and water. The cloud from the impact was so dense, that for years after the Sun’s light was diminished, and the Earth cooled dramatically. Much of the cloud was also sulfurous, which created a multi-year period of acid rain. Those animals that weren’t destroyed by the vast shockwaves, earthquakes, and tsunamis from the initial impact were killed by the changing global climate and the lack of food as plants died off.

Evidence for Chicxulub and the K-Pg connection has accumulated over the years, but so have competing or amending hypotheses. Most of the arguments, which still reflect the gradualist vs catastrophist points of view, center on how long the K-Pg period lasted – and correspondingly, if it was longer (on the order of a million years and therefore gradual), then there must have been other causes at work, such as volcanic eruptions, for the mass extinction.

This brings us to the panel, 41 scientists from institutions all over the globe were assembled to read through the K-Pg extinction literature, discuss their findings, and issue a report. Why they were brought together isn’t explicitly explained, but it seems likely it was because it was believed (someone believed) there was enough evidence for a consensus. In the event, there was consensus – at least among these 41 scientists. The main force behind the consensus involved additional evidence for the effect of the Chicxulub impact, and the evidence that the principle competing cause – the volcanic activity at the Deccan Traps in India – ended 300 million years before the K-Pg boundary date.

In the new study, scientists analysed the work of palaeontologists, geochemists, climate modellers, geophysicists and sedimentologists who have been collecting evidence about the KT extinction over the last 20 years. Geological records show that the event that triggered the extinction destroyed marine and land ecosystems rapidly, according to the researchers, who conclude that the Chicxulub asteroid impact is the only plausible explanation for this.

Despite evidence for relatively active volcanism in Deccan Traps at the time, marine and land ecosystems showed only minor changes within the 500,000 years before the time of the KT extinction. Furthermore, computer models and observational data suggest that the release of gases such as sulphur into the atmosphere after each volcanic eruption in the Deccan Traps would have had a short lived effect on the planet. These would not cause enough damage to create a rapid mass extinction of land and marine species.

[Source: EurekAlert]

Does this settle the issue of competing hypotheses for the K-Pg mass extinction? A consensus does not a settlement make, or, of course not. There will almost always be new evidence, or reinterpretation of existing evidence. It’s professionally advantageous – not to mentioned fundamentally scientific – to be skeptical of consensus opinions, as long as you have something substantive (e.g. more testable evidence) to offer.

What it does mean is that for most scientists and for lay people who are interested – the Earth was struck by an asteroid, a very big asteroid, 65 million years ago, which wiped out half of all living species. It could happen again, although the statistical probabilities are very long indeed (like 1 in every 100 million years). Chicxulub was real, had devastating impact (in all senses of the word), and can be referred to as an iconic event, at least for now.