25 February 2015. A new debate has erupted (sorry!) about the role of volcanism at the Cretaceous-Palaeogene boundary. Lest you think this is a new idea let me tell you that it has been around for 35 years. Read on…!
The Best Bang Since the Big One (with apologies to Douglas Adams and Eccentrica Galumbits)
Abstracted from my book Architects of Eternity available on Amazon here
The Cretaceous-Paleogene boundary (K-Pg) used to be known as the Cretaceous–Tertiary boundary and was commonly abbreviated to ‘K–T boundary’. The ‘K’ comes from the German ‘Kreide’ meaning chalk, for
chalk, as we have seen, is a common sediment of the Cretaceous. The ‘T’ stands for Tertiary – the third era of life’s history.
And yet it was not as though Walter Alvarez had not already been working in the Bottaccione area for years. He and fellow post-doc Bill Lowrie, both of them from the Lamont–Docherty Geological Observatory of ColumbiaUniversity in Ithaca, New York had contributed to some seminal work in this area. Like several other geologists in the seventies, they had become interested in the remarkable limestones of the Bottaccione Gorge. The limestones are remarkable because they were deposited in the deep sea – they were not shallow-water limestones like those that outcrop on Wenlock Edge. These limestones are different, deposited in about the deepest water you can get and still find fundamentally undissolved.
They soon discovered that another group, led by the legendary Al Fischer of Princeton as well as Mike Arthur and Isabella Premoli-Silva, were working on the very same question and by combining forces they produced a seminal memoir on the geology of the gorge. Originally it had been the happy conflation of decent foram stratigraphy and palaeomagnetics that had led geologists to the Bottaccione Gorge. But it was clay horizon at the K-Pg boundary that made them stay.
Walter talked the K–Pg time-duration problem over with his dad and it wasn’t long before the famous Nobel laureate had arrived at a solution. They would use Beryllium-10, an isotope that was formed in the upper atmosphere when incoming cosmic rays collided with oxygen and nitrogen atoms. The half-life (the time needed for half the original quantity to decay to another isotope) for Be-10 had been calculated as 2.5 million years. There would be enough Be-10 still left in the clay for them to measure what the original concentration must have been (assuming the rate of production in the upper atmosphere was constant). And then the whole concept came unglued – the half-life of Be-10 was recalculated and found to be only 1.5 million years – and that one-million-year difference was crucial. There would not now be enough Be-10 left in the clay for them to measure. The idea was abandoned.
But around about this time Walter was appointed to Berkeley where his colleague Rich Muller and his father were already on the faculty.
The problem of the duration of the K–T boundary would simply not go away. And once again it was the father who had the idea, which was related to the Be-10 concept yet different. It was based on another uncommon element – iridium, a member of the platinum group of elements. Iridium is in short supply on the Earth’s surface. It has sunk into the deeper layers of our planet, the mantle and the core. A heavy metal, indeed. But, outside the confines of our own planet, iridium is in much greater supply as it is one of those primordial elements that was flung far and wide after the birth of the universe. Could significant quantities be dropping in a steady rain on to the earth? If so, the amount of iridium in the clay layer was at best going to be in the parts per billion (an American billion) range. To measure this small a quantity, cutting-edge techniques would be required. But on the Berkeley campus there was one man who they knew could help: Frank Asaro.
Asaro listened cordially when they diffidently approached him and then dropped a bombshell. He and a colleague were already working on something very similar, although their emphasis was on measuring the duration of deposition of fossil soils. However, if his co-author agreed, Asaro would be willing to make the measurements for the Alvarez team. A deal was struck and Asaro agreed to help. The technique Asaro was using to count iridium atoms was neutron activation analysis – and its minimum analytical requirement was nothing less than a nuclear reactor.
Neutron activation analysis works by irradiating samples. Neutrons from the reactor collide with the atoms in the sample and stimulate them to emit gamma rays. The electromagnetic spectrum of the resulting ray is as characteristic as a fingerprint and allows the various elements within the sample to be measured at levels down to parts-per-billion (ppb). The technique takes time however if the element being searched for is rare. It is not uncommon for samples to be irradiated for several months.
In this case the wait was even longer than usual for the universal scientific bogeyman – mechanical failure – had stopped by Frank’s lab and killed his machine. It was several months before it was back on line, and not until the early summer of 1978 when Walter got the summons from his dad, ‘Frank’s got the data!’ They attended, full of expectation – and their hopes were dashed. Their calculation was that at most, if the time represented by the Gubbio clay across the K–Pg boundary was their worst- case scenario of a few thousand years, they would expect to find 0.01 ppb of iridium in the rocks. Instead they found 3ppb – three hundred times theamount that they had expected. And after Asaro had realised that a mistake had been made in the chemical preparation of the sample, they discovered that they actually had 9ppb in the sample – nine hundred times the amount that they were predicting.
At moments like these in the life of a scientist your career boils down to two alternatives: either you have long drawn-out hair-tearing sessions in your office until your partner threatens divorce or separation, or you have long drawn-out drinking sessions in a bar someplace which continue until your partner threatens divorce or separation. The Alvarez team had to explain this high concentration of iridium. Either the iridium really did come from meteoritic dust and some extraterrestrial explanation was required, or there was some wrinkle in the way that iridium was deposited in sea water that nobody had yet thought of and the whole deal was a busted flush. To test this it was essential to try again, with another sample. Walter searched the literature for another complete K–T boundary section. Denmark was the only other obvious candidate.
I’ve already mentioned the bar which nestles under the cliff halfway between Gubbio and the K–Pg outcrop in the Bottaccione Gorge. Yet this bar, even if you can find it when it’s open, is not the bar. For the nascent
boundary freak there is but one true bar, and that is the one true bar above the one true section, at Stevns Klint on the eastern coast of Jutland in the tiny hamlet of Hojerup. The bar above the cliff at Stevns Klint is a remarkable and supremely civilised place: a rambling, gabled building built in the Scandinavian summer-house style on stilts, whose wooden floor protrudes far out over the edge of the gently eroding chalk cliff that faces the strait between Jutland and Zeeland. The light Scandinavian beer here is as good as anything that you will find anywhere. The Carlsberg is a work of art. But there is also something eerie about the village of Hojerup.
There’s a stillness to the place as though it is somehow aware of the secret it guards. Go there in summer and this small Danish village slumbers in a limpid summer sunlight that you wouldn’t ordinarily associate with the Baltic states. Go there in winter and the rain falls in whispering sheets out across the strait towards southern Sweden. It’s so quiet that after a few moments you can hear your own heart beating – a few more minutes and you can hear somebody else’s – but there’s nobody there. No one except the solitary barman quietly polishing glasses in the otherwise deserted bar and the hum of the chiller cooling the Carlsberg. The Bottaccione Gorge has a similar quality, a place strangely out of time, where the clay layer at the K–T boundary sleeps between its enfolding limestone ribs. For anyone with any knowledge of palaeontology and any form of empathy these are places of ultimate endings – and beginnings.
Walter must have felt the same thing when he made his first visit to Stevns Klint in 1978. The mission was critical – they knew that if they could not find the iridium anomaly in the Stevns Klint section then they could not prove that the iridium anomaly was at least regional (they were hoping of course that it was global). And in that case, even if they replicated the measurements from Gubbio and came up with the same answer then the likelihood was that the iridium anomaly at Gubbio would turn out to be no more than some strange artefact of sea-water chemistry that might be worth some short article in Geochimica et Cosmochimica Acta, but would not be the stuff that Science articles are made of.
The Stevns Klint material did not look at all like the Bottaccione material. This layer was thicker and blacker and the surrounding rock was friable chalk, not hard limestone alternating with marl bands. This was no surprise to the Alvarez team since any time-equivalent material from the geological column can be
expected to have a different aspect according to the different places that it is found. The rock type depends to a large extent on the environment that it was originally laid down in – the facies or environment of deposition – which will vary according to whether the sediment was laid down in the deep
ocean, a shallow sea or on land. Also, sediments are almost always more or less modified from their original state by the many different diagenetic processes that have occurred during their long passage across the millennia.
The only other credible idea was that the killer was an asteroid which had hit Earth bringing with it sufficient iridium to create the anomaly. But the details of the killing mechanism would not stand up to scrutiny. How could a simple impact, no matter how large, kill off a large percentage of the life on Earth? There was no easy answer. Walter returned, dispirited, to his palaeomagnetic work in Italy. In California however, after a lifetime of pugnacious tenacity, Luis was not ready to give up. He remembered an encounter he had had with an obscure Royal Society publication on the eruption of Krakatoa and the dust pall that had surrounded the globe after the detonation of that corked volcano. And so the idea of the K–Pg winter was born. A dust cloud surrounded the world in the immediate aftermath of the K–T impact event and decreased light intensity to a level where photosynthesis on land and in the oceans was halted. Cut off the fuel and the engine will die – and photosynthesis is ultimately the fuel that powers the world.
The K–Pg boundary and the death of the dinosaurs became front- page news. The old adage of uniformitarianism was finally overturned by the resurgence of the old notion of catastrophes as major turning points in Earth’s history. And this time the architects of eternity were an ambitious young geologist, a physicist and a couple of chemists.
This was a major turning point. The old order – with its unhealthy preoccupation with gradualistic notions – was finally defeated, going over the next several years not gently (in fact fighting tooth and nail), but going nonetheless, into this good night. The hard realities of an outside scientific world where analytical technology and numeracy reigned supreme had scaled yet another of classical palaeontology’s last bastions.
The palaeontological establishment split rapidly into several camps: those who could not accept the idea on any grounds, particularly the older generation whose uniformitarian roots were too deep to shift; those who did not like the idea because they found the evidence unconvincing; and those who loved the idea because it was novel and had a dangerous frisson of cataclysm about it.
In the aftermath of the Alvarez paper, it became clear that they had succeeded in priority of publication only by the skin of their teeth. A Dutchman, Jan Smit, was on their tail with his own story of noble metal enrichment in the K–Pg section at Caravaca in Spain and almost simultaneously other groups confirmed the iridium enrichments in Denmark and New Zealand.
There was in those days – and to an extent there still is, at least among certain die-hard sectors of the palaeontological community – a feeling, so deep that it is never spoken of, that palaeontology should only be practised by those who disdain the vulgar requirements of high technology. These, of course, are the practitioners of the old palaeontology.
Luis Alvarez bounded into the fray with enthusiasm. He and Walter fought hard to gain acceptance of the theory among the reactionary old guard, yet really never made much progress, their success being with the younger members of the community. It was they who went to seek out other boundary sections in a variety of different depositional environments.
Early notable finds by these new acolytes of apocalypse were Carl Orth and Chuck Pillmore who together found several sections in a non-marine depositional environment in New Mexico interpreted as a fossilised coal swamp.
One healthy development that arose very quickly after the new era dawned was the Snowbird Conferences. The first was in the early eighties in Snowbird, Utah (hence the name). The conferences were designed to be interdisciplinary and to allow astronomers, astrophysicists, physicists, chemists, biologists and palaeontologists to learn each other’s language.
During the eighties the K–Pg community – and by about 1985 it seemed that almost every university geology department in the Western world had at least one researcher working on the boundary, such was its importance – was focused on finding the impact structure itself, what Walter Alvarez has called the ‘Crater of Doom’. (See Walter’s book T. Rex and the Crater of Doom (Penguin, 1998) for the full details of the K–Pg story and the hunt for the impact structure.)
But the camp of what we may call the scientific unbelievers had found a suspect of their own which could also account for the iridium anomaly.
Iridium may be a material of the solar system, but it is also a material of the inner earth – the mantle and the core. It was too heavy to hang around on Earth’s crust as it cooled and stabilised and instead sank into the depths of our planet. Quite soon after the Alvarez paper had come out, a group of geologists in the States realised this and understood further that the iridium enrichment could therefore be interpreted quite differently. On the face of it, the Berkeley group’s asteroid theory seemed plausible, but the iridium could also have risen to Earth’s surface via volcanic vents. Dewey McLean at Virginia Tech was among the first to suggest that the iridium anomaly at the K–T boundary could have been caused by volcanic activity and then the idea was taken up and championed by Chuck Officer and Charles Drake of Dartmouth College, New Hampshire. These three proposed that the iridium had come from the centre of our own planet via volcanism – and lots of it.
The west of India is covered by a fossilised sea of basalt. This is an igneous rock, which is to say it is one of a group of rocks that are formed by material coming from the mantle and core. And this igneous rock is of a singular age. On the basis of the existing radiometric dates, the Deccan Traps were known to be approximately the same age as the K–Pg boundary. Officer and Drake reasoned that the volcanism that gave rise to this enormous flood of basalt (covering an area as large as France), would have been sufficient to bring enough excess iridium up from the Earth’s core to account for the iridium anomaly at the K–Pg boundary in the Bottaccione Gorge (and at the other sections around the world where it was rapidly being found as the asteroid impact scenario gained momentum). As radiometric dating of the Deccan Traps continued, pursued most energetic- ally by the French geophysicist Vincent Courtillot, it became clearer and clearer that this remnant of an extreme volcanic episode was indeed contemporaneous with the K–Pg event.
By the mid-eighties the development of a palaeomagnetic reversal stratigraphy (see Chapter 3) had extended all the way back into the Mesozoic and was well-integrated with absolute dates as well as global biostratigraphic datums based on planktonic forams and calcareous nannofossils. This integrated scheme is known formally as the ‘Geo-magnetic Polarity Timescale’ but is usually (and mercifully) shortened to GPTS.
Normal and reversed intervals are given numbers. The K–Pg boundary occurs within reversed polarity interval 29 – or Chron 29R. Courtillot’s dating of the Deccan Traps (the word ‘trap’ by the way comes from the Scandinavian term trappa, which means stairs) showed that they spanned three polarity intervals – from the top of Chron 30N to the beginning of Chron 29N, encompassing Chron 29R and corresponding to an interval of somewhat less than a million years. This was strong evidence in favour of the volcanism hypothesis which was particularly strongly championed in France, in part perhaps, because of Courtillot’s high profile among the French scientific-political establishment.
Where did this new support for a volcanic cause for the K–Pg extinctions leave the Alvarez scenario? The problem was that the Berkeley group were continuing to be embarrassed by one huge missing piece of evidence – the impact crater itself. There were some early red herrings – the Manson crater in Iowa was investigated but found to be the wrong age.
Then the search turned to the ocean. Tiny spherules of a mineral called sanidine had been found in K–T boundary sections in Spain and Italy and a young Italian geochemist called Alessandro Montanari, who had gone to Berkeley to work with the Alvarezes for his PhD, deduced that this was the alteration product of the original minerals olivine, pyroxene and calcium- feldspar. This suite of minerals is characteristic of oceanic, not continental crust. The Alvarez group, on the basis of this geochemical evidence turned their attention to the ocean. They knew that there was a good chance (about 25 per cent) that this search would be fruitless because the sea floor is being continuously consumed at the continental margins in areas known as subduction zones. And so, very early on, the Alvarez team made an assumption that the crater had been subducted and stopped looking for it because they believed that it was no longer there. This in turn meant that for all of the eighties the question of volcanism versus impact went unanswered.
That question was answered when the Chicxulub impact structure on the Yucatan Peninsula was dated as being precisely the same age as the K/Pg boundary in the 1990’s.