by Peter Ward
To paraphrase one of the great twentieth-century paleontologists, David Raup of the University of Chicago: Were the surviving species gifted with good genes—or simply good luck?
RESULTS OF THE PERMIAN EXTINCTION
If intense controversy still exists about the cause or causes of the Permian extinction, on one aspect of that time interval are all in agreement: in the aftermath of the extinction ecosystems were profoundly affected, and extinction recovery was long delayed. It is this latter evidence that readily distinguishes the Permian extinction from the later Cretaceous-Tertiary event. While both caused more than half of the species on Earth to disappear, the world recovered relatively quickly after the “K-T” event. This may have been due to different causes for the two. Asteroid impact on the Earth and the environmental destruction accruing from the impact have for more than a decade been accepted as the cause of the K-T event. But the killing conditions following the impact soon dissipated. This was not the case after the Permian event. As we have seen above, while some Earth scientists believe that the Permian as well as the K-T events were caused by large-body impacts on the Earth, it seems as if the environmental conditions causing the Permian extinction persisted for millions of years after the onset of the extinction. It is not until the Middle Triassic, some 245 million years ago, that some semblance of recovery seemed to be under way.
These results would be expected if some part of the Permian mass extinction were directly or indirectly caused by the reduction in oxygen at the end of the Permian. The newest Berner curves show that oxygen stayed low into the Triassic, and there is even some indication that the oxygen levels did not bottom out and begin rising until near the end of the lower Triassic, which might account for the long delay in the recovery. This evidence suggests that the environmental events producing extinction just kept persisting. If so, and if animals were capable of any sort of adaptation in the face of these deleterious conditions, we would predict that the Triassic would show a host of new species not only in response to the many empty ecological niches brought about by the mass extinction, but might also show new species arising in response to the longer-term environmental effects of the prolonged extinction event itself. This is the pattern that is observed during the Triassic; the world was refilled with many species looking and acting like some of those going extinct (therefore an ecological replacement), but a host of novel creatures also appeared, especially on land. In the next chapter we will postulate that many of the latter new species evolved to counter the continued low oxygen that had begun near the end of the Triassic, but that continued right into the Jurassic, a period of more than 50 million years. The Triassic was truly the crossroads of animals adapted to two different worlds, one of higher oxygen and one of low.
THE CONTROVERSY: IMPACT VS. GREENHOUSE
With the end of the twentieth century and the arrival of the twenty-first, ever more attention was indeed being paid to the Permian extinction, largely because it was the most devastating of all, with the now oft-repeated estimate that as many as 90 percent of all species disappeared. But how fast, which is a clue to how, began to be best appreciated with the work of paleontologists from China and the United States in extensive studies of a thick Permian and Triassic limestone cropping out near Meishan, China.2 Geologists worked to plot the thickness and identity relative to each other of every sedimentary layer. Then fossils were collected from the beds that had been so meticulously measured. Each fossil was carefully identified, and its collection level in the piles of strata noted. The paleontologists made use of Charles Marshall’s new statistical method, called confidence interval methodology,3 which allowed estimates of the ultimate time range a given fossil might have had. The geologists in China had a great advantage going for them. In China there were scattered ash layers that could be dated using sensitive machines to measure uranium/lead isotope ratios, and this was done most recently on samples by MIT’s Sam Bowring.4 The newest work by this group now puts the extinction as lasting no more than sixty thousand years, which is amazing resolution in rocks a quarter billion years in age.
The Chinese effort combined results from five different stratigraphic sections in the Meishan locality, with sampling intervals made every thirty to fifty centimeters. A total of 333 species of marine life were ultimately found in these rocks, belonging to such varied sea creatures as corals, bivalve and brachiopod shellfish, snails, cephalopods, and trilobites among others. Nowhere at any stratigraphic horizon at any time has so thorough a collecting effort, or so rich a fauna, been documented with such precision.
The various environmental conditions in the seas at the end of the Permian included widespread evidence of oceanic anoxia, or low oxygenation in both the shallow and the deep sea. This was worked out beautifully in 1996 by Yukio Isozaki of Tokyo University, who located the boundary in deep-sea bedded cherts that had been thrust onto the Japanese mainland. Precisely around the mass extinction event, the normally red charts turned a deep black, just as everything died. The anoxia was apparently of such magnitude that many marine organisms were rather suddenly killed off, just as they are today in modern red tides. There is also evidence of global warming at the time of the extinction, and the coincidence of the Siberian lava eruptions at the same time as the mass extinction.
There have been various suspects as to the cause of this extinction. First is the possibility that the Siberian flood basalts introduced large volumes of gas into the atmosphere, triggering large-scale climate change and acid rain, as suggested by Berkeley geolochronologist Paul Renne and others. With new information from disparate sources, a sudden methane release into the atmosphere became a viable candidate for the killer. But in spite of no evidence to support impact, the understanding that impact could cause extinction was still on everyone’s mind. The new evidence from China argued for some sort of “quick strike.” Among potential causes of mass extinction, only asteroid impact was thought to be capable of such mass death in so short a time.
At the turn of the century, Earth historians were enamored with large-body extraterrestrial impact as the cause of most, if not all, mass extinctions. In 2000, the Permian extinction looked like nothing known: it was still suspected to be some sort of impact extinction by the geological fraternity, but one seemingly different from the dinosaur-killing K-T event that had made sensational news in 1980. Perhaps the Permian extinction was many impacts, or a single large impact superimposed on some other kind of extinction mechanism. The most puzzling thing was that search as they might, none of the investigators looking at the Chinese rocks in the late twentieth and early twenty-first century could find the well-known clues associated with the impact extinction ending the Cretaceous already by then so well studied at the many K-T boundary sites, such as iridium, glassy spherules, and shocked quartz grains.
In 2001, and then over the next several years, a team led by geochemist Luann Becker reported5 the discovery of high levels of complex carbon molecules given the ridiculous name Buckminsterfullerenes, mercifully shortened to Buckyballs. They used this evidence to argue that like the mass extinction of the end of the Cretaceous, the Permian extinction was also the result of the collision of a large asteroid with the Earth. Only this one hit 251 million years ago.
The Buckyballs described by this team are large molecules that contain at least sixty carbon atoms, and because they have a structure resembling a soccer ball or a geodesic dome, they were named for architect Buckminster Fuller, the inventor of the geodesic dome. The hypothesis is that geodesic-dome-like carbon molecules trapped the gases helium and argon inside their cage structures, and that these new indicators of impact exist in strata of latest Permian age at three different geographic sites scattered around the world. The Becker et al. team interpreted these particular Buckyballs as extraterrestrial in origin, and therefore like iridium (which, pointedly, was not found), because the noble gases trapped inside have an unusual ratio of isotopes. For instance, terrestrial helium is mostly helium-4 and contains only a small
amount of helium-3, while extraterrestrial helium, the kind found in these fullerenes, is mostly helium-3. According to the authors, all this star stuff could have been brought to Earth only by a comet impacting the Earth at the end of the Permian (more correctly, it ended the Permian).
The researchers announced that the comet or asteroid was six to twelve kilometers across, or about the size of the K-T asteroid that left the huge Chicxulub crater near what is now the town of Progreso on Mexico’s Yucatán Peninsula 65 million years ago. But such a large Permian impactor would be expected to have left a monstrous crater, just as the later Chicxulub impact did, and so the Becker team began an earnest search of potentially overlooked or buried impact craters.
Two years later, in 2003, they announced that they had found a giant buried crater in the seabeds off Australia.6 The case for an impact cause for the Permian extinction seemed made. But then problems arose, in both the interpretations of the Buckyballs and the probability that the large underwater structure named the Bedout crater was an impact crater at all.
Science is about replication and prediction (among other things), and on both points the Permian extinction Buckyball hypothesis ultimately collapsed (although, curiously enough, impact and Buckyballs were, in 2012, still the first response spit out by Googling “Permian extinction”). But we workers searching for the causes of this mass extinction had our doubts early, and a certainty that the hypothesis could not be correct.
The original Becket et al. study was based on samples taken in China, Japan, and elsewhere. Later work could not replicate the results from China, and our friend Yukio Isozaki had shown several years earlier that the critical boundary interval that Becker had sampled near Osaka in Japan had actually been removed by low-angle faulting right at the boundary interval—three entire conodont zones on either side of the boundary were missing. Yet they reported that the helium-3 anomaly was there, just where they had been told (erroneously) the boundary should be. Something was fishy. Eventually our colleagues at Caltech demonstrated that helium-3 leaks out of a Fullerene cage in fewer than one million years, so none should have been left after 252 million years. Furthermore, the deep structure interpreted to be the crater that gave rise to all the Buckyballs, helium-3, and death to the world’s biota turned out to be a great emplacement of volcanic rocks unrelated to any sort of asteroid or comet impact.
A group of geologists and organic chemists teamed together and used a fairly new tool to look at latest Permian and early Triassic marine strata. Rather than looking for body fossils, they extracted organic residues from the strata7 in search of chemical fossils, which, if found, are known as biomarkers. The biomarker recovered can come only from a photosynthesizing purple bacteria species that can exist only in shallow water devoid of oxygen and saturated with toxic hydrogen sulfide. Apparently a great biomass of H2S-producing microbes filled the oceans—not just a small area like the Black Sea of today, but most or even all of the world’s oceans—based on newer studies by teams from MIT, who by 2009 had discovered the same biomarker in more than a dozen localities of latest Permian age scattered across the globe.8
A possible solution to the enigma of the cause of the largest of all mass extinctions came from a team of geochemists from Penn State in 2005. Led by Lee Kump of Penn State, one of the world’s foremost experts on the chemistry of the oceans and especially its carbon cycle, along with his longtime colleague Mike Arthur (also of Penn State), their paper suggested the H2S present at the end of the Permian, produced in the sea by microbes (a different species from the purple sulfur bacteria, to be accurate), was directly involved in the extinctions both on land and in the sea.9
THE KUMP HYPOTHESIS—AND THE DAWN OF THE GREENHOUSE EXTINCTION THEORY
The Kump et al. scenario is as follows. If deepwater H2S concentrations increased beyond a critical threshold during oceanic anoxic intervals (times when the ocean bottom and perhaps even its surface regions lose oxygen), then the oceanic conditions (such as those in the modern Black Sea) separating sulfur-rich deep waters from oxygenated surface waters could have risen abruptly to the ocean surface. The horrific result would be great bubbles of highly poisonous H2S gas rising into the atmosphere. This new entry into planetary killing provides a link from the marine to the terrestrial extinctions, because H2S accumulates in the troposphere to lethal levels for plants and animals under relatively modest fluxes of H2S from the ocean. This proposal relates not only to the end of the Permian, but may have occurred at other times in Earth history, and thus was perhaps a dominant perturbation causing mass extinctions.10
Kump and his team did some rough calculations and were astounded to conclude that the amount of H2S gas entering the late Permian atmosphere would be more than two thousand times greater than the small modern flux (this is the toxic killer coming from volcanoes). Enough would have entered the atmosphere to most likely lead to toxic levels.
Moreover, the ozone shield, a layer that protects life from dangerous levels of ultraviolet rays, would also have been destroyed. Indeed, there is evidence that this happened at the end of the Permian, for fossil spores from the extinction interval in Greenland sediments show evidence of the mutation expected from extended exposure to high UV fluxes attendant on the loss of the ozone layer.
Today we see an ozone hole in the atmosphere over Antarctica, under which the biomass of phytoplankton rapidly decreases. If the base of the food chain is destroyed, it is not long until the organisms higher up are perturbed as well. The complete loss of our ozone layer has even been invoked as a way to cause a major mass extinction if the Earth was hit by particles from a nearby supernova, which would also destroy the ozone layer. Finally, an abrupt increase in methane concentrations significantly amplifies greenhouse warming from an associated CO2 buildup and methane levels that would have risen to >100 ppm. As the H2S goes into the atmosphere, at the same time destroying the ozone layer, greenhouse gases also do their work in making the planet hotter. It turns out that the lethality of H2S increases with temperature. Thus a new and plausible alternative to impact was put forth. The extinctions would have been drawn out, or in pulses—a succession of short-term events, killing each time.
Up until now we have looked at evidence from the rocks themselves. But there is a second way to unravel past events, and that is to use some of this data to model what past atmospheres have been like. There are many kinds of such models, and many are relevant to trying to predict what our Earth’s future atmosphere and heat level may be like. For the Permian, levels of oxygen and carbon dioxide, as well as potential global temperatures, have been modeled. First, changes in atmospheric CO2 and O2 have been calculated by Yale’s Bob Berner. He and others have found that there must have been a pronounced spike in CO2 levels accompanied by plunging oxygen levels at the end of the Permian. Second, Lee Kump’s group undertook the difficult job of looking at the potential distribution of H2S emission around the globe. For this they used a global circulation model, or GCM.
These models were originally developed to understand modern-day weather and climate patterns. But because the positions of the continents are known for the critical period at the end of the Permian and into the Triassic, as well as temperatures and levels of oxygen and carbon dioxide in the atmosphere and oceans, the method could be applied to the Permian. Kump and his team reasoned that the critical element to track would be phosphorus. This is a prime component of fertilizer, and if oceanic phosphorus levels were observed to rapidly rise at the end of the Permian, the amount of hydrogen sulfide gas could be calculated as well due to the beneficiaries of the raised phophorus levels—the sulfur microbes.
The emergence of H2S did not happen once; it occurred over and over, as succession of burps clustered around the time that the Permian-Triassic boundary strata were being deposited around the world. Kump finished with the most ominous note. Not only did the model show where the H2S would emerge from the sea into the air, but he also showed new calculations that completely corroborated his earlier 2
005 estimates of how much H2S would have eventually gone into the atmosphere. The results: there would have been more than enough to kill off most land life, and as the nasty stuff also dissolves in seawater, it would have been greatly lethal in shallow marine settings as well, especially among shallow-water organisms that secreted calcium carbonate skeletons, such as corals, clams, brachiopods, and bryozoans—all invertebrate victims of the greatest extinction.
Since the introduction of the Kump et al. interpretation, others including Tom Algeo of the University of Cincinnati have greatly increased our understanding of the chemical aspects of this particular mass extinction, through numerous references.11
ALTITUDINAL COMPRESSION
The study of past mass extinctions is not new; in fact it is one of the very first kinds of research that could actually be called “science” when geology was first stirring as a discipline, in the first years of the nineteenth century. What is new about it is our understanding of the role of microbes in causing one or more of the so-called big five mass extinctions of the Phanerozoic.
Yet if extinctions themselves are not a new topic, the opposite side of the coin—the aftermath to mass extinction—has emerged in the past decade as a major new subdiscipline of evolutionary biology and paleobiology alike. We have learned that the more devastating the mass extinction, the more different the world coming after, not only in the immediate aftermath—the first few hundred thousand to million years later—but for subsequent tens of millions of years, and for some biological lineages, for all time.