The Siberian traps erupt - did greenhouse gases cause the greatest extinction, 250 million years ago?
Definition: Catastrophe - a perturbation of the biosphere that appears to be instantaneous when viewed at the level of detail that can be resolved in the geological record (page 20). In practice, this "minimal detectable duration" is of the order of 20,000 years though it varies according to the nature of the event and the geological strata which record it occurrence. An extinction event could conceivably pass unrecorded except for its subsequent long term impact on earth's biota - one thinks immediately of "bollide impacts", collisions with asteroids, large meteors and comets. Changes in the number, distribution and type of species recorded in the fossil record alone would bear mute witness to such an extinction event (unless, as is the case for bollide impacts, one can find a huge smoking crater at the right "temporal horizon" in the geological record). This places extinction researchers like Prof Hallam in the position of a sleuth like Sherlock Holmes: one has a body, a crime scene and one has to find the culprit on the basis of evidence left at the crime scene. Thus oxygen isotope ratios determine ancient ambient temperatures. Fossil magnetic field variations are used to correlate geological strata of unknown age with geological strata whose age has been determined, thus providing dates for ancient events recorded in mute rock. Radio-isotopes help in the dating of strata as do the presence of certain typical fossil species which serve as chronological markers.
Hallam's discussion of the complex and variagated history of extinction event research reveals the degree to which "pure" science and economic activity are intertwined. The industrial revolution increased mining activity on a global scale, revealing a rich and fantastic fossil record which begged an explanation form the burgeoning materialistic scientific ideology of the time. At times, religionists fought a pitched rearguard action. One argument for the extinction of dinosaurs: they were too big to fit on Noah's ark!
Partly in reaction to religionist opponents, the doctrine of "Gradualism" rejected rapid, catastrophic changes in earth's history - as this might indicate God's miraculous workings - in favor of the belief that all changes were regular, gradual and slow (Sir Charles Lyell). Large variations in land elevation, form, etc resulted from natural forces that had acted for a long time. Lyell and Darwin found natural allies in each other as Darwin held that life "evolved" slowly over time from primitive beginnings and was not the work of any sudden divine act of creation.
In the 20th century, as the assembled fossil / geological history of the earth neared completion, it became evident that some shifts in the biota were, in fact, rapid, "catastrophic". This new understanding has been dubbed "Neocatastrophism" and is now the dominant school among earth scientists. There are now 5 recognized mass extinction events during which between 16 - 51% of marine families of organisms became extinct (marine fossils are the most abundant and usable by the extinction event researcher). There are also a number of minor extinction events, often on a regional - as opposed to planetary - scale. All of these events are characterized by a rapid loss of biodiversity, ecosystem simplification and rapid species turnover (particularly during the recovery phase where active competition for vacant ecological niches is occurring). The "much-publicized" Cretaceous-Tertiary (K-T) extinction event - 65 million years ago - which wiped out the dinos and raised the mammals to dominant terrestrial lifeform is, in reality, the samllest of the 5 major extinction events (at least in terms of marine family loss). The worst was the Late Permian extinction - 250 million years ago - both on land and sea. Some researchers claim that marine evolution was turned back hundereds of millions of years; biodiversity recovery was also particularly slow for that greatest of all extinctions.
What then are causes of mass extinctions? This is a relevant question today because ecologists assert that species are going extinct today - due to human activities and population growth - at a rate comparable or superior to those seen at times of past mass extinctions. Hallam lists as major drivers of extinction events: bollide impacts (asteroid, comet, large meteor), climate change and sea level change.
Contrary to the impression one would get from all those asteroid / comet impact films, bollide impacts are actually the least important cause of mass extinctions. Current understanding of extinction event mechanics leads to the conclusion that the bollide that killed the dinos was merely the coup-de-grâce that finished off an already sick planetary ecosystem. Bollide impacts had occurred before - wreaking continental / regional extinction and provoking high species turnover rates - but these had not triggered mass extinction on a planetary scale.
Climate change, like sea level change, is a complex phenomenon. It includes global cooling and warming and is itself multi-causal in origin. Large scale volcanism spews climate changing gases into the atmosphere. These change the ratio of solar energy entering /leaving the atmosphere resulting in global heating or cooling. Sulfur oxides may cool the atmosphere by creating high level acid mists which reflect incoming sunlight. Carbon dioxide - CO2 - warms the earth by trapping outgoing heat radiated by the earth's surface and oceans. Many scientists consider that CO2 and other Green House Gases - GHGs - released since the industrial revolution are raising global temperature with unpredictable and potentially dangerous consequences.
Climate is also strongly affected by variation in the arrangement of the continents over time (due to the movement of the tectonic plate system). Continental arrangement affects deep ocean currents which, in turn, regulate the earth's global energy balance as well as regional energy flows. Before the current arrangement of continents - which restricts south to north oceanic heat transfers from equatorial waters - crocodiles were found as far north as Ellesmere Island in Canada's north! Rapid climate change is correlated with large scale extinction events. Logical! - once you stop to consider the dependance of living beings on the stability / regularity of their environment.
Sea level changes are another strong correlate with large scale extinction. Sea level changes modify habitats and may cause anoxic - oxygen depleted - zones to spread, resulting in mass die offs, species turnover and / or replacement. Like climate change, sea level change is both complex in action and multiple in origin. It is further complicated by the fact that sea levels may change regionally or globally. Regional changes are caused by land / seabed level changes due to deep volcanism / tectonic movements. Global changes may result from massive volcanic seabed uplift or from changes in global temperature. In the latter case, temperature change causes sea water volume to expand / contract, resulting sea level transgressions / regressions in shallow coastal waters. Sea level changes may affect the distribution of anoxic - oxygen depleted - zones ("dead zones"). Volcanism may also affect dead zone distribution by injecting large volumes of acidifying CO2 with varied effect on local biota. Global warming tends to spread anoxic dead zones, a warning for us living today in our globally warming world. We also note that extinction event causes may interact, reinforcing or canceling one another, regionally or globally.
Global warming (one mode of climate change) is, from the fossil record, a potentially dangerous phenomenon. Mankind may be tweaking the tail of a sleeping dragon by emitting GHGs into the atomosphere. For example, Global Warming - GW - may trigger sudden massive releases of methane from tundra / seabed reservoirs of "methane ice" (methane clathrate). Once the ice begins to melt massive amouts of methane are released; this happened in the earth's past with disastrous results for affected biota. In the fossil record we read another warning for our hubris-inflated industrial societies. Massive mathane releases from tundra or seabed have several, mutually reinforcing destructive effects. Seabed releases poison the water column: oxydizing methane depletes the oxygen store creating acidic dead zones (anoxia). Methane - or the CO2 resulting from its oxydization - is a potent GHG which amplifies the original rise in temperature which caused the methane release in the first place (positive feedback). The temperature rise itself lowers the capacity of oceans to store oxygen, causing dead zones to spread. This is important today if we intend to use the oceans to feed our burgeoning populations - the oceans are being overfished already without any need of anoxic poisoning! In addition, temperature rises reduce the capacity of sea water to hold CO2, causing the oceans themselves to become sources - not sinks - of CO2, further amplifying the original temperature rise. We are entering into a dangerous minefield of multiple, interlocked, mutually re-inforcing feedback loops acting to further destabilize our planet's already damanged "life support systems".
What does contemporary research into extinctions say about the relation between these events and biological evolution. Darwin held a Gradualist position: life evolved sowly under the selective pressure of the "struggle for survival" acting upon random, spontaneously occurring, genetically determined "variations" (in form or function of an organ, for example). The "most fit" forms would outsurvive the lesser and so life would slowly evolve towards "more perfect" forms. Modern biophysical research on dinosaurs does not support the claim they were "inferior" in performance to mammals - rather the contrary, if anything. However, in the shattered world following a bollide impact at the K-T boundary, mamals did have traits which favored them: samll size, high reproductive rate, insectivorous diet.. all desirable traits when the SHTF. Biologists call it the Lilliputian Effect: small organisms survive catastrophes, big ones don't. Also known as the Humpty-Dumpty Effect: the bigger they are, the harder they fall.
The contemporary reading of the fossil record does not support Darwin in other ways. Evolution moves by fits and starts, it is not gradually and regular when view over long time spans. The fossil record is compared to an improvisational theater play divided into Acts (biological eras) - indicating massive ecosystemic turnovers - and Scenes (lower order reconfigurations: disappearances, role changes, appearances of new characters..). The image is a bit forced and artificial but gives a good representation of modern evolutionary chronology. The act and scene changes are extinction events, major and minor. Thus modern paleontology and evolutionary theory may be described as "Neo-catastrophic". Darwin's original theory - of regular, gradual change - is still fit for the day to day "fine tuning" or micro-adaptation. For Hallam, it explains, for example, minor adaptations to small climate changes or to an adaptive mutation in an interacting organism (antelopes become longer legged, more able to outrun the cats that hunt them - the cats, in turn, adapt by becoming quicker or better camouflaged or smarter or begin to hunt in packs..) Hallam argues that the major drivers of evolution are perturbations of the physical environment: climate change (volcanism, tectonics), sea level change and a single more or less confirmed bollide impact (which finished off the dinos and opened the Age of Mammals, 65 million years ago at the Cretaceous-Tertiary boundary).
geological transition between Permian and Triassic, 250 million years ago
Extinction events tend to be complex, multiply caused events: "perfect storms". Thus, "the end-Ordovician event is linked to both climate cooling and warming, and with both marine regression and transgression or anoxia, or both. This is because it is a double event: the earlier one is thought to be related to glacially induced cooling and regression, the later to warming as polar ice disappeared, and to the rise of sea level associated with the spread of anoxic waters" (page 161). One smoking gun does not fit all extinctions! Hallam provides a most informative chart, "summary of the proposed causes of the main phanerozoic mass-extinction events" (page 162) including 14 extinctions. He lists six proposed causes, with the number of times each one is indicated as a "probable" extinction cause or as a "possible" cause. In the following discussion, probables are the first number, possibles, the second. Bollide impact is indicated in 1 / 0 of the 14 extinction events, volcanism 5 / 0, climate cooling 3 / 4, climate warming 3 / 2, oceanic regression 5 / 2 and anoxia / oceanic transgression 10 / 2. Sea level rise is the clear winner, a probable cause in 10 mass extinctions followed by sea level regression and volcanism - 5 apiece, climate cooling and warming - 3 each - and with the Hollywood film favorite, bollide impact, trailing with only 1 probable extinction event to its tally. Only three of the 14 tabulated extinctions are linked to a single probable cause. The remaining 11 extinctions are multicausal with 2 - 5 listed possible or probable causes. They present a varied mix of causes. Aside from the predominance of anoxic seas / sea level rise, no particular pattern emerges. Life is a crap shoot on both the micro and macro levels it seems..
Cretaceous / Tertiary boundary, marking death of the dinos, 65 milllion years ago: mass extinctions leave their marks in rock strata due to shifts in biochemical activity, climate change (effects rock weathering), changes in rate of sediment deposition, etc.
Hallam contends, oddly, that mass extinctions do not significantly alter the course of evolutionary history. His argument: if a type of coral, species A, becomes extinct then after millions of years another form of life, also coral-like, will emerge to fill the ecological niche. True! But this argument ignores, I believe, some major evolutionary emergences "provoked by" - or at least concomitant with - extinctions. Boosted cephalization - increased brain mass / body mass ratio - is a good example for mammals in general and primates in particular. Cephalization increased following mass extinction events. Given that Dr Hallam is a large brained primate, I find this lapsus on his part somewhat curious..
The present geological period, the Quarternary is marked by low extinction rates despite highly variable climate cycles (Glaciation / Interglaciation cycling). Perhaps an innate adaptability of short lived, rapidly breeding, rapidly mutating mammals play a role in the low extinction rate. This picture changes with the arrival of modern man (Cro-magnon). Extinction rates globally are so high today that scientists warn of a "6th major extinction event" in progress. Sadly, it appears that humans - once again! - are to blame: overkilling, habitat destruction, deliberated / unintentended introduction of invasive species.
A philosophical question: is what we do to nature "unatural" or are we simply doing what a nervous system hard wired by 3.5 billion years of Darwinian evolution would be expected to do?
And what about aboriginal peoples and the myth of the "noble savage"? Candidates vary: Hopi Indians, Inuit, Bushmen.. But were these peoples not forced by their environment to "respect nature" - or die? The argument is a Darwinian one: those cultures, like ours, which don't "live within natural bounds" will kill themselves off. The harsh environments of the cited "noble savages", on the other hand, honed their survival skills particularly well: they were forced to live within natural bounds or die off fast. This might be a circular argument though..
Ignorance, of course, can be blissful or lethal (or both??) The seas - upon which future mankind will rely for protein to sustain its "burgeoning numbers" (United Nations report) - may be more vulnerable than the land to mass extinctions. Reason: marine biodiversity is only 1 / 25 that of land. This renders both species and ecosystems less resiliant to disruptive changes. Yet, stupidly, we pollute seas, overfish and modity the climate heedlessly..
The last chapter is a veritable indictment of humanity's "stewardship" of our earth. In tones reminiscent of an old testament prophet, Prof. Hallan declaims, "the current free-for-all in deep sea fishing must cease" (page 201). "There is an ominous warning from the geological past. Global Warming seems to be strongly implicated in the biggest mass-extinction event of all, somme 250 million years ago" (page 202)
Generally, good reading. A bit too jargon-loaded in places. Glossary of limited use.
Update: http://palaeo.gly.bris.ac.uk/Essays/wipeout/default.html
The science of the Permian - Triassic extinction, the greatest extinction event of all, 250 million years ago. Relevence to today's world? Scientists now believe that the P-T extinction was due to CLIMATE CHANGE caused by massive carbon dioxide outgassing from the Siberian Traps (extensive open lava fields). This initial pulse of planetary heating in turn triggered the release of methane sequestered in seabed methane clathrates ("methane ice"). Since methane is an even more potent greenhouse gas than CO2, this release caused further global warming. The combination of the two heat pulses killed off most higher forms of life on earth. Scientists today warn that the methane trapped in permafrost and seabed clathrate deposits is being destabilized by global warming. We may be triggering the killer pulse of heat that will destroy our civilization. This time we won't be able to blame mother nature..
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