Where did all the dinosaurs go? Mass extinctions Why are we asking about the dinosaurs in this book? The reason is that (as we examine in Ch. 17, due to global warming, and in Ch. 26), humanity is causing extinctions at an accelerating rate. The resulting loss of biodiversity could be catastrophic (refer to the discussion in Extension 26.1, Sustainability and value and Extension 26.6, Insects, animals, and people). The study of extinctions that have occurred and their consequences can then inform our approach to the future. At least five mass extinctions have occurred in the past 545 million years. At the Permo- Triassic boundary, 250 million years ago, as much as 95% of life at sea was snuffed out. (288) New evidence has been found in older German and modern Russian literature that 8 of 27 orders of insects didn t get to the Triassic from the Permian. (288) Several ideas could explain the extinctions. Volcanic eruptions could occur cyclically, for example. Massive eruptions could cause long-lasting changes that led to the extinction of so much life. Asteroid impacts, thought to occur on a cyclic basis, could have been responsible. Other sorts of catastrophes could have occurred. The iridium anomaly The so-called Cretaceous/Triassic boundary (K/T, the K for the German spelling of Cretaceous) was long known from the fossil record, but it was unclear over what time the distinctive layer was laid down. The diversity of the lifeforms below the boundary was much greater than in the impoverished layers above the boundary.
Energy, Ch. 16, extension 8 Where did all the dinosaurs go? 2 An iridium anomaly was found in 65-million-year stone in Gubbio, Italy in 1978 at the K/T boundary by geologist Walter Alvarez. Around 1980, an extended analysis around the world revealed a huge discrepancy in the amount of iridium at the K/T boundary everywhere it was examined. The uniformity in the anomaly in the proportion of iridium suggested a worldwide cause for the K/T extinction. The father and son team of Luis and Walter Alvarez suggested that an asteroid about 10 km in diameter traveling at perhaps 30 km/s could have spewed dust and debris far into the stratosphere, blanketing the world and plunging it into sustained darkness. (289) This provided a simple, complete neat explanation for many puzzling findings and was embraced after some hesitation (Lyell s uniformitarian, or gradualist, view was the paradigm for change of most scientists). As a result, asteroid impacts have become the current most popular explanation for the majority of the extinctions. (See Extension 16.6, Mars and nuclear winter for an explanation of what happened to cause the extinction.) The asteroid impact triggered the so-called K/T die-off referred to above. This die-off took place over as few as 100 years (and certainly less than 2 million years). After the K/T impact, computer models suggest fires and dust caused a year of darkness and a smog of nitric oxides, and may have triggered an ice age. (290,291) Many experts, however, believe it must have been a longer-term problem, rather than a short-term one, to have caused such a massive extinction. (292) The long term problem could have stemmed from the aftereffects of the collision. In solving any crime, it is good to have a smoking gun. In this case, a crater in the Yucutan Peninsula of Mexico has been recognized as a record of a particularly large impact 65 Myr
Energy, Ch. 16, extension 8 Where did all the dinosaurs go? 3 BP that dusted the iridium worldwide: (293) The Chicxulub ( Devil s Appendage ) Structure in Yucatan. The date of crater formation was determined by using the argonargon method on some small globules of glass from the impact, determining an age of 65.1 million years. (293) Other catastrophes It is apparent that these extinctions were mostly caused by impacts of extraterrestrial objects, and most of them appear to be associated with an impact ring remnant (Vredefort, South Africa, Sudbury, Ontario, Canada, and Chicxulub, Yucatan, Mexico). (294) Some smaller extinctions have occurred, and most scientists believe that the smaller extinctions are due to catastrophes here on Earth. For example, an extinction event took place 34 million years ago (the Eocene/Oligocene boundary). Apparently, that extinction was due to global cooling of 4 C (the researchers examined tiny earbones of fossil fish to nail down what had happened). (295) An extinction at the Permian-Triassic boundary could be either volcanic in nature or due to an asteroid, but the evidence is consistent with the occurrence of one very bad day. (296) Another extinction that was extensively studied was the Early Toarcian oceanic anoxic event (which took place around 183 million years ago). (297) Consistency in the data about carbon isotopes studied around the world suggest that this extinction was produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments. (298) Of course, if an asteroid or meteroid hit Earth, it could trigger volcanic eruptions as one of its after effects.
Energy, Ch. 16, extension 8 Where did all the dinosaurs go? 4 These gases are the clathrates discussed in Ch. 11 as a possible source of methane fuel. Methane is trapped in structures at high pressure. In this one case at least, a catastrophic release of the gas methane caused a massive worldwide extinction event. As the huge reservoirs of methane trapped beneath the ocean floor escaped explosively, they depleted much of the sea s oxygen, driving some 80% of oceanic species to extinction. An age of global warming followed this event. (298) The ecological niches eventually filled. Most people had automatically assumed that the more species that were wiped out, the longer the recovery would require, and conversely, the fewer the number of extinctions, the shorter the time required for recovery. The new species were expected to fill the vacant ecological niches, and when the old niches were again filled, the recovery would have been complete. Effects of catastrophic extinction on biodiversity One very interesting and important finding that resulted from examining the Early Toarcian extinction in particular, carefully using statistical techniques, is that it became apparent that it takes the ecosystems around 10 million years to recover from any extinction event, whether minor or major. (298) After this time, the creation of new species decreases as the system presumably reaches a new equilibrium. After discovering the 10 million year timeframe for recovery from the Early Toarcian event, the Kirchner and Weil team added all the extinctions and still found 10 million years. Then they removed the five major extinctions from the analysis and the reanalysis still showed a 10 million year recovery time. The authors suggest that the opportunities for filling niches in the ecology come primarily from the evolutionary diversification that grows exponentially with time that is, that the extinctions destroy essentially all niches,
Energy, Ch. 16, extension 8 Where did all the dinosaurs go? 5 and the system regroups itself and determines through evolution which niches are created by a process of progressive elimination as species diversify. (299) Chance determines which niches exist, because the first new species may take a very long time to arrive. That species leaves places for other species to fill. This view is consistent with the role chance plays in recovery as noted in the recovery from the Mt. St. Helens eruption (though this was a much smaller extinction in a restricted region over a much shorter timescale, the role of chance has been apparent here). (300) The apparently constant time interval needed to recover from an extinction event bodes ill for the future, as human beings are in the process of causing (over a several hundred year period) a dieoff akin to these extinction events. For example, there is the documented worldwide loss of amphibians. (301) There is disagreement on when we will reach (or whether we may already have reached) a level at which recovery will take 10 million years. At any rate, there is reason for caution and for thinking in terms of the precautionary principle: we should save all the diversity while we still can.