Extinctions and LIPs
By Larry Larason
Last month I wrote about large igneous provinces (LIPs), and mentioned that they may be linked to mass extinctions. Let’s look at that.
Mass extinctions have been an enigma since they were first noticed in the fossil record. Two main theories of mass extinctions have been proposed: large asteroid impacts and LIPs.
Luis Alvarez was a physicist, who had worked at Los Alamos during the period when the atomic bombs were built. In 1980 when a team led by Luis and his geologist son, Walter, proposed that an asteroid impact caused the demise of the dinosaurs it was met with skepticism. It was only in the middle of the last century that geologist Eugene Shoemaker had convinced other geologists that the meteor crater in Arizona was just that, and that large meteors did strike Earth. The evidence presented by the Alvarez team included high amounts of iridium in a layer of clay at the boundary between the Cretaceous and Tertiary rocks. Since that time similar layers have been found in rocks of the same age at more than 100 sites around the world, including some in New Mexico and southeastern Colorado. [On geological maps, rocks of the Cretaceous are labeled “K” to avoid confusion with Cambrian formations, so the extinction event is known as the K-T boundary.] Iridium is quite rare in Earth’s crust, but common in meteors and comets, so the scientists proposed that it had come from an extraterrestrial object. In 1990 the Chicxulub Crater on the coast of Yucatán was identified as the impact site. After the Alvarez’s proposal was accepted, many more geologists began looking for meteor craters with appropriate dates for mass extinctions.
First, I need to mention that there are more LIPs around the world than there are extinction events. So is it only coincidence that some massive flows of lava happened at about the same time as big die offs? Well, we don’t have much else to blame because not much else was going on at the time of most of the extinctions. Of course, the K-T event is the exception because of the asteroid.
So how could LIPs cause global death? In addition to the huge eruptions of basalt there would have been vast emissions of gases, mostly carbon dioxide, but also including such noxious stuff as fluorine, chlorine, and hydrogen sulfide. Rain falling through such a mix in the atmosphere would have been highly acidic and capable of damaging plants. In addition, the lava flowing to create the Siberian Traps may have ignited a lot of coal.
The Permian event occurred around 250 million years ago, but not all at the same time. There may have been as many as three deadly phases. In short, the earth became a lethal world for a couple of million years. More than 90 percent of ocean life became extinct. For example, trilobites, one of the first animals to evolve in the sea, were seen no more after the Permian. Temperature increased quickly by about 110 F, possibly due to the rise in CO2 from the volcanic eruptions. Atmospheric oxygen became scanty. Ultraviolet radiation in sunlight increased; the chlorine and fluorine belched with the lava may have damaged the ozone layer in the atmosphere, which would normally have deflected it. Seventy percent of terrestrial vertebrate animals died off. Even insects were hard hit during this extinction. As one geologist has written, “. . . Nothing larger than a housecat walked across the Permian-Triassic boundary . . .”
To my mind one of the most telling facts about the Permian/Triassic extinction is the coal gap. Coal is formed when plants growing in lakes and lagoons die and fall to the bottom without being decomposed. For about 15 million years after the plant die-off, the only coal formed was in thin beds and very low grade.
Although most geologists now believe that the Siberian eruptions account for the devastation of the mass extinction, others are still looking for an extraterrestrial cause. One group believes they’ve found a large impact crater on the continental shelf of Australia, though this is considered mere speculation by most geologists. But in July a paper was published describing the dating of a crater in Brazil to 254 million years ago, close to the time of the Permian extinction. It isn’t near the size of Chicxulub, only about 25 miles in diameter as opposed to the 100 miles of Chicxulub, but part of the rock excavated by the impact is oil shale. The shale would have released a lot of methane into the atmosphere thus helping to account for the global warming at the end-Permian.
During the early Triassic Period the earth was still a hostile place. Oxygen remained in short supply. The recovery of life’s diversity required 5-10 million years. And then the planet was hit again with flood basalt, beginning in Morocco about 201 million years ago. The supercontinent of Pangaea was breaking up when the Central Atlantic Magmatic Province [CAMP] began erupting. The extent of this extinction event was not as large as at the end of the Permian, but about half of species alive beforehand disappeared rather suddenly at the time of the first eruptions of CAMP. Recent more precise dating of the eruptions ties CAMP almost directly to the die-offs.
About 75% of life died in the Cretaceous-Tertiary extinction 65 million years ago. The Chicxulub asteroid strike ignited a global firestorm, and threw tons of dust and sulfates into the atmosphere, creating cooling that lasted for years. In addition, because the impact occurred on a coast, there were unprecedented tsunamis. Among the most affected, other than the non-avian dinosaurs, were photosynthetic plants, suggesting that the dust and ash that filled the skies for years after the asteroid struck blocked sunlight. The Deccan Traps were erupting at the same time. Some geologists wondered if the asteroid impact could have triggered the eruptions, but more precise dating has shown that lava was flowing as much as five million years before the K-T boundary. Many geologists now believe that the LIP in India affected the global ecosystem, and the asteroid impact was only the final nail in the dinosaurs’ coffin.
Another LIP I mentioned last month, the Columbia River Basalts, began erupting around 17 million years ago during the Miocene Epoch. Some species became extinct about 14 million years ago. The extent of this event was so minor is it usually called a “disruption” rather than an extinction. The flood basalts in Washington/Oregon are not strongly implicated in this case.
A few years ago I might have favored the asteroid theory of extinction. After more study I believe that Mother Earth is not always benign, and it doesn’t take an ET object to bring about devastation.