The 62 million year extinction cycle

Mena

Right, I'm at my wits end about where to post this... Galactic source (Astronomy) of Mass Extinctions (Palaeontology), so the die rolled and landed on my favourite subject that I know not enough about.

I got this from Slashdot and found it extremely interesting. Can any of our resident experts expand on this?

I like the sounds of the "cosmic ray hypothesis" but does it have any real scientific merit?

Slashdot wrote:

"Cosmologist Adrian Mellott has an article in Seed Magazine discussing his search for the mechanism behind the mass extinctions in earth's history that seem to occur with a period of about 62 million years. Scientists have identified nearly 20 mass extinctions throughout the fossil record, including the end-Permian event about 250 million years ago that killed off about 95 percent of life on Earth. Mellott notes that as our solar system orbits the Milky Way's center, it oscillates through the galactic plane with a period of around 65 million years. 'The space between galaxies is not empty. It's actually full of rarefied hot gas,' says Mellott. 'As our galaxy falls into the Local Supercluster, it should disturb this gas and create a shock wave, like the bow shock of a jet plane,' generating cascades of high-energy subatomic particles and radiation called 'cosmic rays.' These effects could cause enhanced cloud formation and depletion of the ozone layer, killing off many small organisms at the base of the food chain and potentially leading to a population crash. So where is the earth now in the 62-million year extinction cycle? '[W]e are on the downside of biodiversity, a few million years from hitting bottom,' writes Mellott."

Source

Damn, this should really be in Astronomy as well...

Or just maybe AstrophysicPalaentological studies deserves it's own forum

Mena

seedmagazine.com wrote:

SOMETIMES, SOMETHING KILLS NEARLY ALL LIFE ON THE ENTIRE PLANET. BUT IS THERE A REGULAR CYCLE TO THIS CREATION AND DESTRUCTION OF EARTH’S BIODIVERSITY?

Out of all the things that live and die on Earth, a small fraction becomes entombed in mud or sand and fossilizes, forming over time a grand tableau of life’s history. We measure the richness of this history in terms of biodiversity, the number of different kinds of creatures present at a given time. Though life’s story stretches back billions of years, only its somewhat recent chapters are at all clear, and even then mostly in the ocean, where fossils are more easily formed and preserved. Starting with the advent of abundant marine fossils about 500 million years ago, biodiversity’s development over time can be seen as a rising curve; more kinds (genera) of creatures exist now than existed 500 million years ago. But closer scrutiny reveals wiggles along this steady, gradual rise.

Though they may appear minor, some of these short-term fluctuations were the most dramatic events in life’s history, precipitous drops in biodiversity known as mass extinctions. Scientists have identified nearly 20 throughout the fossil record. Some 250 million years ago, one of the worst, the end-Permian event, killed off about 95 percent of life on Earth. Such transitions can be so radical and rapid that the rocks are fundamentally altered, inaugurating an entirely new geological period. The signs of ancient cataclysms are visible on most exposed cliff faces, where the oblivion of whole classes of creatures is often compressed into a centimeters-thin layer of rock.

Other declines occurred gradually, perhaps due to a decreased emergence of new species rather than increases in extinctions. Fortunately, all known dips in biodiversity seem to be followed by periods of rapid diversification called radiations. It was the end-Permian event that allowed the dinosaurs to develop and flourish. Of course, yet another mass extinction ended their reign. In their place, birds and then mammals ascended. And now humans have emerged. But even with all our intelligence and technology, we still don’t really understand what causes extinctions or radiations.

One of the big mysteries associated with these phenomena is also a key question for life’s future: Do they occur with any regularity? If we discovered a cycle to these events, it might suggest what’s driving the changes on Earth, and what linkage, if any, they have to events elsewhere in the universe.

I was always interested in this, but only as a spectator. Most of my scientific career had been in cosmology, working on computational models for the formation of structure after the big bang, dreaming of vast sheets and filaments of dark matter upon which galaxies congealed like dust settling on soap bubbles. But a few years ago I was drawn into a novel field that really doesn’t yet have a name, a blending of astrophysics and paleontology. Each year more evidence shows that past and future cosmic events, such as the deaths of distant stars or the orbits of comets, profoundly shape life on Earth. With this in mind, I began watching the literature for anything related to extinctions or the long-term history of the fossil record.

Soon I was losing sleep over something I’d read in a 2005 issue of Nature. Robert Rohde and his mentor, Richard Muller of UC Berkeley, had reported a fascinating cycle in biodiversity within a major compendium of fossil data sets, a regular 62-million-year rise and fall in the count of all kinds of creatures. They had explored several mechanisms to explain it and found them lacking, but both they and their editors deemed the signal so significant that it was published.

There’s a long history of people seeing cycles in the fossil record, but none of the work has been particularly rigorous, and rock strata don’t come with date labels. Paleontologists have rightly been skeptical of the claims. But the work of Rohde and Muller was different. They had capitalized on a careful study in 2004 that revised the dates for all those ancient environmental transitions seen in layers of rock. They also used a better quantitative method than past researchers, something called Fourier analysis.

Given reasonable limitations, almost any mathematical function—a line you’d graph with Xs and Ys—can be represented as a sum of sines and cosines, which have the familiar squiggly shape of oscilloscopes in old sci-fi movies. These “sinusoids” have different wavelengths, long for the long-term trends and short for sudden changes. Something called the “power spectrum” measures how much energy is in the different wavelengths. By looking at the varying contributions of sinusoids of different lengths to the power spectrum, you can reliably make estimations about the trends within a data set. Music is a good example. If you examined the power spectrum of the sound waves of different instruments playing middle C, each would look unique—which is why you could tell the instruments apart. But each would have a big peak at middle C—which is how you could tell they were all playing that note. So power-spectral analysis is a very effective way to search for regularities; one has to wonder why it wasn’t used sooner on the fossil record.

Using the revised timescales and Fourier analysis, Rohde and Muller looked for a periodic signal in the history of biodiversity. They began by subtracting out biodiversity’s long-term growth—a vital step if you want to find any short-term signal (the wiggles) superimposed upon the rising curve. They were looking for evidence of a 26-millionyear cycle that had been hinted at in the 1980s; the strong peak in their power spectrum indicating a 62-million-year cycle was a surprise. Using the same data, Bruce Lieberman and I checked their results. We estimated the 62-million-year peak had a 1 in 100 probability of arising through random chance. Then, collaborating with paleobiologist Richard Bambach, we found evidence of the same cycle in three more data sets.

Back to my sleepless nights. When a long, nearly regular cycle is found, an astronomical event or interaction may be the source, because orbits under gravity usually maintain regular rhythms for very long times. Some cycles of the Earth’s motion around the Sun are already known. But these have periods of hundreds of thousands of years, possibly a million or two, and no more. Nothing known in the motion of the Earth itself can make a 62-million- year cycle. Further, the laws of celestial mechanics rule out any object orbiting the Sun with such a long period; it would be so distant that the gravity of other stars would pull it away. But other astronomical cycles are still in play.

It takes about 200 million years for the Sun to complete one orbit around the center of our Milky Way galaxy. Moreover, the galaxy is a thin disk, and there is also a motion along a vertical direction. As our solar system slowly orbits the Milky Way’s center, it oscillates through the galactic plane with a period of around 65 million years. When we move up in the disk, we are pulled back down by gravity, coasting past the midpoint, then rising back up again, akin to a weight bobbing up and down on a spring.

Was this the missing mechanism? In fact, Rohde and Muller had considered this and dismissed it, for the same reason almost anyone would: One would think that any effect would occur when we passed through the disk of the galaxy, or perhaps when we got very far away from it. But that would happen twice per cycle, every 30 million years or so, which doesn’t explain the 62-million-year signal.

Trying to understand all this, I did something that in retrospect is fairly obvious: I looked at the phase. That is, how did the cycles of biodiversity and the Sun’s bobbing motion correspond? People had already computed the history of the Sun’s galactic orbit. It turns out that the biodiversity minima of the 62-million- year cycle happens when the Sun is “bobbed up” on only one side of the galaxy, when the solar system is on the disk’s upper, “north” side. So I visited my colleague Barbara Anthony-Twarog in the office next door. She has a beach ball painted with constellations, the Milky Way, and astronomical coordinate systems. It confirmed what I recalled: The galaxy’s north side lies toward the constellation Virgo, as well as the largest concentration of mass in our neighborhood, the Local Supercluster some 60 million light-years away. This supercluster is so massive that its gravity pulls our galaxy toward it at a velocity of about 200 kilometers per second.

This realization was the key for what follows, which I developed with my collaborator Mikhail Medvedev. The space between galaxies is not empty. It’s actually full of rarefied hot gas. As our galaxy falls into the Local Supercluster, it should disturb this gas and create a shock wave, like the bow shock of a jet plane. Shocks in hot gas at such high speeds generate cascades of high-energy subatomic particles and radiation called “cosmic rays.” These should be showering the north side of the galaxy’s disk. We are protected by the galactic magnetic field, much as the Earth’s magnetic field protects our planet. When we rise to the north side, we are less protected—and the ensuing flux of cosmic rays contains particles of such energy that they can reach the Earth’s surface.

So what’s the harm? Of course, radiation can be dangerous. It can lead to mutations, most of which are detrimental, often fatal, to organisms carrying them. The cancer rate would likely rise. Although it takes many mutations to do this, the rate of evolution of new species might rise too, thus assisting the rapid diversifications seen after major extinctions. We also know that cosmic rays ionize the atmosphere, knocking electrons out of atoms, and this as well might have detrimental effects, like enhanced cloud formation and depletion of the ozone layer. More clouds make the Earth more reflective, reducing the amount of solar heat reaching the ground and making the biosphere less productive. Ozone protects us from a dangerous form of ultraviolet light from the Sun. So the Earth would lose a lot of its sunblock, inducing cancers as well as killing off many small organisms at the base of the food chain, potentially leading to a population crash.

We don’t know how severe these effects would be; our research group is actively involved in making better estimates. Cosmic rays may provide only a slow decline, which would still show up in the power spectrum of biodiversity. But even if they are not of extinction-level intensity alone, there is ample evidence that stresses on the biosphere can amplify the negative effects of other events. For instance, a constant low-grade stress, like global warming or an ice age, when combined with a sudden catastrophic event, like massive volcanic eruptions or a large asteroid strike, could result in a mass extinction when neither force alone would be sufficient.

Now the natural question is, where are we in this putative cycle of extinction? Our solar system has just passed the midplane of the Milky Way, on its way up. If the past is any guide, we are on the downside of biodiversity, a few million years from hitting bottom. Our cosmic-ray hypothesis may not be the right mechanism, but it should be testable by simply looking for specific kinds of radiation from gas clouds just on the galaxy’s northern side. We now must try to understand the 62-millionyear cycle itself by seeking correlations with things like the rate of seabed fossil formation or the rates of species origination and extinction. Only by gathering these clues can we fathom the diversity recession that seems to lie in our geological future.

—Adrian Melott is a professor of astronomy and physics at the University of Kansas.

Source

mysterious

62mya is the big one.

There is a smaller one every 20something mya too.

26,000 years
12,5000 years
3,600 years.

There are like a galactic year, month, day, hour and minute. A minute in a galactic day for example could a few hundred years, if that makes sense. It's nature nothing stays the same forever. In a way it further's my belief that humanity was meant to evolve.

Red Hand

Well, there was an article in New Scientist not so long ago that questioned the Gaiea Theory and suggested that some mass extinctions were caused by of evolving life itself. Examples would be "snowball earths" produced through the early history of life on earth due to various innovations that life had evolved.

http://www.newscientist.com/article/mg20227131.400-gaias-evil-twin-is-life-its-own-worst-enemy.html?DCMP=OTC-rss&nsref=endangered-species

mysterious

IT seems totally natural.

It would be stupid to think that our solar system "just so happens" to had a major event 65mya.

It's simply called evolution as the above poster said. Evolve = evolution.

My big question is do you know what animals survived that last one in 65mya:p:D ( can't say it on this forum)

Galvasean

mysterious wrote:

My big question is do you know what animals survived that last one in 65mya:p:D ( can't say it on this forum)

Small mammals, reptiles, birds, fish and a host of others. Pretty much all teh large animals were wiped out, although a few large dinosaurs may have lasted a bit longer than once taught:
http://www.livescience.com/animals/090428-lost-dinosaurs.html

Linguo

I'm meant to be working but got sucked in by these articles, fascinating!! Is it widely accepted that the galaxy is responsible for our mass extinctions?

Galvasean

Linguo wrote: »

I'm meant to be working but got sucked in by these articles, fascinating!! Is it widely accepted that the galaxy is responsible for our mass extinctions?

Not directly. There is not much support for the supernova or cosmic ray theories. However indirectly, there is much evidence to suggest that meteor impacts have contributed to mass extinctions throughout earth's history.