Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!husc6!husc4!gallagher From: gallagher@husc4.UUCP Newsgroups: sci.bio,sci.astro,sci.misc Subject: Re: Mass extinctions Message-ID: <1522@husc6.UUCP> Date: Mon, 30-Mar-87 02:09:18 EST Article-I.D.: husc6.1522 Posted: Mon Mar 30 02:09:18 1987 Date-Received: Tue, 31-Mar-87 04:27:52 EST References: <784@scicom.AlphaCDC.COM> Sender: news@husc6.UUCP Reply-To: gallagher@husc4.UUCP (paul gallagher) Organization: Harvard Univ. Science Center Lines: 148 Xref: utgpu sci.bio:186 sci.astro:771 sci.misc:232 In article <784@scicom.AlphaCDC.COM> rwb@scicom.AlphaCDC.COM (Robert Brumley) writes: > >I have become very interested lately in the subject of mass extinctions >in earth's past. Apparently, with remarkable regularity, about every >26 million years a large percentage of the life forms on our planet >die out. > >I would appreciate any information on this phenomenon as well as >current hypotheses explaining it. Thanks. > This information is from "Periodicity in Marine Extinction Events" by J.J. Sepkoski, Jr., and David M. Raup, in Dynamics of Extinction, ed. by D.K. Elliott: The article presents a statistical analysis of extnction rates over the past quarter billion years and strongly supports the hypothesis that mass extinctions have occurred regularly every 26.2 million years, +/- 1 million years. Since they occur at regular intervals, it is very likely that they result from some single ultimate cause, but there are "no obvious terrestrial or solar processes operating cyclically on time scales of 10^6 to 10^7 years", except for the unproved hypothesis of periodic magnetic reversals. So the ultimate cause may be extraterrestrial. This is supported by: impact debris of apparent extraterrestrial origin at two of the extinction horizons (the Maestrichtian and Late Eocene); meteoric debris and geochemical anomalies within 7 million years of where an extinction event should have occured in the 26 m. yr. cycle but did not; and the observation that "terrestrial cratering may display a periodicity that is in phase with that of extinction events". There have been 4 proposals about the astronomical cause of these events, but they remain basically guesses: "1. Transit of the solar system through the spiral arms of the Galaxy. 2. Vertical oscillation of the solar system about the plane of the Galaxy. 3. Precession of an undetected tenth planet. 4. Orbital dynamics of an unobserved solar companion." In more detail: 1. Napier and Clube (Nature (1979), Vol, 282, pp. 455-459.) pointed out that the solar system passes through the spiral arms of the galaxy every 50 million years and that during this time the solar system may capture interstellar "planetismals" which would increase asteroid bombardment of the earth. Also, the tidal forces from the spiral arms might perturb the cometary cloud surrounding the solar system and in this way increase asteroid bombardment of the earth. Unfortunately, this is a cycle of 50 million years, which is twice the observed cycle of 26 million years. It's possible that 1/2 the extinction events are cause by the passage through the spiral arms and 1/2 are random, but Sepkoski and Raup reject this explanation because the mass extinctions are simply too regular to permit more than one random event. 2. The solar system oscillates vertically through the galactic plane every 52 to 74 millon years. So, it reaches an oscillatory extreme every 33 million years. Schwartz and James (Nature (1984), Vol. 308, pp. 712-713) suggest that the Earth might be subjected to extremes of soft x-rays and hard UV radiation from the galactic center at these extremes. This radiation might disturb the upper atmosphere and thus cause mass extinctions. Alternatively. Rampino and Stothers (Nature, Vol. 308, pp. 709-712, and Science, (1984), Vol. 226, pp. 1427-1431) argued that tidal forces from molecular clouds concentrated near the galactic plane "might perturb the Oort Cloud and inner cometary reservoir as the solar system approached the plane", which could produce comet showers of several million years duration, such that several comets would strike the Earth. However, once again the mass extinction events occur in a 26 million year cycle, not a 33 million year cycle. Also, Thaddeus and Chanan (Nature (1985), Vol. 314, pp. 73-75) argued that the concentration of matter near the galactic plane is not so concentrated as previously assumed and probably would not affect the Oort Cloud significantly. 3. Whitmore and Matese (Nature (1985), Vol. 313, pp. 36-38) argued that "Planet X", the possible tenth planet beyond Pluto, "might be able to produce periodic comet showers if its orbit were highly inclined and if the inner edge of the comet cloud extended as a thin disk almost to the orbit of Neptune." This would cause Planet X to precess through the planetary plane. If the precession period of the planet was 56 million years, the planet, with 1 to 5x the mass of Earth, "would sweep comets out of the inner disk as it passed through the plane every half-period", causing maybe 10^5 comets to cross Earth's orbit and possibly impact. It is questionable whether the approach of the planet would be rapid enough and its mass great enough (it must be small since it has never been observed) to cause a large number of comets to scatter. In any case, the precession period of this possibly non-existent planet is bases entirely on the periodicity of the mass extinction events, so it is "very speculative" to say the least. 4. Davis, Hut, and Muller (Nature, Vol. 308, pp. 715-717) and Whitmire and Jackson (Nature, Vol. 308, pp. 713-715) proposed that the Sun may be part of a binary star system with an undetected companion of low mass and density. If you assume it has a periodicity of 26 million years, it should have a major axis of about 3 light years and a perihelion distance of about 0.3 to 0.5 light years from the Sun, which is within the inner comet reservoir. Its mass is estimated to be 1/10 to 1/1000 of the sun's mass. Thus, this star, called "Nemesis", would perturb about 10^9 comets into the inner solar system at perihelion, of which about 10 to 200 should impact the Earth over a 10^5 to 10^6 year period. Nemesis' orbital periodicity should be irregular, varying 10 to 20%, and the orbital configuration would have a "half-life" of about a billion years. The problem with this hypothesis is, of course, that it's just speculation. In any case, observing anything as small and dark as Nemesis would be very difficult. The mass extinction events over tha past quarter billion years are as follows: 1. Guadalupian. This is the Late Permian mass extinction. More than 50% of marine families and 95% of marine species become extinct. The trilobites, rostroconchs, and blastoids disappeared entirely. Brachiopods, crinoids, cephalopods, and corals lost more than 20 families each. 248-258 million years ago. 2. Rheatian. The Late Triassic mass extinction. Conodonts and conulariids disappeared, and cephalopods, brachiopods, bivalves, gastropods, and marine reptiles suffer major losses. 213-225 million years ago. 3. Pliensbachian. Early Jurassic. Many cephalopod extinctions, and scattered extinctions in other groups. 194-200 million years ago. 4. Tithonian. End of Jurassic. Many cephalopod, bivalve, and dinoflagellate extinctions. 144-147 million years ago. 5. Cenomanian. Extinctions among cephalopods, echinoids, osteichthyan fishes, sponges, dinoflagellates, and globegerinids. 90 to 91 million years ago. 6. Maestrichtian. End of Cretaceous. All ammonoids, rudistids, stromatoporoids, pleisiosaurs disappear. Also, many families of other cephalopods, bivalves, gastropods, bryozoans, echinoids, sponges, osteichthyan fishes, marine reptiles, and microplankton become extinct. Meanwhile on land, the dinosaurs disappear. 64-65 million years ago. 7. Late Eocene (Priabonian). Decline in diversity among many groups, especially microplankton. 38-40 million years ago. 8. Middle Miocene (Kanghian-Serravallian). High rates of extinction among various groups, especially microplankton, but again no taxonomic group is inordinately affected. 2-14.4 million years ago. These do not form a 26 million years cycle, since certain mass extinctions don't occur as expected. There are small extinction peaks in the Olenakian in the Early Triassic and in the Bajocian in the Middle Jurassic, but there are none in the Callovian (Middle Jurassic) or the Hauterivian (Early Cretaceous) as would be expected. However, using statistics Sepkoski and Raup show that the data nonetheless show a cyclic pattern. They treat the "measures of extinction intensity as continuous time series and search for temporal regularities in the fluctuations" using standard Fourier analysis and also "treat the identified extinction peaks as discrete events and test for regularity in their timings" using a nonparametric randomization procedure. (I don't know enough about statistics to evaluate the math, but since this theory has been well-exposed I think you can trust it.) Paul Gallagher