Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!sri-unix!teknowledge-vaxc!dplatt From: dplatt@teknowledge-vaxc.UUCP Newsgroups: sci.bio,sci.astro,sci.misc Subject: Re: Mass extinctions Message-ID: <11235@teknowledge-vaxc.ARPA> Date: Fri, 27-Mar-87 14:59:04 EST Article-I.D.: teknowle.11235 Posted: Fri Mar 27 14:59:04 1987 Date-Received: Sat, 28-Mar-87 15:41:13 EST References: <784@scicom.AlphaCDC.COM> Organization: Teknowledge, Inc., Palo Alto CA Lines: 86 Xref: utgpu sci.bio:177 sci.astro:762 sci.misc:225 In-reply-to: rwb@scicom.AlphaCDC.COM's message of 26 Mar 87 02:44:40 GMT Posting-Front-End: GNU Emacs 18.33.2 of Fri Feb 13 1987 on teknowledge-vaxc (berkeley-unix) Robert Brumley asked for information about the 26-million-year periodicy in mass extinctions that has been reported, and the mechanisms that have been proposed that might account for it. There's a lot of controversy about this issue in the scientific community. Quite a few scientists feel that the apparent periodicy of mass extinctions may not really be there... that it may be a statistical fluke resulting from the mathematical model used to plot the species-extinction rates. Even if there is a periodicy, this doesn't necessarily imply that extraterrestrial influences (e.g. comet showers) are the sole, or even the primary cause. The "Nemesis" (a.k.a. "death star") hypotheses has fallen rather far out of favor recently, for a number of reasons. For one, nobody has located Nemesis; for another, celestial mechanics seem to predict that if our sun had a companion with a 26-million-year orbit, it would be orbiting so far away that gravitational influences from other stars would have disrupted its orbit (and possibly stolen it away) long ago. In any case, if Nemesis is actually out there, the chances are that the IRAS satellite probably recorded its presence, and it'll be identified when the IRAS data is eventually analyzed. One of the things that has influenced the ET-influence-on-extinction issue is the report by Alvarez et al concerning anomalously high levels of iridium in the clay layer corresponding to the time of one of the major extinctions. Alvarez hypothesized that the iridium probably came from an extraterrestrial object (a comet or asteroid) which collided with Earth and raised a very large dust cloud. [I've seen suggestions that the point of collision may have been in the northern Atlantic ocean, on the mid-oceanic ridge... the levels of iridium seem to be higher in that area. It was suggested that the asteroid punched through the thin crust at that point, triggering an upwelling of magma, and voila! Iceland is born!] There's some evidence that other extinction episodes occurred at about the same time as other [possible] asteroid or large-meteoroid impacts [there's a suggestion that the Okefenokee Basin in Florida is the site of one of those impacts]. On the other hand... there have been a large number of extinction episodes throughout history, and the high-levels-of-iridium signature doesn't show up on all of them (or even many of them, I gather). It has also been suggested that the raised levels or iridium could arise from major outburst of volcanic activity, which could bring up large amounts of iridium-enriched magma from the Earth's mantle, and could also lead to ecological disturbances (dust, ash, sulphur dioxide outgassing) just as distruptive as those resulting from an asteroid or comet impact. It's also worth noting that the dinosaurs had been undergoing a long and fairly gradual fall-off (in the number of species, at least) prior to the [apparently] sudden extinction event. There is some reason to believe that while an impact event may have dealt the "coup de grace" to the dinosaurs, they may already have been under substantial survival pressure from some other cause (or causes). Other mechanisms have been proposed to account for some of the non-iridium-related extinctions. In particular, climatic change triggered by the motion of continental plates across the surface of the globe can have a major effect on animal and plant populations. Several of the major extinction episodes have occurred when one or another major continent passed across the south polar region... it has been suggested that this results in massive continental glaciation, leading to a reduction in global temperatures and a change in ocean water levels and circulation patterns, global weather, and such second-order effects. Interestingly, when mass extinctions occur in the ocean, the species most affected seem to be tropical species that can't survive in cooler waters; temperate-zone species tend to survive more readily. For a good summary of the currently-recognized extinction episodes, and an analysis of them (by a scientist who clearly favors the climatic-change hypothesis), I recommend "Extinction", a book recently published by the Scientific American Library. Apologies to all for any mis-statements of fact that I've made in this posting; I'm strictly an enthusiastic amateur. Corrections, clarifications, and opposing points of view are encouraged. Dave Platt Internet: dplatt@teknowledge-vaxc.arpa UUCP: {hplabs|sun|ucbvax|seismo|uw-beaver|decwrl}!teknowledge-vaxc.arpa!dplatt Voice: (415) 424-0500 USnail: Teknowledge, Inc. 1850 Embarcadero Road Palo Alto, CA 94303