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... When The Earth Shifted Gears No one really knows just how the terrestrial magnetic field is generated or why it has reversed its direction so frequently in past geological time. Per-haps there is a clue in the following correlation: "The Mesozoic-Cenozoic histories of reversals in the earth's magnetic field and of periods of widespread anoxia in the ocean basins show a remarkable correlation; periods of black-shale deposition (' anoxic events') occur during lengthy periods without magnetic reversals (' quiet periods'). My assembly of published work indicates a remote connection between quiet periods and anoxic events and suggests its form: Magnetic quiet periods coincide with fast seafloor spreading. During these periods, buoyant spreading ridges displace seawater into broad shelves, thus decreasing earth's albedo and causing global ... . Temperature gradients, and thus density gradients, from pole to equator decrease in surface waters, and the deep ocean currents of oxygenated polar waters wane. Oxygen minimum zones intensify and widen; anoxic conditions throughout entire basins are indicated by black shales deposited in the deep sea. These relations thus suggest that the earth's interior processes and its climates are related and their status recorded by both magnetic polarity and anoxic event chronologies of the earth." (Force, Eric R.; "A Relation among Geomagnetic Reversals, Seafloor Spreading Rate, Paleoclimate, and Black Shales," Eos, 65:18, 1984.) Comment. But what stopped and restarted the magnetic reversals and other concurrent processes? Strangely enough, the quiet, anoxic periods do not seem to coincide with biological extinctions ...
Terms matched: 2 - Score: 314 - 15 May 2017 - URL: /sf032/sf032p12.htm
... first?The advent of complex life keeps getting pushed back farther and farther in time, as evidenced by the following abstract: "Microfossils resembling fecal pellets occur in acid-resistant residues and thin sections of Middle Cambrian in Early Proterozoic shale. The cylindrical microfossils average 50 x 110 microns and are the size and shape of fecal pellets produced by microscopic animals today. Pellets occur in dark gray and black rocks that were deposited in the facies that also preserves sulfide minerals and that represent environments analogous to those that preserve fecal pellets today. Rocks containing pellets and algal microfossils range in age from 0.53 to 1.9 gigayears (Gyr) and include Burgess Shale, Greyson and Newland Formations, Rove Formation, and Gunflint Iron-Formation. Similar rock types of Archaean age, ranging from ... Science Frontiers ONLINE No. 46: Jul-Aug 1986 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects Which came first?The advent of complex life keeps getting pushed back farther and farther in time, as evidenced by the following abstract: "Microfossils resembling fecal pellets occur in acid-resistant residues and thin sections of Middle Cambrian in Early Proterozoic shale. The cylindrical microfossils average 50 x 110 microns and are the size and shape of fecal pellets produced by microscopic animals today. Pellets occur in dark gray and black rocks that were deposited in the facies that also preserves sulfide minerals and that represent environments analogous to those that preserve fecal pellets today. Rocks containing pellets and algal microfossils range in age from 0.53 to 1.9 gigayears ...
Terms matched: 2 - Score: 230 - 15 May 2017 - URL: /sf046/sf046p14.htm
... Science Frontiers ONLINE No. 34: Jul-Aug 1984 Issue Contents Other pages Home Page Science Frontiers Online All Issues This Issue Sourcebook Project Sourcebook Subjects Behind magnetic flip-flops The earth's magnetic field frequently reverses its polarity. Such flips can of-ten be correlated with climate changes, global ice volumes, sea-floor spreading rates, and deposition of black shales, tektite falls, biological extinctions, etc. The frustrating thing is the lack of clear-cut cause and effect; that is, how these phenomena are linked physically to the geomagnetic field. Part of the problem is that we can only guess at how the geomagnetic field is generated. Let us assume that the earth's magnetic field is created by dynamo action in the planet's fluid core. ... . Olson finds analytically that the core dynamo may reverse sign due to fluctuations in core turbulence caused by two competing energy sources: heat loss at the mantle-core boundary and progressive growth of the inner core. In concept, the heat lost at the core-mantle boundary might be linked to climate changes and sea-floor spreading. Taking a different tack, D. Gubbins has investigated the possibility that field reversals are triggered by ice ages and meteorite impacts (tektite falls). The physical mechanism here would be the increase in pressure upon the core, which affects the rate of freezing in the outer core, and thus the power available to the core dynamo. Gubbins found that these externally caused pressure changes were too small to explain the polarity changes. However, the parameters involved are ...
Terms matched: 2 - Score: 228 - 15 May 2017 - URL: /sf034/sf034p15.htm
... origin near Lake Acraman, about 300 kilometers west. (Gostin, Victor A., et al; "Impact Ejecta Horizon within Late Precambrian Shales, Adelaide Geosyncline, South Australia," Science, 233:198, 1986.) Reference. The subject of very large terrestrial craters is discussed in ETC2 in our catalog: Carolina Bays, Mima Mounds. Description here . The Amirante Basin (black circle) lies about 500 kilometers north-east of Madagascar. From Science Frontiers #47, SEP-OCT 1986 . 1986-2000 William R. Corliss ...
Terms matched: 2 - Score: 222 - 15 May 2017 - URL: /sf047/sf047p15.htm
