No. 32: Mar-Apr 1984
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 warming. 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!