Pulsation tectonics as the control of long‐term stratigraphic cycles

Abstract

A new theory of pulsation tectonics involving the cyclic eruption of thermal plumes from the D″ layer in the lowermost mantle explains the long‐term cycles of marine stratigraphy. Heating of the mantle changes conditions at the core/ mantle boundary that result in the quiescence of the earth's magnetic field. The rising plumes, after a phase lag, later induce faster plate spreading and widespread intraplate volcanism. Subsequently, colder rock returning to the lower mantle reestablishes the D″ thermal boundary layer and sets up conditions for more turbulent core convection and frequent magnetic reversals. Meanwhile, lower‐ mantle plumes cease erupting, and the plates spread more slowly. The cycles of fast and slow plate spreading and quiet and reversing magnetic field have a duration of 60–100 m.y. Global changes in spreading rate are one of the main causes of large‐scale (100–300 m) eustatic sea level changes and variations in output of volcanic gases. Consequential changes in albedo, global temperature, humidity, forestation, atmospheric and oceanic CO2, oceanic acidity, and oxygenation lead to variations in the preservation of organic carbon in the oceans and epicontinental seas and in the level of the calcite compensation depth. During rising sea levels, there is an increase in organic carbon preservation and relatively little calcite is preserved; with falling sea levels less organic carbon and more calcite are preserved in the deep sea. The long‐term cycles in carbon and calcite‐bearing sediments correspond generally to the pulsations of Grabau (1934), the long‐term rhythms which nearly equal the lengths of geologic time periods.