SECULAR VARIATIONS IN THE PELAGIC REALM

Abstract

Abstract Diversity of pelagic biotas varies with a rhythm of about 32 million years. “Polytaxic” times of maximal diversity coincide with higher and more uniform oceanic temperatures, with continuous pelagic deposition and with widespread marine anaerobism, eustatic sea-level rises, and heavier carbon isotope values in marine calcareous organisms and organic matter. Pelagic communities reach maximal complexity, expressed in numbers of taxa and in predator size. “Oligotaxic” episodes are characterized by lower marine temperatures and sharper latitudinal and vertical temperature gradients, by interruptions of submarine sedimentation caused by intensified current systems, by marine regression, by a lack of anaerobic marine sedimentation, and by lighter carbon isotope values in marine carbonate skeletons and organic compounds. Degradation of pelagic communities is reflected by loss of large predators and lowered diversity; blooms of opportunistic species occur during these intervals. The fluctuation from oligotaxic to polytaxic conditions is attributed to changes in rates of oceanic circulation as a direct result of climatic variation: during polytaxic times warm, globally equable climates result in reduced oceanic convection rates, causing expansion and intensification of the oxygen minimum layer. Colder climatic intervals lead to increased circulation rates, more efficient oxygenation of ocean waters, and oligotaxy. The climate changes in response to either fluctuations in receipt of solar energy, or to internal causes such as rhythms in mantle convection and associated processes: rates of plate motions, volcanism and orogeny. These cycles directly influence the accumulation patterns of petroleum source rocks, major phosphorite deposits, deep-sea carbonates and biogenic silica.