The dynamics of marine stepwise mass extinction

Mass extinction is characterized by the loss of 50 - 90 + percent of genetically and ecologically diverse species within 1 - 3.5 Myr intervals. Three conflicting theories exist: (1) Graded Mass Extinction; (2) Stepwise Mass Extinction; and (3) Catastrophic Mass Extinction. These can only be adequately tested with high resolution (cm-scale) stratigraphic data spanning the entire mass extinction interval and adjacent strata. Such data are presently available only for the Eocene-Oligocene (E-O), Cretaceous-Tertiary (K-T) and Cenomanian-Turonian (C-T) extinctions. In general, prevalent uniformitarian stratigraphic philosophy and use of the modern Earth/Life system as a model for the Phanerozoic has hindered the search for, and expectations of, high resolution stratigraphic data critica) to mass extinction research. The modern Earth/Life "model" predicts highly variable, environmentally and biologically resilient systems and predominantly autocyclic stratigraphic response to large-scale forcing mechanisms. Yet, present environmental systems are not typical of 90 + percent of Phanerozoic history characterized by more stable and delicately balanced environmental/ecological systems, no permanent polar ice, much higher sealevel, warmer and more equable, maritime-dominated climates, little seasonality, and much broader temperature and habita! gradients. Phanerozoic marine species were largely adapted to widespread warm stable environments, and were predominantly stenotopic and stenothermal. Phanerozoic Earth/Life systems were capable of rapid, widespread response to abrupt enviran mental fluctuations, including mass extinction. A stratigraphic and biological record dominated by near isochronous to short-term depositional events is predicted by dominan! Phanerozoic environments. Testing of the three mass extinction hypotheses with the best high-resolution data available (C-T, K-T, E-O boundary intervals) demonstrates the following phenomena in common: (1) Ali are stepwise extinctions, graded ecologically from stenotopic (first) to eurytopic groups, and from Tropical to Temperate taxa, over 1-3.5 Myr intervals. Steps are abrupt (100 Kyr or less) to catastrophic; background extinction rates and patterns separate them. (2) Ali steps of each mass extinction are contained within an interval of extraordinarily large (2-5 times background), rapid tluctuations in atmospheric/oceanic temperature and chemistry, as depicted from geoche mical profiles. These exceeded the adaptive ranges and evolutionary response rates of many marine taxa, causing steps of extinction al major tluctuations. (3) Each interval has diverse evidence for onc or more comet/asteroid impacts ("storms") on Earth, clustered around the extinction interval. Most well-defined impact events are directly associated with major marine geochemical excursions and mass extjnction steps. A mass extinction theory based on these case histories suggests that initial impacts by comets/asteroids within a storm were in the sea, causing large-scale disruption of temperature, chemistry, stratification, and circulation dynamics within the marine system, and a dense, solar-screening, water vapor cloud in the atmosphere, collectively initiating long-term dynamic feedback processes represented by large-scale geochemical tluctua tions. Many tluctuations were too large and rapid for survival by narrowly adapted, predominantly stenotopic, stenothermal marine taxa; stepwise extinction resulted. Additional impacts exaggerated the oceanic/climatic perturbations, and reset the extinction clock.