A CLOCKWORK EARTH: MIKE ARTHUR, AL FISCHER AND RECURRENT CYCLES IN GEOLOGICAL THOUGHT

The first paper I read by Mike Arthur was Secular Variations in the Pelagic Realm (Fischer & Arthur, 1977). This beautiful product of Mike’s early partnership with his graduate mentor, Al Fischer, introduced the idea of an ~32-million-year (myr) rhythm in diversity of pelagic organisms. In doing so, it linked biological diversity to the long history of previous ideas about recurrent, but not necessarily strictly periodic, geologic cycles. More precisely, it defined the history of pelagic diversity as characterized by repeated alternations between polytaxic” and “oligotaxic” regimes. It explicitly linked those alternations to variation in marine temperature, anoxia and older ideas of repeated variation in such processes as onlap and offlap of marginal seas, marine sedimentation, orogeny, and mantle convection (e.g., Holmes, 1927).

Fischer and Arthur’s identification of ~32-myr periodicity in pelagic diversity inspired Raup & Sepkoski’s 1984 statistical definition of 26-myr periodicity in post-Permian marine extinction. In focusing on extinction, Raup and Sepkoski decoupled periodicity in biological diversity from periodicity of Earth’s geologic cycles. Their study was quickly accompanied by hypotheses that long-timescale periodicity of extinction is driven by cometary perturbations due to (i) an unknown star relatively near the Sun or (ii) periodic passage of our solar system through the galactic plane. Many subsequent studies have attacked these extrasolar hypotheses, whereas others have attacked or defended the hypothesis of regularly periodic mass extinction.

Regardless of the final perspective on statistical periodicity of mass extinctions, two aspects of Fischer & Arthur’s 1977 publication will remain resonant in geologic thought: (i) explicit linkage of pelagic diversity to oceanic and geologic conditions, and (ii) repeated alternation of high-diversity and low-diversity conditions over geological time.