THE COMMON-CAUSE HYPOTHESIS: INITIAL PREDICTIONS OF HABITABLE AREA FROM SIMPLE GEOMETRIC MODELS

Forward simulations have demonstrated many ways in which sea level controls sequence stratigraphic architecture and alters the fossil record, and these biases have been confirmed in numerous field studies. The extent to which sea level might also produce real evolutionary and ecological change (the common-cause hypothesis) remains unknown, although several recent empirical studies offer tantalizing support. Of possible controls of sea level, changes in habitable area are readily investigated through numerical simulations. Realistic basin simulation models can be used to study dynamic topography, topography that changes through geologic time, but simple geometric models can generate baseline predictions for static topography that does not vary over time, and these are explored here.

These geometric models indicate that the relationship of habitable area to sea level is a complex one, and that it depends not only on the amount of sea-level change, but also the starting position of sea level, and whether the coast is straight (a 2D coast) or complex (a 3D coast). 2D coasts, such as modern passive margins, experience significant gains in habitable area only for a narrow range of initial conditions, primarily when sea-level starts at or below the shelf-slope break. Similarly, they experience substantial losses in habitable area when sea-level starts close to but above the shelf-slope break. 3D coasts, such as modern southeast Asia or epicontinental seas, are far more sensitive to sea-level changes and can experience appreciable changes in area for a much greater set of initial conditions. Furthermore, epicontinental seas may also experience a loss of shallow-water habitable area during a sea-level rise, not just during a sea-level fall. The complex response of habitable area to sea-level change is likely a prime reason for the mixed support in previous studies of species-area relationships in the marine fossil record.