STORM AND NORMAL WAVEBASES: A RELEVANT DISTINCTION?

Surface waves are a dominant mover of sediments on shallow marine shelves. These waves are commonly divided conceptually into two distinct classes, normal and storm waves, based on their hydrodynamic characteristics and on their wavelength, which relates to depth of penetration. Here we use over 2 million individual spectral density estimates collected between 1996 and 2008 from 32 buoys from the Caribbean, Gulf of Mexico, and Western Atlantic to test the hypothesis that normal and storm waves constitute two distinct modes in depth of wave penetration. Although we find clear evidence for distinct size classes of waves within individual hourly measurements and over the timescales of individual storms, surface waves in the modern ocean do not exhibit the expected bimodality in wavelength when time averaged over the course of weeks to years. Instead, time averaged wave spectral densities consistently exhibit unimodal distributions. There is, however, significant spatial bimodality in the wavelengths of surface ocean waves; waves in the comparatively protected Gulf of Mexico and Caribbean have a modal wavelength of approximately 75 m whereas waves along the western Atlantic margin have modal wavelengths of ~110 m. Thus, rather than being distinctly bimodal with respect to depth of penetration, there is a uniform and continuously increasing probability that a wave will reach the bottom as water depth decreases and this probability distribution varies significantly and as a function of wave fetch. Bimodality in the hydrodynamics of storm-generated versus normal wind waves can, however, still result in deposition of distinct sedimentary structures. These new quantitative estimates for depth of wave penetration provide important empirical constraints on the paleodepths of ancient sedimentary deposits that preserve evidence of wave-generated sedimentary structures. Our results also highlight important differences between sheltered shelf environments, such as those that characterized ancient epeiric seas, and the narrow open-ocean facing coastlines of many modern shelf environments.