Geology and Geomorphology

Abstract

Geomorphology and geological processes exert fundamental controls on the occurrence, distribution, and makeup of mesophotic coral ecosystems (MCEs). Two broad geomorphic categories are shelves and slopes. Shelves include outer portions of continental and insular shelves that dip gently into mesophotic depths before reaching the shelf break and have very low gradients ( 70°). On shelves, MCEs are best developed on positive relief features elevated above the surrounding seafloor. In slope settings, MCE development is typically favored on steep irregular slopes, where coral cover is concentrated on steep-sided buttresses and sediment is channelized into narrow chutes. Relict features related to past sea levels are critically important MCE habitats on both shelves and slopes. Coral and coralline algae remain the primary frame builders in MCEs. However, accretion at mesophotic depths is likely very slow, such that they form only thin biostromal veneers over relict substrates. Sediments in MCEs are dominantly autochthonous skeletal sands and gravels. Although fluxes of sediments to the seafloor in MCEs are typically lower than in shallow reefs, sedimentary dynamics still play an important role. Low-gradient seafloor has an increased potential for accumulation of sediment detrimental to MCEs. In slope settings, downslope bed-load transport of sediment can be orders of magnitude higher than vertical fluxes and likely exerts an important influence on MCEs.