Holocene, mesophotic, carbonate sedimentation, Bermuda atoll margin; a submersible study

Bermuda is an atoll-like reef system and associated islands bathed by the Gulf Stream at the cool, northern margin of coral reef growth in the North Atlantic Ocean. This study documents observations from and samples acquired by submersible on the shallow slope between 60 and 400 m water depth on the southern side of the structure. The mesophotic bathymetry is like other such carbonate margins, a shallow slope, an escarpment, and a lower slope, but the sectors are deeper than elsewhere, and interpreted to be due to underlying volcanic topography. The seafloor limestones are almost wholly mid-Holocene in age, 4.0 ka to 7.7 ka implying that the present depositional system at these depths is largely moribund. Facies are dominated by intensely bioeroded, encrusting coralline algal boundstones with associated bryozoans, serpulids, and hydrozoans with a wackestone matrix containing abundant bioeroded sponge chips. This facies also forms numerous rhodoliths. There are only two samples of intertidal algal cup reef facies, and both are clearly allochthonous in water depths of 106 and 384 mwd. Their presence and ages of similar to 5.0 ka does, however, confirm the presence of such cup reefs that grew coeval with the mesophotic boundstones. The age of most facies coincides with the mid-Holocene thermal. This is one of the few analyses of Holocene mesophotic deposition at the boundary between the Holocene photozoan and heterozoan depositional realms during the Holocene and illustrates the importance of subtle, if somewhat un-resolved oceanographic change in determining accretion.