Discovery of enigmatic toroidal carbonate concretions on the Rio Grande Rise (Southwestern Atlantic Ocean)

Abstract We present enigmatic toroidal carbonate concretions retrieved from 700 m water depth from two sites in the upper plateau of the Rio Grande Rise. The concretions have a diameter of ~15 cm and a central hole of ~5 cm, and were observed on top of loose bioclastic sand over an area of ~30 m2 at 0.5–1.5 m from one another. They consist of brown, porous, bioclastic grainstone, lacking internal structures. Grains consist of sand (