Orphan Knoll is a bathymetric feature located 550 km northeast of Newfoundland, Canada, close to the area of interaction and mixing of subtropical waters transported by the North Atlantic Current and fresh and cool waters of polar and sub-polar origin. Orphan Knoll is also situated at the exit pathway of the Labrador Sea Water. It occupies an area about 75 km by 190 km, rising to a depth of 1800 m. Near-bottom current measurements provide evidence for anti-cyclonic circulation around the knoll. The Orphan Basin-Orphan Knoll region is biologically rich and provides a productive benthic habitat for a complex diversity of organisms that include deep sea corals and sponges.
Dissolved inorganic carbon and total alkalinity were measured to evaluate calcium carbon saturation states and pH along two sections; one section extends from the north end of Orphan Knoll, across Orphan Basin to Newfoundland Shelf and one perpendicular to it. Aragonite saturation states at the plateau of Orphan Knoll were less than 1.2 with the average and standard deviation of 1.12 and 0.04, respectively, whereas the flanks deeper than 2100 m were undersaturated. Calcite saturation states were >1 throughout. A slight shoaling of saturation horizons from north to south and also from west to east, was observed. This trend corresponds to water mass structure around the topography. The average and standard deviation of pHtotal on the plateau of Orphan Knoll were 7.894 and 0.018, respectively. However, saturation states and pHtotal were lower at equivalent depths along the Newfoundland Slope caused by direct outflow from the Labrador Sea. Due to the observed low aragonite saturation and pHtotal on the plateau and flanks of Orphan Knoll, it is important to monitor the response of the benthic community to future changes in marine carbonate chemistry.