The Romanche Trench, also called the Romanche Furrow or Romanche Gap, is the third deepest of the major trenches of the Atlantic Ocean, after the Puerto Rico Trench and the South Sandwich Trench. It bisects the Mid-Atlantic Ridge (MAR) just north of the equator at the narrowest part of the Atlantic between Brazil and West Africa, extending from 2°N to 2°S and from 16°W to 20°W. The trench has been formed by the actions of the Romanche Fracture Zone, a portion of which is an active transform boundary offsetting sections of the Mid-Atlantic Ridge. It was named after the French navy ship La Romanche, commanded by captain Louis-Ferdinand Martial which on 11 October 1883 made soundings that revealed the trench. The boat was returning to France after spending a few months on a scientific mission near Cape Horn as part of the first International Polar Year. Oceanography The trench has a depth of , is long and has an average width of and allows for a major circulation of deep ocean basin water from the west Atlantic to the east Atlantic basins. Deep water flow through the trench is from west to east with a rate of 3.6 Sverdrups (million m³/s) of water. Lower North Atlantic Deep Water (LNADW) is found around below sea level and flows from the Greenland and Norwegian seas; it brings high salinity, oxygen, and freon concentrations towards the equator. Antarctic Bottom Water (AABW) flows below the LNADW and reaches down to the seafloor. Formed around Antarctica, the AABW is cold, has low salinity, and high silicate concentration. As it flows north, it is constrained by numerous obstacles on the seafloor. In the basin on the eastern side of the MAR the Walvis Ridge blocks the northward passage. For LNADW and AABW, the Romanche and Chain Fracture Zones (just south of the equator) are the only deep passages in the MAR where interbasin exchange is possible. As AABW flows through the Romanche Fracture Zone, salinity and temperature increase significantly. Geology The Romanche Fracture Zone offsets the Mid-Atlantic ridge by , making it the largest equatorial fracture zone in the Atlantic. According to the normal scenario for the opening of the South Atlantic, it is spreading at a rate of and began forming about . North of and parallel to the fracture zone is a transverse ridge which is particularly prominent over hundreds of kilometres east and