west of the MAB of the South Atlantic. The western part of the transverse ridge consists of fragments of uplifted oceanic crust and upper mantle. The summit of the transverse ridge is capped by Miocene shallow water limestones that reached above sea level 20 Ma before subsiding abnormally fast. The eastern part of the transverse ridge, however, consists of a thick sequence of stratified material called the Romanche Sedimentary Sequence (RSS). The RSS includes pelagic material from the early Cretaceous (140 Ma) which, coupled with the thickness of the sequence, does not fit with the normal scenario for the opening of the South Atlantic around the Aptian-Albian (125–100 Ma). The transverse ridge separates the present trench from an -long aseismic valley where the Romanche transform was located until about 10–8 Ma. This transform migration was preceded by a process 25–17 Ma during which the Miocene shallow-water platform mentioned above reached sea level as the transverse ridge was first uplifted, then deformed, and finally buried under the sea. Biological role The hydrothermal vents of the MAR support many life forms. The Romanche and Chain Fracture Zones creates a huge gap in the MAR and can act as a "Subsea Berlin Wall" segregating the North Atlantic communities from those in other oceans. Swarms of hydrothermal shrimps reminiscent of those found from northern MAR vent-sites have been found on the northern part of the southern MAR. Bivalve communities have been reported around vents further south. These species await a formal description and it is not known whether or not they represent communities distinct from those on the northern MAR. The flow of NADW through the Romanche and Chain Fracture Zones may serve as a conduit for larval transport from the western North Atlantic to the eastern South Atlantic. For example, the caridean shrimp Alvinocaris muricola inhabits cold seeps in both the Gulf of Mexico and the Gulf of Guinea, which clearly suggests a gene flow across the Atlantic, but the amount of data is very limited and dispersal pathways are not fully understood. References Notes Sources Fracture zones Oceanic trenches of the Atlantic Ocean Mid-Atlantic Ridge