A radioactive nuclide with a half-life of 53. 3 days produced by cosmic rays (i.e. electron capture decay to ⁷Li) that can be used as a tracer of ventilation processes occurring on a seasonal timescale. It is deposited by rainfall on the ocean surface, and homogenized within the surface mixed layer, with a fraction found to penetrate into the upper thermocline. The extent of penetration before decay depends partly on the strength of vertical mixing and advective processes. The distribution below the mixed layer at any time depends largely on the depth history of the mixed layer, i.e. Beryllium-7 found in the thermocline can be remnant or previous mixed layers formed within several half-lives of the isotope (a seasonal timescale). Thus, if the depth history of the mixed layer is known, then the mixing and advection component affecting the Beryllium-7 distribution can be found. Conversely, given an understanding of these processes, it can be used to interpret mixed layer history on a seasonal timescale.