This paper studies passive-tracer mixing in an idealized, convecting ocean mixed layer. A two-dimensional direct numerical simulation is used to compute an eddy-diffusivity kernel relating the mean vertical flux at one depth to tracer gradients at other depths. The kernel captures local and nonlocal contributions within a single transport representation. The experiment shows how multiple passive tracers can diagnose that representation directly from the simulated flow. Readers get a concrete method for measuring nonlocal mixing and a physical explanation of why a diffusivity defined only at the local depth can be insufficient.