Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 10 / a / iii

For a constant-mass, well-mixed box the mean residence time is stored water mass divided by its total outgoing water-mass transport. Using the source figure’s endpoints,At water steady state, each denominator can equivalently be replaced by that box’s summed inflow. If transports are specified as volume rates, use volume rather than mass in the numerator, or convert with density consistently. The drawn maximum basin depths alone do not supply layer volumes, plan areas or transport magnitudes, so no numerical residence times follow. Internal recycling counts as exit from a particular box; these times are not the time to escape the combined sea system.