The authors consider Lagrangian motion of fluid particles in unsteady gravity currents in geophysical flows. The vertical motion of fluid particles, especially the induced vertical mixing in these currents, is partially responsible for the ocean thermohaline circulation, and thus plays a role in the global climate dynamics. First, a reduced dynamic system for slow variables is derived for a nonautonomous multiscale system. The reduced system, still nonautonomous, is the original system restricted to a centre-like nonautonomous invariant manifold (so-called slow manifold) which holds slow motions of the system. An algorithm is also presented to obtain an approximation of the nonautonomous slow manifold. A novelty here is that the reduction principle applies to nonautonomous multiscale systems which satisfy conditions that are true only locally in space (as in many physical cases). This makes the reduction principle applicable to real physical systems. Then, this invariant manifold reduction principle is applied to an approximate conceptual Lagrangian model of gravity currents and a reduced nonautonomous system for slow vertical motion is obtained. This reduced system may be useful as a conceptual tractable tool for understanding some features of vertical mixing in unsteady gravity currents.