Buoyant particle-laden flows are a major mechanism of particle transport in numerous natural and industrial flows, including fluvial and estuarine plumes, crystallisation and flotation processes, and during the solidification of alloys. When small particles sediment from a surface current generated by an axisymmetric turbulent plume, the concentration of particles in the environment surrounding the plume is larger at higher levels than at lower levels. This distribution of particles in the environment results in an unstable density stratification and as a result, convection in the environment may ensue. We address the onset of this instability and its effect on the dynamics of the plume and on the pattern of sedimentation on the surrounding floor. New laboratory experiments were performed to investigate the convection driven by the settling of particles in the environment surrounding the plume. We use scaling analysis to derive a criterion for the onset of the instability. The sedimentation mechanism present in this new regime involving convection in the environment is described theoretically. Our predictions for the deposition patterns are successfully compared with the new laboratory measurements.