Most existing studies of the effects of rotation on the Rayleigh–Taylor instability consider stratified layers. However, in nature and in the laboratory localized sources of unstable buoyancy are also commonly observed. We report on laboratory-scale direct numerical simulations of isolated buoyancy sources interacting with a pre-existing pycnocline. We demonstrate that for a readily accessible parameter range, long lived, coherent structures that share characteristics of plumes and eddies spontaneously develop. These plumes interact with the pycnocline, but never fully penetrate it. In contrast, strong rotation leads to a surface-trapped response appearing as sheets in laboratory experiments. Finally, we demonstrate that temperature-induced changes in buoyancy behave differently than salt-induced changes, due to the lower diffusivity in the latter that reduces plume dilution and increases penetration into and through the stratified layer. This distinction is important because the majority of experimental literature involves salt-induced stratification.