Lower Mississippian ramp-slope deposits (Paine Member) of southwestern Montana are composed of thin, rhythmically interbedded limestone and argillaceous limestone (argillite). Millimetre-thick graded layers typical of limestone beds represent distal storm deposits,whereas argillite layers containing abundant whole, delicate fossils represent quiet-water deposition during times of little or no storm activity. Spectral analyses of the fluctuating insoluble-residue content (quartz, muscovite-illite, organic matter) indicate a dominant periodicity of 0.6-2.85 ka in the ramp-slope deposits; no spectral peaks corresponding to typical Milankovitch-type periods (~20-100 ka) were observed. Similar ~2.5 ka paleoclimatic periodicities are recorded in Quaternary continental and alpine glaciers, Quaternary deep-sea sediments, 14C variations in Holocene tree rings, and Permian deep-water evaporite varves. These short-term paleoclimatic fluctuations may represent one of several harmonics of the precessional (19-23 ka) or obliquity (41 ka) orbital cycles or may be related to variations in solar activity.