Stable carbon, oxygen, and strontium isotope data from Maastrichtian planktic and benthic foraminifera from southern high-latitude Ocean Drilling Program Sites 690, 750, and 761 reveal major changes in climate and ocean chemistry during the early to late Maastrichtian transition, some 4 to 6 m.y. prior to the Cretaceous-Tertiary (K-T) boundary. The δ18O results suggest accelerated cooling, resulting in the lowest marine temperatures of the Late Cretaceous. The late Maastrichtian rise in the seawater 87Sr/86Sr ratio begins at the time of this change and suggests increased rates of continental weathering and/or runoff. The δ13C values show a negative shift of ∼0.5‰ followed by a positive shift of ∼1‰. These δ13C fluctuations may reflect change in deep water chemistry with cooling, variation in reduced carbon storage in coastal areas caused by eustatic sea-level fluctuation, and increased surface-water productivity. These findings contradict the common representation of stable and climatically equable environmental conditions in the latest Cretaceous before the K-T boundary event.