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 delta (super 18) O results suggest accelerated cooling, resulting in the lowest marine temperatures of the Late Cretaceous. The late Maastrichtian rise in the seawater (super 87) Sr/ (super 86) Sr ratio begins at the time of this change and suggests increased rates of continental weathering and/or runoff. The delta (super 13) C values show a negative shift of approximately 0.5per mil followed by a positive shift of approximately 1per mil. These delta (super 13) C fluctuations may reflect change indeep 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.