Abstract A composite late Maastrichtian (65.5 to 68.5 Ma) marine osmium (Os) isotope record, based on samples from the Southern Ocean (ODP Site 690), the Tropical Pacific Ocean (DSDP Site 577), the South Atlantic (DSDP Site 525) and the paleo-Tethys Ocean demonstrates that subaerially exposed pelagic carbonates can record seawater Os isotope variations with a fidelity comparable to sediments recovered from the seafloor. New results provide robust evidence of a 20% decline in seawater 187Os/ 188Os over a period of about 200 kyr early in magnetochron C29r well below the Cretaceous–Paleogene Boundary (KPB), confirming previously reported low-resolution data from the South Atlantic Ocean. New results also confirm a second more rapid decline in 187Os/ 188Os associated with the KPB that is accompanied by a significant increase in Os concentrations. Complementary platinum (Pt) and iridium (Ir) concentration data indicate that the length scale of diagenetic remobilization of platinum group elements from the KPB is less than 1 m and does not obscure the pre-KPB decline in 187Os/ 188Os. Increases in bulk sediment Ir concentrations and decreases in bulk carbonate content that coincide with the Os isotope shift suggest that carbonate burial flux may have been lower during the initial decline in 187Os/ 188Os. We speculate that diminished carbonate burial rate may have been the result of ocean acidification caused by Deccan volcanism.