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Stable isotopic records across the Cretaceous/Paleogene (K/P) boundary in Maud Rise Holes 689B and 690C indi­ cate that significant climatic changes occurred during the latest Cretaceous, beginning approximately 500 k.y. prior to the mass extinction event and the enrichment of iridium at the K/P boundary (66.4 Ma). An oxygen isotopic decrease of ~ 0.7%o~ 1.0%o is recorded in the Late Cretaceous planktonic and benthic foraminifers between 66.9 and 66.6 Ma. The negative isotope excursion was followed by a positive excursion of similar magnitude between 66.6 Ma (latest Creta­ ceous) and —66.3 Ma (earliest Paleocene). No other isotopic excursions of this magnitude are recorded in the plank­ tonic and benthic microfossil records 1.0 m.y prior to, and for 2.0 m.y following the mass extinction event at the K/P boundary. The magnitude and duration of these isotopic excursions were similar to those at the Paleocene/Eocene and Eocene/Oligocene boundaries. A major 5C excursion occurred 200 k.y. prior to the boundary, involving a positive shift in planktonic and benthic oC of ~0.5%o-0.75%o. Similar changes observed in other deep-sea sequences indicate that this reflected a global change in 5C of the oceanic total dissolved carbon (TDC) reservoir. The magnitude of this inferred carbon reservoir change and its association with high latitude surface-water temperature changes recorded in the 60 records implies that it was linked to global climate change through feedback loops in the carbon cycle. At the K/P boundary, the surface-to-deep water 6C gradient is reduced by approximately 0.6%o-~0.2%o. However, unlike sequences elsewhere, the planktonic-benthic 6C gradient (A5C) was not eliminated in the Antarctic. The sur­ face-to-deep water gradient was re-established gradually during the 400 k.y. following the mass extinction. Full recovery of the A6C occurred by —60.0 Ma. In addition to the reduced vertical 5C gradient across the K/P boundary, there was a negative excursion in both planktonic and benthic 6C beginning approximately 100 k.y. after the boundary (66.3 Ma). This excursion resulted in benthic 5C values in the early Paleogene that were similar to those in the pre-K/P boundary intervals. This negative shift appears to reflect a change in the 5C of the oceanic TDC reservoir shift that may have resulted from reduced carbon burial and/or increased carbon flux to the oceans. Any model that attempts to explain the demise of the oceanic plankton at the end of the Cretaceous should consider the oceanic environmental changes that were occurring prior to the massive extinction event.

The Paleoceanographic and Paleoclimatic Signature of the Cretaceous/Paleogene Boundary in the Antarctic: Stable Isotopic Results from ODP Leg 113