Stable-isotope and planktic foraminiferal analyses across the Cretaceous/Tertiary (K/T) boundary transition at Nye Klov indicate longterm oceanic instability associated with global sea-level fluctuations, a gradual mass extinction, and decreased magnitude of the δ13C shift in high latitudes.

Oceanic instability, which began at least 100 kyr before the K/T boundary and continued for about 300 kyr into the Tertiary, was accompanied by a gradual faunal turnover. The maximum sea-level lowstand during latest Maastrichtian is recorded about 75 cm below the K/T boundary. A sea-level rise first in evidence at 20 cm to 40 cm below the boundary continued into Zone P0 (boundary clay). This sea-level rise was accompanied by 2 °C of cooling in surface and bottom waters prior to the K/T boundary. δ13C values remained relatively stable up to 10 cm below the K/T boundary. A negative shift of 0.5 to 1.0 per mil occurred in the boundary clay in both planktic and benthic foraminifera. The surface to deep δ13C gradient remained nearly unchanged, in contrast to low latitudes, where this gradient is virtually eliminated. No sudden mass extinction occurred in this cosmopolitan, high-latitude fauna, and nearly all Cretaceous taxa thrived well into the Tertiary, when they gradually disappeared. Shallow seas, dominated by Cretaceous survivor taxa and a well-developed oxygen minimum zone, prevailed during the earliest Tertiary. Short-term sea-level lowstands are marked by hiatuses at the top of Zones P0 and P1a about 50 kyr and 230 kyr after the K/T boundary, respectively. Rising sea level reestablished normal marine conditions about 300 kyr to 350 kyr after the K/T boundary, coincident with the first post-K/T boundary recovery of the Tertiary fauna and extinction of Cretaceous survivors. During this time, high-latitude regions temporarily acted as centers of origin and dispersal for planktic foraminifera.

Long-term oceanic instability, gradual faunal turnover, absence of a sudden mass extinction, and greatly diminished δ13C shift in high latitudes suggest that a K/T boundary bolide impact was not the primary cause for the K/T boundary faunal transition. Moreover, these data strongly imply that the destructive effects of the bolide impact would have been greatest in low latitudes and negligible in high latitudes.

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