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Gradual mass extinction, species survivorship, and long-term environmental changes across the Cretaceous-Tertiary boundary in high latitudes

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 δ 13 C 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. δ 13 C 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 δ 13 C 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 δ 13 C 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.