Stable isotope and planktic and benthic foraminiferal analyses of the K/T boundary transition at Stevens Klint, Denmark, indicate a 100-500 kyr long period of instability in oceanic bottom-water temperature and sea level. During the latest Maastrichtian (from 2.7 m below and up to the K/T boundary) bottom-water temperatures gradually cooled about 1.5°C as surface water temperatures remained constant. At the K/T boundary bottom water temperatures decreased additionally 3°C, and then returned to pre-boundary values just above the boundary interval. Coincident with the onset of the first temperature decrease there was a prominent sea-level fall, estimated between 50-100 m as observed in benthic and planktic foraminiferal assemblages as well as in a decreasing surface-to-bottom δ13C gradient. About 40 cm below the iridium-rich K/T boundary clay, sea level began to rise. Poor preservation of foraminifera above the K/T boundary allows only tentative interpretation of Danian events at Stevns Klint, however, sea level generally continued to rise interrupted by short sea level falls. Associated with the observed sea-level changes are three important hiatuses: At the contact between the white chalk and overlying grey-white bryozoan chalk about 2.7 m below the K/T boundary, at or near the top of the Fish Clay (Zone Pla, Parvularugoglobigerina eugubina missing) and at the contact of the Cerithium Limestone and overlying Bryozoan Limestone about 60 cm above the K/T boundary (part of Zone Plc missing). These unconformities are coeval with hiatuses observed in many continental shelf sections and indicate a global eustatic rather than regional isostatic response. Notable is that at Stevns Klint the episode of oceanic instability is registered over approximately the same stratigraphic interval in which supposed extraterrestrial amino acids occur, a circumstance that may indicate a connection. A gradual positive shift in δ13C is registered in both benthic and planktic foraminiferal assemblages in the upper Maastrichtian at Stevns Klint. The positive shift is followed by a prominent negative δ13C shift (1.3%) at the K/T boundary. An upper Maastrichtian positive δ13C excursion followed by a negativeδ13C anomaly at or near the K/T boundary has previously been observed in deep-sea cores from the Pacific Ocean and the Weddell Sea, Antarctica, implying a global event. Previous suggestions of a relation between the uppermost Maastrichtian positive δ13C excursion and sea-level variation are not supported by the data for Stevns Klint.
All Science Journal Classification (ASJC) codes
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes