8th International Nannoplankton Association Conference
Jens Wendler, Kai-Uwe Gräfe, Helmut Willems:
The growing knowledge of calcareous dinoflagellate cysts from the Cretaceous enables palaeoecological reconstruction of long-term cyclic, and abrupt, environmental changes. In cyclic sequences, groups of species which are characteristic of certain lithofacies can be distinguished. The periods of time investigated in this kind of study are in the order of Milankovitch frequencies and are expressed by the prominent light/dark sedimentary rhythms of, for example, the boreal Cenomanian. Based on the facies preferences of certain marker-species, the palaeoceanographic changes that led to the sedimentary cyclicity are reconstructed. Our main conclusion is that enhanced stratification, associated with decreased nutrient availability, due to low seasonality, are the main factors controlling the calcareous dinoflagellate cysts' distribution pattern in the dark layers.Association changes, related to carbonate productivity breakdown, also accompany prominent events such as the K/T boundary. The Danian Fish Clay, an impact-fallout-bearing boundary layer exposed at Stevns Klint, Denmark, is an exceptionally well-suited sediment to study the ecological consequences of this sudden event. Characteristic cyst assemblages can be distinguished for the intervals shortly before, during, and after the impact. Pithonelloideae, the dinoflagellates that dominated the boreal shelf-assemblages in the Maastrichtian, rapidly became extinct in the Danian. Typical bloom abundances directly below the clay confirm that there is no gradual decline in number well before the event. A few specimens can be found above the clay layer. Those may indicate a short-term recovery but more likely are reworked. Despite the mass mortality of Pithonelloideae the clay layer shows a relatively high diversity of calcareous dinoflagellates. Many species appear to have been well adapted to the exceptional environmental conditions. The forms that became dominant in the Danian were already present in the Late Maastrichtian but cannot be found in the Fish Clay.
Copyright © 2000, most recent revision July 28, 2000Tania Hildebrand-Habel (email@example.com)