Response of foraminiferal assemblages to precession-paced environmental variation in a mid-latitude seaway: Late Turonian greenhouse of Central Europe
Abstrakt
Foraminiferal assemblages were studied as part of a multi-proxy dataset from an expanded record of Late Turonian hemipelagic deposition obtained from the Bch-1 research borehole in the Bohemian Cretaceous Basin (Central Europe). Based on fluctuations in the Si/Al ratio, precession-paced cyclicity (18-23 kyr) is recognized in the studied interval and interpreted to reflect changes in seasonality.
This study focuses on relationships between the astronomically controlled lithological cyclicity and the composition of foraminifera assemblages, together with variation in other parameters (geochemical, geophysical, palynological) that may provide further information about palaeoenvironmental change on a precession time scale. Spearman correlation enables the foraminiferal assemblages to be divided into three clusters.
These clusters are interpreted to reflect palaeoenvironmental patterns that result from variation in nutrient supply and in surface water salinity, driven by changes in seasonality. The "Lenticulina - Gyroidinoides - agglutinated foraminifera" cluster dominated during periods of decreased seasonality and alternated with the "Cibicides - spiral unkeeled planktics" cluster (lower part of study interval) or the "Gavellinela - Praebulimina - spiral keeled + biserial planktics" cluster (upper part of study interval) coincident with the presumed seasonality maxima.
Replacement of the Gavelinella cluster by the Cibicides cluster may reflect a longer-term trend of seasonality changes or changes in food supply. The observed low foraminiferal abundance and low planktic/benthic ratio during the seasonality maxima indicates that these were times of more intense rainfall, increased elastic supply and surface water turbidity, and more pronounced salinity fluctuations.