Milankovitch cycles and microfossils: principles and practice of palaeoecological analysis illustrated by Cenomanian chalk-marl Rhythms

Abstract. While standardization of sampling, processing and picking techniques is essential in micropalaeontology, standard counts (and percentages) have three serious disadvantages. They are interdependent, so changes in one taxon affect counts of all others; they can be misleading, e.g. when percentage abundance increases but absolute numbers decrease; and they conceal changes in absolute abundance, which for palaeoecology are often most revealing. A technique which combines a minimum count with estimates of absolute numbers is recommended and has been applied to 12 samples from a mid-Cenomanian chalk/marl rhythm. Data for insoluble residues, stable isotopes of carbon and oxygen and numbers of planktonic and benthonic foraminifera and ostracods all vary through the rhythm. Absolute abundance of planktonic foraminifera correlates best with surface sea water temperatures calculated from oxygen isotopes. Abundances of both calcareous and agglutinated benthonic foraminifera correlate best with percent insoluble residue values and these foraminifera respond passively to changes in sedimentation rate. Ostracods do not correlate well with either control. Ratios of insoluble residues and of benthonic foraminiferal abundances between chalks and marls confirm that the rhythms are productivity cycles. Mid-Cenomanian chalk beds were deposited rapidly, taking at most 5–7000 years of the 21,000 year precession cycle.