PLANKTONIC MICROFOSSIL EVOLUTION AND BIOSTRATIGRAPHY ACROSS THE JURASSIC-CRETACEOUS BOUNDARY

Late Early Jurassic through earliest Cretaceous planktonic foraminifera count fewer than a dozen, relatively long ranging globigerine species in two genera. Biotope stretched along sub-Tethyan and Tethyan continental margins from eastern N. America to W. Australia. Biomass was generally low. Building material of tests is considered to be high-magnesium carbonate (aragonite), but few sites show original aragonitic tests.

Post-Jurassic taxa use low-magnesium carbonate (calcite) for test building. Reasons for this chemical-physical change in building material are not well understood, but may relate to the fact that seawater changed its Mg/Ca mole ratio during early Cretaceous from higher ratio values (2-4) to lower ratio (1-2) values. Aragonite and high-Mg calcite would be the minerals of choice for many carbonate secreters when Mg/Ca in seawater is high (most or all of Jurassic),

Kimmeridgian is the time Jurassic planktonic foraminifera flowered. A dramatic change-over, not well documented and not understood led to the survival of only one taxon in Tithonian time. During Berriasian time six new taxa appeared. Jurassic taxa did not cross the periods boundary. This strongly contrasts with dinoflagellate and nannofossil species diversity trends which show a steady expansion in number of taxa, with only a minor decline for dinoflagelates in earliest Cretaceous. The latter may be due eustatic low sealevel, diminishing coastal niches.

An explosion in productivity in calcareous (nanno-) plankton during the Tithonian, especially the robust nannoconid types, contributed to lowering of the CCD and onset of chalk (‘creta’) deposits that characterize the Lower Cretaceous in all ocean basins. A decrease in pCO₂ during the Tithonian may have been a factor in this evolutionary surge of calcified plankton. It is not understood if this also led to the demise of the (? all aragonitic) Jurassic planktonic foraminifera.