The Aptian - Albian cold snap: Evidence for mid Cretaceous icehouse interludes

Variations in calcareous nannofossil assemblages from both low and high palaeolatitudes of the late Aptian - early Albian Suggest a global cooling. Significant shifts include a decline of Tethyan taxa and a subsequent biogeographic expansion of species of high latitudinal affinities. This cooling is Supported by sedimentological observations (glendonites, ice rafted debris) and oxygen isotope data. The first phase of decreasing surface water temperatures is indicated by a marked decline of Tethyan nannoconids in the late Aptian at both high and low latitudinal sites. This is paralleled by an increase of the cool water nannofossil species Crucibiscutum salebrosum and a biogeographic expansion of the high latitudinal nannofossil taxon Repagulum parvidentatum. The biogeographic expansion of high latitudinal nannofossil cold water belts in the northern and southern hemisphere is followed by the first common Occurrence of diatoms in the high latitudes. This onset of siliceous phytoplankton marks the second phase of the cooling. Diatoms Occur simultaneously both in the high latitudes of the northern and southern hemisphere, suggesting a global climatic cooling. This scenario requires flow and exchange Of Surface waters from the high latitudes. The Equatorial Atlantic Seaway between Africa and South America came into existence only in the late Aptian - Albian allowing for all exchange of biota. If the scenario of cold polar regions in the late Aptian - early Albian is valid, this Must have had also consequences for the formation and flow of cold oceanic deep waters. Our findings Suggest a much more complex climatic history of the 30 Ma lasting Aptian - Turonian greenhouse period than previously thought. This period was obviously punctured by at least one icehouse interlude in the late Aptian - early Albian interval, which effected the evolution of, primary producers severely.