The Barremian–Aptian turnover of biota in northwestern Europe: evidence from belemnites

Abstract

`Neocomian' (Valanginian–Barremian) and `Gault' (Aptian–Albian) are lithostratigraphic terms used in the last century to divide the sub-Chalk Cretaceous of northern Europe. The division is based on significant palaeoceanographic and palaeogeographic changes in the early Aptian. These changes are linked to a major turnover of marine floras and faunas, which occurred simultaneously in the Boreal Realm, the Tethys and the Indo-Pacific Ocean. Various floral and faunal elements became extinct in the Aptian both in the Tethys and in the Boreal Realm, while on the other hand new genera and species evolved. Marked changes in the diversification rate were observed for organisms with a different mode of life (plankton, nekton, benthos). Apart from calcareous nannofossils, foraminifera and ammonites, this turnover also occurs in belemnites, and the suborder Belemnitina was completely replaced by the suborder Belemnopseina. The northwestern European Barremian is characterised by the very successful belemnite family Oxyteuthididae (Belemnitina), which was completely replaced by the genus Neohibolites (Belemnopseina) in early Aptian times. This Aptian turnover, which is expressed by a homogenisation of marine floras and faunas, is best explained by significant palaeoceanographic changes occurring in the early Aptian. The sea-floor spreading rate, which increased at the Barremian–Aptian boundary significantly, peaked in the late early Aptian. A mantle plume in the Pacific possibly caused an additional production of oceanic crust. Both processes caused a major sea-level rise. This in turn resulted in a major change of the palaeoceanographic setting and restricted marginal seas, like the northwestern European basins in Barremian times, became drowned. The more open oceanic setting of the Aptian enabled cosmopolitan floras and faunas to spread worldwide. The Aptian saw the radiation of planktonic foraminifera, which played only a minor role in pre-Aptian times. These palaeoceanographic changes, from restricted marginal seas to a more homogeneous pelagic setting, are considered to be the main cause for the significant changes in the composition of belemnite faunas.