Late Turonian ostracod assemblages record a shift from mesotrophic to oligtrophic hemipelagic deposits in the Bohemian Cretaceous Basin (Czech Republic)
Abstract
The late Turonian marine benthic ostracods have been identified in the hemipelagic marlstones and limestones of the Bohemian Cretaceous Basin situated between the Boreal and Tethyan Realms. Cytherella cf. concava, C. cf. ovata, Cytherelloidea hindei and C. stricta (all Platycopida and survivors of the upper Cenomanian OAE2 low-oxygen level) were dominant in the monotonous poorly fossiliferous marlstone of the Jizera Formation.
The cooling associated with cool water shift to the south and marked by the Hyphantoceras Event enabled the spread of the diverse Boreal faunae and the ostracod fauna of the Teplice Formation, which highly resembles the British Islands ostracod associations. Their biodiversity increased in mesotrophic conditions and the maximum was attained in an oligotrophic system depleted in nutrition.
The ratio of Platycopida/Podocopida changed from 9:1 in hypoxic dark marlstone to 1:2 in times of supposed oligotrophic, well aerated conditions in the grayish and dark marlstones deposited in the period of maximum flooding surface. Although the dinoflagellates and coccolithophorids of appropriate size were very common to abundant for filter feeder platycopids, they were replaced upwards by deposit feeder podocopids.