Nannoplankton productivity fluctuations and rhythmically-bedded pelagic carbonates of the greenhorn limestone (upper cretaceous)

Abstract

Comparison of calcareous nannofossil assemblages from rhythmically-interbedded calcareous shales and adjacent bioturbated chalks of the Greenhorn Limestone (Cenomanian-Turonian) of the Western Interior Basin indicate significant changes in phytoplankton paleocommunity structure between lithotypes. Significantly higher Shannon diversity and evenness values in the chalk assemblages indicate more stable surface water conditions with lower fertility. Three taxa indicative of high surface water fertility are significantly more abundant in marlstone assemblages which suggests that calcareous shale/marlstone deposition corresponded with fluctuating surface water conditions with relatively high fertility. The higher surface fertility and concomitant production of organic matter resulted in sediment anoxia during marlstone deposition, leading to high rates of organic carbon storage. Reduced phytoplankton production during chalk deposition allowed oxygenation of the sea floor and establishment of diverse benthic invertebrate communities.