Bryozoa, foraminifera and calcareous nannoplankton as environmental proxies of the “bryozoan event” in the Middle Miocene of the Central Paratethys (Czech Republic)

Abstract

In order to improve the reliability of using bryozoans as an indicator of the palaeoenvironment, a detailed study of the ancient environmental parameters of the Middle Miocene bryozoan event was carried out using foraminifera, calcareous nannoplankton and oxygen and carbon stable isotope data. Abundant occurrence of Bryozoa in the Middle Miocene of the Central Paratethys was an isochronous event which can be correlated with M6 Zone between FO of Orbulina suturalis and LO of Praeorbulina circularis (14.8–14.58 Ma). The ecosystems of the Central Paratethys marine basins during this time were generally influenced by the Middle Miocene climatic optimum and the large transgression which covered the Central Paratethys basins with tropical–subtropical waters. These events triggered the expansion of Bryozoa which settled in the narrow shallow-water zone of the basin and were accompanied by epiphytic foraminifera such as Asterigerinata, Lobatula and Elphidium and large foraminifera such as Amphistegina, rarely cibicidoids. Although foraminiferal assemblages show only small palaeoenvironmental variations, bryozoan species compositions remarkably vary according to palaeoenvironments. In this study, four clusters with differing bryozoan assemblages can be distinguished in shallow-water environment as follows: 1) Reteporella– Hornera verrucosa assemblage, indicative of a high-energy environment, 2) Buffonellodes– Rhynchozoon assemblage, occurred on carbonate substratum with seagrass meadows influenced by the Middle Miocene warm-water incursion. 3) Smittina– Metrarabdotos assemblage, indicative of toleration of suboxic zone in sediment. 4) Schizomavella tenella– Schizoporella tetragona assemblage, recorded almost in all shallow-water environments. This cluster probably represents species with high adaptation potential. Exceptionally deeper-water conditions were inferred at one section, and represented by a Tervia irregularis dominated assemblage. This assemblage was only present for a short period of time.