Calcareous nannofossil palaeoenvironmental reconstruction and preservation in sapropel S1 at the Eratosthenes Seamount (Eastern Mediterranean)

Abstract

The most recent organic carbon-enriched layer (sapropel S1) deposited at the Eratosthenes Seamount has unique features, such as an early lithological interruption, fine light silt laminae and an exceptional vertical extent that is over 25 cm thick. Here we investigate calcareous nannofossil assemblages to reconstruct very high-resolution palaeoenvironmental and palaeoceanographic variations recorded before, during and after the perturbation episode that involved the eastern Mediterranean Sea, due to the massive freshwater discharge via Nile River. Our results show that the deep chlorophyll maximum development, observed in all micropalaeontological groups from previous studies, is a gradual process that started well before the base of sapropel S1. A high-frequency variability in the nutricline depth is evident at millennial- and/or centennial-scale throughout the sapropel deposition time interval. Also we highlight the poor-preservation of delicate tiny holococcolith crystals while anoxia was occurring at the seafloor and we suggest that such a phenomenon may be used to mark the original thickness of sapropel deposition where oxygen re-ventilation fronts were developed. Finally, calcareous nannofossil reworking peaks shed light on the nature of fine silt laminae within the sapropel S1 at the Eratosthenes Seamount, which may be ascribed to fine sediment plumes from the Nile River deposited during exceptional runoff events.