Geochemical and micropaleontological characterisation of a Mediterranean sapropel S5: A case study from core BAN89GC09 (south of Crete)

Abstract

Several geochemical and micropaleontological proxies were studied in a well developed sapropel S5 from a core collected south of Crete to investigate the causes and the mechanisms of its deposition, to reconstruct the paleoenvironmental and paleoceanographical conditions at time of its formation and to detect the roles played by productivity and anoxia. The geochemical proxies are also used to unravel the sediment provenance and therefore to help tracing water circulation patterns and freshwater sources. The multiproxy approach reveals that the sapropel layer has high internal variability and five different depositional phases can be identified, four within the visible sapropel layer. An increase in temperature and the development of a Deep Chlorophyll Maximum characterise the onset of the sapropel (phase 1), with enhanced productivity being favoured by local riverine input, not clearly ascribable to Nile river freshwater discharge, and by the shoaling of the pycnocline/nutricline. Surface water stratification and productivity develop, together with strong seafloor dysoxia/anoxia. Phase 2 is characterised by lower stratification, slightly lower temperature and/or higher seasonal contrast, and still high productivity. Phase 3 records high productivity, high temperature and high stratification, probably more sustained by the Nile input. A sudden partial re-oxygenation takes place at the end of this phase, changing the main features of the sapropel. In the fourth phase, productivity starts to decrease, water stratification seems to be partially broken down and sediment oxygenation increases. A transitional phase (phase 5) occurs between the end of the visible sapropel and the normal pelagic post-sapropel sedimentation, where productivity is still high and oxygenation improves compared to the visible sapropel layer. The massive occurrence of Polysphaeridium zoharyi, cyst of a red tide forming dinoflagellate, also characterises this phase. The geochemical proxies indicate that pre-sapropel and post-sapropel sediments are quite different from each other, and the provenance shifts from a more mixed to a more markedly southern origin of sediments.