Benthic environmental changes in SE Aegean Sea during the last 26 ky BP: preliminary results

Foraminifera (single-celled protists that secrete a shell-like test) are among the most abundant organisms in the deep sea (the largest habitat on Earth), and are recognized to be highly sensitive to environmental changes due to both natural and man-induced factors in marine and transitional environments. In particular, the potential of benthic foraminifera has long been recognized for their use in marine paleoenvironmental studies.
   The present study focuses on a high resolution analysis of the distribution of benthic foraminifera from one SE Aegean Sea core. The main aim is to describe the impact of the environmental changes on the marine ecosystem through the study of proxies related to the benthic environment. With this goal in mind, one site was selected to investigate spatial and temporal variability as recorded by the benthic microfauna.
   Detailed analysis of the benthic foraminiferal content of the core M22-18 in NE Cretan Sea, allowed its palaeoenvironmental reconstruction. The core, 270 cm long, was drilled at 360 m water depth and 39 samples (1 cm thick) were taken. Each sample was washed, sieved at 125 μm and then dried at 60^oC. Quantitative analysis was carried out on aliquots separated from each sample by means of a microsplitter, in order to obtain at least 250 – 300 benthic foraminiferal specimens. The number of planktonic foraminiferal tests was also recorded during picking. Based on the faunal counts, benthic foraminiferal numbers (BFN; number of specimens per gram dry sediment) were calculated. This number gives information on the taphonomy of the original living assemblage, the oxygen level, the energy level in which the sediments were deposited and to a minor extent the productivity or organic flux. The percentage of planktonic species in the total foraminiferal association (%P) was calculated as 100*P/(P+B). Raw data were transformed into percentages over the total abundance of benthic foraminifera. Reconstruction of bottom water conditions concerning oxygen content was based on the presence of the dysoxic indicators in the assemblage. For this purpose, the percentage occurrence of the well established redox fauna front dwelling taxa (Bulimina, Uvigerina, Fursenkoina, Globobulimina spp. and Bolivina spathulata/dilatata) which is related to disturbance and/or environmental stress was calculated. Two radiocarbon ages indicate that the studied sequence covers the last 26 ky BP.
   BFN remains relatively stable exhibiting low values apart from the basal part of the record where BFN shows an abrupt increase. Planktonic/benthic ratios vary between 16 and 95% of the benthic assemblage.
   A decrease in BFN and an increase of low oxygen indicators (infaunal taxa) suggest a strong decrease in oxygen concentrations. Poor ventilation created dysoxic condition allowing the presence of stressed benthic microfauna.
   The group of low-oxygen taxa shows high percentages from the bottom to 240 cm, but its abundance strongly decreases between 240 and 40 cm. At 30 cm this group increases again and then remains in constant percentage values in the upper part of the core. The strong decrease of BFN and the increase of the benthic foraminifera deep infauna in the lower part of the core suggests extremely low oxygen values on the sea bottom.