Paleoenvironmental interpretation of core BAN88-11GC (Eastern Mediterranean, Pleistocene-Holocene) on the grounds of foraminifera, Thecosomata and calcareous nannofossils

Abstract

This paper deals with the quantitative variations of planktonic Foraminifera, Thecosomata and calcareous nannofossils from Core BAN88-11GC, recovered in the Eastern Mediterranean during the Cruise Bannock 1988. Sediments are Middle Pleistocene to Holocene in age, spanning about the fast 250,000 years. Sapropels S-1 to S-8 (only S-4 is missing) and two tephra layers (Y-5, X-2) are also recorded. Calcareous nannofossils provided the biostratigraphic framework: three biozones (Gephyrocapsa oceanica and Emiliania huxleyi Zones, Emiliania huxleyi Acme Zone) were recognized and correlated to five climatic zones (V, W, X, Y, and Z). Two climatic curves were constructed based on the relative abundance of warm and cold species of Foraminifera and Thecosomata; they are well correlated and point out four warm and three cold intervals. Measurements of maximum diameter of Orbulina universa show that larger sizes of this taxon well correlate with positive peaks of the climatic foraminiferal curve, confirming their paleoclimatic significance. Statistical analyses of planktonic Foraminifera were carried out: Factor 1 and Principal Component 1 can be related to water temperature. The Cluster Analysis shows four groups of species (warm, temperate/warm, temperate, cold species). Globigerinoides gomitulus seems a useful temperate or temperate/warm water indicator. Neogloboquadrina dutertrei, previously considered as low-salinity indicator, seems better related to enhanced circulation and/or high productivity levels. The Pseudothecosomata Gleba cordata and the Gymnosomata Paedoclione doliiformis are more frequent during some temperate/warm intervals. The tephra deposition seems to affect the Thecosomata assemblages. On the basis of the quantitative study on calcareous nannofossils, it was possible to point out significant variations either in the dominant species (E. huxleyi, G. oceanica, G. caribbeanica and "small" Gephyrocapsa) or in other taxa, usually less abundant but showing peculiar behaviour (Helicosphaera carted and Syracosphaera sp. 1). A comparison between the planktonic Foraminifera and Thecosomata climatic curves and the distribution patterns of dominant nannofossil taxa allowed us to infer that, at least in some cases, temperature controlled the percentage variations of the latter species. Moreover, H. carted clearly increases in abundance whereas Syracosphaera sp. 1 sharply decreases or even disappears within all sapropels. Abundance peaks of H. carted, although less striking, are also present in the tephra layers. Such a peculiar behaviour might be explained with turbidity increase in surface waters, due to the Nile runoff in the case of sapropels and to volcanic ash in the case of tephra.