Palaeo-oceanography of the post-glacial Eastern Mediterranean

Abstract

DURING the late Pleistocene the hydrography of the Eastern Mediterranean underwent severe changes in response to climatic fluctuations. Among the most striking of these changes were periods of deep water stagnation. Episodes of oxygen depletion are indicated by the presence of layers of dark sapropelitic muds enriched in organic carbon, devoid of the remains of benthic organisms^1–4. While stagnant conditions prevailed in the deep basin, the water column above 1,000–800 m remained ventilated, and cores raised above this depth do not contain sapropels^4–6. Several theories have been proposed to explain the past stagnations. They all use a mechanism which stabilises the water column by forming a steep density gradient in the upper waters. In this way the vertical mixing, and hence the ventilation, are retarded, oxygen is depleted in the deep water and stagnation commences. Olauson's theory⁷, whereby influx of freshwater leads to reduced surface density and the interruption of normal vertical circulation, is the most widely accepted at present. This theory is supported by recent studies^8–10, which use measurements of oxygen isotope composition in shells of planktonic foraminifera. These studies demonstrate that many of the sapropels correspond to δ¹⁸O minima, suggesting freshwater influx, and thus a reduced upper water salinity at the time of the sapropel formation. Moreover, the epipelagic species record greater depletion of ¹⁸O than the mesopelagic species, indicative of the establishment of salinity and density gradient in the upper waters¹⁰. Using a similar approach we have attempted to investigate in detail the hydrographie changes associated with the deposition of the most recent sapropel, which took place about 7,000–9,000 yr BP⁶.