During Leg 161 a transect of six sites was drilled in the western Mediterranean with the purpose of reconstructing the paleoceanographic and tectonic history of this key area of the basin (Fig. 1). The paleoceanographic target concentrated on the Atlantic-Mediterranean water exchange with a focus on periods of sapropel formation. Sapropels are dark, organic-rich (>2% TOC) layers well known in the eastern Mediterranean (Olausson, 1961; Thunell et al., 1977; Kidd et al., 1978; Cita and Grignani, 1982). Timing of sapropel deposition results independently of climatic conditions (glacial or interglacial intervals) and has been related to astronomical parameters (Hilgen, 1991; Rossignol-Strick, 1985; Lourens et al., 1996). Their occurrence has been interpreted as a response to oceanographic and biogeochemical change: water column stratification leading to bottom water stagnation and/or enhanced productivity (Vergnaud-Grazzini et al., 1977; Thunell et al., 1977; Rossignol-Strick, 1983; Calvert, 1983). Recent micropaleontological (planktonic and benthic assemblages) and geochemical (oxygen and carbon-stable isotope) investigations have shown that, at least during the late Quaternary, organic-rich layers were deposited in the Western Mediterranean Sea (Muerdter 1984; Thunell et al. 1990; Emeis et al., 1991).
Magnetic parameters are extremely sensitive to the anoxic conditions such as those existing at the time of sapropel formation. Several studies show that under suboxic-anoxic environments the deep-sea sediments undergo early diagenesis and lose their original pattern of rock magnetic parameters. Vigliotti (1997) has shown that sapropel-like organic-rich layers recovered in the Japan Sea are characterized by distinctive features such as decrease in concentration of ferrimagnetic minerals, increase in magnetic grain size and coercivity of the remanence, and changes in the magnetic mineralogy with authigenic formation of iron sulfides, such as pyrrothite and/or greigite. Therefore, these layers yield distinctive magnetic features that can be easily identified in magnetic properties profiles.
The aim of this study is to reconstruct the paleoceanographic history of the western Mediterranean during the late Quaternary by comparing the magnetic properties of the sediments with the paleoclimatic record indicated by microfossil analysis. Particular attention will be given to the possible occurrence of the sapropel layers in the westernmost part of the Mediterranean Sea. Rock-magnetic properties and planktonic foraminifer distribution are effective tools for reconstructing the paleoenvironmental record of deep-sea sediments and should be able to test the environmental conditions leading to the sapropel deposition.
A growing number of studies show that variations in the magnetic minerals within deep-sea sediments can be related to paleoceanographic and paleoclimatic changes, such as river influx, variations in carbonate content, the source of the detrital input, sea-level changes, aeolian dust concentration, and oxic-anoxic conditions (Kent 1982; Oldfield and Robinson, 1985; Robinson, 1986; Bloemendal et al., 1988; Doh et al., 1988; Robinson et al., 1995; Vigliotti, 1997). Changes in the concentration, mineralogy, and grain size of magnetic mineral assemblages reflect climatically induced variations within the lithogenic fraction of the sediment.
Planktonic foraminiferal assemblages were analyzed quantitatively, used to characterize variations in surface circulation and sea-surface temperature in the western Mediterranean, and compared with variations in global climate inferred from oxygen isotopes. A combination of proxies, such as magnetic parameters and foraminiferal assemblages, is thus an effective tool for reconstructing the paleoenviromental record of deep-sea sediments.