INTRODUCTION

The occurrence of organic-rich sediments (sapropels) is a common feature of sediment sequences throughout Pleistocene, Pliocene and Miocene times of the eastern Mediterranean (e.g. Kullenberg, 1952; Olausson, 1961; Cita et al., 1977; Vergnaud-Grazzini et al., 1977; Williams and Thunell, 1979; Rossignol-Strick et al., 1982; Fontugne and Calvert, 1992, and many others more). During ODP Leg 107, distinct sapropel intervals of upper Pliocene to Pleistocene age were recovered in the Tyrrhenian Sea. During Leg 161, sapropels and organic rich layers have

The occurrence of organic-rich sediments (sapropels) is a common feature of sediment sequences throughout Pleistocene, Pliocene, and Miocene times of the eastern Mediterranean (e.g., Kullenberg, 1952; Olausson, 1961; Cita et al., 1977; Vergnaud-Grazzini et al., 1977; Williams and Thunell, 1979; Rossignol-Strick et al., 1982; Fontugne and Calvert, 1992; and others). During Ocean Drilling Program (ODP) Leg 107, distinct sapropel intervals of upper Pliocene to Pleistocene age were recovered in the Tyrrhenian Sea. During ODP Leg 161, sapropels and organic-rich layers have been discovered in the Balearic and Alboran Seas (Murat, Chap. 41, this volume; de Kaenel et al., Chap. 13, this volume) which makes a re-evaluation of possible mechanisms of sapropel formation necessary.

Sapropels are defined to be discrete finely laminated (Nesteroff, 1973) layers, more than 1 cm thick, and containing more than 2 wt% organic carbon (Kidd et al., 1978). Hilgen (1991) proposed a concept that explains the formation of sapropels in view of orbitally driven variations in northern hemisphere insolation. According to this concept, sapropels correlate with periods of minimum orbital precession, which are coeval with maximum northern hemisphere insolation. This concept was later confirmed and confined by other workers (e.g., Lourens et al., 1996). Sapropels in this conceptual model are defined as sedimentary units containing higher than background organic carbon concentrations. For Leg 161 we followed this definition to determine the occurrence of sapropels or organic-rich layers (ORL; Murat, Chap. 41, this volume) at the western Mediterranean drill sites.

To determine environmental conditions during sapropel formation requires reconstruction of hydrographic parameters: ambient surface-water paleotemperature and paleosalinity before, during, and after sapropel formation. As demonstrated by Rostek et al. (1993), sea-surface temperature (SST) and paleosalinity can be estimated by using the combined planktonic foraminiferal oxygen isotope and alkenone U^k´₃₇ signals. Reliable application of this strategy, however, requires knowledge about growth season and depth habitats for both planktonic foraminifers used for isotope analysis and of phytoplankton species used for alkenone measurements. For instance, we chose planktonic foraminifer Globigerina bulloides for isotope analysis, which occurs commonly in glacial and interglacial sections of sediment cores in the western Mediterranean Sea. This species has been shown to preferentially grow in spring (April-May; Kallel et al., 1997) at water depths between 50 and 200 m (Hemleben and Spindler, 1983). The primary alkenone-producing prymnesiophyte Emiliania huxleyi, on the other hand, preferentially dwells in the upper 20-50 m of the mixed layer and blooms from March to November (Knappertsbusch, 1993). Therefore, ¹⁸O derived from G. bulloides and SST inferred from U^k´₃₇ index of E. huxleyi both combine hydrographic signals from different seasons and different water depths. This does limit the accuracy of estimated paleosalinity, which must be kept in mind when reconstructing ocean conditions during discrete time intervals. Furthermore, E. huxleyi, which is believed to be the main producer of alkenones in the modern ocean (Volkman et al., 1980; Marlowe et al., 1984) and has been used for calibration of the alkenone paleotemperature equation (Prahl and Wakeham, 1987; Prahl et al., 1988; Ternois et al., 1997), first appears in the geologic record at (268 ka; Thierstein et al., 1977). For earlier Pleistocene periods, alkenones most probably were derived from the prymnesiophyte genus Gephyrocapsa (Marlowe et al., 1990), which, according to Volkman et al. (1995) shows a temperature sensitive U^k´₃₇ pattern similar to that of E. huxleyi. Whereas use of the U^k´₃₇ index to infer SST is thus not well constrained for the period before 268 ka, it can still be used to indicate general trends of SST changes.

Postdepositional oxidation of organic carbon poses additional limits on the use of U^k´₃₇ indices as SST indicators. Oxidation of organic matter has been demonstrated to affect the distribution of long-chain alkenones by preferential degradation of C_37:3 alkenones relative to C_37:2 alkenones (Ficken and Farrimond, 1995; Flügge, 1997; Hoefs et al., 1998). If concentrations of C_37:2 alkenones are high, as in the Madeira Abyssal Plain turbidites, degradation does not change the U^k´₃₇ index significantly across the redox boundary (Hoefs et al., 1998). C_37:3 alkenone concentrations are expected to be low in sediments from the Mediterranean because this compound tends to be depleted relative to C_37:2 alkenones in warm conditions. In such cases, the U^k´₃₇ index is largely determined by the abundance of C_37:2 alkenones. Therefore, preferential degradation of C_37:3 alkenones are of only minor importance for the U^k´₃₇ index.

In this study we use the U^k´₃₇ index at Sites 974 and 975 to reconstruct SST during periods of sapropel formation. To further evaluate hydrographic conditions we use planktonic ¹⁸O in combination with the U^k´₃₇-SST estimates to infer paleosalinity during these events.

Site 975 is located on the South Balearic Margin between Mallorca and Menorca and the South-Balearic-Algerian-Basin. Characteristic circulation features are the Algerian Current and recirculated water that consists of water masses from the Balearic Sea. The Algerian Current is characterized by the occurrence of meso-scale anticyclonic eddies (diameter 100 km) that slowly move along the current axis (Deschamps et al., 1984). The presence of eddies force the Atlantic inflow (Modified Atlantic Water [MAW]) along a flow path close to the coast until near 0°E, where upon it gradually moves more off-shore. Eddies developing near the coast generate upwelling cells that are biologically productive and are advected away from the coast (Arnone and La Violette, 1986). The hydrography of the Algerian Basin is largely defined by the dominance of Atlantic waters and, as such, constitutes a throughflow zone of Atlantic water moving east and north, thereby maintaining a reservoir of Atlantic water (Millot, 1987). The North Balearic front is defined as the northern boundary of this reservoir and closely delineates the extent of the Atlantic layer. Site 975 is within the MAW reservoir south of the Balearic Front.

Site 974 is also influenced by MAW, of which one-third is circulated by the Algerian current into the Tyrrhenian Sea. The Tyrrhenian Sea has two connections to the open Mediterranean, the Corsica Channel (sill depth about 350 m) to the north and the Sardinia Channel (sill depth about 2000 m) to the south, that enable an advection of water masses from the entire western Mediterranean (Astraldi and Gasparini, 1995). Surface waters at Site 974 are further influenced by two seasonally variable gyres. During winter and spring, a cyclonic eddy develops northeast of Corsica, and an anticyclonic eddy develops south off Sardinia. In summer and fall, the northern gyre becomes dominant, and the southern gyre moves further to the south, responding to an intensification of the northern gyre. This gyre circulation is stimulated by annually prevailing west winds in the Tyrrhenian Sea that also promote coastal and mid-gyre upwelling (Astraldi and Gasparini, 1995).

Surface salinity in the Tyrrhenian Sea is low because of the dominant influence of MAW advected by the Algerian Current. Typical MAW characteristics are clear seasonal changes and large horizontal gradients. Salinity values range from 36.8 for the newly incoming water, to 38.2-38.4 in the central parts of the northern and southern gyres, which are influenced by intensive mixing with saline subsurface water (Astraldi and Gasparini, 1995). Along with mid-gyre upwelling, temperature anomalies of 6°C (seasonal range of 13°-16°C in the central gyre opposed to 14°-22° outside the gyre; Marullo et al., 1995) are observed.

The hydrography at the Balearic Rise and in the Tyrrhenian Sea is thus strongly influenced by the advection of Atlantic waters and associated changes in temperature and salinity. Upwelling of intermediate waters that come from eastern Mediterranean sources additionally control surface water conditions. The range of hydrographic change that goes along with this variability must be kept in mind when interpreting estimates of paleotemperature and salinity at both sites.