Water column stratification increased at climatic transitions from cold to warm periods during the late Quaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. Here we report downcore sea surface temperature estimates of gravity core M40-4/67 taken in the region of the Rhodes Gyre (eastern Mediterranean Sea) that elucidate the influence of glacial/interglacial temperature changes in determining density of surface waters and formation of sapropels.
Methods:Samples were obtained from cores taken aboard r/v Meteor either during the expedition or in the Core Repository of the Institute of Baltic Sea Research. Samples were punched out of the working halves with sawn-off syringes, wrapped in plastic, and stored frozen until freeze-drying in the shore-based laboratory.Subsamples (~1-2 g dry weight) of the homogenized bulk samples were used to extract lipids and to estimate the alkenone unsaturation index, a SST indicator (Brassell et al., 1986). Weighted splits of freeze-dried and homogenized sediments were extracted by ultrasonic agitation with 35-mL CH2Cl2 (2 ' 10 min). Elemental sulfur was removed by the addition of copper foil during the extraction. After each extraction step, samples were centrifuged, and the solvent was collected by pipette. The two lipid extracts were combined and dried in a rotary evaporator. The extracts were redissolved in CH2Cl2 and precleaned over a silica gel column (conditioned with 30-mL CH2Cl2) by elution with 14-mL CH2Cl2 and dried again in a rotary evaporator. The extracts were redissolved in n-hexane and subsequently fractionated using high-pressure liquid chromatography (HPLC). The HPLC column, packed with silica gel was MERCK LiChrospher Si 100-5. Four fractions were obtained using 5.5-mL n-hexane, 14-mL n-hexane/dichloromethane, 9-mL dichloromethane, and 9-mL acetone as eluents. The ketones (here: alkenones) were isolated in the second fraction. The first fraction contained the aliphatic hydrocarbons, and the third and fourth fractions yielded the more polar compounds.Gas chromatographic (GC) analyses were carried out on a HR 8000 Fisons gas chromatograph (FID, cold on-column injection) equipped with a 30-m (0.32 mm ID) glass capillary column (DB5HT). Helium was used as a carrier gas (column head pressure 110 kPa). Oven temperature programming conditions were from 35°C to 300°C at 15°C min-1 followed by an isothermal period of 15 min at 300°C and from 300°C to 330°C at 15°C min-1 followed by an isothermal period of 10 min at 330°C. Alkenones were identified by comparing retention times with those of standards. Peak areas were converted to the UK'37 index, and SSTs calculated according to a global coretop calibration (Müller et al., 1998): SST (°C) = (C37:2 / (C37:2 + C37:3) - 0.044) / 0.033. Alkenone concentrations were determined from GC analyses of the ketone fractions based on peak responses relative to 5alpha-cholestane as an external standard. Results of duplicate analyses (n = 10) are also given.
