Meteor 40/4, Site 78 (Strait of Sicily, Mediterranean Sea): AMS 14C ages, carbon concentrations and alkenone sea surface temperature estimates of a multicore and a gravity core

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. Multicore samples were sliced in 1cm samples and frozen on board.

Dating on 4 samples from the gravity core was performed on tests of G. ruber picked from the coarse residues of sieved subsamples and submitted to the Leibniz Labor für Altersbestimmung und Isotopenforschung, Kiel University. The remaining bulk sample was homogenized by mortar and pestle

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 x 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 synthetic 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 = 6) are also given.

From the gravity core, a second subsample was used to determine concentrations of total carbon and inorganic carbon. The total carbon contents were determined by combustion at 1200°C under oxygen and detection of CO2 in an infrared detector using a Metalyt CS 100/1000S and a Heraeus elemental analyzer. Inorganic carbon from carbonates was determined in the same instrument by acidifying subsamples with 17% phosphoric acid under a helium stream and detecting evolving CO2. Organic carbon was assumed to be the difference between total and inorganic carbon.