The relative contribution and the composition of terrestrial organic matter to sediments of the Gotland Basin/Central Baltic Sea were assessed by the analysis of phenolic lignin oxidation products and the delta13C of organic matter. Samples were taken from box core 20048-1 and Multicore 20007-1 taken at the same location.
Methods: Core retrieval and sample processing: A 10.30m long giant kasten core (KaL#20048-1) was recovered (57°23.14'N, 020°15.51'E; water depth 241 m) by r/v Poseidon (1995) and a multicore (MUC#20007-1) at the same position by r/v Alexander von Humboldt in 1994. The kasten core was opened and logged on board, and the entire kasten core was archived in plastic boxes. The multicore was sliced into 1 cm slices, which were frozen on board and freeze-dried in the home laboratory in Warnemuende.
For lignin analyses, 1-cm sub-samples were taken by sawn-off plastic syringes from archived material of box core 20048-1 in intervals of about 10 cm and air dried at 40°C. Subsamples of sediments from the multicore 20007-1 were taken from dried slices.
Lignin analysis: Between 500 to 2000 mg of dried and homogenised sediment were oxidized at 170°C for 2 h in the presence of 2 mol/L NaOH, CuO, and (NH)4Fe(SO4)2. After centrifugation, the supernatants were acidified to pH 2 with 6 mol/L HCl. The humic acids, which precipitated, were removed by centrifugation. The supernatant was further purified by solid phase extraction. The lignin-derived phenols were sorbed from the acidic solution on C18 material and later eluted with ethyl acetate. The solvent was removed by rotary evaporation, and the phenolic oxidation products were transferred to autosampler vials with methanol that was then removed under a flux of N2. Before analysis by GC/MS, the samples were dissolved in acetonitrile and derivatized with N,O-bis-(trimethylsilyl)trifluoroacetamid (BSTFA) for 1 h at room temperature. Thereafter, they were diluted with acetonitrile according to the expected phenol concentrations. One microliter of each sample was injected in splitless mode, and the phenols were separated in a HP 6890 gas chromatograph equipped with a HP5MS column (30 m x 250 micrometer x 0.25 micrometer). The temperature program of the gas chromatograph was 100°C isothermal for 4 min, ramp to 220°C at 4°C min⁻¹ with a 5-min isothermal period at 120°C, isothermal at 220°C for 3 min, ramp to 300°C at 30°C min⁻¹, and final isothermal period for 10 min. The transfer line to the mass spectrometer was kept at 325°C throughout the analysis. The HP 5973 mass spectrometer was operated in the EI mode at 70 eV. The ion source temperature was 230°C, and the quadrupole was kept at 150°C. Compounds were quantified by integration of the base ions and by comparison of the peak areas with those of synthetic standards. Before oxidation, ethylvanillin was added as an internal standard for the determination of recovery. To rule out possible transformations of the internal standard during the oxidation step, blanks containing only ethylvanillin and the reagents were also processed. GC-FID analysis of these blanks displayed a single peak with the retention time of ethylvanillin, and there was no evidence of any transformation of ethylvanillin during the oxidation step under the experimental conditions. The internal standard was added at the beginning of the analysis to ensure that the internal standard and the lignin oxidation products have the same history during the entire analysis. On average, 75% of the added ethylvanillin was recovered after the complete analytical procedure; the range of recoveries was from 50% to 105%.
