Sequential iron extraction, pore water and solid phase analyses for sediment cores from cruises M157 and MSM105

Fe2+ was analysed photometrically (Hach Lange DR 5000 photometer) at 565 nm following the method of Collins et al. (1959). 1 mL of acidified sample (20 L of 1% ascorbic acid) was added to 50 μL of a ferrospectral reagent (Merck Chemicals) in disposable polystyrene cuvettes (Stookey, 1970). Samples with high Fe2+ concentrations were diluted with oxygen-free artificial seawater. Total dissolved hydrogen sulfide (H2S) was measured photometrically using the methylene blue method in samples that were fixed on board with ZnAc (Cline, 1969). Dissolved silicon (Si4+) was determined in samples acidified by suprapure concentrated HNO3 (10 μl per ml of sample) by inductively coupled plasma optical emission spectroscopy (ICP-OES; axial plasma observation; Agilent 720) with a precision of at least 2%. Total organic carbon (TOC) was determined by using an element analyzer (Euro EA, HEKAtech). Total iron (Fe), potassium (K) and aluminium (Al) concentrations in the solid phase were determined by ICP-OES following digestion in nitric acid, hydrofluoric acid and perchloric acid. A combination of the multi-step sequential Fe extraction methods by Poulton and Canfield (2005) and Huerta-Diaz and Morse (1990) was applied to determine sedimentary Fe speciation including pyrite.