Porewater analyses of sediment core M157_41-03

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. PO43- concentrations were determined photometrically using the molybdenum blue method (Grasshoff et al. 1999) and a Hach Lange DR 5000 photometer. Highly sulfidic samples were spiked with 20 μL of 30% HCl and bubbled with argon for one minute to limit sulfide interference for PO43- analysis. NH4+ was measured by flow injection using a PTFE tape gas separator technique after Hall and Aller (1992). The precisions of the Fe2+, PO43- and NH4+ analyses were 1%, 7% and 6% respectively. Sulfate (SO42-), fluorine (F-) and bromine (Br-) was determined by ion chromatography (Metrohm 861 Advanced Compact IC, Metrohm A Supp 5 column, 0.8 mL min-1, conductivity detection after chemical suppression). Total dissolved hydrogen sulfide (H2S) was measured photometrically using the methylene blue method in samples that were fixed on board with ZnAc (Cline, 1969). The analytical precision for SO42- and H2S analysis was 2%. Dissolved calcium (Ca2+), barium (Ba2+), potassium (K+), silicon (Si4+), strontium (Sr2+) and magnesium (Mg2+) were 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%. Dissolved inorganic carbon (DIC) and non-purchable organic carbon (NPOC) were determined by direct injection to a carbon analyzer (analytikJena multiN/​C 2100s).