Solid-phase samples were collected onboard (RV Heincke expedition HE443) using cut-off syringes. The MUC was sliced and sampled. For sampling the GC, small windows were drilled into the liner through which the syringes were introduced. All samples were stored anoxically and frozen at -20°C until processing. The processing involved freeze-drying and grinding. Extracts deriving from the sequential extraction of Fe phases were processed for stable Fe isotope analysis after Henkel et al. (2016).

Methods:Sequential Fe extraction after Poulton and Canfield (2005) and Henkel et al. (2016). Fe aca (1 M Na-acetate, 24 h, pH 4.5); Fe hyam (1 M hydroxylamine-HCl in 25% v/v acetic acid, 48 h); Fe di-ct (Na-dithionite (50 g L-1)/0.02 M Na-citrate, 2 h, pH 4.8); Fe oxa (0.2 M ammonium oxalate/0.17 M oxalic acid, 6 h).Processing after Henkel et al. (2016). Column separation after Schoenberg and von Blanckenburg (2005). Isotope analysis on a ThermoScientific Neptune plus Multicollector-ICP-MS (standard-sample bracketing method with IRMM-014), Isotope Geochemistry Group at MARUM-Centre for Marine Environmental Sciences, University of Bremen.