Total acid digestion: Dissolution of dry sediment in a mixture of 65% HNO3 (3 ml), 30% HCl (2 ml), and 40% suprapur® HF (0.5 ml) in a CEM Mars Xpress microwave system. Elemental contents determined by ICP-OES (Inductively coupled plasma – optical emission spectroscopy; iCAP 7400, Thermo Fisher Scientific Inc.). Sequential Fe extraction: after Poulton & 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). Elemental contents determined by ICP-OES (Inductively coupled plasma – optical emission spectroscopy; iCAP 7400, Thermo Fisher Scientific Inc.)δ56Fe analysis of sequentially extracted Fe pools: Processing after Henkel et al. (2016). Column separation after Schoenberg & von Blanckenburg (2005). Isotope analysis on a ThermoFinnigan Neptune Multicollector-ICP-MS (standard-sample bracketing method with IRMM-014) (Schoenberg & von Blanckenburg 2005).