Table S1 The grain size, TOC and elemental composition of sediment samples in the northern South China Sea

Grain size measurements of the sediments were carried out on a Mastersizer 3000 laser particle size analyzer. Prior to analysis organic matter was removed by leaching the sediment with 15% H2O2. Total organic carbon (TOC) of the sediments was measured in Thermo EA-IsoLink elemental analyzer. Prior to analysis carbon bound to carbonate minerals was removed by leaching the sediment with 10% HCl. Total element concentrations (Al, Ca, Ti, Fe, Mn) of the bulk sediments were measured after acid digestion (HF, HNO3 and HCl) by Thermo iCAP 7400 ICP-OES. Solid phase iron speciation data were measured following sequential Fe extraction (Fecarb, Feox, Femag, Fepy) by Thermo iCAP 7400 ICP-OES.