The sediment cores were sampled with a resolution of 10 cm, resulting in a total of 290 samples (Table 2). Around 250 mg of dried sediments was mechanically crushed through 100–200-μm mesh then processed by an alkaline digestion (Lithium meta-borate) in a Pt crucible at 1000 °C for 1 h. The residue was dissolved over night in a nitric acid matrix and then major and minor elements (Al, Ba, Ca, Fe, K, Mg, Mn, Na, P, S, Si, Sr, Ti) were analysed by Atomic Emission Spectrometry ICP-AES (Thermo Optek Iris Advantage, Royal Museum for Central Africa, Tervuren, Belgium). Y and Au internal standards were used to correct for instrumental drift. For both trace and major elements analyses, external calibrations were performed using artificial standard solutions and dissolved mineralised natural rock standard (e.g., BHVO-1, DWA, CCH-1 SGR-1, JGB-1).
In order to get a complete geochemical signature, 14 P-rich concretions, chosen among the different cores, were acid digested (Table 3a and Table 3b). In a clean laboratory, 1.7 to 36 mg of concretions were digested overnight in a concentrated mixture of Suprapur acid (3 ml HCl/2 ml HNO3/1 ml HF) at 90 °C in sealed Teflon beakers. After evaporation to dryness, the residue was dissolved in 2.5 ml of 2% HNO3 Suprapur and diluted to 12 ml with Milli-Q water. During the same procedure, we have also dissolved and analysed, for comparison, a pure vivianite from Anlua, Cameroon (tubular crystals, MRAC collection).