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Trace element composition of concretions, ICP–MS measurements

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).

Composition of major oxide in bulk sediments from lake Baikal: ICP-AES data from P-rich samples

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).

Major composition (wt.%) of concretions, ICP-AES measurements

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).

Selected electron-microprobe analyses of vivianite and santabarbaraite from Lake Baikal

Electron-microprobe analyses (Table 5) were completed on a selection of grains from 11 samples, with a CAMECA SX-50 instrument (University of Louvain-la-Neuve, Belgium), under an accelerating voltage of 15 kV and a probe current of 20 nA. The standards used were the Kabira graftonite (Fe, Mn, P; Fransolet, 1975), corundum (Al), olivine (Mg), wollastonite (Si), and willemite (Zn).

Unit-cell parameters and Mn(Fe+Mn) ratios of vivianites from Lake Baikal

The unit-cell parameters (Table 4) were calculated with the least-squares refinement program LCLSQ 8.4 (Burnham, 1991), from the d-spacings corrected with an internal standard of Pb(NO3)2.

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