Abstract. Ferruginous lacustrine systems, such as Lake Towuti, Indonesia, can experience restricted primary production due to phosphorus trapping by hydrous ferric iron (oxyhydr)oxides that reduce P concentrations in the water column. The oceans were also ferruginous during the Archean, so understanding the dynamics of phosphorus in modern-day ferruginous analogues may shed light on the marine biogeochemical cycling that dominated much of Earth's history. Here we report the presence of large crystals (> 5 mm) and nodules (> 5 cm) of vivianite – a ferrous iron phosphate – in sediment cores from Lake Towuti, and address the processes of phosphorus retention and iron mineral transformations during diagenesis in ferruginous sediments. Core scans together with analyses of bulk sediment and pore water geochemistry document a 30 m long interval consisting of beds of sideritic and non-sideritic clays and diatomaceous oozes containing diagenetic vivianites. High-resolution imaging of vivianite revealed continuous growth of crystals from tabular to rosette habits that eventually form large (up to 7 cm) vivianite nodules in the sediment. Mineral inclusions like millerite and siderite reflect antecedent diagenetic mineral formation that is related to microbial reduction of iron and sulfate. This implies the formation and growth of vivianite crystals under reducing conditions during diagenesis. Negative ð56Fe values of vivianite indicated reductive dissolution of ferric oxides as the source of Fe in the vivianites with incorporation of microbially fractionated light Fe2+ into the crystals. The size and growth history of the nodules indicate that, after formation, continued growth of vivianite may constitute a significant sink for P in these sediments.
Vivianite formation in ferruginous sediments from Lake Towuti, Indonesia
