Abstract. Ferruginous lacustrine systems, such as Lake Towuti,
Indonesia, are characterized by a specific type of phosphorus cycling in
which hydrous ferric iron (oxyhydr)oxides trap and precipitate phosphorus to
the sediment, which reduces its bioavailability in the water column and
thereby restricts primary production. The oceans were also ferruginous
during the Archean, thus 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 vivianite formation, phosphorus retention by
iron and the related mineral transformations during early diagenesis in
ferruginous sediments. Core scan imaging, together with analyses of bulk sediment and pore water
geochemistry, document a 30 m long interval consisting of sideritic and
non-sideritic clayey beds and diatomaceous oozes containing 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 diagenetic mineral formation antecedent to the one of
vivianite that is related to microbial reduction of iron and sulfate.
Together with the pore water profiles, these data suggest that the
precipitation of millerite, siderite and vivianite in soft ferruginous
sediments stems from the progressive consumption of dissolved terminal
electron acceptors and the typical evolution of pore water geochemistry
during diagenesis. Based on solute concentrations and modeled mineral
saturation indices, we inferred vivianite formation to initiate around 20 m
depth in the sediment. Negative δ56Fe values of vivianite
indicated incorporation of kinetically fractionated light Fe2+ into the
crystals, likely derived from active reduction and dissolution of ferric
oxides and transient ferrous phases during early diagenesis. The size and
growth history of the nodules indicate that, after formation, continued
growth of vivianite crystals constitutes a sink for P during burial,
resulting in long-term P sequestration in ferruginous sediment.