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 30m long interval consisting of sideritic and non-sideritic clayey beds and diatomaceous oozes containing vivianites. Highresolution 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 20m depth in the sediment. Negative 56Fe values of vivianite indicated incorporation of kinetically fractionated light Fe2C 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.
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Acknowledgements. This research was carried out with partial support from the International Continental Scientific Drilling Program (ICDP); the U.S. National Science Foundation (NSF); the German Research Foundation (DFG); the Swiss National Science Foundation (SNSF); PT Vale Indonesia; the Ministry of Research, Education, and Higher Technology of Indonesia (RISTEK); Brown University; the University of Minnesota; the University of Geneva; GFZ German Research Centre for Geosciences; the Natural Sciences and Engineering Research Council of Canada (NSERC); and Genome British Columbia. This study was financially and logistically supported by the DFG ICDP priority program through grants to Jens Kallmeyer (KA 2293/8-1) and Aurèle Vuillemin (VU 94/1-1), an SNSF grant to Aurèle Vuillemin (P2GEP2_148621), and an NSERC Discovery grant (0487) to Sean A. Crowe.
Financial support. This research has been supported by the Swiss
National Science Foundation (grant no. P2GEP2_148621), the Deutsche Forschungsgemeinschaft (grant nos. VU 94/1-1 and KA 2293/8-1), and the Natural Sciences and Engineering Research Council of Canada (grant no. 0487).
© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.
Copyright 2020 Elsevier B.V., All rights reserved.
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