Mineral biosignatures record pore water geochemistry during microbial diagenesis – modern Lake Towuti as a ferruginous case study

Ferruginous conditions prevailed in the oceans through much of Earth’s history. However, past biogeochemical cycling inferred from mineral components identified in ancient iron formations remain poorly understood in terms of microbial processes prior to lithification. In Lake Towuti, Indonesia, ferruginous sediments sink through a stratified water column and are deposited under anoxic conditions that mimic the Earth’s early oceans, thereby allowing the study of both geochemical conditions in pore waters and long-term diagenetic evolution of its 1 Ma stratigraphic record.

We combined detailed pore water geochemistry and stratigraphic proxies with scanning electron microscopy imaging of authigenic phases. Although variability in elemental profiles attests to climate- and tectonic-driven processes along the 100-m-long sediment sequence, deposition of ferruginous minerals appears transient as particulate iron, reworked from surrounding lateritic soils, undergoes partial dissolution-precipitation during sinking and after burial. Minerals found to form in situ included magnetite (Fe₃O₄), millerite (NiS), siderite (FeCO₃) and vivianite (Fe₃[PO₄]₂ ∙ 8H₂O). Acicular millerite aggregates overgrown by siderite and vivianite indicate that they directly precipitated from saturated pore waters (Ostwald ripening). This also suggests that these mineral phases may constitute a diagenetic sequence stemming from the progressive consumption of terminal electron acceptors with sediment organic matter remineralization during shallow burial. Thus, we consider that these minerals act as biosignatures of redox processes driven by autochthonous sedimentary microbial populations that actively control pore water geochemistry after deposition, thereby differentially imprinting the stratigraphy of bulk sediment during burial.