Subsurface biosphere, pore water geochemistry and mineral biosignatures along the 1 Ma sediment archive of ferruginous Lake Towuti, Indonesia

Ferruginous conditions prevailed in the oceans through much of Earth’s history. However, past biogeochemical cycling inferred from mineral components in ancient iron formations remain poorly constrained in terms of microbial processes prior to lithification. In Lake Towuti, Indonesia, ferruginous sediments are deposited under stratified conditions that mimic the Earth’s early oceans. Over geologic time, Lake Towuti experienced dynamic redox conditions, resulting in variable ferric and organic matter fluxes feeding microbial life at the lake floor. Although environmental conditions exert control over microbial assemblages at the time of deposition, geochemical evolution of these substrates select for specific groups of microorganisms capable of maintaining metabolic activity during entombment.

The 100 m long core retrieved by the ICDP Towuti Drilling Project allowed for investigations of the subsurface biosphere, pore water geochemistry and diagenesis of iron minerals. We established the abundance and phylogenetic distribution of microorganisms along the 1 Ma stratigraphic record, and created integrated environmental and geochemical datasets in order to identify the main taxa and metabolic features involved in sediment mineralization. Ferruginous conditions predominantly selected for Bathyarchaeia. Relevant metabolisms identified from metagenome-assembled genomes indicated sulfur transformation and (homo)acetogenesis, suggesting that heterotrophic dark carbon fixation and cryptic sulfur cycling linked to iron minerals may be prominent features of microbial life in this ferruginous system.

Changes in environmental processes and conditions lead to variability in metal and organic substrate concentrations with depth, while sustaining different microbial processes in various depth intervals. Geochemical profiles reflect microbial activity after deposition and demonstrated mineral precipitation induced by microbial mineralization. Precipitation of magnetite (Fe₃O₄), millerite (NiS), siderite (FeCO₃), and vivianite (Fe₃[PO₄]₂ · 8H₂O) from pore water constitute biosignatures of microbial iron and sulfate reduction, fermentation and methanogenesis. For example, oxygen, iron, and carbon isotopes measured on siderites enabled us to differentiate between depositional and diagenetic signals. Siderite δ¹⁸O signatures reflected in-lake hydrological fluctuations. Low negative δ⁵⁶Fe values recorded periods of water column stratification and oxygenation events, with minor diagenetic redistribution. Negative δ¹³C signatures reflected incorporation of biogenic HCO₃^- during organic matter fermentation, whereas positive δ¹³C excursions indicated mass balance due to increased production of biogenic methane.