A one-million-year isotope record from siderites formed in modern ferruginous sediments

Ancient iron formations hold important re-cords of environmental conditions during the Precambrian eons. Reconstructions of past oceanic systems require investigation of mod-ern ferruginous analogs to disentangle water column and diagenetic signals recorded in iron-bearing minerals. We analyzed oxy-gen, iron, and carbon isotopes in siderite, a ferrous carbonate phase commonly used as an environmental proxy, from a 100-m-long record spanning a 1 Ma depositional his-tory in ferruginous Lake Towuti, Indonesia. Combining bulk sediment and pore water geochemistry, we traced processes control-ling siderite isotope signatures. We show that siderite oxygen isotope compositions (delta 18O) reflect in-lake hydrological and depositional conditions. Low iron isotope values (delta 56Fe) record water column oxygenation events over geological timescales, with minor diage-netic partitioning of Fe isotopes by microbial iron reduction after deposition. The carbon isotope compositions (delta 13C) reflect the incor-poration of biogenic HCO3-, which is consis-tent with sediment organic matter reminer-alization lasting over ca. 200 ka after burial. Positive delta 13C excursions indicate an increased production of biogenic methane that escaped the sediment during low lake levels. Diffusion across the sediment-water interface during initial formation of siderites tends to align the isotope signatures of bottom waters to those of pore waters. As microbial reduction of ferric iron and oxidation of organic matter proceed and saturate pore water conditions with respect to siderite, overgrowth on nu-clei partially mutes the environmental signal inherited from past bottom waters over ca. 1 Ma. Because high depositional fluxes of fer-ric iron and organic matter in early oceans would have promoted similar microbial pro-cesses in ferruginous deposits prior to lithifi- cation, the environmental record contained in siderite grains can successively integrate depositional and early diagenetic signals over short geological timescales.