Abstract
Deposition of ferruginous sediment was widespread during the Archaean and Proterozoic Eons, playing an important role in global biogeochemical cycling. Knowledge of organic matter mineralization in such sediment, however, remains mostly conceptual, as modern ferruginous analogs are largely unstudied. Here we show that in sediment of ferruginous Lake Towuti, Indonesia, methanogenesis dominates organic matter mineralization despite highly abundant reactive ferric iron phases like goethite that persist throughout the sediment. Ferric iron can thus be buried over geologic timescales even in the presence of labile organic carbon. Coexistence of ferric iron with millimolar concentrations of methane further demonstrates lack of iron-dependent methane oxidation. With negligible methane oxidation, methane diffuses from the sediment into overlying waters where it can be oxidized with oxygen or escape to the atmosphere. In low-oxygen ferruginous Archaean and Proterozoic oceans, therefore, sedimentary methane production was likely favored with strong potential to influence Earth’s early climate.
Highlights
Deposition of ferruginous sediment was widespread during the Archaean and Proterozoic Eons, playing an important role in global biogeochemical cycling
Precipitation of Fe from these oceans resulted in the widespread deposition of ferruginous siliciclastic sediments—which are by definition Fe-rich and contain highly reactive Fe that is not pyritized—and, in cases, nearly pure chemical sediments like many banded iron formations (BIFs)[8]
The pSRR rates suggest that sulfate reduction may occur throughout the upper 12 m of sediment, but here it makes a minor, even insignificant, contribution to total organic carbon mineralization
Summary
Deposition of ferruginous sediment was widespread during the Archaean and Proterozoic Eons, playing an important role in global biogeochemical cycling. More than 90% of the produced methane is consumed through anaerobic oxidation of methane (AOM) with sulfate as terminal electron acceptor precluding appreciable fluxes to the overlying water and atmosphere[5]. Oxygen exposure in these sediments directly controls sedimentary organic carbon preservation and bottom water anoxia increases carbon burial efficiency[6]. Near, absence of oxygen, nitrate and sulfate, organic matter mineralization in ferruginous sediments would be expected to proceed anaerobically via the energetically most favorable terminal electron acceptors available—in this case ferric iron, followed by CO2 through methanogenesis[4]. The Fe (oxyhydr)oxide flux to Lake Towuti is comprised mainly of poorly- to nanocrystalline goethite with lesser amounts of hematite and magnetite, all of which may have been reworked and recrystallized to some extent during transport or through redox cycling in the uppermost sediments before burial[16]
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