Organic matter mineralization in modern and ancient ferruginous sediments

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.

Methanogen Productivity and Microbial Community Composition Varies With Iron Oxide Mineralogy

“…Additional experiments by Roden and Urrutia (2002) showed similar results with Geobacter metallireducens and their work also suggests that Fe(II) sorption onto iron-reducing bacteria may further inhibit their ability to carry out iron reduction. Friese et al (2021) observed methanogenesis dominating over iron reduction in sediments in ferruginous Lake Towuti, Indonesia despite the presence of reactive iron(III) phases and attribute this to surface passivation of reactive iron. Our observed increase in Fe(II) content of the ferrihydrite (Figure 3D) over time is also consistent with surface passivation, which may have contributed to our observed results showing that methanogens can outcompete iron-reducing bacteria on longer timescales in the presence of ferrihydrite.…”

“…Indeed, many studies have shown that iron reduction outcompetes methanogenesis in wetlands and soils when iron is present as a poorly crystalline iron oxide (Sahrawat, 2004;Laanbroek, 2010). However, there are also many instances where methanogenesis and iron reduction occur concomitantly (Slomp et al, 2013;Vigderovich et al, 2019), or where methanogenesis dominates in zones of ferruginous lakes with persistent iron oxides (Lambrecht et al, 2020;Friese et al, 2021). A decreased relative ability of iron-reducers to out-compete methanogens in the presence of well-crystalline iron oxides may explain overlapping zones of iron reduction and methanogenesis in the environment.…”

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

“…Sedimentary OM is mainly autochthonous (Friese et al, 2021), albeit at low concentrations, with some contribution of fluvially-derived material (Morlock et al, 2019;Hasberg et al, 2019). TOC and siderite concentrations display opposite trends, with highest TOC concentrations found in diatom oozes and green clays (Fig.…”

“…One was used to determine the partial pressure of methane using a Thermo Finnigan Trace gas chromatograph (Thermo Fisher Scientific, Waltham, United-States) equipped with flame ionization detector (GC-FID), and the other one was used for carbon isotopic analysis of methane (δ 13 CH 4 ) and carbon dioxide (δ 13 CO 2 ) by isotope-ratio-monitoring gas chromatography/mass spectrometry (irm-GC/MS) on a Trace GC Ultra gas chromatograph (Thermo Fisher Scientific) coupled to a continuous-flow isotope ratio mass spectrometer (Delta V Plus, Thermo Fisher Scientific) as described in Friese et al (2021). Results are reported as δ-values in per mil relative to V-PDB.…”

“…We attempt to disentangle the isotope signatures of siderites in Lake Towuti's sediment that are inherited from past environmental conditions (Severmann et al, 2006(Severmann et al, , 2008Wittkop et al, 2014) from those imprinted during early post-depositional processes (Mozley and Wersin, 1992;Johnson et al, 2013;Posth et al, 2014). We use isotope signatures presently recorded in siderites to infer hydrological changes in Lake Towuti, such as stratification events and lake level fluctuations, in relation to climatic cycles (Russell et al, 2020;Ulfers et al, 2021;Morlock et al, 2021), and trace the influence of the biogeochemistry (Vuillemin et al, 2016;Bauer et al, 2020) and geomicrobiology (Bray et al, 2017;Vuillemin et al, 2017Vuillemin et al, , 2018Friese et al, 2021) in these modern ferruginous sediments with detailed pore water geochemistry, bulk reactive iron and organic carbon. Based on combined siderite O-Fe-C isotopes, we reconstruct biotic and abiotic processes of fractionation controlling siderite isotope signatures arising from ~1 Ma of depositional history in Lake Towuti and exposure of the corresponding ferruginous sequence to microbial Fe reduction and OM oxidation.…”