Microbial Control on Siderite Formation in Ferruginous Sediments of Lake Towuti, Indonesia.
Abstract
Authigenic minerals can be formed in the water column and sediments, either abiotically or triggered by biological activity, and have been used as proxies for paleosalinity, paleoproductivity and diagenetic conditions. Siderite crystals (FeCO3) from ferruginous Lake Towuti were investigated to infer mineral formation associated with processes of organic matter remineralization and microbial Fe3+ reduction. Lake Towuti is a deep tectonic basin on Sulawesi, Indonesia. Erosion of lateritic soils in the catchment supplies the lake with considerable amounts of iron oxy(hydr)oxides but little sulfate, making Lake Towuti an outstanding natural laboratory to study mineral formation under ferruginous conditions. Lake Towuti is currently stratified with anoxic conditions below 130 m, allowing microbial reduction processes to take place in the hypolimnion. Samples were taken from a 120-meter core recovered by the ICDP Towuti Drilling Project in summer 2015. SEM and TEM imaging showed that siderite crystals grow from micritic phases into mosaic monocrystals, developing into aggregates with increasing burial depth. Elemental mapping revealed Mn/Fe zonation in the siderites, reflecting diagenetic evolution of pore water chemistry. Intervals with persistent light δ13C and δ56Fe compositions are interpreted to reflect long periods of bottom water anoxia with little siderite formation. Concomitant occurrence of vivianite argues for accumulation of methane and dissolved iron in anoxic bottom waters. In contrast, we observe siderite-rich intervals with heavier δ13C and δ56Fe compositions and a lack of vivianite, which point towards increased burial of amorphous Fe3+ due to bottom water oxygenation. During and after burial, this amorphous Fe3+ is reduced and reacts with the inorganic carbon dissolved in pore water to form siderite. We therefore argue that siderite isotopic compositions reflect microbial sedimentary processes, not bottom water conditions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.B11F0530V
- Keywords:
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- 0419 Biomineralization;
- BIOGEOSCIENCESDE: 1039 Alteration and weathering processes;
- GEOCHEMISTRYDE: 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRYDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRY