ARE SIDERITE VARVES DEEPWATER MICROBIALITES?

Siderite (FeCO₃) laminations are known from Cenozoic lacustrine sequences, including annually laminated (varved) Holocene occurrences. These carbonates share many features with microbialities including mm-scale organic-rich laminae, and crystal morphologies associated with organominerals such as dumbbells and smooth rhombs. Siderite is usually interpreted as a diagenetic mineral, but new geochemical data supports earlier field evidence suggesting it may occur in ferruginous lakes as a primary, biologically induced, deep water (>20 m) precipitate. Carbon- and oxygen-stable isotopic analysis of Holocene siderite varves from Otter Lake, Michigan (OL) suggests they precipitated in equilibrium with lake water and an alkalinity source influenced by methanogenesis. Siderite genesis in this context requires intense water column stratification and a carbon pump driven by photosynthetic communities in shallow water. Deep water or lake floor iron reducers and methanogens help overcome the thermodynamic barriers to siderite precipitation, and enable seasonal cycles of mineral genesis. Bulk sediment geochemistry determined from X-ray fluorescence (XRF) suggests OL sediments are significantly enriched in iron (2.7 to 26 Fe:Al) and manganese (0.14 to 3.4 Mn:Al) with generally low sulfur (6.3 to 58 Fe:S). We also examined sediments from two lakes known to contain siderite (Lake Malawi and Lake of the Clouds), and one lake whose waters are ferruginous (Brownie Lake) but does not host siderite in its sediments. The siderite precipitating systems have elevated Fe:Al and Mn:Al ratios within the range of those measured in OL sediments. Brownie Lake has lower Fe:Al, Mn:Al, and Fe:S, suggesting that siderite precipitation in small ferruginous lakes may be inhibited by allochtonous sediment geochemistry among other variables. New stable isotope, electron microprobe, and X-ray diffraction work is underway to further develop criteria for distinguishing between iron carbonates precipitated in deep water versus those which precipitated diagenetically. Further exploration for siderite precipitating lakes may provide opportunities to study microbialite processes in a ferruginous geochemical context similar to that which dominated the Archean oceans but is exceptionally rare today.