LACUSTRINE PALEOENVIRONMENT OF HOLOCENE SIDERITE CYCLES, OTTER LAKE, MICHIGAN USA

Endogenic siderite (FeCO₃) occurs in the annually-laminated sediments of Otter Lake in southeast lower Michigan. A Holocene chronology was established in a 7-m sediment core using lamination counts and radiocarbon dating. The presence of siderite was confirmed via powder x-ray diffraction, and thin sections demonstrate that siderite is concentrated in the summer layer of the lamination couplet.

Siderite abundance was quantified by x-ray diffraction using a relative-intensity-ratio (RIR) method with external corundum standard at 20-cm intervals throughout the core, and at 10-cm intervals in areas of detectable siderite. Siderite is difficult to quantify using traditional carbonate abundance determination methods, though our RIR method appears to be accurate to ±5% by weight. Carbonates do not occur in the surface sediments of Otter Lake, and siderite is present only in sediments older than 2500 cal yr BP. Peak siderite abundance of 19% dry sediment weight occurs at 5800 cal yr BP, with abundance fluctuating on millennial-scale cycles to the base of the core at 6900 cal yr BP.

Previous work characterized the abundance of major elements and dissolved inorganic carbon (DIC) in modern lake water, and determined δ¹³C isotope values of lake DIC with depth. Previous work also determined the δ¹³C isotope values of sediment carbonates and revealed that sediments with abundant siderite also contained positive δ¹³C values, up to 7.5‰. Equilibrium modeling of lake water in Geochemists Work Bench software suggests that siderite would only precipitate in waters with an order-of-magnitude more dissolved Fe than was measured in the Otter Lake hypolimnion.

We propose that siderite precipitated in summer from an Fe-enriched monomolimnion by mixing of DIC-enriched hypolimnetic water with Fe-rich monomolimnetic water containing DIC sourced from acetate fermentation. Siderite precipitated during millennial-scale periods of meromixis when Fe accumulated in the anoxic monomolimnion. Changes in lake level driven by regional climatic fluctuations influenced iron supply and lake mixing dynamics, while siderite precipitation created a sediment iron sink; hence the siderite cycles in the sediments of Otter Lake were driven by a combination of auto- and allocyclic controls.