Iron‐rich rhythmically laminated sediments in Lake of the Clouds, northeastern Minnesota1

Abstract

Sediments from the deepest part of Lake of the Clouds (31 m) are rhythmically laminated throughout all but the basal part of the column. The laminations can be correlated with events in the lake during the annual cycle. Ferrous iron (maximum 620 ppm) accumulates in a layer of water about 2 m thick in the deepest part of the lake. The laminations form in the sediments only where overlain by this stagnant iron‐rich water. The light‐colored portion of the couplet presumably forms from iron oxides precipitated from partially oxygenated bottom water during fall and spring overturns, the dark layer from organic remains that settle out during summer.Chemical analysis and electron‐microprobe studies reveal a much higher concentration of iron and manganese in the light layers than in the dark. Vivianite [Fe₈(PO₄)₂ · 8H₂O] and probably limonite (Fe₂O₃ · nH₂O) occur extensively in nodules throughout the core. Siderite (FeCO₃) was identified by X‐ray diffraction. Numerous orange spherical aggregates occur in the light layers. An iron and manganese profile of the sediment column suggests that the most distinct laminae are found where the iron content of the sediment is at least 4%.