Major chemical and mineralogical components of profundal surface sediments of Minnesota lakes

Minnesota lakes, characterized primarily on t&e basis of water chemistry, can also be classified according to the proportions of organic matter, CaCOs, and elastic minerals in their profundal sediments. The northeastern (group 1) lakes are characterized by shallow to very deep basins in Precambrian crystalline rocks, active outlets, and dilute surface waters. Profundal sediments of these lakes contain no sedimentary calcium carbonate and can be subdivided on the basis of organic content. Highly organic sediments of group 1 lakes contain more than 30% organic matter (as loss on ignition), but most group 1 lake sediments contain less than this. The central group 2 lakes occupy shallow to moderately deep basins in calcareous glacial till, have inactive outlets as a result of net evaporation, contain more concentrated salts than group 1 lakes, and commonly precipitate CaCOs during the summer. These lakes can be subdivided on the basis of sedimentary CaCOs. Sediments of group 2 lakes with more than 30% organic material contain little or no CaCOs. Southern group 2 lake sediments are characterized by 1030% CaC03 and 10-300/o organic matter. Sediments of western group 2 lakes contain 30-5070 CaCOs and about twice as much elastic as organic materials. Group 3 lakes occupy shallow depressions in calcareous, sulfur-rich glacial drift of the arid southwestern prairie region and contain even more concentrated salts than group 2 lakes. Sediments in these lakes are mainly elastic, averaging about 12% organic matter and 20% CaCOs. Variations in water and sediment chemistry, and in sediment mineralogy of these lakes are clearly related to an increase in climatic aridity and aquatic productivity from northeast to southwest. Differences in nature of the underlying glacial drift and in degree of erosion also affect the composition of the lake sediments.