Deposition and diagenesis of organic matter and calcium carbonate in modern north temperate lakes

The three most important components in the sediments of modern north temperate lakes are detrital material, organic matter, and calcium carbonate. The detrital material is derived from whatever materials are available in the drainage basin, with modifications by weathering and diagenesis. Organic matter is a mixture of both allochthonous material (pollen grains, seeds, needles, organic detritus, etc) from the drainage basin and autochthonous organic matter (largely debris from planktonic algae and aquatic macrophytes). Recent studies of sedimentary pigments in lake sediments indicate that most of the organic matter in sediments of productive (eutrophic) lakes is derived from algae. Sediments in these lakes usually are olive gray and contain more than 20% dry weight organic matter (10% organic carbon). High concentrations of organic matter also accumulate in meromictic lakes in which organic matter is protected from oxidation by permanently anoxic bottom waters. Low-magnesian calcite is the most common carbonate mineral in those lakes that are saturated with respect to calcium and/or magnesium carbonates. Dolomite and high-magnesian calcite can form even under humid, temperate conditions if the Mg:Ca ratio in the water is greater than about 8. Aragonite is mostly derived from mollusk debris, but may form as a primary precipitate if the Mg:Ca ratio in the water is greater than about 12. Assimilation of carbon dioxide by phytoplankton photosynthesis is an important aspect of carbonate precipitation in hard-water lakes. In some lakes, the rate of precipitation of calcium carbonate from waters that are supersaturated with respect to calcium carbonate is directly proportional to rates of assimilation of calcium carbonate by plankton.