Lake SedimentsCategories of lake sediment are defined according to where and how the sediment is sampled.
For Lakes, sediment may be composed of:
Various sizes of rock fragments and soils ranging from clays and silts to sand, gravels and boulders;
Chemical precipitations of compounds including snarl, tufa, ferric hydroxide, ferric carbonate, and silicon dioxide;
Organic matter deposits such as peat.
The coarser sediments are widespread in lakes. As a rule, the larger sized material is found in the shallow zones, while clay and silt may occur over any part of the bottom at any depth.
The color of the clay and silt may grade from white through shades of blue and green to black. Sands may range from gray to black.
Lake marl is essentially calcium carbonate. It is often precipitated by certain bacteria and algae and is mostly a characteristic of small lakes and ponds (and some streams). Marl formation occurs in high carbonate waters chiefly as a result of photosynthesis. The typical color of marl is white to grayish.
Another carbonate that precipitates in lakes is tufa. Tuft is a porous lime carbonate formed primarily by some of the algae. Also, the reddish compound ferric hydroxide is found in the bottom sediments particularly as limonite.
In some of the organically poor and in some relatively productive but clear lakes, the sediments are composed of a gray or red-gray, highly viscous material known as gyttja. Gyttja is a fine textured mud that is essentially a mixture of the remains of plants and animals, chemical precipitations, and mineral substances. Decomposition of gyttja under aerobic conditions contributes much of the elemental nutrients to organic production within the lake. Fossilization of gyttja leads to the formation of anthracite.
Under anaerobic conditions in regions of peat formation, lake sediments may be brown or blackish-brown due to the addition of brown humus to gyttja-like material. During anaerobic conditions, particularly during summer, organically rich sediments may decompose to form sapropel. Sapropel is s blue-black substance containing hydrogen sulfide and methane. Fossilization of sapropel could result in the formation of oil.
Peat in various stages of compaction and decomposition occurs rather commonly in lakes. Peat is of plant origin and is normally thickest near the shore in areas of extensive plant growth. Lignite and medium ranked bituminous coal are derived through peat decomposition.
Whatever the factors and materials concerned in the composition of lake sediments, there usually exists a mixture of substances. This happens because the rate and amount of sedimentation in lakes is not uniform. It varies with seasonal inflow of sediment bearing waters and with fluctuations in internal currents.
In cold climates, melting snow in the spring flushes large amounts of coarse sediments into a lake. These sediments are distributed by lake currents over the bottom. In winter when there is little stream discharge into the lake and lake currents are reduced, fine sediments settle to cover the coarser sediments of summer. These layers represent the seasonal fluctuations in rate and extent of sedimentary deposition.
Remains of plants and animals are often contained in the depositional strata of the basin. Since these are deposited chronologically, the development and evolution of the lake basins together with the succession of biologically different associations can provide pertinent details about the history of a lake.
Although inorganic sedimentation alone plays an important role in the filling and the subsequent “death” of lakes, living plants and animals can also contribute to the fate of lakes.