Varve

A Warve ( "Annual layer " of Swedish varvig lera: " layered clay " ) includes the sedimentation of a year in lakes or in the sea.

Basics

A Warve built up of several distinct sub- layers. For the formation of varves changing Sedimentationsverhältnisse are required in annual rhythm. In the presence of varves in the sediment can be determined precisely as in tree rings, by counting the varves (see Warvenchronologie ) age ( the sediment ). Furthermore vary the widths of the annual layers and their internal structure as a function of applied external parameters, such as the climate.

History

The term Warve ( Hvarfig lera ) was first in 1862 on a map of the Swedish Geological Survey ( Sveriges Geologiska Undersökning ) used. The formal definition was given in 1910 by the Swedish geologist Gerard Jacob De Geer ( 1858-1943 ).

Warventypen

There are different Warventypen.

Clastic varves

Clastic varves can be formed in waters where the substance input from the catchment dominated the deposited sediment on the lake bottom. This is especially true on periglacial and alpine lakes. Here it is in the summer when the water present in the environment of the water ice thaws, by strong outflows coarse material ( sand, often bright) deposited on the lake bottom. In winter, when the lake is frozen, which came into the lake during the year fine material (clay ) decreases and there is a fine-grained (often dark) position. This sequence forming a year represents the frequent repetition of this sequence leads to the formation of a Bändertons.

Evaporitic varves

Evaporitic varves can be deposited in the aquatic semi-arid areas. There, the solubility concentration of various minerals (carbonates, sulfates, chlorides ) is exceeded by heating and evaporation in regular -like manner each year, while these minerals fall out one by one and are deposited on the lake bottom. The evaporation process is called evapotranspiration. The repetition of these characteristic sequence can be classified depending on the whole situation as sedimentation of a year or considered, under other circumstances as a sequence of alternating rhythms of mineral Schwebstoffeinträgen and salt crystallization. The latter option applies to the formation of some alabaster deposits.

Examples of such waters are the Dead Sea or the bulkhead in North Africa.

Organogenic or biogeochemical varves

Organogenic or biogeochemical varves can be deposited in dimictic eutrophic lakes of the meso- high latitudes. These varves reflect here represent a choice of the past in the lake life. In spring, when the nutrient-rich deep water can be mixed into the surface water during the spring circulation, enter into such lakes often mass blooms of planktonic diatoms on. Some of these algae comes after the death of the lake bottom. Their shells are often the spring location. In the summer of little material gets to the bottom of the lake, since the stable thermal stratification ( thermocline ) impedes the mixing of surface and deep water. That remained after the spring bloom in the surface water nutrients circulate in short cycles ( nutrient - producer - consumer - nutrient ). An exception are calcium crystals, which in summer algal blooms ( CO2 removal by algae ) and because of the warming of the surface water ( outgassing of CO2 in the atmosphere) are precipitated. This lime forms in hard water lakes, the summer situation. If, in the autumn for recirculation of the entire water body, then could more blooms of planktonic diatoms occur and the shells are deposited on the lake bottom. In addition, in the autumn layers and benthic - littoral diatoms and organic detritus are found. In winter, a clay layer is deposited, if there has been during the year sufficiently high entries from the catchment area and over a longer duration provides a stable ice sheet for complete calm body of water. The consequence of these featured sub- layers represents the sedimentation and the life one year. Examples of long sequences with this type of annual layer are the Eifel maars (without, or with little lime ) and the Lake Belau in Schleswig- Holstein ( kalkführende varves ).