Warve
A warve ("annual layer ", from Swedish varvig lera : "layered clay") comprises the sedimentation of a year in lakes or in the sea .
Basics
A warve is made up of several distinguishable sub-layers. For the formation of varves, changing sedimentation conditions are necessary every year. If varvae are present in the sediment, as with tree rings , the age (of the sediment) can be precisely determined by counting the varves (see varven chronology ). Furthermore, the thicknesses of the annual layers and their internal structure fluctuate depending on external parameters , such as the climate.
history
The term warve ( Hvarfig lera ) was first used in 1862 on a map of the Swedish Geological Service (Sveriges Geologiska Undersökning). The formal definition was given in 1910 by the Swedish geologist Gerard Jakob De Geer (1858–1943).
Warvent types
There are different types of warfare.
Clastic varves
Clastic varvae can be formed in waters in which the input of substances from the catchment area dominates the sediment deposited on the lake bed. This is especially true of periglacial and high mountain lakes. Here in summer, when the ice around the water thaws, coarse material (sandy, often light) is deposited on the lake bed due to strong runoff. In winter, when the lake is frozen over, the fine material ( clay )that got into the lake during the year sinksand a fine-grained (often dark) layer is created. This sequence together represents a year. The frequent repetition of this sequence leads to the formation of a band tone .
Evaporative varves
Evaporative varvae can bedepositedin the waters of semi-arid areas. There,the solubility concentration ofvarious minerals ( carbonates , sulfates , chlorides ) is exceededin a regular way every yeardue to heating and evaporation ; these minerals precipitate one after the other and are deposited on the lake bed. The evaporation process is called evaporation . The repetition of this characteristic sequence can, depending on the overall situation, be classified as sedimentation of a year or, under other circumstances, viewed as a sequence of alternating rhythms of mineral sedimentation and salt crystallization. The latter variant applies to the formation of some alabaster deposits .
Examples of such waters are the Dead Sea or the Schotts in North Africa.
Organogenic or biogeochemical varves
Organogenic or biogeochemical varvae can bedepositedin dimictic meso- eutrophic lakes at higher latitudes. These varves reflect a part of the life that has expired in the lake. In spring, when the nutrient-rich deep water can be mixed into the surface water during the spring circulation, mass blooms of planktonic diatoms often occur in such lakes. Some of these algae reach the bottom of the lake after they die. Their shells often form the spring layer. Little material reaches the bottom of the lake in summer, as the stable thermal stratification ( thermocline ) hinders the mixing of surface and deep water. The nutrients remaining in the surface water after the spring bloom circulate in short cycles (nutrient-producer-consumer-nutrient). An exception are lime crystals , which areprecipitatedduring summer algal blooms ( CO 2 removal by algae) and because of the warming of surface water ( outgassing of CO 2 in the atmosphere ). This lime forms the summer location in hard water lakes. If the entire body of water recirculates in autumn, more planktonic diatoms could appear and the shells could be deposited on the lake bed. In addition, benthic - littoral diatom shells and organic detritus are foundin the autumn layers . A layer of clay is deposited in winter when there has been sufficiently high input from the catchment area during the year and a stable layer of ice ensures that the water body is completely calm over a longer period of time. The sequence of these presented sub-layers represents the sedimentation and life for a year. Examples of long sequences with this type years layer are the Eifel - Maare (without or with little lime) and the Belauer lake in Schleswig-Holstein (Calciferous varves).
See also
Web links
- Varves In: GEOPOLAR. Institute for Geography, University of Bremen