Another method of determining absolute age is by looking at varves. Seasonal fluctuations in particle size and speed of sedimentation take place. Tree rings and varves are accurate methods for determining the absolute age of geological events, but neither one is useful for dating very far back in time. In a year with a long growing season the tree ring for that year will be thick. Varve dating has a greater significance than just for local dating, since frequently there is enough organic material to allow radiocarbon dates to be calculated. Consequently, varves commonly form under anoxic conditions. Tree Rings The first method of finding the absolute age of an object is by examining tree rings. However, this discovery led De Geer and many of his co-workers into making incorrect correlations, which they called 'teleconnections', between continents, a process criticised by other varve pioneers like Ernst Antevs. There is therefore the possibility of using the calendrical varve chronology to calibrate radiocarbon dates. But as with tree rings see dendrochronology the varves will vary from year to year, depending on the rapidity of the thaw, quantity of summer rain, winter snow, etc. The light layer usually comprises a coarser laminaset, a group of conformable laminae, consisting of silt and fine sand deposited under higher energy conditions when meltwater introduces sediment load into the lake water. When the meltwater reaches a lake the heaiest sediments sand and silt sink to the bottom quickly and eventually formed thick layers of light colored rock.
Its use for archaeological dating is rather limited in that sites have to be related to the geological changes the ice-sheet moraines or changing Baltic sea-levels before their dates can be determined. But as with tree rings see dendrochronology the varves will vary from year to year, depending on the rapidity of the thaw, quantity of summer rain, winter snow, etc. Swedish pioneer Baron Gerard de Geer discovered in the late 19th century that these could be counted and correlated or linked over long distances, which gave him a timescale of 12, years and fixed the end of the Ice Age at about 10, years ago. Despite these earlier forays, the chief pioneer and populariser of varve research was Gerard De Geer. The first varve chronology was constructed by De Geer in Stockholm in the late 19th century. Another method of determining absolute age is by looking at varves. Varve dating has a greater significance than just for local dating, since frequently there is enough organic material to allow radiocarbon dates to be calculated. By , De Geer had discovered that it was possible to compare varve sequences across long distances by matching variations in varve thickness, and distinct marker laminae. Each ring is also different, and the thickness of each ring is representative of the length of the growing season. A well-known marine example of varve sediments are those found in the Santa Barbara basin, off California. When the meltwater reaches a lake the heaiest sediments sand and silt sink to the bottom quickly and eventually formed thick layers of light colored rock. The supply of sediment to the ice-marginal lake varies with the season. During the summer, melting accelerates, melt-water streams flow faster and carry more material. Consequently, varves commonly form under anoxic conditions. In the s, Edward Hitchcock suspected laminated sediment in North America could be seasonal, and in Warren Upham postulated that light-dark laminated couplets represented a single year's deposition. Each spring or summer when the glacier was melting the glacier deposits a ton of sediment it was carrying into strems of water that are melting off of it. A varve chronology, similar to a tree-ring chronology may be set up. If there wasa very long winter then there will be a thinner tree ring. De Geer and his co-workers and students made trips to other countries and continents to investigate varved sediments. The varved sediments exposed in these sites had formed in glaciolacustrine and glacimarine conditions in the Baltic basin as the last ice sheet retreated northwards. They are most commonly found in glacial lake beds. The light layer usually comprises a coarser laminaset, a group of conformable laminae, consisting of silt and fine sand deposited under higher energy conditions when meltwater introduces sediment load into the lake water. By this stage, other geologists were investigating varve sequences, including Matti Sauramo who constructed a varve chronology of the last deglaciation in Finland. Radioactive Isotopes For really old dates we need to use radioactive isotopes. In , the Geochronological Institute, a special laboratory dedicated to varve research was established. At present, the Swedish varve chronology is based on thousands of sites, and covers 13, varve years.
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