|
A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (varved clay) on the first map produced by the Geological Survey of Sweden in 1862. Initially, "varve" referred the separate components of annual layers in glacial lake sediments, but at the 1910 Geological Congress, the Swedish geologist Gerard De Geer (1858–1943) proposed a new formal definition, where varve means the whole of any annual sedimentary layer.〔De Geer, G. (1912). A geochronology of the last 12,000 years. Proceedings of the International Geological Congress Stockholm (1910),1, 241–257.〕 More recently introduced terms such as 'annually laminated' are synonymous with varve. Of the many rhythmites in the geological record, varves are one of the most important and illuminating in studies of past climate change. Varves are amongst the smallest-scale events recognised in stratigraphy. An annual layer can be highly visible because the particles washed into the layer in the spring when there is greater flow strength are much coarser than those deposited later in the year. This forms a pair of layers—one coarse and one fine—for each annual cycle. Varves form only in fresh or brackish water, because the high levels of salt in normal sea water coagulate the clay into coarse grains. Since the saline waters leave coarse particles all year, it is nearly impossible to distinguish the individual layers in salt waters. Indeed, clay flocculation occurs at high ionic strength due to the collapse of the clay electrical double layer (EDL), which decreases the electrostatic repulsion between negatively charged clay particles. == History of varve research == Although the term varve was not introduced until the late nineteenth century, the concept of an annual rhythm of deposition is at least two centuries old. In the 1840s, Hitchcock suspected laminated sediment in North America could be seasonal, and in 1884 Warren Upham postulated that light-dark laminated couplets represented a single year's deposition. Despite these earlier forays, the chief pioneer and populariser of varve research was Gerard De Geer. While working for the Geological Survey of Sweden, De Geer noticed a close visual similarity between the laminated sediments he was mapping, and tree-rings. This prompted him to suggest the coarse-fine couplets frequently found in the sediments of glacial lakes were annual layers. The first varve chronology was constructed by De Geer in Stockholm in the late 19th century. Further work soon followed, and a network of sites along the east coast of Sweden was established. 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. By 1914, 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. 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. In 1924, the Geochronological Institute, a special laboratory dedicated to varve research was established. De Geer and his co-workers and students made trips to other countries and continents to investigate varved sediments. Ernst Antevs studied sites from Long Island, U.S.A. to Lake Timiskaming and Hudson Bay, Canada, and created a North American varve chronology. Carl Caldenius visited Patagonia and Tierra del Fuego, and Erik Norin visited central Asia. By this stage, other geologists were investigating varve sequences, including Matti Sauramo who constructed a varve chronology of the last deglaciation in Finland. 1940 saw the publication of a now classic scientific paper by De Geer, the ''Geochronologia Suecica'', in which he presented the Swedish Time Scale, a floating varve chronology for ice recession from Skåne to Indalsälven. Lidén made the first attempts to link this time scale with the present day. Since then, there have been revisions as new sites are discovered, and old ones reassessed. At present, the Swedish varve chronology is based on thousands of sites, and covers 13,200 varve years. In 2008, although varves were considered likely to give similar information to dendrochronology, they were considered "too uncertain" for use on a long-term timescale. However, by 2012, “missing” varves in the Lake Suigetsu sequence were identified in the Lake Suigetsu 2006 Project by overlapping multiple cores and improved varve counting techniques, extending the timescale to 52,800 years.〔 〕〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Varve」の詳細全文を読む スポンサード リンク
|