Differenze tra le versioni di "Varva"

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Delle molte ''ritmiti'' <ref>Con ritmite s'intende ognuna delle serie ritmiche di fondi sedimentari; sinonimo di varva</ref> trovate nelle registrazioni geologiche, le varve sono uno degli studi più importanti ed illuminanti sui mutamenti climatici del passato. Le varve sono tra gli eventi nella più piccola scala riconosciuti nella [[stratigrafia]].
 
== Storia della ricerca sulla varva ==<!--
AlthoughSebbene theil termtermine varvevarva wasnon notvenne introducedintrodotto untilse thenon latenel nineteenthtardo centuryXIX secolo, theil conceptconcetto ofdi anun annualritmo rhythmannuale ofdi depositionsedimentazione isè atdi leastalmeno twodue centuriessecoli oldvecchio. InNegli theanni 1840s[[1840|'40]], [[Edward Hitchcock|Hitchcock]] suspectedintuì laminatedche sedimentil insedimento Northlaminato nel Nord America couldpotesse beessere seasonalstagionale, ande innel 1884 [[Warren Upham]] postulatedpostulò thatche lightl'''accopiamento'' (''couplet'') laminato chiaro-darkscuro laminatedrappresentasse coupletsun representeddeposito adi singleun yearssingolo depositionanno. DespiteNonostante thesequesti earlybrevi foraystentativi, theil chiefprincipale pioneerpioniere andche populariserrese ofpopolare varvela researchricerca wassulla varva fu Gerard De Geer. WhileMentre workinglavorava forper thela Geological''Ricerca SurveyGeologica of SwedenSvedese'', De Geer noticednotò auna closesimilarità visualvisiva similarityravvicinata betweentra thei laminatedsedimenti sedimentslaminati heche wasegli mapping,stava mappando e andgli [[dendrochronologydendrocronologia|tree-ringsanelli dell'albero ]]. ThisQuesto promptedlo himspinse toa suggestsupporre theche coarse-finegli coupletsaccoppiamenti frequently''grossolano-fine'' foundfrequentemente introvati thenei sedimentssedimenti ofdei glaciallaghi lakesglaciali werefossero annualstrati layersannuali.
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TheLa first varveprima [[chronologycronologia]] wasdella constructedvarva byvenne realizzata da De Geer ina [[StockholmStoccolma]] in thenell latetardo 19thXIX centurysecolo. 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 to making incorrect correlations, which they called 'teleconnections', between continents, a process criticised by other varve pioneers like Ernst Antevs.
 
In 1924 a special laboratory dedicated to varve research - the Geochronological Institute - 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]].
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