Geochronology of the last 12,000 years
Geology is the history of the earth but until now it has been a history without ages in years. Various attempts have been made to estimate the lengths of time involved in individual parts of this history. However, none could be proven to be strictly corre
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Geochronology of the last 12,000 years Geol Rundsch 3:457-471
Translation reeeived: 22 May 2002 © Springer-Verlag 2002
Geology is the his tory of the earth but until now it has been a history without ages in years. Various attempts have been made to estimate the lengths of time involved in individual parts of this history. However, none could be proven to be strict1y correct. The outstanding authors of one of OUf modern teaching books in geologyl state: "The desire to date the major events of the geological past in years increases as the events approach our own time and touch on human history. However, the difficulties of these attempts are huge and the results are of doubtful value. In the best case, they do little more than portray the order of magnitude of distinct periods. Geological processes are very complicated and each single factor involved is variable in itself, so that the connection of such uncertain variables creates a huge uncertainty in the results". Under these circumstances, it is appropriate to introduce a new and precise method of investigation which enables the establishment of areal geochronology based on counts of annual sediment layers covering the last 12,000 years. The basis for this chronology is provided by late glacial and post-glacial, periodically layered sediments, in which the deposit of each year can be clearly recognised. Initially, we counted the annual layers having regular intervals at a large number of sites along a line profile stretching from the southernmost to the central part of Sweden. This resulted not only in enumeration of the entire sequence of centuries during which the margin of the continental ice sheet retreated for more than 800 km, but also in an estimate of the time span of the post-glacial period since the disappearance of the ice sheet. Leeture presented on the oeeasion of the geologie al eongress in Stoekholm 1910 Translated by Christian Dullo and William W Hay
w.w. Hay (~) Forsehungszentrum GEOMAR, Wisehhofstr. 1-3,24148 Kiel, Germany e-mail: [email protected] W.-C. Dullo (ed.), Milestones in Geosciences © Springer-Verlag Berlin Heidelberg 2003
The most important sediment within these post-glacial deposits is a glacio-marine clay, termed "varvig lera"2 in Swedish for its varves or periodic layers which differ in colour and structure. Already during my first geological field work in the year 1878, I was surprised by the regular pattern of these layers, which very much resemble the annual rings of trees. Hence, the year after, I started detailed investigations and measurements of these layers in various regions of Sweden and continued this in the following years. It turned out that these layers were so regular and continuous that no periodicity other than annual could be ascribed to them. This led to my contention in the year 1882 that there is a very close relationship between the periodic layering of clay and the annual retreat of ice on land 3 . Two years later the investigations had progressed to the point that I considered my idea of the annual nature
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