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Tephrochronology and the dating of the Settlement

<< Previous: "The Vikings in Iceland"

Until recently, there has been much discussion over the archaeological dating of the settlement of Iceland. Both the Íslendingabók and the Landnámabók date the founding of Reykjavík by Ingólfur Arnarson and Hjörleifr Hróðmarsson to 874.

There were sever discrepancies between the historical date and the dates given by C14 samples from various excavations in Iceland which gave dates as early as the later 8th century.

Recent work comparing c14 samples from barley grain and birch charcoal have shown interesting differences. Although the charcoal dates centre around 780, the barley grain dates centre around 890, much closer to the traditional date of settlement. It appears that much of the firewood used by the settlers was old, dead wood that has skewed the dating (see table here).

Because of the volcanic nature of Iceland, another form of dating can also be applied on many archaeological sites.This is tephrochronology - the use of layers of volcanic ash as a dating medium. These ash layers have different chemical signatures, and so can be identified by their makeup.

Of particular importance is the Landnám Tephra - a layer of volcanic ash that precedes the earliest settlement and covers much of Iceland.

This distinctive tephra layer was recognised in ice core samples taken from Greenland.

Throughout each year, layers of snow fall over the ice sheets in Greenland. Each layer of snow differs in chemistry and texture. Summer brings 24 hours of sunlight to the polar regions, and the top layer of the snow changes in texture—not melting exactly, but changing enough to be different from the snow it covers. The season turns cold and dark again, and more snow falls, forming the next layers of snow.

By distinguishing these annual differences, layers within the ice can be dated by counting back from the present. Using this method, it was possible to date the Landnám Tephra to 871±2.

The photgraph below shows a number of datable trephra layers. The Landnám Tephra is the one at the bottom, dated 872.

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