Senglasiale vulkanske askelag i innsjøsedimenter på Vestlandet

Abstract

A sediment core from a peat bog on Dimnøy at Sunnmøre, western Norway has been investigated in order to search for volcanic ash horizons. A new volcanic ash with a geochemistry more or less identical to the rhyolitic component of the Vedde Ash, but some 2500 14C-years older was found. The ash geochemistry was determined by wavelength dispersive spectral electron microprobe analysis (WDS-EMPA). The ash is here named the Dimna Ash. It was found in a palaeo-lake with typical late glacial stratigraphy with grey sand and silt at the base followed by brownish Allerød mud below grey Younger Dryas silt. The Vedde Ash bed is very pronounced in the sediment sequence and is up to 48 cm in thickness near a basin inlet. A series of AMS dates from terrestrial plant remnants confirms the age of the late glacial stratigraphy. The Dimna Ash is invisible to the naked eye and was detected through the application of a density separation technique. It was found near the base of the lacustrine sequence, about 120 cm below the base of the Vedde Ash. The Dimna Ash is concentrated in a zone with two distinct peaks, 20 cm apart. The lowermost peak has an ash particle density of 10 600 and the uppermost 12 900 grains pr. cm3. The lowermost AMS dates, yielding the ages 12 780 ± 60 and 12 840 ± 70 14C BP (approx. 15 100 calendar years BP) lie 25 cm above the uppermost density peak. These datings are minimum ages for the volcanic eruption that produced the Dimna Ash. Ash grains with similar geochemical signature have also been found in the sediments between the Dimna and the Vedde Ash, although in much smaller quantities. These ash grains might represent reworking of the Dimna Ash. The possibility that the Dimna Ash could have been deposited on the inland ice and then washed out into the basin during deglaciation can not be ruled out. Nine shards of volcanic ash of a different geochemical composition were analyzed from a zone more than 50 cm high. This zone was stratigraphically above the Dimna Ash, but below the Vedde Ash. The geochemistry of the shards is similar to the Borrobol Ash previously found in Scotland and Sweden. Even though a well defined Borrobol Ash layer was not found in this work this proves that there might be a potential for one or more such layers existing also at Sunnmøre.