ASH LAYERS FROM THE LAST GLACIAL TERMINATION IN THE NGRIP ICE CORE

Deposits from explosive eruptions leave marker horizons – important for correlating terrestrial, marine and ice records. Ice cores contain the chemical trace of eruptions, but in the absence of historical records these are difficult to correlate. Where ash particles are present, chemical analysis of the juvenile material (glass) makes correlation more significant and source volcanoes can often be identified.

A number of ash layers were identified in the GRIP core and some correlated with other occurrences. More recently a systematic study was done on the transition zone (ca 16-10 thousand years BP) in the NGRIP core. Based on continuous sulphate, calcium and dust profiles, 95 samples were selected, each covering an area of about 1 cm2 and spanning 1-2 year accumulation. Samples were examined and analysed using SEM-EDS and WDS methods. Twelve samples had a number of ash grains up to 50 micron in diameter and eight samples had one ash particle. Two of the layers, Saksunarvatn 10180 and Vedde 11980 ice core years in age, can be correlated with the GRIP core.

All the layers appear to be of Icelandic origin – providing a valuable extension of Icelandic tephrochronology with 5 rhyolitic, 4 basaltic and 3 intermediate layers. The frequency of ash layers found in the NGRIP core is 2,4 –3,5 layers/ky in Bølling-Allerød and Younger Dryas, but only one layer was found in the first 1300 years of the Holocene. This may indicate a significant activity decrease in explosive volcanism due to the decreased Iceland ice cover or change in the prevailing wind patterns in the early Holocene.

There is marked difference in the excess sulphate in the ice – considered to be the volcanic signal – between the B/A and the colder periods, GS-2 and Y-D. During the B/A the excess sulphate is low for both basaltic and rhyolitic eruptions, but during the colder periods it is significantly higher. In the light of the importance of sulphates in climatic forcing it is important to establish, if this is genuine or a reflection of how excess sulphate is estimated.