Prof. Emeritus Mike Baillie in the cold
store of the Niels Bohr Institute, Copenhagen
Dr Sarah Coulter preparing bottles to collect
ice samples for Holocene tephra analysis
Refining a late Holocene tephrochronology for the Northern Hemisphere using Greenland ice-cores
The central Greenland ice cores are probably the world’s most important repositories of palaeoenvironmental data for the last glacial-interglacial cycle, the dating of which attains a precision that surpasses most sedimentary sequences over this time span. The correlation of the ice core records with each other, as well as with those from terrestrial and marine sequences in the northern hemisphere, can be greatly facilitated by the identification of geochemically-distinct tephra layers within the ice. The recognition of tephras in the ice also provides a means by which the source volcanic eruption for acid layers in the ice can be identified, and the age of the non-historic eruptions can be greatly refined. Furthermore, the wider environmental impact of particular eruptions can be examined by looking at the ice core proxy records at the time of the tephra.
Tephrochronologists in GAP, led by Professors Emeriti Valerie Hall, Jonathan Pilcher and Mike Baillie, and involving former research assistants Dr Gill Plunkett and Dr Sarah Coulter, are collaborating with glaciologists in the Niels Bohr Institute in Copenhagen to look for tephras in sections of the Dye-3, GRIP and NGRIP ice cores. The team has targeted its research (funded by 14Chrono) at a number of specific Late Holocene tephras, including those from historic eruptions such as Katmai AD 1912, Laki 1783-1784, Öraefajökull AD 1362 and Hekla AD 1104, as well as some whose precise dates are not known (e.g. Bronze Age eruption of Thera, Hekla 4). The Katmai and Öraefajökull tephras have been successfully isolated in ice of the same age as their respective eruptions, and their major element compositions confirmed by single shard microprobe analysis. These confirm the precision of the ice core chronologies over the last millennium. The research has revealed 11 other tephras, the results of which have recently been published in Journal of Geophysical Research – Atmospheres.
The collaboration with the Danish glaciologists has also involved research on ice from the more recent NEEM coring project in north Greenland. Continuous Flow Assessment (CFA) samples from ice spanning the first millennium AD have been examined for tephra content by Jonathan Pilcher. CFA samples are collected at the coring site by melting narrow (3.3 x 3.3 mm) rods of ice which are simultaneously analysed for chemical and isotopic properties, as well as the concentration of dust particles. Despite the small surface area represented by these samples, tephra shards have been found in 18 samples, and these will form the basis for a more detailed sampling of the cores for tephra analysis. More samples are eagerly awaited…