Ocean-transported pumice in the North Atlantic
Anthony Newton (1999) Ocean-transported pumice in the North Atlantic. University of Edinburgh, Unpublished PhD thesis. 414 pp.
The overall aims of this study are to identify the sources of the widespread Holocene pumice deposits found along the coasts of the North Atlantic region and establish the ages of the source eruptions. In order to tackle this, it is necessary to determine whether it is possible to “fingerprint” the pumice of individual eruptions and link ocean-transported material with the established tephrochronological framework based on the stratigraphy of airfall deposits. Over 1500 electron probe microanalyses and over 200 Secondary Ion Mass Spectrometry analyses have been undertaken on pumice and tephra samples. These are the first high quality grain specific analyses carried out on ocean-transported pumice in the North Atlantic.
Current knowledge of the extent of pumice distribution in the North Atlantic region is assessed for both shoreline (natural) and archaeological contexts. Pumice pieces have been recovered from Holocene raised shorelines of north-west Iceland for the first time. Further original fieldwork in Norway has confirmed the presence of multiple levels of brown, black and grey pumice on mid-Holocene Norwegian raised beaches and white pumice on early- Holocene shorelines. Archaeological pumice, donated by collaborators, from sites in the British Isles has also been analysed. The number of archaeological sites where pumice has been recorded has been doubled to 150.
All of the analysed pumice can be correlated to volcanic activity in Iceland. These analyses establish that the majority of the mid- to late-Holocene pumice found in the North Atlantic area is dacitic and produced from Katla. A collaborative project identified 17 silicic tephra layers (SILK layers) produced by the Katla, ten of which are linked to pumice production between c. 6600 and 1626 14C years BP. Geochemically different and older pumice also occurs in Mesolithic archaeological sites in Scotland and this was also produced by Katla. Some of this older Mesolithic pumice was probably erupted by Katla c. 7000 14C years BP. The remainder of the pumice was erupted by early Holocene activity at Katla, which also deposited pumice on the flanks of the volcano. In addition, early Holocene activity from Öræfajökull produced pumice found on a raised shoreline in Norway. The 1362 AD eruption of the same volcano produced the white pumice found in three medieval archaeological sites in Scotland. The pumice found on raised shoreline in Svalbard was produced by eruptions from both Katla and the island of Jan Mayen.
Crucially, the most prolific Icelandic producer of distal tephra layers, Hekla, is not the source of any of the pumice found around the North Atlantic. It is suggested that this could be because of the fragile nature of the Hekla pumice. This work shows that high quality geochemical data is essential if correlations are to be made between pumice deposits and sources, and highlights both the potential and limitations of the use of pumice as a tephrochronological tool.