Ocean-transported pumice in the North Atlantic
Anthony Newton © 1999
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. |