Distribution, sediment magnetism and geochemistry of the Saksunarvatn (10,180 ± 60 cal. yr BP) tephra in marine, lake, and terrestrial sediments, northwest Iceland

In 1997, seismic surveys in the troughs off northwest and north Iceland indicated the presence of a major, regional sub-bottom reflector that can be traced over large areas of the shelf. Cores taken in 1997, and later in 1999 on the IMAGES V cruise, penetrated through the reflector. In core MD99-2269 in Hunafloaall, this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the North Atlantic-wide Saksunarvatn tephra. We trace this tephra throughout northwest Iceland in a series of marine and lake cores, as well as in terrestrial sediments; it forms a layer 1 to 25 cm thick of fine- to medium-grained basaltic volcanic shards. The base of the tephra unit is always sharp but visual inspection and other measurements (carbonate and total organic carbon weight %) indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking. Off northwest Iceland the Saksunarvatn tephra has distinct sediment magnetic properties. This is evident as a dramatic reduction in magnetic susceptibility, an increase in the frequency dependant magnetic susceptibility and 'hard' magnetisation in a -0.1T IRM backfield. Geochemical analyses from 11 sites indicate a tholeiitic basalt composition, similar to the geochemistry of a tephra found in the Greenland ice-core that dates to 10180 +/- 60 cal. yr BP, and which was correlated with the 9000 C-14 yr BP Saksunarvatn tephra. We present accelerator mass spectrometry C-14 dates from the marine sites, which indicate that the ocean reservoir correction is close to ca. 400 yr at 9000 14C yr BP off northwest Iceland. Copyright (C) 2002 John Wiley Sons, Ltd.