Jökull - 01.01.2012, Side 65

Holocene marine tephrochronology on the Iceland shelf
South- and west Iceland shelf
Little research on other areas around the shelf of
Iceland has been undertaken in terms of identifying
tephra layers systematically. On the western part
of the shelf only two tephra layers, the well known
tephra markers Saksunarvatn ash and Vedde ash, have
been identified until now (Table 2) (Andrews et al.,
2002; Geirsdóttir et al., 2002; Jóhannsdóttir, 2003).
This is not to say there are no tephra layers in ma-
rine sediments in the west but rather they have not
been looked for in detail. However it would be ex-
pected that relatively fewer tephra layers would be
identified on the western shelf compared to the north-
ern shelf, due to prevailing wind directions in Ice-
land. The east flowing jet stream dominates at 10–
15 km heights (Jónsson, 1990) and most explosive
eruptions in Iceland produce plumes reaching such
heights (Gudmundsson et al., 1992; Lacasse et al.,
2004; Oddsson et al., 2012) promoting east and north-
eastward distribution. This is evident from terres-
trial archives in western Iceland where tephra studies
have revealed a marked decrease/drop in number of
tephra layers compared to north, east and south Ice-
land (Jóhannsdóttir, 2007; Thordarson and Höskulds-
son, 2008). South of the Iceland shelf nine tephra
layers have been described in the sediments spanning
late glacial and early Holocene and one with an age of
about 23,400 years (Thornalley et al., 2011). Among
these are the tephra markers Saksunarvatn ash and
Vedde Ash (Table 2). No tephra stratigraphical in-
vestigation on marine cores has yet been carried out
for the Holocene time period. Hitherto no cores have
been collected on the East Iceland shelf and no infor-
mation is available on tephra layers there.
Sources of tephra layers on the Iceland shelf
The vast majority of the marine tephra layers iden-
tified are basaltic originating from the most active
volcanic systems in Iceland; Grímsvötn, Veiðivötn-
Bárðarbunga and Katla (Figure 2 and Table 2). The
Grímsvötn volcanic system has been the most produc-
tive during the Holocene with about seven eruptions
per century followed by Veiðivötn-Bárdarbunga and
Katla (Óladóttir et al., 2005, 2008, 2011a). This is re-
flected in the marine tephra stratigraphy where tephra
layers from Grímsvötn are the most common and then
Veiðivötn-Bárdarbunga and Katla volcanic systems
(Gudmundsdóttir et al., 2012 and unpublished data).
Other systems that have produced tephra deposited on
to the Iceland shelf are Hekla, Eyjafjallajökull, Ör-
æfajökull, Askja, Snæfellsjökull and Kolbeinsey ridge
(KOL) or the Tjörnes fracture zone (TFZ) (Figure 2).
Tephra layers from the KOL/TFZ are found in sedi-
ment cores; MD992269, -2271 and -2275; HM107-
01, -01, -03, -04 and -05 (Eiríksson et al., 2000,
2004; Knudsen et al., 2004, 2008; Kristjánsdóttir et
al., 2007; Gudmundsdóttir et al., 2012).
TEPHRA MARKERS
One of the applications of tephra studies, as men-
tioned previously, is dating and correlations. For
that purpose tephra markers are used. Tephra mark-
ers should be relatively easily recognizable, either
by macro- and microscopic characteristics (such as
colour, grain type, crystal content) or chemical char-
acteristics, be traceable over large areas and well
dated (e.g. Larsen and Eiríksson, 2008a). Ideally a
tephra marker should have all these qualities. Tephra
markers are the foundation of a tephrochronological
framework. Examples of tephra layers that have been
used as markers in Icelandic tephra stratigraphy are
the silicic tephra layers, H1104, Hekla 3, Hekla 4 and
Hekla 5, large explosive eruptions from the Hekla
volcanic system. These tephra layers have specific
characteristics such as color, grain morphology and
chemical composition and have been found in vari-
ous environments, soil, lakes, marine and glacial ice
within and out side terrestrial Iceland (e.g. Thorarins-
son, 1967, 1971; Larsen and Thorarinsson, 1977;
Dugmore et al., 1995b; Dugmore and Newton, 1998;
Eiríksson et al., 2000, 2004; van den Bogard and
Schmincke, 2002; Zillén et al., 2002; Boygle, 2004;
Pilcher et al., 2005; Wastegård, 2005, Jóhannsdóttir,
2007; Kristjánsdóttir et al., 2007; Sejrup et al., 2011;
Larsen et al., 2011, 2012; Gudmundsdóttir et al.,
2012). These tephra layers have been used for dating
the marine shelf sediments off north Iceland and for
correlation between areas on the shelf and terrestrial
Iceland as will be discussed later.
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