Jökull - 01.01.2012, Side 66

E. R. Guðmundsdóttir et al.
The majority of the known tephra markers in Ice-
landic tephra stratigraphy are either silicic or interme-
diate in chemical composition. They generally have
more distinct characteristics, they often have wider
distribution due to the nature of the eruptions and are
not as abundant as basaltic tephra layers. The vast
number of basaltic tephra, the similar appearance and
chemical composition of layers from the same sys-
tem makes it challenging to use them as marker lay-
ers or isochrones. Moreover the basaltic tephra layers
tend to be more spatially confined making them less
suitable as marker layers. Nevertheless widespread
Icelandic basaltic tephra markers do exist such as
Saksunarvatn ash, Mjáuvötn tephra, Settlement layer,
Hrafnkatla, V1477 and V1717 (Table 2). Due to their
limited number in marine sediments, basaltic tephra
layers from Katla have a potential as tephra markers
in marine sediments north off Iceland. Basaltic tephra
layers also have the potential to be tephra markers as a
part of a spectra or series of tephra layers (Gudmunds-
dóttir et al., 2012). The age of the marine tephras on
the North Iceland shelf is based on correlation with
terrestrially dated tephras and dated volcanic events.
POTENTIAL TEPHRA MARKERS IN
THE MARINE ENVIRONMENT
One of the aims of tephra and tephrochronological
studies is to increase the number of tephra layers
that can be used as marker horizons. Identification
of a tephra layer with distinct characteristics, found
in several locations that can be absolutely dated has
the potential to become a tephra marker. Known lo-
cal and regional markers in terrestrial tephrochronol-
ogy in Iceland are about 40 (e.g. Larsen et al., 1999;
Larsen and Eiríksson, 2008a; Óladóttir et al., 2011a).
A portion of these markers has been traced into the
marine sediments on the Iceland shelf (e.g. Larsen et
al., 2002; Kristjánsdóttir et al., 2007; Gudmundsdótt-
ir et al., 2012) (Table 2). In general tephra layers
with volumes more than 0.1 km3 can be expected to
be deposited in marine sediments around Iceland and
overseas. Large tephra layers exceeding volumes of
10 km3, such as the Hekla 4 and 3, can be expected
to have regionally extensive deposition. Less volu-
minous tephra layers can be expected to have lim-
ited distribution depending on the weather conditions
at the time of the eruption. They commonly tend to
be strongly limited laterally downwind (Larsen and
Eiríksson, 2008b). Holocene silicic, intermediate and
basaltic tephra layers in the 0.1–1 km3 category that
are likely to occur sporadically outside Iceland and
have not been reported in marine sediments on the
shelf are shown in Table 3. All of these tephra lay-
ers have been used as tephra markers in Icelandic ter-
restrial tephrochronology and most of them are silicic
or intermediate. Holocene basaltic tephra layers that
have been mapped fall within the 0.01–1 km3 cate-
gory but a few reach a volume of 10–20 km3 (Larsen
and Eiríksson, 2008b and references therein). Recent
additions to the marine tephra marker assemblage are
Hekla 1947, V1797, Hrafnkatla, Hekla DH, Hekla Ö
and Askja S (Eiríksson et al., 2011; Gudmundsdóttir
et al., 2011, 2012).
CORRELATING MARINE AND
TERRESTRIAL TEPHRA
STRATIGRAPHY
High-resolution studies have enabled the establish-
ment of a detailed marine tephrochronological frame-
work, which has been correlated to high-resolution
terrestrial tephra stratigraphy in Iceland. Land-sea
correlation between marine core MD99-2275 and
three soil sections in north and east Iceland has re-
sulted in correlation of over 30 tephra layers within
the last 7,050 years (Gudmundsdóttir et al., 2012).
For the last 15,000 years 40 tephra layers from shelf
sediments around Iceland have been correlated to
their terrestrial counterparts both in Iceland and over-
seas. Thereof 26 are absolutely dated tephra lay-
ers, i.e. tephra markers. The tephra markers Hekla
1947, V1717, KOL 1372, H1104, Sn-1, Hekla 3,
Hekla 4, Suduroy tephra, Saksunarvatn ash, Vedde
Ash and tephra with Borrobol affinity have been
identified in more than one core on the North Ice-
land shelf in addition to four tephra layers from the
Kolbeinsey Ridge (KOL) or Tjörnes Fracture Zone
(TFZ); KOL3-2269/3373, KOL2-2269/2912, KOL1-
2269/2780 and KOL-GS-2 (Eiríksson et al., 2000;
64 JÖKULL No. 62, 2012