Jökull - 01.01.2012, Side 60

E. R. Guðmundsdóttir et al.
tographs have proven not to be very effective in de-
tecting tephra layers in marine sediments (Kristjáns-
dóttir et al., 2007; Gudmundsdóttir et al., 2011a)
whereas grain counting and grain size analyses have
been effective (Gudmundsdóttir et al., 2011a). Origin
of Icelandic tephra layers can be traced to individual
volcanic systems based on their chemical composi-
tion (e.g. Jakobsson, 1979; Larsen, 1981, 1982; Lar-
sen et al., 1999; Óladóttir et al., 2008, 2011b) (Fig-
ure 2). Moreover, microprobe analyses have proven
to be very useful to distinguish between a primary
tephra layer and reworked tephra in the marine sed-
iments. A primary tephra layer manifests itself by a
dominant glass composition whereas reworked tephra
consists of grains of variable chemical composition
(Gudmundsdóttir et al., 2011a, 2012).
The distinction between a primary tephra layer
and reworked tephra is the prerequisite to be able
to use tephra layers as tool for dating and synchro-
nization. Primary tephra layers are formed by trans-
port and fall-out during a volcanic event. Rework-
ing and secondary deposition of tephra in marine en-
vironments are caused by processes such as tephra
fall-out on sea ice and ice sheets that causes a time
lag between eruption and deposition from months to
years or centuries to millennia respectively (e.g. Bond
Figure 2. Map showing the volcanic systems of Iceland (modified from Óladóttir et al., 2011; Jóhannesson
and Sæmundsson, 1998). The relevant volcanic systems are shown in colors that refer to specific chemical
composition of each system. Offshore volcanic systems except Vestmannaeyjar are not shown. Red dots indi-
cate locations of cores MD99-2269 and MD99-2275 and the Svartárkot and Kárahnjúkar soil sections. – Kort
sem sýnir gosbelti og eldstöðvakerfi á Íslandi (breytt frá Óladóttur o.fl., 2011; Jóhannessyni og Sæmundssyni,
1998). Eldstöðvakerfum á landgrunninu er sleppt nema Vestmannaeyjum. Eldstöðvakerfi sem gjóska hefur
fundist frá í sjávarseti eru sýnd í litum sem vísa til einkennandi efnasamsetningar gosefna. Rauðir punktar sýna
staðsetningu kjarna MD99-2269 and MD99-2275 og jarðvegssniða við Svartárkot og Kárahnjúka.
58 JÖKULL No. 62, 2012