Jökull - 01.01.2014, Blaðsíða 27

Volume estimate of nine prehistoric Katla tephra layers
Table 1. Tephra marker layers around the Katla volcano, their origin, age and characteristics. – Gjóskuleiðarlög
umhverfis Kötlu, uppruni þeirra, aldur og einkenni.
Marker Volcanic 14C Calendar Main characteristics Main Reference
layer system age yrs1 in the study area thickness
axes
Eldgjáa Katla (B) 934 AD Coal-bluish black, S, SE Hammer et al., 1980;
rather coarse east of the Katla volcano Larsen, 2000
V-871a Bárðarbunga (B) 871 AD Greenish-grey to greenish black, NW Grönvold et al., 1995;
abundant plagioclase crystal fragments Larsen, 1984
Hrafnkatla Katla (B) ?760 AD Coal-black, rather thick NE Óladóttir et al., 2011a
Ey-Ha Eyjafjallajökull 1540±5 BP 520 AD Light greyish yellow Smith and Haraldsson, 2005;
Dugmore et al., 2013
YNa Katla (S) 1676±12 BP 380 AD Grey, fine grained NW Larsen et al., 2001
small needle shaped grains
Grákolla (G)aTorfajökull (S) 1995±30 BP 10 AD Light grey, sometimes mixed with NE Larsen and Eiríksson,
Askja tephra unpubl. data
Hx Hekla (S) ?280 BC Light-brown or beige, coarse grained thin layer E, ENE Óladóttir et al., 2011a;
Larsen and Vilmundard., 1992
Coarse suite Unknown (I) ?∼420 BC Greyish-black, coarse grained, no fine material This study
Blue layer Hekla? (B) ?610 BC Bluish-black very fine grained, well sorted This study
Hy Hekla (S) ?600 BC Light-yellowish, coarse grained, E, ENE Óladóttir et al., 2011a;
sometimes black grains mixed within it Larsen and Eiríksson, 2008a
UNa Katla (S) 2660±50 BP 850 BC Greyish-olive green, needle shaped grains E, NE Larsen et al., 2001
Hm Hekla (S) ?950 BC Two coloured, bluish-black and light-brown, SE This study;
often mixed but not layered Larsen and Eiríksson, 2008a
Hn Hekla (S) ?1000 BC Two coloured, light-brown and greenish/bluish-black, SE This study;
often mixed rather than layered Larsen and Eiríksson, 2008a
K-E Katla (B) Black, very thick west of the Katla volcano, NW Róbertsdóttir, 1992
not a marker east of the volcano
MNa Katla (S) 2975±12 BP 1210 BC Light olive-green, needle shaped grains S, SE Larsen et al., 2001
LNa Katla (S) 3139±40 BP 1430 BC Greyish-olive green, NE Larsen et al., 2001;
needle shaped grains Larsen and Eiríksson, 2008b
K-N Katla (B) Black, very thick west of the Katla volcano, W, SW Róbertsdóttir, 1992
not a marker east of the volcano
HSa Hekla (S) 3515±55 BP 1855 BC Two coloured in most outcrops, layered, E, SE Larsen et al., 2001
lower part yellowish, upper part greyish/bluish-black
N4 Katla (S) ∼3600 1970 BC Olive green, needle shaped grains NW Larsen et al., 2001
aTephra marker layer used for SAR age calculations. (B), (S) and (I) indicate basaltic, silicic and intermediate
composition. 1Mean calendar age, calibrated (2s) after Stuiver et al. 1998. ?Calculated SAR age.
marker layers originate from the volcanic systems
Katla and Hekla but Bárðarbunga–Veiðivötn, Torfa-
jökull and Eyjafjallajökull have also produced im-
portant marker layers. Additionally one intermediate
tephra suite of unknown origin is used as a marker.
Stratigraphy is important for tephra identification
and correlation but all marker layers have some ob-
vious characteristics. The silicic markers are eas-
ily recognizable based on their colour and internal
stratigraphy and other layers have distinctive macro-
scopic characteristics such as specific grain morphol-
ogy, crystal content or thickness (Table 1). Correla-
tion was mainly based on field observation (Figure 3)
but supported by (1) major element analyses of se-
lected key layers confirming their origin (see methods
in Óladóttir et al., 2011b) and (2) age calculations of
the tephra layers. If a correlated tephra layer had a
calculated age in one outcrop that differed substan-
tially from other calculated ages, both the age model
and correlation were re-examined and corrected ac-
cordingly.
Age calculation Age calculation is based on soil
accumulation rate (SAR) between previously C-14
dated tephra marker layers. Several tephra markers
were used as cornerstones, varying between outcrops
(Tables 1 and 2). The V-871 basaltic tephra, the sili-
cic Katla layers, UN, MN, LN, and the silicic HS
layer were present in most outcrops. Additionally, the
basaltic Eldgjá tephra, the youngest silicic Katla layer,
YN, and the silicic Eyjafjallajökull, Ey-H layer, were
used (Tables 1–4).
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