Jökull - 01.01.2014, Blaðsíða 34

B. A. Óladóttir et al.,
Electron microprobe results
Of the 80 analysed samples, 40 are from Katla, 16 are
of the coarse suite, 10 of the Hekla Blue layer, 10 of
an unknown Hekla layer about 100 years older than
Hn, and 4 of the Hekla-S layer.
All analysed Katla layers are evolved Fe-Ti basalts
(Figure 5). The coarse suite turned out to be three lay-
ers of intermediate composition, all very similar al-
though the compositional range is greater in the older
two. The Blue layer has a similar composition as∼2–
4 ka older tephra layers named F and T that have been
suggested to be produced by eruptions in the vicinity
of the Hekla volcano (Harðardóttir et al., 2001; Jó-
hannsdóttir, 2007).
The unknown Hekla layer (Table 2) is of an inter-
mediate origin (Figure 5). Its soil accumulation rate
(SAR) age, 1110±40 BC (∼3110 b2k) is similar to
the H3 age ∼1050 BC (2879±34 14C years BP; Dug-
more et al., 1995) and it could either be part of the
H3 layer or of another Hekla layer with an eastward
deposition erupted between H3 and Hn.
0
1
2
3
4
5
6
1.6 1.8
Ti
O
2
w
t%
K2O wt%
0.4 0.6 0.8 1 1.2 1.4
Katla this study
Katla Óladóttir et al. 2008
Hekla unknown this study
Figure 5. Concentrations of TiO2 versus K2O in tephra
glass from the Katla layers (grey triangles) and the
Hekla unknown layer (red points). Purple points are
samples from Óladóttir et al., 2008. – Línurit sem
sýnir styrk títanoxíða (TiO2) sem fall af styrk kalíoxíða
(K2O) í gjóskugleri frá Kötlu (gráir þríhyrningar) og
óþekktu Heklulagi (rauðir tíglar). Fjólubláir tíglar eru
sýni greind af Bergrúnu Óladóttur o.fl. (2008).
Age calculations
In total nine tephra marker layers (Eldgjá, V-871, Ey-
H, YN, Grákolla, UN, MN, LN, HS; Table 1) were
used to calculate age based on soil accumulation rate
(SAR). The 36 soil profiles show different SAR peri-
ods as the marker layers are not always the same (Ta-
ble 3). The Langanes outcrop has seven dated tephra
markers which is the highest number of marker lay-
ers in an outcrop whereas Álftavatn and Herjólfsstaðir
have only two (Table 3). In 21 out of the 36 outcrops,
4-5 marker layers of known age were found.
The SAR is lowest, 0.11 mm/year, between V-
871 and UN in Leiðvöllur outcrop and highest, 0.95
mm/year, between UN and HS in the same outcrop.
This outcrop is close to Kúðafljót, a glacial river that
has influenced the soil sections by variable sediment
influx, resulting in extremely variable SAR values.
The average SAR ranges from 0.14–0.70 mm/year in
Skuggafjallakvísl and Herjólfsstaðir, respectively, and
the overall average is 0.38 mm/year (Table 3).
The age of the eight prehistoric Katla tephra lay-
ers, mapped during this study, was calculated based
on these SAR values. The oldest layer, K-IX, was
erupted around 1150±40 BC and the youngest one,
K-I, around 270±60 BC (Table 5). In addition, im-
proved SAR ages of two Hekla layers, Hm and Hn,
were calculated from these sections giving the age of
950 (950±30) and 1000 (1000±40) BC, respectively,
with previous published ages being 960 and 1030 BC
(Óladóttir et al., 2012).
Volume estimates
Nine prehistoric Katla tephra layers have been
mapped (Figure 6) and their volumes on land esti-
mated. Seven out of the nine tephra layers have
dominant eastward dispersion. The minimum un-
compacted (UCP) volume on land ranges from 0.2
to 2.7 km3 (0.1–1.6 km3 compacted volume) with an
average size of 0.9 km3 and the area covered by the
tephra fall within the 1 cm isopach on land ranges
from 1700–10,400 km2 (Table 5). The volume un-
certainty within the mapped area is ∼5% calculated
after x/
√
n where x is the uncertainty on an indi-
vidual measurement, estimated 30% (Engwell et al.,
2013), and n is the number of measurements, or 36
34 JÖKULL No. 64, 2014