Jökull - 01.01.2016, Page 71

Grain characteristics of silicic Katla tephra layers
Table 1. Known silicic Katla tephra layers erupted in the last 8100 years. – Þekkt súr Kötlu gjóskulög frá síðustu
8100 árum.
SILK Tephra 14C age BP 14C yrs cal b2k Rounded age References
and SAR age
YN 1676±12 1622±40 ∼1600 Dugmore et al., 2000
UN 2660±50 2850±70 ∼2800 Larsen et al., 2001
MN 2975±12 3230±30 ∼3200 Larsen et al., 2001
LN 3139±40 3440±75 ∼3400 Larsen et al., 2001
N4 ∼3920 ∼3900 Estimated after Óladóttir et al., 2008
N3 ∼4050 ∼4100 G. Larsen unpublished data
N2 ∼4960 ∼5000 After Óladóttir et al., 2005, Table 1b
N1* ∼5830 ∼5800 After Óladóttir et al., 2005, Table 1b
A1* ∼6010 ∼6000 After Óladóttir et al., 2005, Table 1b
A2 ∼6700 ∼6700 Estimated after Óladóttir et al., 2005, 2008
A3 ∼6900 ∼6900 Estimated after Óladóttir et al., 2005, 2008
A5 ∼7100 ∼7100 Directly above Hekla-5, 7125 cal b2k
A7 ∼7180 ∼7200 Estimated after Óladóttir et al., 2008
A8 ∼7400 ∼7400 Estimated after Óladóttir et al., 2008
A9 ∼7500 ∼7500 Estimated after Óladóttir et al., 2008
A11 ∼8000 ∼8000 Estimated after Óladóttir et al., 2008
A12 ∼8100 ∼8100 Estimated after Óladóttir et al., 2008
*N1 and A1 have also been called T1 and T2 (Larsen et al., 2005).
Silicic Katla eruptions during the last 8100 years
Published research about the Holocene silicic tephra
from the Katla volcanic system is scant (e.g. Larsen et
al., 2001; Wastegård, 2002) and most research has fo-
cused on Katla’s basaltic volcanism (e.g. Jakobsson,
1979; Thorarinsson, 1980; Larsen, 2000; Thordar-
son et al., 2001; Óladóttir et al., 2005, 2008). The
best known silicic tephra from Katla, the wide-spread
pre-Holocene Vedde Ash (e.g. Lacasse et al., 1995;
Wastegård et al., 2000 a,b, Blockley et al., 2007;
Davies et al., 2010) is outside the scope of this paper.
Of the 17 known silicic tephra layers, four have
been dated by radiocarbon measurements on peat im-
mediately below the tephra and 12 have been given an
approximate age using soil accumulation rates (SAR)
between tephra layers of known age (Table 1 and ref-
erences therein). All are prehistoric. The most re-
cent tephra layer, SILK-YN, is about 1620 years old
(1676±12 14C BP).
The axes of thickness of six silicic layers intersect
more or less inside the caldera (Figure 2). Although
this points to a source of the silicic eruptions within
the Katla caldera, it is also possible that the some oc-
cur at the caldera fracture (Larsen et al., 2001). There
are silicic rocks that outcrop along the caldera mar-
gin, e.g. at Austmannsbunga, but the geochemistry of
the outcrops differs from that of the Holocene silicic
tephra (Lacasse et al., 2007).
Figure 2. Axes of thickness of the silicic tephra layers
from Katla and sampling locations (Larsen, 2000). –
Þykktarásar súrra gjóskulaga frá Kötlu og sýnatöku-
staðir (Larsen, 2000).
Óladóttir et al. (2007) have shown that during the
last 8400 years the sulphur degassing of the basaltic
Katla tephra has been arrested, indicating that the
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