Jökull - 01.12.1988, Page 89

the Videy island offReykjavík soil sections in bogs
have a quite even rate of soil thickening. Two age
estimates were done by comparing soil thicknesses
between the Medieval tephra and the Landnám and
Katla 1485 layers, respectively. An average age
obtained is 1199 which is in reasonably good
agreement with the historical records of 1226.
Our results suggest the year 1226 as the age for
the Medieval tephra layer.
AGE OF A TURF WALL AT
THE OLD KRÍSUVÍK FARM
A section was made across one of the turf walls
(Fig. 6) which is partly covered by the Ögmundar-
hraun lava flow. The wall is older than 900 AD
because the Landnám tephra layer is not found
beneath the wall but traces of it were found in a
dugout alongside it.
AGE OF THE ÖGMUNDARHRA UN LAVA FLOW
Two 14 C datings have been done on charcoal
(from fire site) from the ruins of the old Krísuvík
farm (Einarsson, pers. comm.) which was overrun by
the Ögmundarhraun lava flow. The datings give a
mean age of 970 years BP and if corrected accord-
ing to the correction diagram of Stuiver and Becker
(1986) the age is 1027 AD (Fig. 4). Jónsson (1982)
dated three charcoal samples from the lower
surfaces of the Ögmundarhraun flow giving a mean
age of 923 BP. Correction by the Stuiver and Becker
diagram gives five dates 1044, 1096, 1122, 1141 and
1156 (Fig. 4). This is due to an irregular content of
14C in the atmosphere at that time. Einarsson’s dat-
ings give a slightly higher age as can be expected.
He proposed that the samples could be bumt drift-
wood or peat.
A few soil sections were measured at the margins
of the Ögmundarhaun lava flow and near the craters
where scoria is preserved in soil left uncovered by
the flow. The Settlement layer is usually about 2-7
cm below the flow but the Medieval layer is 0-4 cm
above it (Fig. 3). The Ögmundarhraun flow is thus
closer to the Medieval layer than the Landnám
layer.
The old annals (Storm, 1888) mention two er-
uptions in Trölladyngjur which is a mountain in the
centre of the Reykjanes peninsula and the Ogmund-
arhaun crater row lies just east of it. The reference is
made to eruptions in the years 1151 and 1188. The
annals also mention a strong earthquake in the year
1151. This is to be expected because faults and
fractures related to the South Iceland Seismic Zone
have been traced west along the Reykjanes
peninsula.
We suggest that the Ögmundarhraun lava flow
was erupted in the year 1151. This agrees
reasonably well with the tephrochronology and the
records of the old annals.
JÖKULL, No. 38, 1988 87