Jökull - 01.01.2005, Page 60

Bergrún Arna Óladóttir et al.
Figure 3. Photographs showing the soil sections measured during this study. a) The four subsections of the
larger Atley outcrop (AT). The total thickness of the soil profile (arrowed) is approximately 5 m. b) Close-up
at the tephra succession in subsection 2 in the Atley outcrop. Shovel is 1 m. c) Central part of the profile at
the other Atley outcrop (HA) showing a 22–26 cm thick tephra layer from Katla volcano, ∼5900 years old.
Scale is 80 cm. d) Upper-most part of the Rjúpnafell profile (RF) showing partly cemented tephra layers in the
oldest soils encountered in this study. The thin yellow layer near the top of the photograph is the key tephra
marker layer A6 from Torfajökull volcano (∼7130 years). – Ljósmyndir af jarðvegssniðum með forsögulegum
gjóskulögum. a) Snið AT í Atley, mælt í fjórum hlutum. Þykkt jarðvegs og gjósku (hvít ör) er um 5 metrar.
b) Nærmynd af hluta 2. Skóflan er um 1 m á lengd. c) Snið HA í Atley, miðhluti. Neðarlega á myndinni er
22–26 cm þykkt gjóskulag frá Kötlu, ∼5900 ára gamalt. Mælikvarðinn er 80 cm langur. d) Efsti hluti sniðs í
Rjúpnafelli sem sýnir hálfhörðnuð gjóskulög í elsta jarðveginum sem rannsóknin náði til. Þunna gulleita lagið
sem örin bendir á er leiðarlagið A6,∼7130 ára gamalt, ættað frá Torfajökli.
Microprobe analyses were obtained on a Cameca
SX 100 at the Laboratoire Magmas et Volcans,
Clermont-Ferrand. The instrument was calibrated on
natural and synthetic mineral standards and raw data
were corrected by an improved ZAF procedure. The
elements Na, Mg, Al, Ca and Si were measured us-
ing 15 kV accelerating voltage, 8 nA beam current,
10 µm beam diameter and counting time was 10 s on
the peak and background. The elements Ti, P and Fe
were measured under the same conditions but over a
longer counting time: 30 s for Ti and 40 s for P and
Fe. The elements K, Mn, S and Cl were measured us-
ing a beam current of 50 nA during 50 s (S), 100 s (K
and Cl) and 150 s (Mn), with same accelerating volt-
age and beam diameter as for previous elements. Total
analysis time per sample was 6 min and 20 s. An ex-
ternal glass standard, ALV981R23 (Metrich and Cloc-
ciatti, 1989), was analysed several times during each
60 JÖKULL No. 55