Jökull - 01.01.2014, Blaðsíða 26

B. A. Óladóttir et al.,
METHODS
Field work
Soil sections around the Katla volcano, 26 east of the
volcano and 10 west of it (Figure 1), were measured in
order to evaluate tephra volume of nine selected pre-
historic Katla layers, named K-I to K-IX. The strati-
graphy of the time period between tephra marker lay-
ers V-871 (Settlement layer, Table 1) and Hekla-S
(HS; Figure 2, Table 1) was measured. Sections in
slopes and below steep hills were excluded to avoid
downhill creep that may skew the thickness of tephra
layers (Figure 2c-d). Tephra was sampled for chemi-
cal analyses to facilitate correlation between sections.
Analyses
Major-element concentration of individual glass
grains was analysed with a WDS Cameca SX100 elec-
tron microprobe in Clermont-Ferrand, France. The
microprobe set up was as described in Óladóttir et al.,
(2011b) except for increased counting times for Fe, up
to 50 s. Concurrently, Mn counting time was reduced
to 20 s. The beam size ranged from 10–20 µm. Raw
data were corrected by the X-PHI correction proce-
dure (Merlet, 1994).
Correlation and tephra marker layers
The soil profiles were correlated using local and re-
gional tephra marker layers (Table 1). Most of the
Figure 2. Soil sections. a) The section measured in this study in Rjúpnafell (Figure 1), from V-871 in the top
and down to HS. Shovel for scale is ∼80 cm. b) Selfjall outcrop showing the same as a). c) Soil section on flat
land, ideal location for volume estimates. d) A soil section on a steep slope. Downhill creep skews measured
tephra thicknesses; such sections were not used in this study. Photo: d) Hafdís Roysdóttir. – a) Jarðvegur frá
tímabilinu frá V-871 niður að HS í Rjúpnafelli (sjá 1. mynd). Skófla er∼80 cm löng. b) Jarðvegsopna í Selfjalli
nær yfir sama tímabil og a). c) Jarðvegsopna á jafnsléttu og þar með hentug fyrir mælingu fyrir rúmmálsmat
gjóskulaga. d) Jarðvegsopna í miklum halla, tilflutningur skekkir þykktir gjóskulaga og gerir opnur í þessu
umhverfi óæskilegar þegar unnið er með rúmmálsmat gjóskulaga. Ljósmynd d) Hafdís Roysdóttir.
26 JÖKULL No. 64, 2014