Íslenskar landbúnaðarrannsóknir - 01.09.1975, Blaðsíða 43

MYNDUN MÓAJARÐVEGS í SKAGAFIRBI 41
SUMMARY
Soil formation in Skagafjörður,
northern Iceland.
Grétar Guðbergsson
Agricultural Research Institute,
Keldnaholt, Reykjavík.
Loessial soil formation during the Holocene
in Skagafjördur is investigated. Twenty seven
soil profiles have been measured and grain
size and mineralogical analysis have been
made. Tephrochronology, based on four light
tephra layers (H5, H4, H3, Hi) from the vol-
cano Hekla, provides a time scale for the
history of the soil formation.
The grain size analysis show that the mean
grain size in three soil profiles (8, 14, 24) is
0,07 mm, i.e. fine sand. The raatio between
fine sand (d > 0,063 mm) and fines
(d < 0,063 mm) is used to reveal changes
in grain size within profiles and between pro-
files. This ratio decreases from the beginning
of soil formation at the end of the Last
Glacial up to the later Bog Period (Subat-
lantic).
With increasing vegetation the movement
of windblown material will be reduced and
formation of loessial soils consequently limi-
ted. This reflects the climatic deterioration
at the beginning of the later Bog Period.
Loessial soil in the Skagafjördur area is
mainly composed of dark brown and altered
brown volcanic glass, derived from the Ice-
landic Moberg Formation. Rhyolithic tepra
is only 17% and rock fragments and crystals
contribute about 15% of the grains. This soil
material, apart from rock fragments and
crystals contrasts within the bedrock of the
area, and must largely have been transported
by wind into the area during the soii forma-
tion period. Clay minerals 'have not been de-
tected. As said above the rhyolithic tephra
layers originate mainly from the volcano
Hekla. The oldest is H-, (7100 B.P.) and after
that rhyolithic tephra grains begin to appear
in the soil and these are markedly enriched
after the fall of the tepra layer Hi (2900
B.P.) which is the thickest in the area, 3 —
12 cm.
Accumulation of soil has been relatively
slow up to Hi (4500 B.P.), averaging about
0,05 mm/year. During the time between H (
and H3 (4500—2900 B.P.), the accumula-
tion decreases to 0,04 mm/year. During the
later Birch Period climatic conditions have
been more favorable than at any other time
during the Holocene. During that period
vegetation covered larger areas than at any
other time in Holocene times. Thus the area
exposed to wind erosion was reduced. Rate
of soil accumulation was then doubled during
the period 2900 B.P. (H3) to the Settlement
(874 A.D.) to become 0,07 mm/year. This
is due deteriorating climate during the earlier
half of the later Bog Period. Since the be-
ginning of the Settlement the accumulation
rate increases greatly, probably due to influ-
ence of dense population and subsequent cle-
aring of shrubland for farming and grazing.
From 874 A.D. to 1104 A.D. (Hi) the accu-
mulation rate is 0,74 mm/year, and from
1104 A.D. to the present the rate has been
0,5 mm/year. It is noteworthy how close the
relationship is between the soil formation and
the changes in vegetation cover during pre-
historic time as can be seen by the low sand/
fines ratio. This relationship is greatly distur-
bed by human activity in historical time.