Jökull - 01.12.1953, Blaðsíða 14

Table I. Pits dug on Vatnajökull 28/6—4/7 1953.
Tafla I. Gryfjur grafnar á Vatnajökli 28/«—4/7 1953.
Date of measure- ment Pit. No. Approx. height m above Sea level Depth of pit cm Accumul. autumn 1952-1 July 1953 cm Average Spec. gravity Accumul. autumn 1952-1 July 1953 mm water Correspond. annual precipit. mm water
28/e I 1080 230 230 0.61 1395 >1680
30/e II 1420 385 345 0.57 1980 2400
3°/e III 1595 510 452 0.53 2385 2860
Vt IV 1445 427 405 0.55 2350 2820
2/t V 1660 435 420 0.55 2300 2760
2h VI 1805 430 426 0.52 2215 2660
*h VII 1988 455 230 0.51 1165 1400
the intake area of Dyngjujökull was 2400 mm
(cf. Rist. 1953).
As already mentioned, 7 pits were dug in
western Vatnajökull by our expedition to meas-
ure the accumulation since the previous
autumn. In each pit some determinations of
the specific gravity of snow and firn were made
in order to find the water equivalent of the
measured accumulation. Fig. 1 shows the situat-
ion of the profiles and the accumulation since
autumn 1952, expressed in mm of water. Fig.
4 shows the profiles themselves. The first pro-
file was measured on 28 June and the last one
on 4 July, and as there was no precipitation in
the area the first four days of July, the accu-
mulation measured represents the period aut-
umn 1952 — end of June 1953, or roughly 10
months. The approximate height of the pro-
files above sea level is shown in Table I.
In profile I, the thickness of the accumulation
layer was easily determined, as the profile was
situated below the firn limit. In profile II, I
was somewhat dubious about the boundary be-
tween the 1952/53 and 1951/52 accumulation,
until I had studied profile III, situated near to
the dirt cone area W of the Grímsvötn depres-
sion, wherefrom dust has been spread sufficient-
ly enough to form a distinct dust layer at the
end of the 1952 ablation period. In profile VII
the limit between the 1952/53 and 1951/52
accumulation layers was not determined with
absolute certainty, as no dust was found there,
but in all probability tlie limit is rightly placed.
No temperature measurements were made in
the firn profiles, but I had the impression that
in both the highest pits (VI and VII) the firn
was frozen to the bottom of the pits.
The layer marked with an X in some of the
profiles was in all probability formed during
the very cold days about Palm Sunday (29/3)
1953, which succeeded a warm period. In pro-
files I—IV, the surface layers were very wet, so
that although the firn was so loose that we
sank deep in it wlien walking, the specific
gravity was 0.55—0.60. From 1200 to 1500 m
height this loose surface layer was very coarse-
grained (grain diam. 2—5 mm).
In Table I, I have summed up the results
of the pit measurements. In the last column I
have calculated roughly the corresponding an-
nual precipitation at the respective pits. In
doing so I have presupposed that the measured
accumulation represents 10 months and that,
except for profile I, practically all the melt-
water from the ablation before 1 July (which
in most of the profiles is rather insignificant),
was still contained within the accumulation
layer when measured. Further, I have not
reckoned with the evaporation, which in this
case is practically negligible. The accumulation
and precipitation values from profile VII at
the highest point of Bárðarbunga are in all
probability minimum values, as it is more likely
that more snow has drifted away from than
towards that spot. In profile III, the opposite
is the case, as the fine-grained layer in that
proíile seemed to be partly a driftsnow layer.
Profile VIII (Fig. 5) situated 2.5 km N of
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