Jökull - 01.12.1967, Page 72

formation of the sediments. If the mixing of
fresh ground-water with interstitial sea-water
is the case, the ð-value of this fresh ground-
water would have to be — 120%c. The lowest
8-value ever reported in precipitation in the
south-western part of Iceland is — 94%0 (Arna-
son and Sigurgeirsson 1967) and north of Vatna-
jökull — 98%0 (Arnason, pers. comm.). It seems
therefore impossible that this explanation is
valid. The same would also be valid for mixing
of interstitial fluvial water with sea-water. An-
other possible explanation is that the fresh
ground-water derived from precipitation on the
mainland originated during a period of time,
when the climate was colder than it is at pre-
sent. The sediments cut by the hole have
formed under very variable climatic conditions.
It seems, however, very unlikely that intersti-
tial fluvial water with such a low 8-value has
mixed with sea-water to give a final 8-value
of about — 60%o.
Clayton et. al. (1966) and Graf et. al. (1965)
have shown that brines from the Illinois,
Míchigan and Alberta basins have the same or
similar 8-values as the local precipitation. Tliey
concluded therefore that the water was of
meteoric origin but the original interstitial sea-
water had all been flushed out of the sedi-
ments, leaving its salt content behind, and re-
placed by rain-water as a result of the íilter
properties of the sediments. The deep water
from the Vestmann Islands coulcl have origin-
ated in the same way. Thus the water woulcl
be derived from precipitation on the mainland.
Two cross sections from the Vestmann Islands
to the mainland are presented in Figs. 6 and 7.
The 8-value of the precipitation is superimpos-
ed (see Arnason and Sigurgeirsson 1967).
It does not seem unreasonable that ground-
water flow from the mainland with a 8-value
of — 62%0 strikes the well at a depth of 900 m
and more. The 8-value of the water in the
sediments can therefore be explained by
ground-water flow from the mainland that per-
colates through the sediments and replaces the
original sea-water molecule for molecule but
lets the salt behincl. This exchange is most
Fig. 6. A cross section showing conceptual ground-water flow from the mainland to the Vest-
mann Islands. The figures indicate the 8-value of the precipitation according to Arnason and
Sigurgeirsson (1967) and the conceptual 8-value of the ground-water. The location of the cross
section is shown in Fig. 8.
Mynd 6. Myndin sýnir sniö frá Vestmannaeyjum til Langjökuls. Merkt er inn á sniðið b-gildi
úrkomunnar í sniðinu, sjá Arnason og Sigurgeirsson (1967). Örvarnar sýna, hvernig höfundur
hugsar sér, að hluti úrkomunnar verði að grunnvat.nsstreyrni, b-gildi grunnvatnsstreymisins við
Vestmannaeyjar er merkt með sömu gildum og mœlt var í vatnssýnum frá holunni. Lega sniðs-
ins er sýnd á Mynd 8.
306 JÖKULL 17. ÁR