Jökull - 01.12.2006, Side 37

The stable isotopic (C and N) composition of modern plants and lichens from northern Iceland
become apparent when the C/N of the sediment is
examined. The C/N values of the surface sediments
are much lower (4.7–11.4) than the terrestrial plants
(∼50). Thus, the low C/N values for the surface
sediments are consistent with those reported for
phytoplankton or aquatic plants (Figure 1), indicating
that organic materials in sediments from those three
lakes are mostly of aquatic origin. Lake Torfadalsvatn
had a slightly higher C/N of 11.4, which may
indicate that the organic matter in the sediments
was derived from a mixture of aquatic and higher
plant material. Very high C/N values were seen for
some of the graminoid (grass and sedge) specimens
analyzed (Table 1). These are consistent with the high
C/N values (>80) resulting from leaf senescence in
some plants (e.g. Wooller et al., 2003). Despite the
decrease in %N with senescence Wooller et al. (2003)
found that the δ15N and δ13C of senescent leaves
from mangroves did not alter significantly from the
green leaves. Collection of a wider variety of plant
specimens in summer would be helpful in order to
obtain a more comprehensive isotopic survey in the
future. Stable isotopic analyses of bulk sediments for
the cores from each lake would also provide insight
into changes in sediment composition over time.
CONCLUSIONS
The δ13C values of the Icelandic plants analyzed
were consistent with values expected for terrestrial
plants using C3 photosynthesis. The δ13C values
of Potamogeton sp. (submerged aquatic macrophyte)
analyzed were consistent with reported values for
submerged aquatic plants. The δ15N values of the
terrestrial plants and lichens from three of the study
sites were in some cases surprisingly negative, which
could be related to phosphorus limitation or uptake
of atmospheric ammonia. The surface sediments
analyzed from the four lake sites had C/N and
δ13C values indicative of a primarily composition of
aquatic plants, notably phytoplankton.
Acknowledgements
This project was funded by the National Science
Foundation (grant number NSF ESH-0317766 to
Matthew J. Wooller). We thank our collaborators
Yarrow Axford, Jason Briner, and Gifford Miller
of the Institute of Arctic and Alpine Research and
Áslaug Geirsdóttir of the Department of Geosciences
at University of Iceland for assistance and comments.
We thank our Icelandic coring perfectionists Þorsteinn
Jónsson and Sveinbjörn Steinþórsson for their hard
work and novel ideas for coring lake sediments. We
also appreciate Tim Howe and Norma Haubenstock
from the Alaska Stable Isotope Facility for their
diligent work and valuable comments on the data.
ÁGRIP
Samsætur kolefnis og köfnunarefnis í setkjörnum.
Lýst er mælingum á stöðugum samsætum kolefnis-
og köfnunarefnis úr setkjörnum úr Arnarvatni stóra
á Arnarvatnsheiði, Torfadalsvatni á Skaga, Stífluvatni
í Fljótum og Litla-Viðarvatni á Melrakkasléttu.
Hlutfall kolefnis (δ13C) mældist á milli -30.9 og
-23.3 prómill í landplöntum. Þetta hlutfall er
einkennandi fyrir plöntur sem hafa C3 ljóstillífun.
Vatnaplöntur höfðu töluvert lægri δ13C gildi. Hlutfall
köfnunarefnis (δ15N) í háplöntum sem og lágplöntum
(fléttum) var mjög breytilegt, frá u.þ.b. -12.4‰
til +5.5‰, þar af höfðu mörg sýnanna lægri
köfnunarefnisgildi en -6.0‰.
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