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Geoscientists have succeeded in reconstructing sea ice cover extend in
the past by use of two different fossil algae species found in the sediment
cores from Fram Strait. The results show also how rapidly the changes were
occurring.
A group of scientists from Alfred-Wegener Institute
and Plymouth University used a new method to reconstruct sea ice conditions
for the past 30 000 years. The discovered fossilized remains of two
different algae species: one typical for sea ice cover and cold climate and
the other one occurring in warmer waters with no sea ice. Moreover they
found out that Arctic sea ice reacts drastically even to short-term
fluctuations of climate.
By means of fossil organic molecular remains,
so-called biomarkers which are contained in the layers of the sediment
cores, the researchers were able to verify the dates when the Fram Strait
was either ice-covered or ice-free. The biomarker IP25, a complex molecule
produced by algae living in the sea ice, was found in varying concentrations
in the sediment; it was used as an indicator for the ice cover. A second
biomarker called brassicasterol which is produced by algae living in the
open water was used as counterpart in the analyses. The presence of
brassicasterol in the sediments indicates ice-free periods. The combination
of these two parameters enables the researchers to reconstruct different ice
conditions.
For a period of extremely cold climate conditions, the
last glacial maximum of about 20,000 years ago, the absence of these two
biomarkers indicates permanent ice cover in the northern Fram Strait. The
lasting lack of light and nutrients under the thick ice shield minimized the
growth of the ice algae. A short but significant warming of the climate
about 15,000 years ago, the early Bølling, caused the Arctic sea ice to melt
so far that the Fram Strait remained ice-free during the winter months. The
ice marker IP25 is absent in the sediment layers of this period, while the
content of brassicasterol is highly increased. The simultaneous occurrence
of both biomarkers in sediments of the past 5,000 years, the late Holocene,
shows that the strait was only ice-covered during the winter and spring
months. This seasonal change between ice-covered and ice-free water surface
therefore enabled the growth of both algal species.
Contact: Prof. Dr. Rüdiger Stein, AWI:
Ruediger.Stein@awi.de
Source: AWI Press release, 23 Oct 2009:
New fossil finds as witnesses for fluctuations of Arctic sea ice cover
during the past 30,000 years
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Ice algae under microscope.
Scheme showing the deposition of different species of algae in various
sea surface conditions (both illustrations: Juliane, Müller, AWI)
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