March 23, 2007
MICROFOSSILS
UNRAVEL
CLIMATE HISTORY OF TROPICAL AFRICA
Scientists from the Royal
Netherlands Institute for Sea Research obtained for the first time a
detailed
temperature record for tropical central Africa
over the past 25,000 years. They did this in cooperation with a German
colleague from the University of Bremen.
The scientists
developed an entirely new method to reconstruct the history of land
temperatures based on the molecular fossils of soil bacteria. They
applied the
method to a marine sediment core taken in the outflow of the Congo River. This core contained
eroded land material and microfossils
from marine algae. The results show that the land environment of
tropical
Africa was cooled more than the adjacent Atlantic
Ocean
during the last ice-age. This large temperature difference between land
and
ocean surface resulted in drier conditions compared to the current
situation,
which favors the growth of a lush rainforest. These findings provide
further
insight in natural variations in climate and the possible consequences
of a
warming earth on precipitation in central Africa.
The results will be published in this week's issue of Science.
One of the techniques currently used to estimate past sea
surface
temperatures, is based on organic molecules from algae growing in the
surface
layer of the Ocean. These organisms adapt the molecular composition of
their
cell membranes to ambient temperature to maintain constant
physiological properties.
When such molecules sink to the sea floor and are buried in sediments
where
oxygen does not penetrate, they can be preserved for thousands of
years. The
ratios between the different molecules from the algal cell membrane can
be used
to approximate the past temperature of the sea surface. These
techniques are
therefore called 'proxies'.
New
Method to Measure Soil Temperatures
Reconstructing continental temperature history is more
difficult than for
the oceans, because soils on the continent do not form a continuous
archive but
are often eroded. The researchers developed an entirely new proxy for
the
annual mean air temperature on land, based on molecules from the cell
membrane
of soil inhabiting bacteria. They analyzed eroded soil material in a
sediment
core in the outflow area of the river Congo
in the South Atlantic Ocean
at a depth of
almost 1000 meters. Since the Congo River drains a large part of
tropical
central Africa,
the land derived material
gives an integrated signal for a very large area.
Cool
Tropical Africa
The new proxy was used in this sediment core to obtain both a
continental
and a sea surface temperature record. A comparison of both records
shows that
ocean surface and land temperatures behaved differently during the past
25,000
years. During the last ice age, temperatures over tropical Africa were
21°C, or
about 4°C lower than today, whereas the tropical Atlantic Ocean was only about
2.5°C colder. By comparing this temperature
difference with existing records of continental rainfall variability,
lead
author Johan Weijers and his colleagues concluded that the land-sea
temperature
difference has by far the largest influence on continental rainfall.
This can
be explained by the strong relationship of air pressure to temperature.
When the
temperature of the sea surface is higher than that of the continent,
stronger
offshore winds reduce the flow of moist sea air onto the African
continent.
This occurred during the last ice age and, as a consequence, the land
climate
in tropical Africa
was drier than it is in
today's world, where it favours the growth of a lush rainforest. These
results
provide further insight into the natural variation of climate and the
possible
consequences of a warming earth on precipitation in central Africa.
This research project was funded by the division of Earth and
Life Sciences
of the Netherlands Organization for Scientific Research (NWO-ALW).
##
Contact:
Dr.
Jan Boon
Netherlands Organization for
Scientific Research
31-022-236-9466
jan.boon@nioz.nl
This text derived from:
http://www.nwo.nl/nwohome.nsf/pages/NWOA_6ZKCZR_Eng
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