Sheet nº 239
March 2006
Pollen grains indicate palaeoclimatic change
Climates of the past provides clues to climate changes of the present. Pollen
grains held in sediments give valuable
information on the climate that prevailed
at the time they were buried.
Palynological investigations can yield
ways of reconstructing climate fluctuations with time. Such reconstructions,
pieced together to represent lengthy periods, are valuable for unravelling the
mechanisms by which the Earth’s cycles
evolve, and for attempts to forecast future climate changes. An IRD team recently published the results of the analysis of
a new sediment drill-core taken from
Colônia meteorite crater, now situated in
the Atlantic rainforest of Brazil. The surPresent-day distribution of the Atlantic rainforest in Brazil
(in grey) and location of the Colônia crater
vey covered three periods, two intergla©IRD/ Marie-Pierre LEDRU
cials and one glacial period. It brought
after data from the foundation SOS Mata Atlantica.
out some highly irregular frequencies of
arboreal pollen input. These reflect variations in the extent of the forest and consequently in the levels of relative humidity and temperature over the past 100 000 years.
Results revealed unexpected differences between the climatic behaviour of the
Northern and Southern Hemispheres. Climatic fluctuations of the Southern and
Northern hemispheres were considered to be identical. However, the new interhemispheric comparisons corroborate the idea that the Southern Hemisphere changes
and develops differently from the Northern Hemisphere. The tropical zone is a prime
area to observe for gaining an understanding of the interactions at work between the
two hemispheres.
he Earth’s climatic history has been
marked by a series of cold, glacial, periods
and warm, interglacial, periods.
A team from the IRD research unit Great Ice,
working jointly with São Paulo University in
Brazil, has examined a continuous sedimentary
record covering two interglacials and one glacial
period. The researchers took drill-cores from a
sediment accumulation in the Colônia meteoritic
crater, situated at the heart of the Atlantic rainforest region in Brazil.
T
Each forest is associated with a particular type
of climate, which is itself characterized by annual
rainfall distribution and an average temperature.
At Colônia, the presence of the Atlantic rainforest
is the result of frequent incursions of air from the
Antarctic polar region up to tropical latitudes. The
convergence of this cold polar air with warm
humid tropical air in the Intertropical
Convergence Zone, which occurs throughout the
year, generates abundant rainfall and cloud
cover. It maintains the moist climatic conditions
../...
and allows dense forest to develop. Pollen grains disseminated by the forest plants become trapped in the
sediments. Practically unchangeable, their identification helps work out the plant associations characteristic
of the climatic period at the time they were covered by
sediment.
Analysis of the sedimentary record makes it necessary
to establish a time-scale for the observed variations.
Concerning the method, 14C dating cannot provide
reliable dates further back than 30 000 years B.P.
Results obtained on the sediments must therefore be
compared with those of other well-dated records in
other palaeoenvironments, in order to determine the
nature of the climatic events recorded. The research
team therefore used ice-cores and a marine record
from a core from the Atlantic floor, which are the only
currently available representative records for the
Southern Hemisphere that cover long periods. This
comparison shows that the pollen frequency fluctuations are linked to changes in humidity. A strong
increase in tree pollen abundance reflects high levels of
relative humidity and signifies a phase of forest expansion. A rise in the amount of herbaceous-plant pollen is
a sign of a drier climate phase.
In the Colônia survey, the research team observed
abrupt changes in arboreal pollen abundance, suggesting sudden changes in the seasonality and distribution
of rainfall. These climate modifications appear to result
from variability in the geographical limits of the polar air
mass as it rises up towards these latitudes, either
removing or increasing the permanent cloud cover and
the abundant precipitation. Over the past 100 000
Redaction – IRD : Aude Sonneville
Translation : Nicholas Flay
years, the Atlantic tropical forest has thus seen
substantial variations in its extent. Significant among
these were three phases of regression (dry periods)
and phases of expansion (moist phases). Globally, this
sequence of changes and developments is synchronous with the great climatic trends of the Earth.
Differences were observed, however, between this
situation in the Southern Hemisphere and the chronological scale derived from the Northern Hemisphere
records. For example, records indicating the Last
Glacial Maximum, dated in the Northern Hemisphere at
between 25 and 19 000 years BP, give an earlier date
for the Southern Hemisphere, between 27 and 21 000
years BP. At Colônia, this period is expressed by an
expansion of the forest and therefore by a maximum of
humidity, whereas the temperate latitudes were extremely cold and dry at that particular time. These climatic
characteristics are similar to the present-day situation,
with the movement up the latitudes of Antarctic polar
air. This time gap in the development of glaciation could
mean that the conditions in the South exerted an
influence on those in the North. Climate reconstruction
models should be able to demonstrate this.
The results from the Colônia survey corroborate
others obtained from palaeoclimatic records on glacial
moraines in South America and ice-cores from the
Antarctic. It is commonly thought that the climatic variability of the Northern Hemisphere, concerning in particular temperature fluctuations in the North Atlantic, is
the source of climate variability in the Southern
Hemisphere. The Great Ice research unit’s findings
support the hypothesis of an atmospheric system that
functions in an independent way, or even of a possible
influence of the climatic dynamics of the South on that
of the North. These new data on the changes and development of the Atlantic tropical forest bring improvements to our knowledge on the Earth’s climatic cycles.
For futher information
CONTACTS:
Marie-Pierre LEDRU, IRD UR 032 - Glaciers et ressources en eau d'altitude - Indicateurs climatiques et environnementaux (GREAT ICE)
IRD - MSE, BP 64501, 34394 Montpellier - France
Tel. : 33 (0)4 67 14 90 32, Fax : 33 (0)4 67 14 90 87, Email: ledru@msem.univ-montp2.fr
Site : http://www.maisondeleau.org
IRD Communication :
Aude Sonneville (editor), Tel.: +33 (0)1 48 03 76 07, Email: fichesactu@paris.ird.fr ;
Sophie Nunziati (press officer), Tel.: +33 (0)1 48 03 75 19, Email: presse@paris.ird.fr
REFERENCES
LEDRU M.-P., ROUSSEAU D.-D., CRUZ F.W.J., KARMANN I., RICCOMINI C., MARTIN L. (2005). Paleoclimate changes during the last 100ka from a
record in the Brazilian Atlantic rainforest region and interhemispheric comparison. Quaternary Research 64, p. 444-450.
LEDRU M.-P., MOURGUIART P., CECCANTINI G., TURCQ B., SIFEDDINE A. , 2002. Tropical climates in the game of two hemispheres revealed by abrupt
climatic change. Geology, vol. 30 no 3, 275-278.
LEDRU M.-P., BERTAUX J., SIFEDDINE A. AND SUGUIO K., 1998. Absence of Last Glacial Maximum records in lowland tropical forests, Quaternary
Research, 49: 233-237.
ILLUSTRATIONS
Contact Indigo Base, IRD picture library, Claire Lissalde or Danièle Cavanna, Tel.: +33 (0)1 48 03 78 99, Email : indigo@paris.ird.fr
The illustrations can be viewed on: www.ird.fr/us/actualites/fiches/2006/fiche239.htm