Sheet nº 232
January 2006
New light on the tropical climate of the past:
20 000 years ago the Amazon basin
was wetter than predicted
Ice cores from Andean glaciers constitute
excellent records of the tropical climate. Their
water isotopic compositions (in deuterium
and 18O, i.e. _D or _18O) yield information
valuable for quantifying past climatic variations. Nevertheless, interpretation of such
information is still a point of controversy
among scientists. IRD researchers decided
therefore to calibrate the _D signal on the
basis of present-day precipitation. They
consequently deployed a rain gauge network
©IRD/Denis Wirrmann
in the Zongo Valley of Bolivia, around 50 km
from the Illimani glacier where an ice core
was extracted that corresponded to the past
20 000 years. Rain sample analysis, combined with results from climatic modelling for
the South American region, showed regional precipitation count and origin to be the
dominant control factor of isotopic composition. At interannual scale, the Andean
glaciers would therefore contain the record of regional humidity variations rather
than temperature variations which had been assumed to be the case up to now.
he Andean glaciers represent only a
small proportion of the world’s continental
ice packs. Yet they are excellent as
archives of climatic history. The water isotopic
composition (in 18O and deuterium, i.e. _D or
_18O) of the ice they contain is a powerful
analytical instrument for reconstructing past
climates. However, interpretation of these
geochemical indices in tropical ice is still difficult
and has been the subject of controversy in the
scientific community for over ten years. For
polar ice-caps, variations in _D signal can be
interpreted in terms of temperature variations
but this is not the case for tropical situations.
The hydrological cycle is more complex than at
the poles, hence isotopic fractioning (1) between
T
the different water phases (ice, liquid water,
vapour) during changes that are taking place in
a mass of air cannot be linked simply to temperature.
Some researchers have assumed that precipitation quantities exerted the dominant control on
_D composition of Andean ice cores. However,
no calibration to verify this had yet been conducted using observations on present-day precipitation. A sampling programme therefore became
necessary in order to interpret correctly the
water isotopic composition of a drilling core
taken from the Illimani glacier in Bolivia. This ice
core, obtained in 1999, yields particularly interesting climate data accumulated over the past
../...
20 000 years. In collaboration with the Bolivian electricity company COBEE (2) and the country’s hydrological institute IHH (3), scientists from IRD’s research unit
Great Ice (4) set up a survey system to record the _D
signal of present-day rainfall in the Zongo Valley, about
50 km from Illimani glacier. This valley is in a strategic
position in that it is the point of passage for air masses
moving in from the Amazon Basin, before they are
lifted up on to the Andean tops where they deposit the
snow which accumulates with time.
Rain gauges were deployed at different altitudes
along this valley. From September 1999 to August
2004, five of them collected monthly rain samples, and
three others captured samples after each bout of rainfall. The meteorological conditions were measured in
parallel. Analysis of each precipitation sample gave its
oxygen and hydrogen isotopic composition. These
isotopic compositions were first correlated with both
local and regional meteorological parameters (ground
temperature, rainfall count, humidity), then with those
reconstructed from re-analysis of existing meteorological data from tropical South America as a whole (rainfall count, convection system intensity, atmospheric
temperature). This process indicated that, on the
seasonal and interannual scale, surface temperature
was not a key control parameter for explaining the
isotopic contents found. It was rather the rainfall count,
both local and regional, that appeared to be the dominant control. With a more detailed study of the interannual changes in these compositions a objective, the
researchers could not rely only on five years’ data.
Redaction – IRD : Béatrice Le Brun
Translation : Nicholas Flay
They applied climate modelling in order to achieve a
92-year simulation of rainfall isotopic compositions for
the entire globe. Evidence emerged that the isotopic
composition of Andean ice packs is linked to precipitation in upstream reaches of the Andes, along the
trajectory of air masses that move over the Amazon
Basin from their origin over the tropical Atlantic. The
water isotopic composition of these ice packs therefore
indeed depends much more on regional humidity
variations than on temperature variations, which was
previously taken to be the case.
The tropical Atlantic currently constitutes the main
zone of origin of the air masses that precipitate on the
Andes, before they cross the Amazon Basin. The calibration obtained with the present-day precipitation
figures was applied to the _D signal record obtained
from the Illimani ice core, using the assumption that the
air-mass trajectories changed little in the past. The
results showed that the atmosphere over the Amazon
Basin and the northern tropical Atlantic was wetter
during the last glacial maximum at 20 000 years B.P.
than previously thought, by a few per cent. These interpretations must now be refined by studying the potential impact of second-order climatic parameters on the
isotopic compositions (net condensation temperature
in convection clouds for example) and investigating the
climatic mechanisms that could explain these variations in humidity in tropical South America at the timescale of glacial-interglacial transition.
________________________
(1) Redistribution of heavy and light molecules between the different
water phases
(2) Compañia Bolivia de Energia Eléctrica (Bolivia)
(3) Instituto de Hidraulica e Hidrologia (Bolivia)
(4) Glaciers et Ressources en Eau d’Altitude – Indicateurs
Climatiques et Environnementaux
For futher information
CONTACTS:
Françoise Vimeux –IRD researcher, Great Ice Research Unit (UR032 – www.greatice.ird.fr), hosted by the Laboratoire des Sciences
du Climat et de l'Environnement (LSCE) CEA Saclay Bâtiment 709- Orme des Merisiers 91191 Gif-sur-Yvette, France.
Tel.: +33 (0)1 69 08 57 71. Fax : +33 (0)1 69 08 77 16, E-Mail: Vimeux@lsce.saclay.cea.fr
Great Ice web site:
http://www.mpl.ird.fr/hydrologie/greatice/PagesGREATICE/Presentation.htm
IRD Communication :
Sophie Nunziati (press officer), Tel.: +33 (0)1 48 03 75 19, Email: presse@paris.ird.fr
REFERENCES
Françoise Vimeux, Robert Gallaire, Sandrine Bony, Georg Hoffman, John C.H. Chiang - What are the climate
controls on dD in precipitation in the Zongo valley (Bolivia) ? Implications for the Illimani ice core intrepretation, Earth
and Planetary Science Letters, Volume 240, Issue 2, 1 December 2005, Pages 205-220.
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/2004/fiche232.htm