Pollen-inferred precipitation time-series from
equatorial mountains, Africa, the last 40 kyr BP

Pollen-inferred precipitation time-series from
equatorial mountains, Africa, the last 40 kyr BP

Global and Planetary Change, v.26, pp 25-50, November 2000

Raymonde Bonnefille and Françoise Chalié
CEREGE, CNRS-UMR 6635, Europôle Méditerranéen de l'Arbois
BP 80- F. 13 545 Aix-en-Provence Cedex 04 France

Palaeoclimatic estimates of mean annual rainfall in the equatorial highlands of Central East Africa have been established for the last 40 kyr. The values are inferred from nine fossil pollen sequences, collected from six peat bogs located between 2° and 4°S latitude, in the forest belt, from 1800 to 2240 m a.s.l. The transformation of pollen data into climatic parameters is achieved by the best analogues statistical method, using a modern pollen data set of East and Central Africa and calibration by meteorological data. The climatic reconstructions are first performed for each individual sequences. They are transformed into time-series using 88 of the 125 available radiocarbon dates (including 41 AMS dates). The synthesis of the results is presented as a single curve, illustrating the precipitation values obtained for 682 dated stratigraphic layers plotted on a radiocarbon timescale. The precipitation changes are presented at about a century resolution during the Holocene, and about a millennium for the glacial period.

The most characteristic feature of the synthetic curve is the changing variability through time.

During the last glacial period (30 to 15 kyr BP interval), our results indicate a ca. 450 mm/yr (32%) precipitation decrease relative to the present. Two maxima decrease periods are registered at ca. 19 kyr BP (700 mm/yr, ca. 45%) and between 18 and 16 kyr BP (608 mm/yr, ca. 42%). Several abrupt positive peaks of higher precipitation are noticed pre 35 kyr BP and ca. 22-20 kyr BP.

During the lower Holocene (10-7 kyr BP), the mean calculated precipitation estimate is 30 mm/yr (2%) below the present-day value, with several abrupt positive shifts; the maximum at ca. 8 kyr BP reaches 600 mm/yr (42%). Great variations between low and high precipitation values are expressed post ca. 4 kyr BP.

These results are discussed in light of other climatic indicators extracted either from the same sedimentary record, isotopes, microfauna, or from other geological palaeoclimatic proxy in the concerned region, and pattern of the monsoon record in marine cores. Anthropogenic disturbance on past vegetation, as well as humidity signal as a better response of forest to climatic forcing, and possible CO2 effect need more investigation in the future. We feel confident that the greater variability at the century scale registered in the equatorial highlands provides a good explanation for increased aridity in the desertic region of Sudan and Egypt during the last 4 kyr. The possibility that such greater variability could be induced by high variability or instability of the Atlantic Ocean or global sea surface temperature instability should be explored.

Download the reconstruction time series for Burundi ( text or Microsoft Excel format)
and for South Tanganyika ( text or Microsoft Excel format), plus the Data Description
from the WDC Paleo Archive.

The pollen data has been submitted to the African Pollen Database.

Synthetic curve of precipitation estimates expressed as deviation from the present-day mean annual rainfall value for the Burundi highlands and for the South Tanganyika basin (compiled from 8 sequences from Burundi and 2 from South Tanganyika basin). In bold, three points filtered curve provides 1 kyr average resolution for glacial period and five points filter, 450 yr resolution for the Holocene.

To read or view the full study, please visit the Elsevier Science website.
It was published in Global and Planetary Change Volume 26, pp. 25-50, November 2000.

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16 March 2001