WESTERN SAHEL HYDROLOGY AND LAND USE OVER THE LAST THREE MILLENNIA: SEPARATING NATURAL VARIABILITY FROM ANTHROPOGENIC INDUCED CHANGES

The African Sahel is a semiarid ecosystem extremely prone to precipitation fluctuations and therefore one of the most vulnerable regions of the world with respect to changes during the Anthropocene. However, the reasons for severe droughts in the 1970s and 1980s and most recently in 2010 are not fully understood. These decadal-scale variations seem to be related to temperature variations in the Atlantic and Indian Ocean and may be overprinted by anthropogenic activities. We therefore need a better understanding of past Sahelian climate variability.

With our study, we aim at disentangling land-use effects from natural variations during the Late Holocene. We present a record of the past 3100 years from a marine site off Mauritania using a combination of terrestrial and marine proxies. Pollen grains are used to reconstruct vegetation changes on the continent whereas the organic-walled dinoflagellate cysts (dinocysts) reflect local oceanographic conditions including terrigenous input. Variations between 1100 BC and 1700 AD are used as a baseline and deviations from this natural variability within the last 300 years are interpreted as anthropogenic influence.

From 1100 BC to ~ 1700 AD pollen and dinocyst associations suggest rather small changes in continental rainfall and terrestrial input. From ~ 1700 AD onward, relative abundances of the dinocyst species Lingulodinium machaerophorum increase continuously. This species is typical for river plume areas where fluvial input is influenced by agricultultural or industrial activities. Its increase coincides with increasing dust and river fluxes recorded at the same core site which have been attributed to the onset of the commercial agriculture in the Sahel by Mulitza et al., 2010. At the same time, an increase of Saharan elements in the pollen associations point to an increase in aridity but might also be interpreted as a land-use signal.

Within the time-interval of the last 70 years, a comparison with precipitation data is possible. Our data show that relative abundances of Savannah pollen as an indicator for more humid conditions decrease after the onset of the Sahel droughts in the 1970s and 1980s. This suggests that the natural hydrological variability in the Sahel region led the vegetation change during that time.