investigated in a marine sediment core covering the AHP in high-temporal resolution.
The results were compared to the simulated rainfall amounts and its isotopic com-
position for selected Holocene time-slices. The simulations were performed using an
atmospheric general circulation model containing explicit stable water isotope
diagnostics coupled to a dynamic vegetation model. These observations and simu-
lations provide insights into amplitudes of past climate variations and allow a
quantitative assessment of precipitation changes in NW Africa during the Holocene.
2 Materials and Methods
Marine surface sediments from coastal areas around NW Africa (between 31
W, Fig. 1 a) were investigated to assess how the isotopic
gradient in rainfall on the continent is re
N and 10
D compositions of terrigenous
plant lipids deposited in the sediments. To understand how rainfall variations on
short time-scales, i.e., during the late twentieth century Sahel drought, are re
D compositions of plant-derived n-alkanes we analyse multi-core GeoB9501-4,
taken near the Senegal River mouth and covering the past 100 years (Mulitza et al.
2010 ; Fig. 1 c, d). These data allow a quantitative calibration of compound-speci
D changes against meteorological data. Sediment core GeoB7920-2 (Tjallingii
et al. 2008 ; Fig. 1 c, d), taken at the same location as ODP 658C (20
analyses to investigate the termination phase of the AHP and reconstruct precipi-
tation and vegetation changes. All sediment samples were extracted with organic
solvents, and fractions containing plant-wax lipids were quanti
W; deMenocal et al. 2000 ), was analyzed by compound-speci
ed by gas-chro-
matography and measured for their compound-speci
c stable carbon and hydrogen
isotope compositions (details of the methods are in Schefu
et al. 2011 ).
For isotope modeling, stable isotopes H 18 O and HDO have been incorporated in
the hydrological cycle of the coupled atmosphere-land surface model ECHAM5-
JSBACH (Haese et al. 2013 ). With this newly developed setup it is possible to
distinguish between evaporation and transpiration
fluxes, and separately simulate
the relevant fractionation processes for both. To investigate the hydrological evo-
lution of NW Africa during the last 50 years a simulation with prescribed observed
sea-surface temperature and atmospheric conditions nudged to ERA-40 reanalyses
data was performed. For the Holocene, four different time-slice experiments (pre-
industrial (PI), 2,000 years BP, 4,000 years BP, 6,000 years BP) were conducted
and analyzed (Haese 2014 ).
3 Key Findings
n-Alkane distributions and concentrations in marine surface sediments from the
coast of NW Africa indicate the input of terrestrial plant waxes at all locations
(Fig. 1 a).
D compositions of plant waxes in the sediments show lowest values