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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
°
Nto
8
W, Fig.
1
a) were investigated to assess how the isotopic
gradient in rainfall on the continent is re
°
N and 10
°
Wto20
°
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
ected in
δ
ected
in
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
δ
c
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
δ
°
45
′
N, 18
°
35
′
13
C
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
c
D and
δ
δ
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
ß
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
δ
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