offshore the Sahel and highest offshore the Sahara desert (Fig. 1 a) in accordance
with the latitudinal trend of
D in precipitation (Bowen and Revenaugh 2003 ). The
D of plant waxes resembles the isotopic range
of continental precipitation. The observed spatial isotopic gradient is also found in
pre-industrial climate simulations with the ECHAM5-JSBACH model (Fig. 1 b).
D values of plant waxes extracted from multi-core GeoB9501-4 near the
Senegal River mouth reveal temporal variations linked to hydrologic conditions
over the western Sahel during the last century. The Sahel drought in the late-1960s
to mid-1990s can be traced in a+10
correspond to a rainfall index for the western Sahel (Fink et al. 2010 ). Comparison
with precipitation-isotope data provides an estimate of a 30 % reduction in rainfall
during the Sahel drought, in accordance with meteorological observations. In
agreement with this data, a maximum isotopic shift of +15
shift of the
D signal. Changes in
D in precipitation
is detected in the nudged ECHAM5-JSBACH simulation between wet and dry
years of the period 1957
2011. However, the simulated timing of the Sahel drought
is erroneous (Haese 2014 ).
To study the ending of the AHP, we analyze core GeoB7920-2. Terrestrial
versus marine elemental ratios of GeoB7920-2 con
rm an abrupt increase (within
centuries) of terrigenous material around 5,500 years BP (Fig. 2 a), in accordance
findings by deMenocal et al. ( 2000 ). Sedimentary concentrations of plant
waxes increase after the AHP as expected from increased eolian transport (Fig. 2 b).
D compositions of plant waxes (Fig. 2 d) reveal a gradual
change following the declining summer insolation (Fig. 2 f). Plant-wax
the AHP are about 20
depleted relative to the latest Holocene. Using the
observed modern local precipitation-isotope relation, these data suggest a doubling
of rainfall during the AHP relative to today. Compound-speci
13 C data indicate
higher C 4 plant coverage during the AHP (Fig. 2 c) and also reveal a gradual
vegetation change. The latter is, however, not contemporaneously with the pre-
cipitation change arguing against strong bio-geophysical feedback mechanisms.
findings are supported by the ECHAM5-JSBACH isotope time-slice
experiments as well as a 6,000-year long (6,000 years BP to 0 years BP) transient
simulation with a comprehensive climate model (Fischer and Jungclaus 2011 ). The
latter experiment also reveals a gradual decrease of precipitation from approx.
5,500 years BP until 1,500 years BP in the area between 5
N (Fig. 2 e).
Due to the simulated evolution of precipitation the modeled vegetation cover in NW
Africa is reduced, leading to a gradual increase of the desert fraction. The four
isotope time-slice ECHAM5-JSBACH experiments, forced with oceanic and veg-
etation boundary conditions derived from the transient simulation by Fischer and
Jungclaus ( 2011 ), indicate a change of
N and 30
6,000 years BP and pre-industrial. These modelled values agree well with the
D in precipitation of about 15
findings from the measured compound-speci
D compositions of plant waxes
(Fig. 2 d).
In consequence, we infer from this new data-model study that continental
rainfall diminished gradually over NW Africa during the Holocene leading to
find no parallel changes in vegetation type, arguing