North-West African Hydrologic Changes in the Holocene: A Combined Isotopic Data and Model Approach - Integrated Analysis of Interglacial Climate Dynamics (INTERDYNAMIC) - page 106

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

δ

approximately 25 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).
δ

‰ amplitude in

δ

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

D closely
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

δ δ

D in precipitation
is detected in the nudged ECHAM5-JSBACH simulation between wet and dry
years of the period 1957

‰ in

δ

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
with the findings by deMenocal et al. ( 2000 ). Sedimentary concentrations of plant

waxes increase after the AHP as expected from increased eolian transport (Fig. 2 b).
Compound-speci

D compositions of plant waxes (Fig. 2 d) reveal a gradual
change following the declining summer insolation (Fig. 2 f). Plant-wax

c

δ

D during

δ

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.
These

c

δ

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 °

between
6,000 years BP and pre-industrial. These modelled values agree well with the

δ D in precipitation of about 15 ‰

findings from the measured compound-speci c δ 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
progressive aridi cation. We find no parallel changes in vegetation type, arguing