Gradual aridification of the Sahara during the last 11,000 years revealed by plant wax δD analyses of Lake Yoa (Chad)

Abstract

It is still an ongoing debate whether the transition of the last ‘green Sahara’ period to today’s large desert during
the Holocene, the African Humid Period (AHP), was a progressive or an abrupt change in hydrological conditions.
Several climate records mainly from East Africa suggest a rapid decline of moisture availability at the end of the
AHP including new data from a marine sequence off the Horn of Africa (Tierney & deMenocal, 2013). Other
archives including sedimentological, geochemical and palynological data from the central North African Lakes
Chad and Lake Yoa point to a gradual rather than an abrupt transition near 5,000 years ago (Amaral et al., 2013;
Kröpelin et al., 2008). The discrepancy of the available paleo-hydrological reconstructions underline the impor-
tance of proxy parameters directly related to hydrological conditions for accurate assessment of continental rainfall
changes.
Here, we present the first molecular-isotopic data from Lake Yoa documenting the hydrologic evolution over the en-
tire Holocene. Compound-specific carbon and hydrogen isotope analyses were performed on long-chain n-alkanes.
Our data indicate relative high but variable contributions of plant-derived long-chain n-alkanes carrying a distinct
leaf-wax signature, i.e. a high Carbon Preference Index (CPI). A trend towards higher CPI values since 7,300
years ago suggests declining soil degradation and vegetation cover under increasingly drier conditions. In parallel,
the average-chain-length of the long-chain n-alkanes increases gradually towards the present implying higher rel-
ative contributions from grasses. Compound-specific carbon isotope data confirm this finding, indicating a mixed
C3/C4 contribution in the early and mid-Holocene changing towards a C4-grass dominated vegetation in the late
Holocene. Most importantly, compound-specific hydrogen isotope data reveal a continuous increase from 8,100
years ago towards the present, reflecting a gradual aridification. The large amplitude of about 40 per mil change
from the early to late Holocene reflects the retreating influence of moisture derived from the West Africa monsoon
from the area, in accordance with isotopic modelling studies (Tierney et al., 2011).
Our data thus indicate a gradual hydrological evolution in the eastern central Sahara over the Holocene, controlled
by insolation forcing, and argue against strong bio-geophysical feedback mechanisms. Site-specific local thresh-
olds might exist in other regions, e.g., the Horn of Africa, leading to abrupt rainfall changes.

Resources

• Rethemeyer et EGU2014-12391.pdf

Bibliography

Rethemeyer, J., Kröpelin, S., Karls, J., Thienemann, M., Melles, M., Schefuß, E. (2014): Gradual aridification of the Sahara during the last 11,000 years revealed by plant wax δD analyses of Lake Yoa (Chad). Geophysical Research Abstracts