Orbital- and millennial-scale changes in the hydrologic cycle and vegetation in the western African Sahel: insights from individual plant wax δD and δ^(13)C

Abstract

To reconstruct variability of the West African monsoon and associated vegetation changes on precessional and millennial time scales, we analyzed a marine sediment core from the continental slope off Senegal spanning the past 44,000 years (44 ka). We used the stable hydrogen isotopic composition (δD) of individual terrestrial plant wax n-alkanes as a proxy for past rainfall variability. The abundance and stable carbon isotopic composition (δ^(13)C) of the same compounds were analyzed to assess changes in vegetation composition (C_(3)/C_(4) plants) and density. The δD record reveals two wet periods that coincide with local maximum summer insolation from 38 to 28 ka and 15 to 4 ka and that are separated by a less wet period during minimum summer insolation. Our data indicate that rainfall intensity during the rainy season throughout both wet humid periods was similar, whereas the length of the rainy season was presumably shorter during the last glacial than during the Holocene. Additional dry intervals are identified that coincide with North Atlantic Heinrich stadials and the Younger Dryas interval, indicating that the West African monsoon over tropical northwest Africa is linked to both insolation forcing and high-latitude climate variability. The δ^(13)C record indicates that vegetation of the western Sahel was consistently dominated by C_4 plants during the past 44 ka, whereas C_(3)-type vegetation increased during the Holocene. Moreover, we observe a gradual ending of the Holocene humid period together with unchanging ratio of C_3 to C_4 plants, indicating that an abrupt aridification due to vegetation feedbacks is not a general characteristic of this time interval.

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