Geochemistry of ODP Hole 108-658C sediments

During the African Humid Period (AHP), much of the modern hyperarid Saharan desert was vegetated and covered with numerous lakes. In marine sediments off northwestern Africa, the AHP is represented by markedly reduced siliciclastic sediment flux between ~ 12.3 and 5.5 ka. Changes in the origin of this terrigenous sediment fraction can be constrained by sediment chemistry and radiogenic isotope tracers. At Ocean Drilling Program (ODP) Site 658, Hole C (20°44.95'N, 18°34.85'W, 2263 mbsl), the neodymium (Nd) isotope composition of terrigenous detritus shows little variability throughout the last 25 kyr, indicating that the contributing geological terranes have not changed appreciably since the last glacial period. In contrast, there were large and abrupt changes in strontium (Sr) isotope ratios and chemical compositions associated with the AHP, during which 87Sr/86Sr ratios were markedly less radiogenic, and sediments show higher chemical indices of alteration. We show that sediment geochemical changes during the AHP cannot be attributed to changes in the source terranes, physical sorting, or intensity of chemical weathering. The low 87Sr/86Sr and high Sr concentrations of AHP-age samples also conflict with the interpretation of increased fine-grained, fluvially derived sediments. We propose that the most significant compositional changes at ODP 658C are due to the addition of an aluminosilicate component that has a highly altered major element signature but is enriched in soluble elements like Sr and magnesium (Mg) compared to aluminum (Al) and has low 87Sr/86Sr relative to local terrigenous source areas. We interpret these characteristics to reflect authigenic sediment supply from extensive North African paleolake basins that were prevalent during the AHP.

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