Hydrological and vegetation changes in North Africa over the past 23 000 years: a comparative study of watershed areas of the Nile River using remote sensing and compound-specific δ2H and δ13C from the Eastern Mediterranean Sea

Abstract

Hydroclimate variation and vegetation changes of the Nile River watershed area in northeast Africa since the Last Glacial Maximum (LGM) were reconstructed based on n-alkanes, their carbon isotope ratios (δ¹³C_n-alkanes), and their hydrogen isotope ratios (δ²H_n-alkanes) in sediments from ODP Site 967 in the Eastern Mediterranean Sea. The results were compared with the present vegetation cover in the watershed areas using ArcGIS. The average proportion of current grassland in the Equatorial Lake and Ethiopian Highland Plateaus watershed areas was 45.8 and 64.7%, respectively. δ²H_n-alkanes ranged from −199 to −127‰ and co-varied with insolation change response to orbital forcing. Depleted δ²H_n-alkanes were found from deglaciation to the middle Holocene, suggesting increased precipitation during the African Humid Period (AHP) from 15 to 5 ka caused by northward migration of the Intertropical Convergence Zone. However, lower precipitation was inferred by enriched δ²H_n-alkanes during the LGM and late Holocene. δ¹³C_n-alkanes at Site 967 did not show a trend in harmony with δ²H_n-alkanes and instead exhibited millennial-scale variations ranging from –25.9 to –33.2‰. These δ¹³C_n-alkanes values consistently indicated a C4 grass-dominated environment in the watershed areas of the River Nile since the LGM, persisting through the AHP and into the present. Reconstructions demonstrated orbital and abrupt forcing of hydroclimate variability while maintaining generally grass-dominated vegetation with weak precipitation feedback over the late Quaternary.