Variations of the Nile suspended discharges during the last 1.75 Myr

Abstract

Planktonic foraminiferal oxygen isotopes, carbonate and organic carbon contents, as well as XRF core scanning Ti, V and Ba intensities have been analyzed on sediments of a Mediterranean marine core (Core MD90-964) to reconstruct changes in the Nile discharges during the last 1.75 Ma. Chronology of Core MD90-964 is established by correlating the planktonic foraminiferal δ 18O record to the Mediterranean Globigerinoides ruber δ 18O stack. A total number of 42 dark layers are observed under visual observation, among which 21 of them are identified as sapropels by organic carbon contents (> 1 wt.%). It is observed that oxidation of sapropels is very active during 1000 and 240 kyr, indicating that the local climate is much colder and dryer during this interval. Our results provide new evidences that oxidation of organic material play a major role in determining the organic carbon contents in sapropels. Carbonate contents and normalized titanium and vanadium contents in bulk sediments are suggested to reflect mostly changes in the Nile discharge. It is found that variation of the African monsoon intensities can strongly affect changes in the Nile suspended discharge via both runoff and drainage precipitation. Elevated Nile discharges are generally observed during African monsoon maxima (deposition of sapropelic layers). Our results suggest that oscillations in the Nile suspended discharges are more the result of river transport capability than that of erosion potential in source areas. It is also observed that cyclicity of the variation of the Nile suspended discharge is generally paced by a 78-kyr cycle, the physical meaning of which remains hitherto unclear. It is probably a bundling of 2–3 obliquity cycles, as a result of nonlinear responses of Atlantic SST to orbital forcing.