Nd isotopic compositions of eastern Mediterranean sediments: tracers of the Nile influence during sapropel S1 formation?

Nd isotopic composition of Eastern Mediterranean sediments are used to investigate the influence of Nile flux variations during the formation of the youngest sapropel S1. Four box cores located on an east–west transect (around 35°N) at increasing distance from the Nile source were studied. Nd(0) signatures were measured in the lithogenic (alumino-silicate residue) fraction and the authigenic/biogenic (carbonates, organic matter and Fe, Mn oxyhydroxides) fraction of the sediment after 1 M HCl leaching. The Nd(0) profiles of the residual fraction show more radiogenic values in the sapropel layers than in the adjacent sediments. These profiles follow the Gaussian-shaped Ba/Al profile shape which defines S1. The Nd(0) variations can be related to changes in the proportions of two continental sources of particulate material: Nile particles and Saharan aerosols. A simple mixing model reveals that the contribution of Nile particulates to the sediments increases relative to Saharan aerosols during sapropel formation, indicating a higher Nile discharge and a decrease in eolian input at that time. For the cores located closest to the Nile fan, the Nd(0) signatures of the leachable fraction are always higher in the sapropel layer (−5 to −4) than in the adjacent sediments (~−6). The sapropel layer Nd(0) values increase with decreasing distance to the Nile towards values close to the Nile particulates. All the results obtained on the leachable fraction point towards a massive input of Nile freshwater during S1 deposition.